The user segment is composed

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, Sony VAIO VGN-SR140D/B Battery

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. Sony VAIO VGN-SR140D/P Battery

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.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Sony VAIO VGN-SR140D/S Battery

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. Sony VAIO VGN-SR140E Battery

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),^[58] references to this protocol have been compiled from public records, Sony VAIO VGN-SR140E/B Battery

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. Sony VAIO VGN-SR140E/P Battery

While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications.

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. Sony VAIO VGN-SR140E/S Battery

GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.

Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer. Sony VAIO VGN-SR140N/S Battery

Cartography: Both civilian and military cartographers use GPS extensively.

Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. Sony VAIO VGN-SR165E/B Battery

The first handsets with integrated GPS launched in the late 1990s. The U.S.Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter. Sony VAIO VGN-SR165E/P Battery

Clock synchronization: The accuracy of GPS time signals (±10 ns)^[59] is second only to the atomic clocks upon which they are based.Disaster relief/emergency services: Depend upon GPS for location and timing capabilities.

Fleet Tracking: The use of GPS technology to identify, locate and maintain contact reports with one or more fleet vehicles in real-time. Sony VAIO VGN-SR165E/S Battery

Geofencing: Vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices are attached to the vehicle, person, or the pet collar. The application provides continuous tracking and mobile or Internet updates should the target leave a designated area.^[60] Sony VAIO VGN-SR190EBJ Battery

Geotagging: Applying location coordinates to digital objects such as photographs and other documents for purposes such as creating map overlays.GPS Aircraft Tracking

GPS tours: Location determines what content to display; for instance, information about an approaching point of interest. Sony VAIO VGN-SR190EBQ Battery

Navigation: Navigators value digitally precise velocity and orientation measurements.Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to computephasors.

Recreation: For example, geocaching, geodashing, GPS drawing and waymarking. Sony VAIO VGN-SR190EEJ/C Battery

Robotics: Self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.

Surveying: Surveyors use absolute locations to make maps and determine property boundaries.

Tectonics: GPS enables direct fault motion measurement in earthquakes. Sony VAIO VGN-SR190NAB Battery

Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems

The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometres (11 mi) altitude and 515 metres per second (1,001 kn) or designed, Sony VAIO VGN-SR190NBB Battery

modified for use with unmanned air vehicles like e.g. ballistic or cruise missile systems are classified as munitions(weapons) for which State Department export licenses are required.^[61]

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code and cannot correct for Selective Availability (SA), etc. Sony VAIO VGN-SR190NDB Battery

Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule refers to operation at both the target altitude and speed, but some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach 30 kilometres (19 mi). Sony VAIO VGN-SR190NEB Battery

These limits only apply to units exported from (or which have components exported from) the USA - there is a growing trade in various components, including GPS units, supplied by other countries, which are expressly sold as ITAR-free.

As of 2009, military applications of GPS include: Sony VAIO VGN-SR190NGB Battery

Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant. Sony VAIO VGN-SR190PAB Battery

Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, Sony VAIO VGN-SR190PCB Battery

use GPS to find targets (for example,gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be viewed with specialized software).

Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and Artillery projectiles. Sony VAIO VGN-SR190PFB Battery

Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s² have been developed for use in 155 millimetres (6.1 in) howitzers.^[66]

• Search and Rescue: Downed pilots can be located faster if their position is known.

Reconnaissance: Patrol movement can be managed more closely. Sony VAIO VGN-SR19VN Battery

GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System. Sony VAIO VGN-SR19VN Battery

The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Sony VAIO VGN-SR19VRN Battery

Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military.

