ABOUT GLOBAL POSITIONING SYSTEM.

GPS receivers are now integrated in many mobile phones.
The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing. It is the only fully functional GNSS in the world, can be used freely, and is often used by civilians for navigation purposes. It uses a constellation of between 24 and 32 medium Earth orbit satellites that transmit precise radiowave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS. Although NAVSTAR is not an acronym,[1] a few backronyms have been created for it.[2]
Since it became fully operational in 1993, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, and hobbies such as geocaching. Also, the precise time reference is used in many applications including the scientific study of earthquakes. GPS is also a required key synchronization resource of cellular networks, such as the Qualcomm CDMA air interface used by many wireless carriers in a multitude of countries.[citation needed]
Contents
• 1 History 
o 1.1 Timeline
• 2 Basic concept of GPS 
o 2.1 Position calculation introduction
o 2.2 Correcting a GPS receiver's clock
• 3 System detail 
o 3.1 System segmentation 
 3.1.1 Space segment
 3.1.2 Control segment
 3.1.3 User segment
o 3.2 Navigation signals
o 3.3 Satellite frequencies
o 3.4 C/A code 
 3.4.1 Demodulation and decoding
 3.4.2 Carrier phase tracking (surveying)
 3.4.3 Position calculation advanced
o 3.5 P(Y) code
o 3.6 Error sources and analysis 
 3.6.1 Signal arrival time measurement
 3.6.2 Atmospheric effects
 3.6.3 Multipath effects
 3.6.4 Ephemeris and clock errors
 3.6.5 Geometric dilution of precision computation (DOP) 
 3.6.5.1 Derivation of DOP equations
 3.6.6 Selective availability
 3.6.7 Relativity
 3.6.8 Sagnac distortion
• 4 Possible sources of interference 
o 4.1 Natural sources
o 4.2 Artificial sources
• 5 Accuracy enhancement 
o 5.1 Augmentation
o 5.2 Precise monitoring
o 5.3 Timekeeping
o 5.4 Modernization
• 6 Applications 
o 6.1 Military
o 6.2 Civilian
• 7 Awards
• 8 Other systems
• 9 Multidimensional Newton-Raphson for GPS
• 10 See also
• 11 References
• 12 External links 
o 12.1 Government links
o 12.2 Introductory / tutorial links
o 12.3 Technical, historical, and ancillary topics links

[edit] History
The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system.
The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
After Korean Air Lines Flight 007 was shot down in 1983 after straying into the USSR's prohibited airspace,[3] President Ronald Reagan issued a directive making GPS freely available for civilian use as a common good.[4] The satellites were launched between 1989 and 1993.
Initially the highest quality signal was reserved for military use, while the signal available for civilian use was intentionally degraded ("Selective Availability", SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.
Of crucial importance for the function of GPS is the placement of atomic clocks in the satellites, first proposed by Friedwardt Winterberg in 1955.[5] Only then can the required position accuracy be reached.
[edit] Timeline
• In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.
Satellite numbers[6][7][8]

Block Launch Period Satellites launched Currently in service
I 1978–1985 10+11 0
II 1985–1990 9 0
IIA 1990–1997 19 13
IIR 1997–2004 12+11 12
IIR-M 2005–2009 7+12 6
IIF 2009–2011 0+102 0
IIIA 2014–? 0+123 0
IIIB 0+83 0
IIIC 0+163 0
Total 59+21+122+363 31
1Failed
2In preparation
3Planned.
(Last update: 16 December 2008)
• In 1978 the first experimental Block-I GPS satellite was launched.
• In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace due to navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS would be made available for civilian uses once it was completed.[9][10]
• By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
• On February 14, 1989, the first modern Block-II satellite was launched.
• In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
• By December 1993 the GPS achieved initial operational capability.[11]
• By January 17, 1994 a complete constellation of 24 satellites was in orbit.
• Full Operational Capability was declared by NAVSTAR in April 1995.
• In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[12] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
• In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.
• On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.
• In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.
• In 2004, U.S. President George W. Bush updated the national policy and replaced the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.
• November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile phones.[13]
• In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.
• On September 14, 2007, the aging mainframe-based Ground Segment Control System was transitioned to the new Architecture Evolution Plan.[14]
• The most recent launch was on March 15, 2008.[15] The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.[16]
[edit] Basic concept of GPS
A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac). The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.
It might seem three satellites are enough to solve for position, since space has three dimensions. However, even a very small clock error multiplied by the very large speed of light[17]—the speed at which satellite signals propagate—results in a large positional error. Therefore receivers use four or more satellites to solve for x, y, z, and t, which is used to correct the receiver's clock. While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer, traffic signal timing, and synchronization of cell phone base stations.
Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible (see [18], Chapters 7 and 8 of [19], and [20]).
[edit] Position calculation introduction
To provide an introductory description of how a GPS receiver works, measurement errors will be ignored in this section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . Knowing the distance from GPS receiver to a satellite and the position of a satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres, if they intersect in more than one point, intersect in a circle. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below.
 
 
Two sphere surfaces intersecting in a circle
The article, trilateration, shows mathematically that the surfaces of two spheres, intersecting in more than one point, intersect in a circle.
A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect at zero points, one point, or in the very special case in which the centers of the three spheres are colinear (i.e. all three on the same straight line) the sphere surface could intersect the entire circumference of the circle. Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, shows this intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically. The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. (The two intersections are symmetrical with respect to the plane containing the three satellites. If the three satellites are not in the same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near-Earth intersection will be the correct position for the case of a near-Earth vehicle. The intersection which is farthest from Earth may be the correct position for space vehicles.)
[edit] Correcting a GPS receiver's clock
The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.
It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . Note that is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of
(correct time) - (time indicated by the receiver's on-board clock), and the GPS receiver clock can be advanced if is positive or delayed if is negative.
[edit] System detail
 
 
Unlaunched GPS satellite on display at the San Diego Aerospace museum
[edit] System segmentation
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).[21]
[edit] Space segment
See also: GPS satellite and List of GPS satellite launches
 
 
A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.
The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,[22] but this was modified to six planes with four satellites each.[23] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.[24] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[25] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[26]
Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within line of sight (12,900 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day.[27] The ground track of each satellite therefore repeats each (sidereal) day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008,[28] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.[29]
[edit] Control segment
The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).[30] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.[31]
Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.
[edit] User segment
 
 
GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).
The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.[32]
 
 
A typical OEM GPS receiver module measuring 15×17 mm.
GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.
 
 
A typical GPS receiver with integrated antenna.
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol, or the newer and less widely used NMEA 2000.[33] Although these protocols are officially defined by the NMEA,[34] references to these protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.
[edit] Navigation signals
 
 
GPS broadcast signal
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-week, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.
Transmission of each 30 second frame begins precisely on the minute and half minute as indicated by the satellite's atomic clock according to Satellite message format. Each frame contains 5 subframes of length 6 seconds and with 300 bits. Each subframe contains 10 words of 30 bits with length 0.6 seconds each.
Words 1 and 2 of every subframe have the same type of data. The first word is the telemetry word which indicates the beginning of a subframe and is used by the receiver to synch with the navigation message. The second word is the HOW or handover word and it contains timing information which enables the receiver to identify the subframe and provides the time the next subframe was sent.
Words 3 through 10 of subframe 1 contain data describing the satellite clock and its relationship to GPS time. Words 3 through 10 of subframes 2 and 3, contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate.
The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC). Words 3 through 10 of subframes 4 and 5 contain a new part of the almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite.[35] The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS (called GPS time) to the international time standard of UTC. Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. An important thing to note about navigation data is that each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites.
All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[36] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.
[edit] Satellite frequencies
• L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
• L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
• L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
• L4 (1379.913 MHz): Being studied for additional ionospheric correction.
• L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009.[37]
[edit] C/A code
[edit] Demodulation and decoding
 
