Artist's conception of GPS satellite in orbit

Civilian GPS receiver in a marine application.

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Global Navigation Satellite System (GNSS is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time. A group of electronic Satellites working in concert is known as a satellite constellation. Medium Earth Orbit (MEO, sometimes called Intermediate Circular Orbit (ICO is the region of space around the Earth above low Earth orbit ( and below Geostationary orbit This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 This article is about a radio receiver for other uses see Radio (disambiguation. A geographic coordinate system enables every location on the Earth to be specified in three coordinates using mainly a spherical coordinate system. Other similar systems are the Russian GLONASS (incomplete as of 2008), the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Galileo is a Global navigation satellite system currently being built by the European Union (EU and European Space Agency (ESA The Compass system (also known as Beidou-2) is a project by China to develop an independent global Satellite navigation system. The Indian Regional Navigational Satellite System ( IRNSS) is an autonomous regional Satellite navigation system being developed by Indian Space Research Organisation

Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by John Walsh, a key decision maker when it came to the budget for the GPS program). The United States Department of Defense ( DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government Acronyms, initialisms, and alphabetisms are Abbreviations that are formed using the initial components in a phrase or name ^[1] The satellite constellation is managed by the United States Air Force 50th Space Wing. The 50th Space Wing ( 50 SW) is a wing of the United States Air Force under the major command of Air Force Space Command (AFSPC The cost of maintaining the system is approximately US$750 million per year,^[2] including the replacement of aging satellites, and research and development. The United States dollar ( sign: $; code: USD) is the unit of Currency of the United States; it has also been

Following the shooting down of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good. Korean Air Lines Flight 007, also known as KAL 007, was a Korean Air Lines civilian Airliner shot down by Soviet jet interceptors Common goods are defined in Economics as goods which are Rivalrous and Non-excludable. ^[3] Since then, 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. Radio navigation or radionavigation is the application of Radio frequencies to determining a position on the Earth. Surveying is the technique and science of accurately determining the terrestrial or three-dimensional space Position of points and the distances and angles between Geocaching is an outdoor treasure-hunting game in which the participants use a Global Positioning System (GPS receiver or other navigational techniques GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks. Time transfer describes methods for transferring reference clock Synchronization from one point to another often over long distances An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer

Simplified method of operation

A typical GPS receiver calculates its position using the signals from four or more GPS satellites. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for four variables: x, y, z, and t. These values are then turned into more user-friendly forms, such as latitude/longitude or location on a map, then displayed to the user.

Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health (the almanac). An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of The signals travel at the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry (see trilateration^[4])

Although four satellites are required for normal operation, fewer may be needed in some special cases. Trilateration is a method of determining the relative positions of objects using the Geometry of Triangles in a similar fashion as Triangulation. If one variable is already known (for example, a sea-going ship knows its altitude is 0), a receiver can determine its position using only three satellites. Also, in practice, receivers use additional clues (doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, and so on) to give degraded answers when fewer than four satellites are visible. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Dead reckoning ( DR) is the process of estimating one's current position based upon a previously determined position or fix, and advancing that position based upon An Inertial Navigation System (INS is a Navigation aid that uses a Computer and motion sensors to continuously track the position orientation and Velocity

Technical description

Unlaunched GPS satellite on display at the San Diego Aerospace museum

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). ^[5]

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. A GPS satellite is a Satellite used by the NAVSTAR Global Positioning System (GPS This is a history of GPS satellite launches Launch dates are UTC 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,^[6] but this was modified to six planes with four satellites each. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded. ^[7] The orbital planes are centered on the Earth, not rotating with respect to the distant stars. ^[8] 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). Inclination in general is the Angle between a Reference plane and another plane or axis of direction The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere An orbital node is one of the two points where an Orbit crosses a Plane of reference which it is inclined to ^[2] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. ^[9]