Each GPS satellite continuously broadcasts a navigation message on L1 C/A and L2 P/Y frequencies at a rate of 50 bits per second (see bitrate). Sony VAIO VGN-SR19XN Battery

Each complete message takes 750 seconds (12 1/2 minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are subcommutated 25 times each, so that a complete data message requires the transmission of 25 full frames. Sony VAIO VGN-SR19XN Battery

Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50 bps, this gives 750 seconds to transmit an entire almanac message. Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.^[69] Sony VAIO VGN-SR210J/S Battery

The first subframe of each frame encodes the week number and the time within the week,^[70] as well as the data about the health of the satellite. The second and the third subframes contain the ephemeris - the precise orbit for the satellite. The fourth and fifth subframes contain the almanac, Sony VAIO VGN-SR21M/S Battery

which contains coarse orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, in order to obtain an accurate satellite location from this transmitted message the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. Sony VAIO VGN-SR21RM/H Battery

In order to collect all the transmitted almanacs the receiver must demodulate the message for 732 to 750 seconds or 12 1/2 minutes.^[71]

All satellites broadcast at the same frequencies. Signals are encoded using code division multiple access (CDMA) allowing messages from individual satellites to be distinguished from each other based on unique encodings for each satellite (that the receiver must be aware of). Sony VAIO VGN-SR21RM/S Battery

Two distinct types of CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise P(Y) code, which is encrypted so that only the U.S. military can access it.

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. Sony VAIO VGN-SR220J/B Battery