 
Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code.
Since all of the satellite signals are modulated onto the same L1 carrier frequency, there is a need to separate the signals after demodulation. This is done by assigning each satellite a unique pseudorandom sequence known as a Gold code, and the signals are decoded, after demodulation, using modulo 2 addition of the Gold codes corresponding to satellites n1 through nk, where k is the number of channels in the GPS receiver and n1 through nk are the pseudorandom numbers associated with the satellites. The results of these modulo 2 additions are the 50 bit/s navigation messages from satellites n1 through nk. The Gold codes used in GPS are a sequence of 1023 bits with a period of one millisecond. These Gold codes are highly mutually orthogonal, so that it is unlikely that one satellite signal will be misinterpreted as another. As well, the Gold codes have good auto-correlation properties.[38]
There are 1025 different Gold codes of length 1023 bits, but only 32 are used. These Gold codes are quite often referred to as pseudo random noise since they contain no data and are said to look like random sequences[39]. However, this may be misleading since they are actually deterministic sequences.
If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern.
The receiver uses the C/A Gold code with the same PRN number as the satellite to compute an offset, O, that generates the best correlation. The offset, O, is computed in a trial and error manner. The 1023 bits of the satellite PRN signal are compared with the receiver PRN signal. If correlation is not achieved, the 1023 bits of the receiver's internally generated PRN code are shifted by one bit relative to the satellite's PRN code and the signals are again compared. This process is repeated until correlation is achieved or all 1023 possible cases have been tried (see "How a GPS Receiver Gets a Lock"). If all 1023 cases have been tried without achieving correlation, the frequency oscillator is offset to the next value and the process is repeated.
Since the carrier frequency received can vary due to Doppler shift, the points where received PRN sequences begin may not differ from O by an exact integral number of milliseconds. Because of this, carrier frequency tracking along with PRN code tracking are used to determine when the received satellite's PRN code begins (see "How a GPS Receiver Gets a Lock"). Unlike the earlier computation of offset in which trials of all 1023 offsets could potentially be required, the tracking to maintain lock usually requires shifting of half a pulse width or less. To perform this tracking, the receiver observes two quantities, phase error and received frequency offset. The correlation of the received PRN code with respect to the receiver generated PRN code is computed to determine if the bits of the two signals are misaligned. Comparisons with correlation computed with receiver generated PRN code shifted half a pulse width early and half a pulse width late (see section 1.4.2.4 of [19]) are used to estimate adjustment required. The amount of adjustment required for maximum correlation is used in estimating phase error. Received frequency offset from the frequency generated by the receiver provides an estimate of phase rate error. The command for the frequency generator and any further PRN code shifting required are computed as a function of the phase error and the phase rate error in accordance with the control law used. The Doppler velocity is computed as a function of the frequency offset from the carrier nominal frequency. The Doppler velocity is the velocity component along the line of sight of the receiver relative to the satellite.
As the receiver continues to read successive PRN sequences, it will encounter a sudden change in the phase of the 1023 bit received PRN signal. This indicates the beginning of a data bit of the navigation message (see section 1.4.2.5 of [19]). This enables the receiver to begin reading the 20 millisecond bits of the navigation message. Each subframe of the navigation frame begins with a Telemetry Word which enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. Also each subframe of the navigation frame is identified by bits in the handover word (HOW) thereby enabling the receiver to determine which subframe (see section 1.4.2.6 of [19] and section 2.5.4 of "Essentials of Satellite Navigation Compendium"). There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data before computing the intersections of sphere surfaces.
After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.
Then the orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.[40]
This process is repeated for each satellite to which the receiver is listening.
[edit] Carrier phase tracking (surveying)
Utilizing the navigation message to measure pseudorange has been discussed. Another method that is used in GPS surveying applications is carrier phase tracking. The period of the carrier frequency times the speed of light gives the wave length, which is about 0.19 meters for the L1 carrier. With a 1% of wave length accuracy in detecting the leading edge, this component of pseudorange error might be as low as 2 millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P code.
However, this 2 millimeter accuracy requires measuring the total phase, that is the total number of wave lengths plus the fractional wavelength. This requires specially equipped receivers. This method has many applications in the field of surveying.
We now describe a method which could potentially be used to estimate the position of receiver 2 given the position of receiver 1 using triple differencing followed by numerical root finding, and a mathematical technique called least squares. A detailed discussion of the errors is omitted in order to avoid detracting from the description of the methodology. In this description differences are taken in the order of differencing between satellites, differencing between receivers, and differencing between epochs. This should not be construed to mean that this is the only order which can be used. Indeed other orders of taking differences are equally valid.
The satellite carrier total phase can be measured with ambiguity as to the number of cycles as described in CARRIER PHASE MEASUREMENT and CARRIER BEAT PHASE. Let denote the phase of the carrier of satellite j measured by receiver i at time . This notation has been chosen so as to make it clear what the subscripts i, j, and k mean. In view of the fact that the receiver, satellite, and time come in alphabetical order as arguments of and to strike a balance between readability and conciseness, let so as to have a concise abbreviation. Also we define three functions, : which perform differences between receivers, satellites, and time points respectively. Each of these functions has a linear combination of variables with three subscripts as its argument. These three functions are defined below. If is a function of the three integer arguments, i, j, and k then it is a valid argument for the functions, : , with the values defined as
 ,
 , and
 .
Also if are valid arguments for the three functions and a and b are constants then is a valid argument with values defined as
 ,
 , and
 ,
Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch as shown in BETWEEN-SATELLITE DIFFERENCING. This difference is designated as  
Double differencing can be performed by taking the differeces of the between satellite difference observed by receiver 1 with that observed by receiver 2. The satellite clock errors will be approximately eliminated by this between receiver differencing. This double difference is designated as .
Triple differencing can be performed by taking the difference of double differencing performed at time with that performed at time . This will eliminate the ambiguity associated with the integral number of wave lengths in carrier phase provided this ambiguity does not change with time. Thus the triple difference result has eliminated all or practically all clock bias errors and the integer ambiguity. Also errors associated with atmospheric delay and satellite ephemeris have been significantly reduced. This triple difference is designated as .
Triple difference results can be used to estimate unknown variables. For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for the three components of position of receiver 2. This may require the use of a numerical procedure such as one of those found in the chapter on root finding and nonlinear sets of equations in Numerical Recipes [41]. Also see Preview of Root Finding. To use such a numerical method, an initial approximation of the position of receiver 2 is required. This initial value could probably be providd by a position approximation based on the navigation message and the intersection of sphere surfaces. Although multidimensional numerical root finding can have problems, this disadvantage may be overcome with this good initial estimate. This procedure using three time pairs and a fairly good initial value followed by iteration will result in one observed triple difference result for receiver 2 position. Greater accuracy may be obtained by processing triple difference results for additional sets of three independent time pairs. This will result in an over determined system with multiple solutons. To get estimates for an over determined system, least squares can be used. The least squares procedure determines the position of receiver 2 which best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares.
[edit] Position calculation advanced
Before providing a more mathematical description of position calculation, the introductory material on this topics is reviewed. To describe the basic concept of how a GPS receiver works, the errors are at first ignored. Using messages received from four satellites, the GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as where the subscript i denotes which satellite and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . Knowing the distance from GPS receiver to a satellite and the position of a satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres if they intersect in more than one point intersect in a circle. We are here excluding the unrealistic case for GPS purposes of two coincident spheres. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below depicting this which hopefully will aid the reader in visualizing this intersection. Two points at which the surfaces of the spheres intersect are clearly marked on the figure. The distance between these two points is the diameter of the circle of intersection. If you are not convinced of this, consider how a side view of the intersecting spheres would look. This view would look exactly the same as the figure because of the symmetry of the spheres. And in fact a view from any horizontal direction would look exactly the same. This should make it clear to the reader that the surfaces of the two spheres actually do intersect in a circle.
 
 
Two sphere surfaces intersecting in a circle
The article, trilateration, shows mathematically how the equation for a circle is determined. A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect in 0 or 1 point. We are here excluding the unrealistic case for GPS purposes of three colinear (lying on same straight line) sphere centers. Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, is shown below to aid in visualizing this intersection. Again trilateration clearly shows this mathematically. The correct position of the GPS receiver is the one that is closest to the fourth sphere. This paragraph has described the basic concept of GPS while ignoring errors. The next problem is how to process the messages when errors are present.
 
 
Surface of a sphere intersecting a circle (i.e., the edge of a disk) at two points
Let denote the clock error or bias, the amount by which the receiver's clock is slow. The GPS receiver has four unknowns, the three components of GPS receiver position and the clock bias . The equation of the sphere surfaces are given by
 , Another useful form of these equations is in terms of the pseudoranges, which are simply the ranges approximated based on GPS receiver clock's indicated (i.e. uncorrected) time so that . Then the equations becomes:
 . Two of the most important methods of computing GPS receiver position and clock bias are (1) trilateration followed by one dimensional numerical root finding and (2) multidimensional Newton-Raphson calculations. These two methods along with their advantages are discussed.
• The receiver can solve by trilateration followed by one dimensional numerical root finding.[41] This method involves using trilateration to determine the intersection of the surfaces of three spheres. It is clearly shown in trilateration that the surfaces of three spheres intersect in 0, 1, or 2 points. In the usual case of two intersections, the solution which is nearest the surface of the sphere corresponding to the fourth satellite is chosen. The surface of the earth can also sometimes be used instead, especially in the case of civilian GPS receivers since it is illegal in the United States to track vehicles of more than 60,000 feet (18,000 m) in altitude. The bias, is then computed as a function of the distance from the solution to the surface of the sphere corresponding to the fourth satellite. To determine what function to use for computing see the chapter on root finding in [41] or the preview. Using an updated received time based on this bias, new spheres are computed and the process is repeated. This repetition is continued until the distance from the valid trilateration solution is sufficiently close to the surface of the sphere corresponding to the fourth satellite. One advantage of this method is that it involves one dimensional as opposed to multidimensional numerical root finding.
• The receiver can utilize a multidimensional root finding method such as the Newton-Raphson method.[41] Linearize around an approximate solution, say from iteration k, then solve four linear equations derived from the quadratic equations above to obtain . The radii are large and so the sphere surfaces are close to flat.[42][43] This near flatness may cause the iterative procedure to converge rapidly in the case where is near the correct value and the primary change is in the values of , since in this case the problem is merely to find the intersection of nearly flat surfaces and thus close to a linear problem. However when is changing significantly, this near flatness does not appear to be advantageous in producing rapid convergence, since in this case these near flat surfaces will be moving as the spheres expand and contract. This possible fast convergence is an advantage of this method. Also it has been claimed that this method is the "typical" method used by GPS receivers.[44][45][46] A disadvantage of this multidimensional root finding method as compared to single dimensional root findiing is that according to,[41] "There are no good general methods for solving systems of more than one nonlinear equations." For a more detailed description of the mathematics see Multidimensional Newton Raphson.
• Other methods include:
1. Solving for the intersection of the expanding signals form light cones in 4-space cones
2. Solving for the intersection of hyperboloids determined by the time difference of signals received from satellites utilizing multilateration,
3. Solving the equations in accordance with .[44][45][47]
• When more than four satellites are available, a decision must be made on whether to use the four best or more than four taking into considerations such factors as number of channels, processing capability, and geometric dilution of precision. Using more than four results in an over-determined system of equations with no unique solution, which must be solved by least-squares or a similar technique. If all visible satellites are used, the results are always at least as good as using the four best, and usually better. Also the errors in results can be estimated through the residuals. [48] With each combination of four or more satellites, a geometric dilution of precision (GDOP) vector can be calculated, based on the relative sky positions of the satellites used.[49][48] As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more estimates to the location and clock offset. The receiver then determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system such as latitude and longitude, using the WGS 84 geodetic datum or a local system specific to a country.[50]
• Finally, results from other positioning systems such as GLONASS or the upcoming Galileo can be used in the fit, or used to double check the result. (By design, these systems use the same bands, so much of the receiver circuitry can be shared, though the decoding is different.)
[edit] P(Y) code
Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.[citation needed] In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.
[edit] Error sources and analysis
 
 
Sources of User Equivalent Range Errors (UERE)
Source Effect
Signal Arrival C/A ± 3 m
Signal Arrival P(Y) ± 0.3 m
Ionospheric effects ± 5 m
Ephemeris errors ± 2.5 m
Satellite clock errors ± 2 m
Multipath distortion ± 1 m
Tropospheric effects ± 0.5 m
 C/A ± 6,7 m
 P(Y) ± 6,0 m
User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e. RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error. Electronics errors are one of several accuracy-degrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally.(see summary table near end of "Sources of Errors in GPS")
 