Orbiting at an altitude of approximately 20,200 kilometers (12,600 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. A nautical mile or sea mile is a unit of Length. It corresponds approximately to one minute of Latitude along any meridian. Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox ^[10] 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 September 2007, there are 31 actively broadcasting satellites in the GPS constellation. September 2007 is the ninth month of that year It began on a Saturday A group of electronic Satellites working in concert is known as a satellite constellation. 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. ^[11]

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). The State of Hawaii ( or həˈwaɪʔiː Hawaiian: Mokuāina o Hawaii) is a state in the United States located on an Archipelago in the Kwajalein Atoll ( Marshallese: Kuwajleen; kʷuwːɔ͡ɛt̪ʲl̪ʲɪn̪ʲ common English pronunciation ˈkwɑːʤəlɨ̞n often nicknamed Kwaj Ascension Island is an island in the South Atlantic Ocean, around from the coast of Africa Diego Garcia is the largest Atoll, in terms of land area in Chagos Archipelago, part of the British Indian Ocean Territory. The City of Colorado Springs is a Home Rule Municipality that is the County seat and most populous city of El Paso County, Colorado, The National Geospatial-Intelligence Agency ( NGA) is an agency of the United States Government with the primary mission of collection analysis and ^[12] 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). Air Force Space Command ( AFSPC) is a major command of the United States Air Force. Schriever Air Force Base (Schriever AFB is a base of the United States Air Force located approximately 10 miles (16 kilometers east of Peterson AFB near Colorado The 2d Space Operations Squadron ( 2 SOPS) is a unit of the United States Air Force at Schriever Air Force Base, Colorado. 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. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs. The Kalman filter is an efficient Recursive filter that estimates the state of a Dynamic system from a series of noisy measurements Space weather is the concept of changing environmental conditions in Outer space. ^[13]

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.

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). Trimble Navigation Ltd ( based in Sunnyvale California, is a provider of advanced positioning equipment most notably GPS receivers Laser rangefinders Garmin Ltd ( incorporated in George Town, Cayman Islands, is the parent company of a group of companies founded in 1989 by Gary Burrell and Min Leica Geosystems (formerly known as Wild Heerbrugg or just Wild) based in eastern Switzerland produces products and systems for Surveying

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). A crystal oscillator is an Electronic circuit that uses the mechanical Resonance of a vibrating Crystal of piezoelectric material to create an 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. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. ^[14]

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 15×17 mm, and used in many products. An original equipment manufacturer, or OEM is typically a company that uses a component made by a second company in its own product or sells the product of the second company SiRFstarIII is a range of high sensitivity GPS Microcontroller chips manufactured by SiRF Technology.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. The Radio Technical Commission for Maritime Services ( RTCM) is an international standards organization This is typically in the form of a RS-232 port at 4,800 bit/s speed. In Telecommunications, RS-232 (Recommended Standard 232 is a standard for serial binary data signals connecting between a DTE ( Data Terminal Equipment 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. WAAS Objectives Accuracy The WAAS specification requires it to provide a position accuracy of 7

SiRFstar III receiver and integrated antenna from UK company Antenova. SiRFstarIII is a range of high sensitivity GPS Microcontroller chips manufactured by SiRF Technology. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located This measures just 49 x 9 x 4 mm.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 0183 (or NMEA for short is a combined electrical and data specification for communication between marine electronic devices such as Echo sounder, sonars NMEA 2000^[15] is a newer and less widely adopted protocol. NMEA 2000 is a combined electrical and data specification for a marine Data network for communication between marine electronic devices such as Depth finders Both are proprietary and controlled by the US-based National Marine Electronics Association. The word proprietary indicates that a party or proprietor exercises private Ownership, control or use over an item of Property. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. NMEA 0183 (or NMEA for short is a combined electrical and data specification for communication between marine electronic devices such as Echo sounder, sonars gpsd is a daemon that receives data from a GPS receiver and provides the data back to multiple applications such as Kismet. Intellectual property ( IP) is a legal field that refers to creations of the mind such as musical literary and artistic works inventions and symbols names Other proprietary protocols exist as well, such as the SiRF and MTK protocols. SiRF Technology Holdings Inc ( is a publicly traded company that manufactures a range of patented GPS chipsets and software for consumer navigation devices and systems Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth. Bluetooth is a wireless protocol utilizing short-range communications technology facilitating data transmission over short distances from fixed and/or mobile devices creating wireless