The almanac is updated typically every 24 hours. Additionally data for a few weeks following is uploaded in case of transmission updates that delay data upload. All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). Sony VAIO VGN-NW21EF/S Battery The satellite network uses a 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 must be aware of the PRN codes for each satellite to reconstruct the actual message data. Sony VAIO VGN-NW21JF Battery The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for relativistic effects that make observers on Earth perceive a different time reference with respect to the transmitters in orbit. Sony VAIO VGN-NW21MF Battery The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.^[74] The P code can be encrypted as a so-called P(Y) code that 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. Sony VAIO VGN-NW21MF/W Battery The L3 signal at a frequency of 1.38105 GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space.^[75]One usage is the enforcement of nuclear test ban treaties. Sony VAIO VGN-NW21ZF Battery The L4 band at 1.379913 GHz is being studied for additional ionospheric correction. The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. Sony VAIO VGN-NW31EF/W Battery The first Block IIF satellite that would provide this signal is set to be launched in 2009.^[76] The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, Sony VAIO VGN-NW31JF Battery will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."^[77] A conditional waiver has recently been granted to LightSquared to operate a terrestrial broadband service near the L1 band. Sony VAIO VGN-NW320F/B Battery Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issue that might arise due to the larger signal power from the LightSquared terrestrial network. Sony VAIO VGN-NW320F/TC Battery The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, Sony VAIO VGN-NW35E Battery essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10 MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). Sony VAIO VGN-NW380F/S Battery The upper 10 MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this will seriously degrade the GPS signal for many consumer uses. Aviation Week magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system. Sony VAIO VGN-NW380F/T Battery Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary sequence known as a Gold code. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver. Sony VAIO VGN-NW50JB Battery If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range 1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. Sony VAIO VGN-NW51FB/N Battery 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. Sony VAIO VGN-NW51FB/W Battery 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. Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. Sony VAIO VGN-NW70JB Battery For more information see Demodulation and Decoding, Advanced. The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent are designated as [x_i, y_i, z_i, t_i] where the subscript i denotes the satellite and has the value 1, 2, ..., n, where  Sony VAIO VGN-NW71FB/N Battery When the time of message reception indicated by the on-board clock is  , the true reception time is   where   is receiver's clock bias (i.e., clock delay). The message's transit time is  . Assuming the message traveled at the speed of light,  , the distance traveled is  . Sony VAIO VGN-NW71FB/W Battery Knowing the distance from receiver to satellite and the satellite's position implies that the receiver is on the surface of a sphere centered at the satellite's position with radius equal to this distance. Thus the receiver is at or near the intersection of the surfaces of the spheres if it receives signals from more than one satellite. Sony VAIO VGN-NW91FS Battery In the ideal case of no errors, the receiver is at the intersection of the surfaces of the spheres. Bancroft's method involves an algebraic as opposed to numerical method and can be used for the case of four or more satellites.^.Bancroft's method provides one or two solutions for the four unknowns. Sony VAIO VGN-NW91GS Battery However when there are two solutions, only one of these two solutions will be a near earth sensible solution. When there are four satellites, we use the inverse of the B matrix in section 2 of.^[84] If there are more than four satellites then we use the pseudoinverse of the B matrix since in this case the B matrix is no longer square. Sony VAIO VGN-NW91VS Battery The receiver can use trilateration^[85][86] and one dimensional numerical root finding.^[87] Satellite position and pseudorange determines a sphere centered on the satellite with radius equal to the pseudorange. Trilateration is used to estimate receiver position based on the intersection of three sphere surfaces so determined. Sony VAIO VPC CW2MFX/PU Battery In the usual case of two intersections of three sphere surfaces, the point nearest the surface of the sphere corresponding to the fourth satellite is chosen. Let d denote the signed distance from the current estimate of receiver position to the sphere around the fourth satellite. The notation,d(correction) denotes this as a function of the clock correction. Sony VAIO VPC S11V9E/B Battery The problem is to determine the correction such that d(correction) = 0. This is the familiar problem of finding the zeroes of a one dimensional non-linear function of a scalar variable. Iterative numerical methods, such as those found in the chapter on root finding in Numerical Recipes can solve this type of problem.^[87] Sony VAIO VPCB119GJ/B Battery Alternatively, multidimensional root finding methods such as the Newton-Raphson method can be used.^[87] The approach is to linearize around an approximate solution, say   from iteration k, then solve the linear equations derived from the quadratic equations above to obtain  . Sony VAIO VPCB11AGJ Battery Although there is no guarantee that the method always converges due to the fact that multidimensional roots cannot be bounded, when a neighborhood containing a solution is known as is usually the case for GPS, it is quite likely that a solution will be found.^[87] It has been shown^[88]that results are comparable in accuracy to those of Bancroft's method. Sony VAIO VPCB11AVJ Battery When more than four satellites are available, the calculation can use the four best or more than four, considering number of channels, processing capability, and geometric dilution of precision (GDOP). Using more than four is an over-determined system of equations with no unique solution, which must be solved byleast-squares or a similar technique. Sony VAIO VPCB11V9E Battery If all visible satellites are used, the results are as good as or better than using the four best. Errors can be estimated through the residuals. With each combination of four or more satellites, a GDOP factor can be calculated, based on the relative sky directions of the satellites used. Sony VAIO VPCB11X9E Battery As more satellites are picked up, pseudoranges from various 4-way combinations can be processed to add more estimates to the location and clock offset. The receiver then takes the weighted average of these positions and clock offsets. After the final location and time are calculated, Sony VAIO VPCCW18FJ/P Battery the location is expressed in a specific coordinate system such as latitude and longitude, using the WGS 84 geodetic datum or a country-specific system.^[90] GPS error analysis which is found in Error analysis for the Global Positioning System is an important aspect for determining what errors and their magnitude are to be expected. Sony VAIO VPCCW18FJ/R Battery GPS errors are affected by geometric dilution of precision and depend on signal arrival time errors, numerical errors, atmospherics effects, ephemeris errors, multipath errors and other effects. Variability in solar radiation pressure^[91] has an indirect effect on GPS accuracy due to its effect on ephemeris errors. Sony VAIO VPCCW18FJ/W Battery Main article: GPS augmentation Integrating external information into the calculation process can materially improve accuracy. Such augmentation systems are generally named or described based on how the information arrives. Sony VAIO VPCCW19FJ/W Battery Some systems transmit additional error information (such as clock drift, ephemera, or ionospheric delay), others characterize prior errors, while a third group provides additional navigational or vehicle information. Examples of augmentation systems include the Wide Area Augmentation System (WAAS), Sony VAIO VPCCW1AFJ Battery European Geostationary Navigation Overlay Service (EGNOS), Differential GPS,Inertial Navigation Systems (INS) and Assisted GPS. Accuracy can be improved through precise monitoring and measurement of existing GPS signals in additional or alternate ways. Sony VAIO VPCCW1AHJ Battery The largest remaining error is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason 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, Sony VAIO VPCCW1S1E Battery so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.