 
Error Diagram Showing Relation of Indicated Receiver Position, Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudorange Errors, PDOP, and Numerical Errors
The term user equivalent range error (UERE) refers to the standard deviation of a component of the error in the distance from receiver to a satellite. The standard deviation of the error in receiver position, , is computed by multiplying PDOP (Position Dilution Of Precision) by , the standard deviation of the user equivalent range errors. is computed by taking the square root of the sum of the squares of the individual component standard deviations.
PDOP is computed as a function of receiver and satellite positions. Consider the unit vectors pointing from the receiver to the satellites. Connecting the tails of these unit vectors forms a tetrahedron. PDOP is sometimes approximated as being inversely proportional to the tetrahedron volume[43]. Also a more detailed description of how to calculate PDOP is given in the section, Geometric dilution of precision computation (DOP).
 is given by = 6.7 meters for the C/A code. The standard deviation of the error in estimated receiver position, , is given by for the C/A code. The error diagram to the right shows the inter relationship of indicated receiver position, true receiver position, and the intersection of the four sphere surfaces.
[edit] Signal arrival time measurement
The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about one percent of a bit pulse width, , or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate at the speed of light, this represents an error of about 3 meters.
This component of position accuracy can be improved by a factor of 10 using the higher-chiprate P(Y) signal. Assuming the same one percent of bit pulse width accuracy, the high-frequency P(Y) signal results in an accuracy of or about 30 centimeters.
[edit] Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer (see airmass). Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
Because ionospheric delay affects the speed of microwave signals differently depending on their frequency — a characteristic known as dispersion - delays measured on two or more frequency bands can be used to measure dispersion, and this measurement can then be used to estimate the delay at each frequency.[51] Some military and expensive survey-grade civilian receivers measure the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. (see Atmospheric Effects in "Sources of Errors in GPS")
The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as WAAS (available in North America and Hawaii), EGNOS (Europe and Asia) or MSAS (Japan), which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required.
Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.[citation needed]
Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.[citation needed]
[edit] Multipath effects
GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas (e.g. a choke ring antenna) may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.
[edit] Ephemeris and clock errors
While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. Data up to four hours old is considered valid for calculating positions, but may not indicate the satellite's actual position. If a fast Time To First Fix (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices.[52] See also Assisted GPS.
The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.
These problems tend to be very small, but may add up to a few meters (10s of feet) of inaccuracy.[53]
[edit] Geometric dilution of precision computation (DOP)
As a first step in computing DOP, consider the unit vector from the receiver to satellite i with components,
 where the distance from receiver to the satellite, , is given by
 and where denote the position of the receiver and denote the position of satellite i. Formulate the matrix A as
 
The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The elements in the fourth column are c where c denotes the speed of light. Formulate the matrix, Q, as
 
This computation is in accordance with "Section 1.4.2 of PRINCIPLES OF SATELLITE POSITIONING" where the weighting matrix, P, has been set to the identity matrix.
The elements of the Q matrix are designated as
 
The Greek letter is used quite often where we have used d. However the elements of the Q matrix do not represent variances and covariances as they are defined in probability and statistics. Instead they are strictly geometric terms. Therefore d as in dilution of precision is used. PDOP, TDOP and GDOP are given by
 ,
 , and
 in agreement with "Section 1.4.9 of PRINCIPLES OF SATELLITE POSITIONING".
The horizontal dilution of precision, , and the vertical dilution of precision, , are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical, x, y, and z should denote positions in either a North, East, Down coordinate system or a South, East, Up coordinate system.
[edit] Derivation of DOP equations
The equations for computing the geometric dilution of precision terms have been described in the previous section. This section describes the derivation of these equations. The method used here is similar to that used in "Global Positioning System (preview) by Parkinson and Spiker"
Consider the position error vector, e, defined as the vector from the intersection of the four sphere surfaces corresponding to the pseudoranges to the true position of the receiver. where bold denotes a vector and denote unit vectors along the x, y, and z axes respectively. Let denote the time error, the true time minus the receiver indicated time. Assume that the mean value of the three components of e and are zero.
 
where are the errors in pseudoranges 1 through 4 respectively. This equation comes from linearizing the equation relating pseudoranges to receiver position, satellite positions, and receiver clock errors as shown in [2] . Multiplying both sides by there results
 .
Transposing both sides
 .
Post multiplying the matrices on both sides of equation (2) by the corresponding matrices in equation (3), there results
 .
Taking the expected value of both sides and taking the non-random matrices outside the expectation operator, E, there results
 .
Assuming the pseudorange errors are uncorrelated and have the same variance, the covariance matrix on the right side can be expressed as a scalar times the identity matrix. Thus
 
since  
Note: since  
Substituting for there follows
 
From equation (7), it follows that the variances of indicated receiver position and time are
 and
 .
The remaining position and time error variance terms follow in a straightforward manner.
[edit] Selective availability
GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the US military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.
Before it was turned off in 2000, typical SA errors were 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.
Another restriction on GPS, antispoofing, remains on. This encrypts the P-code so that it cannot be mimicked by an enemy transmitter sending false information. Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code is so much better than originally expected (especially with DGPS) that the antispoof policy has relatively little effect on most civilian users. Turning off antispoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking the motion of a tectonic plate.
DGPS services are widely available from both commercial and government sources. The latter include WAAS and the US Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.
During the 1990-91 Gulf War, the shortage of military GPS units caused many troops and their families to buy readily available civilian units. This significantly impeded the US military's own battlefield use of GPS, so the military made the decision to turn off SA for the duration of the war.
In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero"[54] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the US military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.[54]
Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[55] have stated that it is not intended to be reintroduced.
One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10-13 (one in five trillion). This represented a significant improvement over the raw accuracy of the clocks.[citation needed]
On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[56] eventually making the policy permanent.[57]
[edit] Relativity
 
 
Satellite clocks are slowed by their orbital speed but sped up by their distance out of the Earth's gravitational well.
According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 microseconds (μs) per day, because they have a higher gravitational potential than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day. When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 1010.[58] To account for this, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.[59] Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was first proposed by Friedwardt Winterberg in 1955.[60]
[edit] Sagnac distortion
GPS observation processing must also compensate for the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. A Lorentz transformation is thus applied to convert from the inertial system to the ECEF system. The resulting signal run time correction has opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.[61]
[edit] Possible sources of interference
[edit] Natural sources
Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.
Space weather degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band [62] or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation [63]. Both forms of degradation follow the 11 year solar cycle and are a maximum at sunspot maximum although they can occur at anytime. Solar radio bursts are associated with solar flares and their impact can affect reception over the half of the Earth facing the sun. Scintillation occurs most frequently at tropical latitudes where it is a night time phenomenon. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to scintillation [64]. In addition to producing scintillation, magnetic storms can produce strong ionospheric gradients that degrade the accuracy of SBAS systems [65].
[edit] Artificial sources
In automotive GPS receivers, metallic features in windshields,[66] such as defrosters, or car window tinting films[67] can act as a Faraday cage, degrading reception just inside the car.
Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well documented case, the entire harbor of Moss Landing, California was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.[68][69] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in the online magazine Phrack.[70]
The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy six GPS jammers during the Iraq War, including one that was destroyed ironically with a GPS-guided bomb.[71] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007.[72]
Some countries allow the use of GPS repeaters to allow for the reception of GPS signals indoors and in obscured locations, however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that may receive data from both GPS satellites and the repeater.
Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference. The first is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction".[73] Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock.[74]
[edit] Accuracy enhancement
[edit] Augmentation
Main article: GNSS Augmentation
Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.
Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.
[edit] Precise monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.
After SA, which has been turned off, the largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.
Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.
A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period one one-thousandth of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.
Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).
[edit] Timekeeping 
While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (TAI - GPS = 19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.
The GPS navigation message includes the difference between GPS time and UTC, which as of 2009 is 15 seconds due to the leap second added to UTC December 31 2008. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, given the current rate of change of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately year 2300.
As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21 1999 (00:00:19 TAI on August 22 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field, which only repeats every 8,192 weeks (157 years), thus lasting until year 2137 (157 years after GPS week zero).
[edit] Modernization
Main article: GPS modernization
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Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,[75] the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS. Announcements from the U.S. Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.
The project aims to improve the accuracy and availability for all users and involves a new control segment (called GPS OCX), new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008.[76] A goal of 2013 has been established for the entire program, with incentives offered to the contractors if they can complete it by 2011 (See GPS signals).
[edit] Applications
The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.
[edit] Military
The military applications of GPS span many purposes:
• Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies. The GPS-receivers that commanders and soldiers use are respectively called the Commanders Digital Assistant and the Soldier Digital Assistant.[77][78][79][80]
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile.[citation needed] These weapon systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately. Military aircraft, particularly those used in air-to-ground roles use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be looked up in Google Earth[citation needed]).
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions. Artillery projectiles with embedded GPS receivers able to withstand accelerations of 12,000G have been developed for use in 155 mm howitzers.[81]
• Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.
• Reconnaissance and Map Creation: The military use GPS extensively to aid mapping and reconnaissance.
• The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor (W-sensor) which form a major portion of the United States Nuclear Detonation Detection System.[82][83]
[edit] Civilian
See also: GNSS applications and GPS navigation device
 
 
This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.
Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.
The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a GPS timing receiver to synchronize its spreading codes with other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning of mobiles for emergency calls and other applications. Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.
The US Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 km (60,000 ft) altitude and 515 m/s (1,000 knots) [84] are classified as munitions (weapons) for which US State Department export licenses are required. These parameters are clearly chosen to prevent use of a receiver in a ballistic missile. It would not prevent use in a cruise missile since their altitudes and speeds are similar to those of ordinary aircraft.
This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A code and cannot correct for SA, etc.
Disabling operation above these limits exempts the receiver from classification as a munition. Different vendors have interpreted these limitations differently. The rule specifies operation above 18 km and 515 m/s, but some receivers stop operating at 18 km even when stationary. This has caused problems with some amateur radio balloon launches as they regularly reach 100,000 feet (30km).
GPS tours are also an example of civilian use. The GPS is used to determine which content to display. For instance, when approaching a monument it would tell you about the monument.
GPS functionality has now started to move into mobile phones en masse. The first handsets with integrated GPS were launched already in the late 1990’s, and were available for broader consumer availability on networks such as those run by Nextel, Sprint and Verizon in 2002 in response to US FCC mandates for handset positioning in emergency calls. Capabilities for access by third party software developers to these features were slower in coming, with Nextel opening up those APIs upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.