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). In Telecommunications and Computing, bitrate (sometimes written bit rate, data rate or as a Variable R or f b An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.

The first 6 seconds of every frame contains data describing the satellite clock and its relationship to GPS time. The next 12 seconds 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). A new part of the almanac is received for the last 12 seconds in each 30 second frame. Each frame contains 1/25th of the almanac, so 12. 5 minutes are required to receive the entire almanac from a single satellite^[16]. 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. WAAS Objectives Accuracy The WAAS specification requires it to provide a position accuracy of 7 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 only its own ephemeris, but transmits an almanac for all satellites.

Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random (PRN) code at 1. In digital communications a chip is a pulse of a Direct-sequence spread spectrum (DSSS code such as a pseudo-noise code sequence used in direct-sequence Code division A pseudorandom number generator ( PRNG) is an Algorithm for generating a sequence of numbers that approximates the properties of random numbers 023 million chips per second so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P-code is a 10. 23 megachip per second PRN code that repeats only every week. When the "anti-spoofing" mode is on, as it is in normal operation, the P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Frequencies used by GPS include

• 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. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second.
• L2 (1227. 60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites. Global Positioning System (GPS satellites broadcast radio signals to enable GPS receivers to determine location and synchronized time
• 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). The United States' Global Positioning System (GPS having reached Fully Operational Capability on July 17[[ 995]] completed its original design goals 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^[17].

Calculating positions

Using the C/A code

To start off, the receiver picks which C/A codes to listen for by PRN number, based on the almanac information it has previously acquired. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern, then measures the received time for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number, referenced to its local clock, starting at the same time the satellite sent it. A random seed (or seed state, or just seed) is a Number (or vector) used to initialize a Pseudorandom number generator. It then computes the offset to the local clock that generates the maximum correlation. This offset is the time delay from the satellite to the receiver, as told by the receiver's clock. Since the PRN repeats every millisecond, this offset is precise but ambiguous, and the ambiguity is resolved by looking at the data bits, which are sent at 50 Hz (20 ms) and aligned with the PRN code.

This data is used to solve for x,y,z and t. Many mathematical techniques can be used. The following description shows a straightforward iterative way, but receivers use more sophisticated methods. (see below)

Conceptually, the receiver calculates the distance to the satellite, called the pseudorange^[18]. The pseudorange (from pseudo- and Range) is a first-approximation measurement for the distance between a Satellite and a navigation satellite receiver—for

Overlapping pseudoranges, represented as curves, are modified to yield the probable position

Next, the orbital position data, or ephemeris, from the Navigation Message is then downloaded to calculate the satellite's precise position. An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of A more-sensitive receiver will potentially acquire the ephemeris data more quickly than a less-sensitive receiver, especially in a noisy environment. ^[19] Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. Receivers can substitute altitude for one satellite, which the GPS receiver translates to a pseudorange measured from the center of the Earth.

When pseudoranges have been determined for four satellites, a guess of the receiver's location is calculated. Dividing the speed of light by the distance adjustment required to make the pseudoranges come as close as possible to intersecting results in a guess of the difference between UTC and the time indicated by the receiver's on-board clock. With each combination of four satellites, a geometric dilution of precision (GDOP) vector is calculated, based on the relative sky positions of the satellites used. Dilution of precision (DOP or Geometric Dilution of Precision (GDOP is a GPS term used in Geomatics engineering to describe the geometric strength As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more guesses 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, e. g. latitude/longitude, using the WGS 84 geodetic datum or a local system specific to a country. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation. This article describes a concept from Surveying and Geodesy. For other meanings see Datum (disambiguation.