[edit] Awards
Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:
• Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
• Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force.
One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13, 2006 at the White House.[85]
On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."
[edit] Other systems
Main article: Global Navigation Satellite System
Other satellite navigation systems in use or various states of development include:
• Beidou – China's regional system that China has proposed to expand into a global system named COMPASS.
• Galileo – a proposed global system being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia, South Korea, and Ukraine, planned to be operational by 2013.
• GLONASS – Russia's global system which is being restored to full availability in partnership with India.
• Indian Regional Navigational Satellite System (IRNSS) – India's proposed regional system.
• Template:Country data PN QZSS – philippines proposed regional system, adding better coverage to the Japanese Islands.
[edit] Multidimensional Newton-Raphson for GPS
This section provides a more detailed discussion of the equations used in the second method described in Position calculation advanced. The linearized equations are developed using the appropriate partial derivatives and the algorithm is described. In [43] the same method is discussed but the equations are not shown. Let and denote the true coordinates of GPS receiver position at time, . Let denote the unknown clock error or bias, the amount by which the receiver's clock is slow. Let the coordinates of each satellite, and the time the message was sent, be , let the GPS clock's indicated received time be and c be the speed of light. The pseudorange is computed as . Assume the message travels at the speed of light, then the pseudorange satisfies the equation,
  
When an approximate solution, rather than the exact solution, is used in equation 1, there is a residual, . Transforming to the right hand side of the equation there results,
 
 
A solution will have been found when is zero or sufficiently close to zero for .
In order to linearize equation 2, the partial derivatives are computed as
 
 
where
 .
Linearizing the right hand side of equation 2 about the approximate solution, there results
 
 
where is the residual due to linearization which is in addition to the residual, , due to an approximate solution.
In order to drive closer to zero choose the values such that
 
 
That is choose the values
 
such that the residual in equation 2 changes by approximately .
Let
 
 
Substituting and transposing to the left hand side of the equation, there results
 
 
Equations 6 provide a set of four linear equations in four unknowns, the delta terms. They are in a form for solution. Using the values of and determined by this linear equation solution,
 
is evaluated using
 
 
Then set in equations 2 through 6, plug the terms
 
from equations 7 into equations 2, set in equations 7, and reevaluate the residuals in equations 2. This procedure is repeated until the are sufficiently small in magnitude.

history of united states of america

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The first known inhabitants of modern-day United States territory are believed to have arrived over a period of several thousand years beginning sometime prior to 15,000 - 50,000 years ago by crossing Beringia into Alaska.[1][2] Solid evidence of these cultures settling in what would become the US is dated to around 14,000 years ago.[3]

Research has revealed much about the early Native American settlers of North America as indicated by Cyrus Thomas.[4] Columbus' men were the first documented Old Worlders to land in the territory of what is now the United States when they arrived in Puerto Rico during their second voyage in 1493.[5] Juan Ponce de León, who arrived in Florida in 1513,[6] is credited as being the first European to land in what is now the continental United States, although some evidence suggests that John Cabot might have reached what is presently New England in 1498.[7][8]

In its beginnings, the United States of America consisted only of the Thirteen Colonies, which consisted of states occupying the same lands as when they were British colonies. American colonists fought off the British army in the American Revolutionary War of the 1770s and issued a Declaration of Independence in 1776. Seven years later, the signing of the Treaty of Paris officially recognized independence from Britain.[9] In the nineteenth century, westward expansion of United States territory began, upon the belief of Manifest Destiny, in which the United States would occupy all the North American land east to west, from the Atlantic to the Pacific Oceans. By 1912, with the admission of Arizona to the Union, the U.S. reached that goal. The outlying states of Alaska and Hawaii were both admitted in 1959.

Ratified in 1788, the Constitution serves as the supreme American law in organizing the government; the Supreme Court is responsible for upholding Constitutional law. Many forms of social progress started in the nineteenth century; those advancements have been widely reflected in the Constitution. Slavery was abolished in 1865 by the Thirteenth Amendment to the United States Constitution; the following Fourteenth and Fifteenth Amendments respectively guaranteed citizenship for all persons naturalized within U.S. territory and voting for people of all races. In later years, civil rights were extended to women and black Americans, following effective lobbying from social activists. The Nineteenth Amendment prohibited gender discrimination in voting rights; later, the Civil Rights Act of 1964 outlawed racial segregation in public places.

The Progressive Era marked a time of economic growth for the United States, advancing to the Roaring Twenties. However, the Wall Street Crash of 1929 led to the Great Depression, a time of economic downturn and mass unemployment. Consequently, the U.S. government established the New Deal, a series of reform programs that intended to assist those affected by the Depression. The New Deal had varied success. However, once the U.S. entered World War II in December 1941, the economy quickly recovered, so much that the U.S. became a world superpower by the dawn of the Cold War. During the Cold War, the U.S. and the Soviet Union were the world's two superpowers, but with the end of the Cold War and the collapse of the Soviet Union, United States became the world's only superpower.Contents [hide]
1 Pre-Columbian period
2 Colonial period
3 Formation of the United States of America (1776–1789)
4 Westward expansion (1789–1849)
5 Civil War era (1849–1865)
6 Reconstruction and the rise of industrialization (1865–1890)
7 Progressivism, imperialism, and World War I (1890–1918)
8 Post-World War I and the Great Depression (1918–1940)
9 World War II (1941–1945)
10 Cold War beginnings and the Civil Rights Movement (1945–1964)
11 The Counterculture Revolution and Cold War Détente (1964–1980)
12 The Reagan Revolution and the end of the Cold War (1980–1991)
13 The World Superpower (1991-present)
14 References
15 Notes
16 Further reading
17 External links


[edit]
Pre-Columbian period
Main article: Pre-Columbian

The earliest known inhabitants of what is now the United States are thought to have arrived in Alaska by crossing the Bering land bridge, at least 14,000 – 30,000 years ago.[10] Some of these groups migrated south and east, and over time spread throughout the Americas. These were the ancestors to modern Native Americans in the United States and Alaskan Native peoples, as well as all indigenous peoples of the Americas.

Many indigenous peoples were semi-nomadic tribes of hunter-gatherers; others were sedentary and agricultural civilizations. Many formed new tribes or confederations in response to European colonization. Well-known groups included the Huron, Apache Tribe, Cherokee, Sioux, Delaware, Algonquin, Choctaw, Mohegan, Iroquois (which included the Mohawk nation, Oneida tribe, Seneca nation, Cayuga nation, Onondaga and later the Tuscarora tribe) and Inuit. Though not as technologically advanced as the Mesoamerican civilizations further south, there were extensive pre-Columbian sedentary societies in what is now the US. The Iroquois had a politically advanced and unique social structure that was at the very least inspirational if not directly influential to the later development of the democratic United States government, a departure from the strong monarchies from which the Europeans came.[citation needed]

[edit]
North America's Moundbuilder Culture

A Mississippian priest, with a ceremonial flint mace. Artist Herb Roe, based on a repousse copper plate.

Mound Builder is a general term referring to the American Indians who constructed various styles of earthen mounds for burial, residential and ceremonial purposes. These included Archaic, Woodland period (Adena and Hopewell cultures), and Mississippian period Pre-Columbian cultures dating from roughly 3000 BC to the 16th century AD, and living in the Great Lakes region, the Ohio River region, and the Mississippi River region.

Mound builder cultures can be divided into roughly three eras:
Archaic era

Poverty Point in what is now Louisiana is perhaps the most prominent example of early archaic mound builder construction (c. 2500 – 1000 BC). An even earlier example, Watson Brake, dates to approximately 3400 BC and coincides with the emergence of social complexity worldwide.
Woodland period

The Archaic period was followed by the Woodland period (c. 1000 BC). Some well-understood examples would be the Adena culture of Ohio and nearby states and the subsequent Hopewell culture known from Illinois to Ohio and renowned for their geometric earthworks. The Adena and Hopewell were not, however, the only mound building peoples during this time period. There were contemporaneous mound building cultures throughout the Eastern United States.
Mississippian culture
Main article: Mississippian Culture

Around 900 – 1450 AD the Mississippian culture developed and spread through the Eastern United States, primarily along the river valleys. The location where the Mississippian culture is first clearly developed is located in Illinois, and is referred to today as Cahokia.

[edit]
Colonial period
Main article: Colonial history of the United States

The Mayflower, which transported Pilgrims to the New World

After a period of exploration by people from various European countries, Spanish, Dutch, English, French, Swedish, and Portuguese settlements were established.[11] Christopher Columbus was the first European to set foot on what would one day become U.S. territory when he came to Puerto Rico on November 19, 1493, during his second voyage. In the 15th century, Europeans brought horses, cattle, and hogs to the Americas and, in turn, took back to Europe corn, potatoes, tobacco, beans, and squash.[11]

[edit]
Spanish colonization

Coronado Sets Out to the North (1540) by Frederic Remington, oil on canvas, 1905.
See also: New Spain

Spanish explorers came to what is now the United States beginning with Christopher Columbus' second expedition, which reached Puerto Rico on November 19, 1493.[12] The first confirmed landing in the continental US was by a Spaniard, Juan Ponce de León, who landed in 1513 on a lush shore he christened La Florida.[6]

Within three decades of Ponce de León's landing, the Spanish became the first Europeans to reach the Appalachian Mountains, the Mississippi River, the Grand Canyon[13] and the Great Plains. In 1540, De Soto undertook an extensive exploration of the present US and, in the same year, Francisco Vázquez de Coronado led 2,000 Spaniards and Mexican Indians across the modern Arizona-Mexico border and traveled as far as central Kansas.[14] Other Spanish explorers include Lucas Vásquez de Ayllón, Pánfilo de Narváez, Sebastián Vizcaíno, Juan Rodríguez Cabrillo, Gaspar de Portolà, Pedro Menéndez de Avilés, Álvar Núñez Cabeza de Vaca, Tristán de Luna y Arellano and Juan de Oñate.[15]

The Spanish sent some settlers, creating the first permanent European settlement in the continental United States at St. Augustine, Florida in 1565.[16] Later Spanish settlements included Santa Fe, Albuquerque, San Antonio, Tucson, San Diego, Los Angeles and San Francisco. Most Spanish settlements were along the California coast or the Santa Fe River in New Mexico.

[edit]
Dutch colonization
Main article: New Netherland

Nieuw-Nederland, or New Netherland, was the seventeenth century Dutch colonial province on the eastern coast of North America. The claimed territory were the lands from the Delmarva Peninsula to Buzzards Bay, while the settled areas are now part of New Jersey, New York, Connecticut, Delaware, and Pennsylvania. Its capital, New Amsterdam, was located at the southern tip of the island of Manhattan on the Upper New York Bay.

[edit]
French colonization
See also: New France and Fort Caroline

New France was the area colonized by France in North America during a period extending from the exploration of the Saint Lawrence River, by Jacques Cartier in 1534, to the cession of New France to Spain and Britain in 1763. At its peak in 1712 (before the Treaty of Utrecht), the territory of New France extended from Newfoundland to the Rocky Mountains and from Hudson Bay to the Gulf of Mexico. The territory was divided in five colonies, each with its own administration: Canada, Acadia, Hudson Bay, Newfoundland and Louisiana.

Also during this period, French Huguenots, sailing under Jean Ribault, attempted to found a colony in what became the southeastern coast of the United States. Arriving in 1562, they established the ephemeral colony of Charlesfort on Parris Island in what is now South Carolina. When this failed, most of the colonists followed René Goulaine de Laudonnière and moved south, founding the colony of Fort Caroline at the mouth of the St. Johns River in what is now Jacksonville, Florida on June 22, 1564. Fort Caroline was destroyed in 1565 by the Spanish under Pedro Menéndez de Avilés, who moved in from St. Augustine, founded to the south earlier in the year.

[edit]
British colonization

In 1607, the Virginia Company of London established the Jamestown Settlement on the James River, both named after King James I
Main article: Colonial America

The strip of land along the eastern seacoast was settled primarily by English colonists in the 17th century, along with much smaller numbers of Dutch and Swedes. Colonial America was defined by a severe labor shortage that gave birth to forms of unfree labor such as slavery and indentured servitude,[17] and by a British policy of benign neglect (salutary neglect) that permitted the development of an American spirit distinct from that of its European founders.[18] Over half of all European migrants to Colonial America arrived as indentured servants.[19]

The first successful English colony was established in 1607, on the James River at Jamestown. It languished for decades until a new wave of settlers arrived in the late 17th century and established commercial agriculture based on tobacco. Between the late 1610s and the Revolution, the British shipped an estimated 50,000 convicts to its American colonies.[20] One example of conflict between Native Americans and English settlers was the 1622 Powhatan uprising in Virginia, in which Native Americans had killed hundreds of English settlers. The largest conflict between Native Americans and English settlers in the 17th century was King Philip's War in New England,[21] although the Yamasee War may have been bloodier.[22]

The Plymouth Colony was established in 1620. The area of New England was initially settled primarily by Puritans who established the Massachusetts Bay Colony in 1630.[16] The Middle Colonies, consisting of the present-day states of New York, New Jersey, Pennsylvania, and Delaware, were characterized by a large degree of diversity. The first attempted English settlement south of Virginia was the Province of Carolina, with Georgia Colony the last of the Thirteen Colonies established in 1733.[23] Several colonies were used as penal settlements from the 1620s until the American Revolution.[24] Methodism became the prevalent religion among colonial citizens after the First Great Awakening, a religious revival led by preacher Jonathan Edwards in 1734.[16]

[edit]
Political integration and autonomy

Join, or Die: This 1756 political cartoon by Benjamin Franklin urged the colonies to join together during the French and Indian War.

The French and Indian War (1754–1763) was a watershed event in the political development of the colonies. The influence of the main rivals of the British Crown in the colonies and Canada, the French and North American Indians, was significantly reduced. Moreover, the war effort resulted in greater political integration of the colonies, as symbolized by Benjamin Franklin's call for the colonies to "Join or Die". Following Britain's acquisition of French territory in North America, King George III issued the Royal Proclamation of 1763 with the goal of organizing the new North American empire and stabilizing relations with the native Indians. In ensuing years, strains developed in the relations between the colonists and the Crown. The British Parliament passed the Stamp Act of 1765, imposing a tax on the colonies to help pay for troops stationed in North America following the British victory in the Seven Years' War. The British government felt that the colonies were the primary beneficiaries of this military presence, and should pay at least a portion of the expense. The colonists did not share this view. Rather, with the French and Indian threat diminished, the primary outside influence remained that of Britain. A conflict of economic interests increased with the right of the British Parliament to govern the colonies without representation being called into question.

Nathaniel Currier's 1846 depiction of the Boston Tea Party.[25]

The Boston Tea Party in 1773 was a direct action by colonists in the town of Boston to protest against the taxes levied by the British government. In the following two years, the relations came to a boiling point with the Intolerable Acts being passed by the British Parliament in 1774. The acts sparked outrage and resistance in the Thirteen Colonies, which formed the Continental Association passing on October 20, 1774 the Articles of Association with the aim to boycott trade with Great Britain. The First Continental Congress hoped that by imposing economic sanctions, Great Britain would be pressured to redress the grievances of the colonies, and in particular repeal the Intolerable Acts. The Congress aimed to alter Britain's policies towards the colonies without severing allegiance. Personal gain was also a notable motivation of members of the Continental Association, made up mostly of those who had economic interests that would be served by forbidding imports from Britain. In response, the British government took punitive measures aimed at making an example of Massachusetts, in order to reverse the trend of colonial resistance to parliamentary authority that had begun with the 1765 Stamp Act. Rather than give in, the Colonists boycott became operative on December 1, 1774 resulting in a sharp fall in trade with Great Britain. The British responded with the New England Restraining Act of 1775. The outbreak of the American Revolutionary War effectively superseded the American attempt to boycott British goods.

[edit]
Formation of the United States of America (1776–1789)
Main article: History of the United States (1776–1789)

Washington's crossing of the Delaware River, one of America's first successes in the Revolutionary war

The Thirteen Colonies began a rebellion against British rule in 1775 and proclaimed their independence in 1776. They subsequently constituted the first thirteen states of the United States of America, which became a nation state in 1781 with the ratification of the Articles of Confederation and Perpetual Union. The 1783 Treaty of Paris represented Great Britain's formal acknowledgement of the United States as an independent nation.[9]

The United States defeated the Kingdom of Great Britain with help from France and Spain in the American Revolutionary War. The colonists' victory at Saratoga in 1777 led the French into an open alliance with the United States. In 1781, a combined American and French Army, acting with the support of a French fleet, captured a large British army led by General Charles Cornwallis at Yorktown, Virginia. The surrender of General Cornwallis ended serious British efforts to find a military solution to their American problem.[9] Seymour Martin Lipset points out that "The United States was the first major colony successfully to revolt against colonial rule. In this sense, it was the first 'new nation'."[26]

Trumbull's Declaration of Independence

Side by side with the states' efforts to gain independence through armed resistance, a political union was being developed and agreed upon by them. The first step was to formally declare independence from Great Britain. On July 4, 1776, the Second Continental Congress, still meeting in Philadelphia, declared the independence of "the United States of America" in the Declaration of Independence. Although the states were still independent entities and not yet formally bound in a legal union, July 4 is celebrated as the nation's birthday. The new nation was dedicated to principles of republicanism, which emphasized civic duty and a fear of corruption and hereditary aristocracy.[9]

A Union of the states with a constitutional government, the Congress of the Confederation first became possible with the ratification of the Articles of Confederation and Perpetual Union. The drafting of the Articles began in June 1776 and the approved text was sent to the States on November 15, 1777 for their ratification. While most States passed laws to authorize their representatives in Congress to sign the document by 1778, Maryland refused to do so until a dispute between the states concerning Western land claims had been resolved. After Virginia passed a law ceding its claims on January 2, 1781, Maryland became the 13th and final state to pass an Act to ratify the Articles on February 2, 1781. The formal signing of the Articles by Maryland was completed on March 1, 1781 in Philadelphia[27] and on the following day Samuel Huntington became the first President of the United States in Congress Assembled.[28] However, it became apparent early on that the new constitution was inadequate for the operation of the new government and efforts soon began to improve upon it.[29]

The territory of the newly formed USA was much smaller than it is today. A French map showing Les Etats Unis in 1790

A series of attempts to organize a movement to outline and press reforms culminated in the Congress calling the Philadelphia Convention in 1787. The structure of the national government was profoundly changed on March 4, 1789, when the American people replaced the confederation type government of the Articles with a federation type government of the Constitution. The new government reflected a radical break from the normative governmental structures of the time, favoring representative, elective government with a weak executive, rather than the existing monarchical structures common within the western traditions of the time. The system of republicanism borrowed heavily from the Enlightenment ideas and classical western philosophy: a primacy was placed upon individual liberty and upon constraining the power of government through a system of separation of powers.[29] Additionally, the United States Bill of Rights was ratified on December 15, 1791 to guarantee individual liberties such as freedom of speech and religious practice and consisted of the first ten amendments of the Constitution.[30] John Jay was the first Chief Justice of the Supreme Court, whose membership was established by the Judiciary Act of 1789; the first Supreme Court session was held in New York City on February 1, 1790.[31] In 1803, the Court case Marbury v. Madison made the Court the sole arbiter of constitutionality of federal law.[32]

[edit]
Foundations for American government

Treaty of Penn with Indians by Benjamin West painted in 1827.

Native American societies reminded Europeans of a golden age only known to them in folk history.[33] The idea of freedom and democratic ideals was born in the Americas because "it was only in America" that Europeans from 1500 to 1776 knew of societies that were "truly free."[33]“ Natural freedom is the only object of the policy of the [Native Americans]; with this freedom do nature and climate rule alone amongst them ... [Native Americans] maintain their freedom and find abundant nourishment . . . [and are] people who live without laws, without police, without religion. ”

—- Jean Jacques Rousseau, Jesuit and Savage in New France[33]


The Iroquois nations' political confederacy and democratic government has been credited as one of the influences on the Articles of Confederation and the United States Constitution.[34][35] However, there is heated debate among historians about the importance of their contribution. Although Native American governmental influence is debated, it is a historical fact that several founding fathers had contact with the Iroquois, and prominent figures such as Thomas Jefferson and Benjamin Franklin were involved with their stronger and larger native neighbor-- the Iroquois.“ As powerful, dense [Mound Builder] populations were reduced to weakened, scattered remnants, political readjustments were necessary. New confederacies were formed. One such was to become a pattern called up by Benjamin Franklin when the thirteen colonies struggled to confederate: "If the Iroquois can do it so can we", he said in substance. ”

—- Bob Ferguson, Choctaw Government to 1830[36]


[edit]
Westward expansion (1789–1849)
Main article: History of the United States (1789–1849)

Economic growth in America per capita income

Territorial expansion of the United States, omitting Oregon and other claims.

George Washington—a renowned hero of the American Revolutionary War, commander in chief of the Continental Army, and president of the Constitutional Convention—became the first President of the United States under the new U.S. Constitution. The Whiskey Rebellion in 1794, when settlers in the Pennsylvania counties west of the Allegheny Mountains protested against a federal tax on liquor and distilled drinks, was the first serious test of the federal government.[37] He announced his resignation from the presidency in his farewell address, which was published in the newspaper Independent Chronicle on September 26, 1796. In his address, Washington triumphed the benefits of federal government and importance of ethics and morality while warning against foreign alliances and formation of political parties.[38] His vice president John Adams succeeded him in presidency; Adams was a member of the Federalist Party. However, the Federalists became divided after Adams sent a peace mission to France despite ongoing disputes with that nation. Thomas Jefferson, a Democratic-Republican, defeated Adams for the presidency in the 1800 election.[39]

The Louisiana Purchase, in 1803, removed the French presence from the western border of the United States and provided U.S. settlers with vast potential for expansion west of the Mississippi River.[40] Slave importation from Africa became illegal beginning in 1808, despite a growing plantation system in many southern states such as North Carolina and Georgia.[41] In response to continued British impressment of American sailors into the British Navy, the Congress declared war on Britain in 1812.[42] The United States and Britain came to a draw in the War of 1812 after bitter fighting that lasted until January 8, 1815, during the Battle of New Orleans. The Treaty of Ghent, officially ending the war, essentially resulted in the maintenance of the status quo ante bellum;[43] however, crucially for the U.S., some Native American tribes had to sign treaties with the U.S. government in response to their losses in the war.[42] During the later course of the war, the Federalists held the Hartford Convention in 1814 over concerns that the war would weaken New England. There, they proposed seven constitutional amendments meant to strengthen the region politically, but once the Federalists delivered them to Washington, D.C., the recent American victories in New Orleans and the signing of the Treaty of Ghent undermined the Federalists' arguments and contributed to the downfall of the party.[44]

The Monroe Doctrine, expressed in 1823, proclaimed the United States' opinion that European powers should no longer colonize or interfere in the Americas. This was a defining moment in the foreign policy of the United States.[16] The Monroe Doctrine was adopted in response to American and British fears over Russian and French expansion into areas of the Western Hemisphere. It was not until the Presidential Administration of Teddy Roosevelt that the Monroe Doctrine became a central tenet of American foreign policy. The Monroe Doctrine was then invoked in the Spanish-American War as well as later in the proxy wars between the United States and Soviet Union in Central America and has also essentially given developing nations in the Americas support from the United States and warned the powers in Europe to steer clear of far western affairs.[45]

In 1830, Congress passed the Indian Removal Act, which authorized the president to negotiate treaties that exchanged Indian tribal lands in the eastern states for lands west of the Mississippi River. This established Andrew Jackson, a military hero and President, as a cunning tyrant in regards to native populations. The act resulted most notably in the forced migration of several native tribes to the West, with several thousand Indians dying en route, and the Creeks' violent opposition and eventual defeat. The Indian Removal Act also directly caused the ceding of Spanish Florida and subsequently led to the many Seminole Wars.[46]

Pioneers Crossing the Plains of Nebraska.