There are many other alternatives and improvements to this process. If at least four satellites are visible, for example, the receiver can eliminate time from the equations by computing only time differences, then solving for position as the intersection of hyperboloids. Also, with a full constellation and modern receivers, more than four satellites can be seen and received at once. Then all satellite data can be weighted by GDOP, signal to noise, path length through the ionosphere, and other accuracy concerns, and then used in a least squares fit to find a solution. The method of least squares is used to solve Overdetermined systems Least squares is often applied in statistical contexts particularly Regression analysis. In this case the residuals also give an estimate of the errors. In Statistics and optimization, the concepts of statistical error and residual are easily confused with each other 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. Galileo is a Global navigation satellite system currently being built by the European Union (EU and European Space Agency (ESA (By design, these systems use the same bands, so much of the receiver circuitry can be shared, though the decoding is different).

Using the 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. 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. RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, a technology developed to assess the integrity of Global Positioning System (GPS signals in

Accuracy and error sources

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 1% of a bit time, 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.

Position accuracy can be improved by using the higher-chiprate P(Y) signal. Assuming the same 1% bit time accuracy, the high frequency P(Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several accuracy-degrading effects outlined in the table below. 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.

Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation 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). The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates For air mass in Meteorology, see Air mass. In Astronomy, airmass is the optical path length through Earth's 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 more frequency bands can be used to measure dispersion, and this measurement can then be used to estimate the delay at each frequency^[20]. In Optics, dispersion is the phenomenon in which the Phase velocity of a wave depends on its frequency 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. In Telecommunications, a carrier wave, or carrier is a Waveform (usually Sinusoidal) that is modulated (modified with an input signal In Telecommunications, modulation is the process of varying a periodic Waveform, i 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.

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 such as WAAS, which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required. Augmentation of a Global Navigation Satellite System (GNSS is a method of improving the navigation system's attributes such as accuracy reliability and availability through WAAS Objectives Accuracy The WAAS specification requires it to provide a position accuracy of 7

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its Water vapor and 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.

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.

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. In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving 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. A choke ring antenna is a particular form of Omnidirectional antenna for use at high frequencies 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.

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 satellites actual position. If a fast TTFF is needed, it is possible to upload 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. ^[21]

The satellite's atomic clocks experience noise and clock drift errors. Clock drift refers to several related phenomena where a Clock does not run at the exact right speed compared to another clock 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. ^[22]

Selective availability

GPS includes a (currently disabled) feature called Selective Availability (SA) that can introduce intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, the guidance of long range missiles to precise targets. When enabled, the accuracy is still available in the signal, but in an encrypted form that is only available to the United States military, its allies and a few others, mostly government users. Even those who have managed to acquire military GPS receivers would still need to obtain the daily key, whose dissemination is tightly controlled. In Cryptography, a key is a piece of information (a Parameter) that determines the functional output of a cryptographic algorithm

Prior to being turned off, SA typically added signal errors of up to about 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly. For instance, the entire eastern U. S. area might read 30 m off, but 30 m off everywhere and in the same direction. To improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy. Differential Global Positioning System (DGPS is an enhancement to Global Positioning System that uses a network of fixed ground-based reference stations to broadcast the difference