In its mission to end slavery, the abolitionist movement also gained a larger following of participants from both black and white races. The American Anti-Slavery Society was politically active from 1833 to 1839 for the government to abolish slavery, but Congress imposed a "gag rule" that rejected any citizen's request against slavery.[47] William Lloyd Garrison, formerly associated with the Society, then began publication of the anti-slavery newspaper The Liberator in Boston, Massachusetts in 1831, and Frederick Douglass, a black ex-slave, began writing for that newspaper around 1840 and started his own abolitionist newspaper North Star in 1847.[48]

The Republic of Texas was annexed by president John Tyler in 1845.[49] The U.S., using regulars and large numbers of volunteers, defeated Mexico in 1848 during the Mexican-American War. Public sentiment in the U.S. was divided as Whigs[50] and anti-slavery forces[51] opposed the war. The 1848 Treaty of Guadalupe Hidalgo ceded California, New Mexico, and adjacent areas to the United States, which composed about thirty percent of former Mexican land. Westward expansion was enhanced further by the California Gold Rush following the discovery of gold in that state in 1848. Numerous "forty-niners" trekked to California in pursuit of gold; land-demanding European immigrants also contributed to the rising Western population.[16] In 1849 cholera spread along the California and Oregon Trail. It is believed that over 150,000 Americans died during the two cholera pandemics between 1832 and 1849.[52]

[edit]
Civil War era (1849–1865)

The Battle of Gettysburg, the bloodiest battle and turning point of the American Civil War
Main article: History of the United States (1849–1865)

In the middle of the 19th century, white Americans of the North and South were unable to reconcile fundamental differences in their approach to government, economics, society and African American slavery. The issue of slavery in the new territories was settled by the Compromise of 1850 brokered by Whig Henry Clay and Democrat Stephen Douglas; the Compromise included admission of California as a free state and the passage of the Fugitive Slave Act to make it easier for masters to reclaim runaway slaves.[49] In 1854, the proposed Kansas-Nebraska Act abrogated the Missouri Compromise by providing that each new state of the Union would decide its stance on slavery.[53] After Abraham Lincoln won the 1860 Election, eleven Southern states seceded from the union between late 1860 and 1861, establishing a rebel government, the Confederate States of America, on February 8, 1861.[54]

By 1860, there were nearly four million slaves residing in the United States, nearly eight times as many from 1790; within the same time period, cotton production in the U.S. boomed from less than a thousand tons to nearly one million tons per year. There were some slave rebellions - including by Gabriel Prosser (1800), Denmark Vesey (1822), and Nat Turner (1831) - but they all failed and led to tighter slave oversight in the south.[55] White abolitionist John Brown tried and failed to free a group of black slaves held in Harpers Ferry, Virginia and was therefore executed for his actions.[56] Harriet Beecher Stowe, daughter of minister Lyman Beecher, published her novel Uncle Tom's Cabin in 1852 in response to the passage of the Fugitive Slave Act. The novel intended to express her views of the cruelty of slavery and sold nearly 300,000 copies during its first year of publication.[49] Numerous slaves also escaped their masters through the Underground Railroad, a term defining secret routes where abolitionists confidentially transported runaway slaves to "free state" territory; its most famous leader was Harriet Tubman.[57]

The Union: blue, yellow, gray; The Confederacy: brown

The Civil War began when Confederate General Pierre Beauregard opened fire upon Fort Sumter, in the Confederate state of South Carolina.[58] Along with the northwestern portion of Virginia, four of the five northernmost "slave states" did not secede and became known as the Border States.[54] Emboldened by Second Bull Run, the Confederacy made its first invasion of the North when General Robert E. Lee led 55,000 men of the Army of Northern Virginia across the Potomac River into Maryland.[59] The Battle of Antietam near Sharpsburg, Maryland, on September 17, 1862, was the bloodiest single day in American history.[60] At the beginning of 1864, Lincoln made General Ulysses S. Grant commander of all Union armies. General William Tecumseh Sherman marched from Chattanooga, Tennessee, to Atlanta, Georgia, defeating Confederate Generals Joseph E. Johnston and John Bell Hood.[54] Sherman's army laid waste to about 20% of the farms in Georgia in his "March to the Sea", and reached the Atlantic Ocean at Savannah in December 1864.[61] Lee surrendered his Army of Northern Virginia on April 9, 1865, at Appomattox Court House.[54] Based on 1860 census figures, 8% of all white males aged 13 to 43 died in the war, including 6% in the North and an extraordinary 18% in the South.[62]

[edit]
Reconstruction and the rise of industrialization (1865–1890)
Main article: History of the United States (1865–1918)

Completion of the Transcontinental Railroad (1869) at First Transcontinental Railroad, by Andrew J. Russell

Reconstruction took place for most of the decade following the Civil War. During this era, the "Reconstruction Amendments" were passed to expand civil rights for black Americans. Those amendments included the Thirteenth Amendment, which outlawed slavery, the Fourteenth Amendment that guaranteed citizenship for all people born or naturalized within U.S. territory, and the Fifteenth Amendment that granted the vote for all men regardless of race. While the Civil Rights Act of 1875 forbade discrimination in the service of public facilities, the Black Codes denied blacks certain privileges readily available to whites.[63] In response to Reconstruction, the Ku Klux Klan (KKK) emerged around the late 1860s as a white-supremacist organization opposed to black civil rights. Increasing hate-motivated violence from groups like the Klan influenced both the Ku Klux Klan Act of 1870 that classified the KKK as a terrorist group[64] and an 1883 Supreme Court decision nullifying the Civil Rights Act of 1875; however, in the Supreme Court case United States v. Cruikshank the Court interpreted the Fourteenth Amendment as regulating only states' decisions regarding civil rights.[65] The case defeated any protection of blacks from terrorist attacks, as did the later case United States v. Harris.[66] During the era, many regions of the southern U.S. were military-governed and often corrupt; Reconstruction ended after the disputed 1876 election between Republican candidate Rutherford B. Hayes and Democratic candidate Samuel J. Tilden. Hayes won the election, and the South soon re-entered the national political scene,[16] firmly under white control.

Following was the Gilded Age, a term that author Mark Twain used to describe the period of the late nineteenth century when there had been a dramatic expansion of American industry. Reform of the Age included the Civil Service Act, which mandated a competitive examination for applicants for government jobs. Other important legislation included the Interstate Commerce Act, which ended railroads' discrimination against small shippers, and the Sherman Antitrust Act, which outlawed monopolies in business. Twain believed that this age was corrupted by such elements as land speculators, scandalous politics, and unethical business practices. By century's end, American industrial production and per capita income exceeded those of all other world nations and ranked only behind Great Britain. In response to heavy debts and decreasing farm prices, farmers joined the Populist Party.[67] Later, an unprecedented wave of immigration served both to provide the labor for American industry and create diverse communities in previously undeveloped areas. Abusive industrial practices led to the often violent rise of the labor movement in the United States.[68] Influential figures of the period included John D. Rockefeller and Andrew Carnegie.

[edit]
Progressivism, imperialism, and World War I (1890–1918)
Main article: Progressive Era

Mulberry Street, along which Manhattan's Little Italy is centered. Lower East Side, circa 1900.

After the Gilded Age came the Progressive Era, whose followers called for reform over perceived industrial corruption. Viewpoints taken by progressives included greater federal regulation of anti-trust laws and the industries of meat-packing, drugs, and railroads. Four new constitutional amendments—the Sixteenth through Nineteenth—resulted from progressive activism.[69] The era lasted from 1900 to 1918, the year marking the end of World War I.[70]

U.S. Federal government policy, since the James Monroe Administration, had been to move the indigenous population beyond the reach of the white frontier into a series of Indian reservations. Tribes were generally forced onto small reservations as White farmers and ranchers took over their lands.

Ellis Island in 1902, the main immigration port for immigrants entering the United States in the late 19th and early 20th centuries.

The United States began its rise to international power in this period with substantial population and industrial growth domestically and numerous military ventures abroad, including the Spanish-American War, which began when the United States blamed the sinking of the USS Maine on Spain. Also at stake were U.S. interests in acquiring Cuba, an island nation fighting for independence from Spanish occupation; Puerto Rico and the Philippines were also two former Spanish colonies seeking liberation. In December 1898, representatives of Spain and the U.S. signed the Treaty of Paris to end the war, with Cuba becoming an independent nation and Puerto Rico, Guam, and the Philippines becoming U.S. territories.[16][71] In 1900, Congress passed the Open Door Policy that at the time required China to grant equal trading access to all foreign nations.[16]

President Woodrow Wilson declared U.S. entry into World War I in April 1917 following a yearlong neutrality policy; the U.S. had previously shown interest in world peace by participating in the Hague Conferences. American participation in the war proved essential to the Allied victory. Wilson also implemented a set of propositions titled the Fourteen Points to ensure peace, but they were denied at the 1919 Paris Peace Conference. Isolationist sentiment following the war also blocked the U.S. from participating in the League of Nations, an important part of the Treaty of Versailles.[16]

[edit]
Post-World War I and the Great Depression (1918–1940)
Main article: History of the United States (1918–1945)

Following World War I, the U.S. grew steadily in stature as an economic and military world power. The United States Senate did not ratify the Treaty of Versailles imposed by its Allies on the defeated Central Powers; instead, the United States chose to pursue unilateralism, if not isolationism.[72] The aftershock of Russia's October Revolution resulted in real fears of communism in the United States, leading to a three-year Red Scare and the U.S. lost 675,000 people to the Spanish flu pandemic in 1918.[73]

Prohibition agents destroying barrels of alcohol in Chicago, 1921

In 1920, the manufacture, sale, import and export of alcohol was prohibited by the Eighteenth Amendment to the United States Constitution. Prohibition encouraged illegal breweries and dealers to make substantial amounts of money selling drugs illegally. The Prohibition ended in 1933, a failure. Additionally, the KKK re-formed during that decade and gathered nearly 4.5 million members by 1924, and the U.S. government passed the Immigration Act of 1924 restricting foreign immigration.[74] The 1920s were also known as the Roaring Twenties, due to the great economic prosperity during this period. Jazz became popular among the younger generation, and thus was also called the Jazz Age.

During most of the 1920s, the United States enjoyed a period of unbalanced prosperity: farm prices and wages fell, while new industries, and industrial profits grew. The boom was fueled by an inflated stock market, which later led to the a crash on October 29, 1929.[75] The Hawley-Smoot Tariff, the Dust Bowl, and the ensuing Great Depression led to government efforts to restart the economy and help its victims with Franklin D. Roosevelt's New Deal. The recovery was rapid in all areas except unemployment, which remained fairly high until 1940.

[edit]
World War II (1941–1945) This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (September 2008)

Main articles: World War II and Homefront-United States-World War II

As with World War I, the United States did not enter World War II until after the rest of the active Allied countries had done so. The United States's first contribution to the war was simultaneously to cut off the oil and raw material supplies desperately needed by Japan to maintain its offensive in Manchuria, and to increase military and financial aid to China. Its first contribution to the Allies came in September 1940 in the form of the Lend-Lease program with Britain.

On December 7, 1941 Japan launched a surprise attack on the American naval base in Pearl Harbor, citing America's recent trade embargo as justification. The following day, Franklin D. Roosevelt successfully urged a joint session of Congress to declare war on Japan, calling December 7, 1941 "a date which will live in infamy". Four days after the attack on Pearl Harbor, on December 11, Nazi Germany declared war on the United States, drawing the country into a two-theater war.

[edit]
Battle against Germany
Further information: Europe first

Upon entering the war, the United States and its allies decided to concentrate the bulk of their efforts on fighting Hitler in Europe, while maintaining a defensive position in the Pacific until Hitler was defeated. The United States's first step was to set up a large airforce in Britain to concentrate on bombing raids into Germany itself. The American Air force relied on the B-17 Flying Fortress as its primary heavy bomber. Britain had ceased its daylight bombing raids, due to heavy casualties inflicted by the Luftwaffe. The USAAF suffered similar high losses until the introduction of the P-51 Mustang as a long range escort fighter for the bombers.

Landing at Normandy at Battle of Normandy, by Robert F. Sargent, United States Army

The American army's first ground action was fighting alongside the British, Australian and New Zealand armies in North Africa. By May 1943, the British 8th Army had expelled the Germans from North Africa and the Allies controlled this vital link until the end of the war. The American navy also played an active role in the Atlantic protecting the convoys bringing vital American war material to Britain. By midway through 1943, the Allies were fighting the war from Britain with unbroken supply lines, while at the same time Hitler's armies were very much on the back foot, with heavy bombing taking its toll on production.

By early 1944, a planned invasion of Western Europe was underway. What followed on June 6, 1944, was Operation Overlord, or D-Day. The largest war armada ever assembled landed on the beaches of Normandy and began the penetration of Western Europe that eventually overthrew Hitler and Nazi Germany. Following the landing at Normandy, the Americans contributed greatly to the outcome of the war, with dogged fighting in the Battle of the Ardennes and the Battle of the Bulge resulting in Allied victories against the Germans. The battles took a heavy toll on the Americans, who lost 19,000 men during the Battle of the Bulge alone. The allied bombing raids on Germany increased to unprecedented levels after the D-Day invasion, with over 70% of all bombs dropped on Germany occurring after this date. On April 30, 1945, with Berlin completely overrun with Russian forces and his country in tatters, Adolf Hitler committed suicide. On May 8, 1945, the war with Germany was over, following its unconditional surrender to the Allied forces.

[edit]
Battle against Japan
Main article: Pacific War

Due to the United States commitment to defeating Hitler in Europe, the first years of the war against Japan was largely a defensive battle with the United States Navy attempting to prevent the Japanese Navy from asserting dominance of the Pacific region. Initially, Japan won the majority of its battles in a short period of time. Japan quickly defeated and created military bases in Guam, Thailand, Malaya, Hong Kong, Papua New Guinea, Indonesia and Burma. This was done virtually unopposed and with quicker speed than that of the German Blitzkrieg during the early stages of the war. This was important for Japan, as it had only 10% of the homeland industrial production capacity of the United States.

Douglas MacArthur lands at the Battle of Leyte, by U.S. Army Signal Corps

The turning point of the war was the Battle of Midway in June 1942. Following this, the Americans began fighting towards China where they could build an airbase suitable to commence bombing of mainland Japan with its B-29 Superfortress fleet. The Americans began by selecting smaller, lesser defended islands as targets as opposed to attacking the major Japanese strongholds. During this period, they inadvertently triggered what would become their most comprehensive victory in the entire war.

The Pacific war became the largest naval conflict in history. The American Navy emerged victorious after at one point being stretched to almost breaking point with almost complete destruction of the Japanese Navy. The American forces were then poised for an invasion of the Japanese mainland, to force the Japanese into unconditional surrender. On April 12, 1945, President Franklin Delano Roosevelt died and Vice President Harry S. Truman was sworn in as the 33rd President of the United States. The decision to use nuclear weapons to end the conflict has been one of the most controversial decisions of the war. Supporters of the use of the bombs argue that an invasion would have cost enormous numbers of lives, while opponents argue that the large number of civilian casualties resulting from the bombings were still unjustified. The first bomb was dropped on Hiroshima on August 6, 1945, and the second bomb was dropped on Nagasaki on August 9. On August 15, 1945, the Japanese surrendered unconditionally.

[edit]
Cold War beginnings and the Civil Rights Movement (1945–1964)

President Kennedy's address on Civil Rights, June 11, 1963. This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (September 2008)

Main article: History of the United States (1945–1964)

Martin Luther King gives his I Have a Dream speech at the 1963 March on Washington for Jobs and Freedom

Following World War II, the United States emerged as one of the two dominant superpowers. The U.S. Senate, on December 4, 1945, approved U.S. participation in the United Nations (UN), which marked a turn away from the traditional isolationism of the U.S. and toward more international involvement. The post-war era in the United States was defined internationally by the beginning of the Cold War, in which the United States and the Soviet Union attempted to expand their influence at the expense of the other, checked by each side's massive nuclear arsenal and the doctrine of mutual assured destruction. The result was a series of conflicts during this period including the Korean War and the tense nuclear showdown of the Cuban Missile Crisis. Within the United States, the Cold War prompted concerns about Communist influence, and also resulted in government efforts to encourage math and science toward efforts like the space race.

In the decades after World War II, the United States became a global influence in economic, political, military, cultural and technological affairs. At the center of middle-class culture since the 1950s has been a growing obsession with consumer goods.

John F. Kennedy was elected President in 1960. Known for his charisma, he is so far the only Roman Catholic to be President. The Kennedys brought a new life and vigor to the atmosphere of the White House. During his time in office, the Cold War reached its height with the Cuban Missile Crisis in 1962. He was assassinated in Dallas, Texas, on November 22, 1963.

Meanwhile, the American people completed their great migration from the farms into the cities and experienced a period of sustained economic expansion. At the same time, institutionalized racism across the United States, but especially in the American South, was increasingly challenged by the growing Civil Rights movement and African American leaders such as Martin Luther King, Jr. During the 1960s, the Jim Crow laws that legalized racial segregation between Whites and Blacks came to an end.

[edit]
The Counterculture Revolution and Cold War Détente (1964–1980)
Main article: History of the United States (1964–1980)

Amid the Cold War, the United States entered the Vietnam War, whose growing unpopularity fed already existing social movements, including those among women, minorities and young people. President Lyndon B. Johnson's Great Society social programs and the judicial activism of the Warren Court added to the wide range of social reform during the 1960s and 1970s. Feminism and the environmental movement became political forces, and progress continued toward civil rights for all Americans. The Counterculture Revolution swept through the nation and much of the western world in the late sixties, dividing the already hostile environment but also bringing forth more liberated social views.

United States Navy F-4 Phantom II intercepts a Soviet Tu-95 Bear D aircraft in the early 1970s

Johnson was succeeded by President Richard Nixon in 1969, who intitially escalated the Vietnam War but soon was able to negotiate a peace treaty in 1973, effectively ending American involvement in the war. The war had cost the lives of 58,000 American troops and millions of Vietnamese. Nixon used a conflict in the Eastern Bloc between the Soviet Union and China to the advantage of the United States, bolstering relations with the People's Republic of China.[76] A new era of Cold War relations known as détente (cooperation) was begun.[77] The OPEC oil embargo led to a period of slow economic growth in 1973. Nixon's administration was brought to an ignominious close with the political scandal of Watergate in August 1974. During the years of his successor, Gerald Ford, the American-backed South Vietnamese government collapsed.

Jimmy Carter was elected in 1976 on the notion that he was not a part of the Washington political establishment.[78] The U.S. was afflicted with a recession, an energy crisis, slow economic growth, high unemployment, and high inflation coupled with high interest rates (the term stagflation was coined). On the world stage, Carter brokered the Camp David Accords between Israel and Egypt. In 1979, Iranian students stormed the U.S. embassy in Tehran and took 52 Americans hostage. Carter lost the 1980 election to Republican Ronald Reagan, whose campaign message advertised that his presidency would bring "Morning in America."[79]

[edit]
The Reagan Revolution and the end of the Cold War (1980–1991)
Main article: History of the United States (1980–1991)

In the 1984 election, Ronald Reagan won 49 states in one of the largest ever election victories.

Ronald Reagan at the Brandenburg Gate challenges Gorbachev to tear down the Berlin Wall in 1987, shortly before the end of the Cold War

Ronald Reagan produced a major realignment with his 1980 and 1984 landslides. In 1980, the Reagan coalition was possible because of Democratic losses in most social-economic groups. "Reagan Democrats" were those who usually voted Democratic, but were attracted by Reagan's policies, personality and leadership, notably his social conservatism and hawkish foreign policy. Widely regarded as a hard-line conservative, Reagan's economic policies (dubbed "Reaganomics") and the implementation of the Economic Recovery Tax Act of 1981 lowered income taxes from 70% to 28% over the course of seven years. Reagan continued to downsize government taxation and regulation.[80] The U.S. experienced a recession in 1982; unemployment and business failures soon entered rates close to Depression-era levels. These negative trends reversed the following year, when the inflation rate decreased from 11% to 2%, the unemployment rate decreased to 7.5%, and the economic growth rate increased from 4.5 to 7.2%.[81]

Reagan took a hard line against the Soviet Union, proclaiming it to be the Evil Empire. Reagan ordered a massive buildup of the U.S. military, incurring a costly budget deficit. Reagan introduced a complicated missile defense system known as the Strategic Defense Initiative (dubbed "Star Wars" by opponents) in which the U.S. could, in theory, shoot down missiles by means of laser systems in space. Though it was never fully developed or deployed,[82] the Soviets were genuinely concerned about the possible effects of the program[83] and the research and technologies of SDI paved the way for the anti-ballistic missile systems of today.[84] The Reagan administration also provided covert funding and assistance to anti-Communist resistance movements worldwide. Reagan's interventions against Grenada and Libya were popular in the U.S., though his backing of the Contra rebels was mired in controversy.[85] The arms-for-hostages scandal led to the convictions of such figures as Oliver North and John Poindexter.[86] He shared many common views and goals with friend and ally Margaret Thatcher, the Prime Minister of the United Kingdom.[87]

Reagan met with Soviet Leader Mikhail Gorbachev, who ascended to power in 1985, four times, and their summit conferences led to the signing of the INF Treaty. Gorbachev tried to save Communism in the Soviet Union first by ending the expensive arms race with America,[88] then by shedding the East European empire in 1989. The Soviet Union collapsed in 1991, ending the US-Soviet Cold War.

[edit]
The World Superpower (1991-present)
Main article: History of the United States (1991 - present)

After the fall of the Soviet Union, the United States emerged as the world's sole remaining superpower and continued to involve itself in military action overseas, including the 1991 Gulf War. Following his election in 1992, President Bill Clinton oversaw unprecedented gains in securities values, a side effect of the digital revolution and new business opportunities created by the Internet (see Internet bubble). The 1990s saw one of the longest periods of economic expansion. Under Clinton an attempt to universalize health care, led by First Lady Hillary Rodham Clinton failed after almost two years of work on the controversial plan.[89]

In 1993, Ramzi Yousef, a Kuwaiti national, planted explosives in the underground garage of One World Trade Center and detonated them, killing six people and injuring thousands, in what would become the beginning of an age of terrorism. Yousef would be subsequently captured.[90] In 1995, a domestic terrorist bombing at the federal building in Oklahoma City killed 168 people.

During the 1990s, the United States and allied nations found themselves under attack from Islamist terrorist groups, chiefly Al-Qaida. The regime of Saddam Hussein in Iraq proved a continuing problem for the UN and Iraq's neighbors in its refusal to account for previously known stockpiles of chemical and biological weapons, its violations of UN resolutions, and its support for terrorism against Israel and other countries. After the 1991 Gulf War, the US, French, and British militaries began patrolling the Iraqi no-fly zones to protect Iraq's Kurdish minority and Shi’ite Arab population – both of which suffered attacks from the Hussein regime before and after the 1991 Gulf War – in Iraq's northern and southern regions, respectively.[91] In the aftermath of Operation Desert Fox during December 1998, Iraq announced that it would no longer respect the no-fly zones and resumed its efforts in shooting down Allied aircraft.[92]

The 1993 World Trade Center bombing by Al-Qaida was the first of many terrorist attacks upon Americans during the same period. Later that year in the Battle of Mogadishu, US Army Rangers engaged Somali militias supported by Al Qaeda in an extended firefight that cost the lives of 19 soldiers. President Clinton subsequently withdrew US combat forces from Somalia (there originally to support UN relief efforts),[93] a move described by Al-Qaida leader Osama bin Laden as evidence of American weakness. These attacks were followed by others including the 1996 Khobar Towers bombing in Saudi Arabia, and the 1998 United States embassy bombings in Tanzania and Kenya. Next came the 2000 millennium attack plots which included an attempted bombing of Los Angeles International Airport, followed by the USS Cole bombing in Yemen in October 2000, which the government associated with Osama bin Laden's al-Qaeda terrorist network.[94]

US responses to these attacks included limited cruise missile strikes on Afghanistan and Sudan (August 1998), which failed to stop Al-Qaida's leaders and their Taliban supporters. Also in 1998, President Clinton signed the Iraq Liberation Act which called for regime change in Iraq on the basis of Saddam Hussein's possession of weapons of mass destruction, oppression of Iraqi citizens and attacks upon other Middle Eastern countries.[95]

In 1998, Clinton was impeached for charges of perjury and obstruction of justice that arose from an inappropriate sexual relationship with White House intern Monica Lewinsky and a sexual harassment lawsuit from Paula Jones. He was the second president to have been impeached. The House of Representatives voted 228 to 206 on December 19 to impeach Clinton,[96] but on February 12, 1999, the Senate voted 55 to 45 to acquit Clinton of the charges.[97]

The presidential election in 2000 between George W. Bush (R) and Al Gore (D) was one of the closest in the U.S. history, and helped lay the seeds for political polarization to come. Although Bush won the majority of electoral votes, Gore won the majority of the popular vote. In the days following Election Day, the state of Florida entered dispute over the counting of votes due to technical issues over certain Democratic votes in some counties.[98] The Supreme Court case Bush v. Gore was decided on December 12, 2000, ending the recount with a 5-4 vote and certifying Bush as president.[99]

New York under attack in the September 11 attacks

George W. Bush in a televised address from the USS Abraham Lincoln thanking members of the US armed services.

At the beginning of the new millennium, the United States found itself attacked by Islamic terrorism, with the September 11, 2001 attacks in which 19 extremists hijacked four transcontinental airliners and intentionally crashed two of them into the twin towers of the World Trade Center and one into the Pentagon. The passengers on the fourth plane, United Airlines Flight 93, revolted causing the plane to crash into a field in Somerset County, Pennsylvania. According to the 9/11 Commission Report, that plane was intended to hit the US Capitol Building in Washington. The twin towers of the World Trade Center collapsed, destroying the entire complex. The United States soon found large amounts of evidence that suggested that a terrorist group, al-Qaeda, spearheaded by Osama bin Laden, was responsible for the attacks.

In response to the attacks, under the administration of President George W. Bush, the United States (with the military support of NATO and the political support of some of the international community) launched Operation Enduring Freedom which overthrew the Taliban regime which had protected and harbored bin Laden and al-Qaeda. With the support of large bipartisan majorities, the US Congress passed the Authorization for Use of Military Force Against Iraq Resolution of 2002. With a coalition of other countries including Britain, Spain, Australia, Japan and Poland, in March 2003 President Bush ordered an invasion of Iraq dubbed Operation Iraqi Freedom which led to the overthrow and capture of Saddam Hussein. Using the language of 1998 Iraq Liberation Act and the Clinton Administration, the reasons cited by the Bush administration for the invasion included the spreading of democracy, the elimination of weapons of mass destruction[100] (a key demand of the UN as well, though later investigations found parts of the intelligence reports to be inaccurate)[101] and the liberation of the Iraqi people.[102] This second invasion fueled protest marches in many parts of the world.

In August 2005, Hurricane Katrina flooded parts of the city of New Orleans and heavily damaged other areas of the gulf coast, including major damage to the Mississippi coast. The preparation and the response of the government were criticized as ineffective and slow.[103]

By 2006, rising prices saw Americans become increasingly conscious of the nation's extreme dependence on steady supplies of inexpensive petroleum for energy, with President Bush admitting a U.S. "addiction" to oil.[104] The possibility of serious economic disruption, should conflict overseas or declining production interrupt the flow, could not be ignored, given the instability in the Middle East and other oil-producing regions of the world. Many proposals and pilot projects for replacement energy sources, from ethanol to wind power and solar power, received more capital funding and were pursued more seriously in the 2000s than in previous decades. The 2006 midterm elections saw Congresswoman Nancy Pelosi become Speaker of the United States House of Representatives and the highest ranking woman in the history of the U.S. government.[105]

In addition to military efforts abroad, in the aftermath of 9/11 the Bush Administration increased domestic efforts to prevent future attacks. A new cabinet level agency called the United States Department of Homeland Security was created to lead and coordinate federal counterterrorism activities. The USA PATRIOT Act removed legal restrictions on information sharing between federal law enforcement and intelligence services and allowed for the investigation of suspected terrorists using means similar to those in place for other types of criminals. A new Terrorist Finance Tracking Program monitored the movements of terrorist's financial resources but was discontinued after being revealed by The New York Times.[106] Telecommunication usage by known and suspected terrorists was studied through the NSA electronic surveillance program.

Since 9/11, Islamic extremists made various attempts to attack the US homeland, with varying levels of organization and skill. For example, in 2001 vigilant passengers aboard a transatlantic flight to Miami prevented Richard Reid (shoe bomber) from detonating an explosive device. Other terrorist plots have been stopped by federal agencies using new legal powers and investigative tools, sometimes in cooperation with foreign governments. Such thwarted attacks include a plan to crash airplanes into the U.S. Bank Tower (aka Library Tower) in Los Angeles; the 2003 plot by Iyman Faris to blow up the Brooklyn Bridge in New York City; the 2004 Financial buildings plot which targeted the International Monetary Fund and World Bank buildings in Washington, D.C., the New York Stock Exchange and other financial institutions; the 2004 Columbus Shopping Mall Bombing Plot; the 2006 transatlantic aircraft plot which was to involve liquid explosives; the 2006 Sears Tower plot; the 2007 Fort Dix attack plot; and the 2007 John F. Kennedy International Airport attack plot.

After months of brutal violence against Iraqi civilians by Sunni and Shi’ite terrorist groups and militias -- including Al-Qaeda in Iraq –- in January 2007 President Bush presented a new strategy for Operation Iraqi Freedom based upon Counter-insurgency theories and tactics developed by General David Petraeus. The Iraq War troop surge of 2007 was part of this "new way forward"[107] and has been credited by some[who?] with a dramatic decrease in violence and an increase in political and communal reconciliation in Iraq.

As of 2009, debates continue over abortion, gun control, same-sex marriage, immigration reform, and the ongoing war in Iraq. Although the new Democratic Congressional majority promised to withdraw U.S. forces from Iraq, Congress continues to fund efforts in both Iraq and Afghanistan, a withdrawal agreement has been agreed upon between the US and Iraqi government. In the area of foreign policy, the U.S. maintains ongoing talks with North Korea over its nuclear weapons program, as well as with Israel and the Palestinian Authority over a two-state solution to the Israeli-Palestinian conflict; the Palestinian-Israeli talks began in 2007, an effort spearheaded by United States Secretary of State Condoleezza Rice.[108] The George W. Bush administration also stepped up rhetoric implicating Iran and more recently Syria in the development of weapons of mass destruction.

In the presidential election of 2008, Senator Barack Obama ran on a platform of "Change". This, coupled with the September 2008 economic crisis, helped aid his victory against Senator John McCain. On November 4, Obama became the first African American to be elected President of the United States; he was sworn into office as the 44th President on January 20, 2009.