During the Gulf War, the shortage of military GPS units and the ready availability of civilian ones caused many troops to buy their own civilian GPS units: their wide use among personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy—but the assumption underlying this policy was that all U. S. troops and enemy troops would have military-specification GPS receivers and that civilian receivers would not exist in war zones. But since many American soldiers were using civilian devices, SA was also denying the same accuracy to thousands of friendly troops; turning it off (by removing the added-in error) presented a clear benefit to friendly troops.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. The Federal Aviation Administration ( FAA) is an agency of the United States Department of Transportation with authority to regulate and oversee all aspects of This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. Radio navigation or radionavigation is the application of Radio frequencies to determining a position on the Earth. The amount of error added was "set to zero"^[23] at midnight on May 1, 2000 following an announcement by U. Events 305 - Diocletian and Maximian retire from the office of Roman Emperor. 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. S. President Bill Clinton, allowing users access to the error-free L1 signal. William Jefferson "Bill" Clinton (born William Jefferson Blythe III, August 19 1946 served as the forty-second President of the United States 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. ^[23]

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,^[24] have stated that it is not intended to be reintroduced. The Federal Aviation Administration ( FAA) is an agency of the United States Department of Transportation with authority to regulate and oversee all aspects of

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). Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 Rubidium (ruːˈbɪdiəm /rəˈbɪdiəm/ is a Chemical element with the symbol Rb and Atomic number 37 An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element This represented a significant improvement over the raw accuracy of the clocks.

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,^[25] eventually making the policy permanent. Events 335 - Dalmatius is raised to the rank of Caesar by his uncle Constantine I. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The United States Department of Defense ( DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government ^[26]

Relativity

Satellite clocks are slowed by its orbital speed but sped up by its 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 inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element 9 microseconds (μs) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units 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. ^[27]. To account for this, the frequency standard onboard 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. ^[28] 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.

Sagnac distortion

GPS observation processing must also compensate for the Sagnac effect. The Sagnac effect (also called Sagnac Interference) named after French physicist Georges Sagnac, is a phenomenon encountered in Interferometry that is 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. In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest ECEF stands for Earth-Centered Earth-Fixed and is a Cartesian coordinate system used for GPS, and is sometimes known as a "conventional terrestrial" system A Lorentz transformation is thus applied to convert from the inertial system to the ECEF system. In Physics, the Lorentz transformation converts between two different observers' measurements of space and time where one observer is in constant motion with respect to 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. ^[29]

GPS interference and jamming

Natural sources

Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible. For telecommunications desensitization is a form of Electromagnetic Interference where a radio receiver is unable to receive the full strength of a radio signal

Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field. A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see ^[30] GPS signals are also subjected to interference from Van Allen Belt radiation when the satellites pass through the South Atlantic Anomaly. The Van Allen radiation belt is a Torus of energetic Charged particles ( plasma) around Earth, held in place by Earth's Magnetic The South Atlantic Anomaly (or SAA is the region where Earth 's inner Van Allen radiation belt makes its closest approach to the planet's surface

Artificial sources

Metallic features in windshields^[31], such as defrosters, or car window tinting films^[32] can act as a Faraday cage, degrading reception just inside the car. A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material

Man-made EMI can also disrupt, or jam, GPS signals. Radio jamming is the (usually deliberate transmission of Radio signals that disrupt communications by decreasing the Signal to noise ratio In one well documented case, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier. ^[33] 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. Phrack is an Underground ezine written by and for hackers first published November 17, 1985. ^[34]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U. The federal government of the United States is the central United States Governmental body established by the United States Constitution. The War in Afghanistan, which began on October 7 2001 as the U S. military claimed to destroy a GPS jammer with a GPS-guided bomb during the Iraq War. The Iraq War, also known as the Second Gulf War, the Occupation of Iraq, or the War in Iraq, is an ongoing Military campaign ^[35] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. An anti-radiation missile (ARM is a Missile which is designed to detect and home in on an enemy radio emission source The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007. ^[36]

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". ^[37] 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. RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, a technology developed to assess the integrity of Global Positioning System (GPS signals in The U. S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). An SAASM, for Selective Availability / Anti-Spoofing Module, is used by military Global Positioning System receivers to allow decryption of precision GPS coordinates The Defense Advanced GPS Receiver (DAGR is a handheld GPS receiver used by the United States Department of Defense and select foreign military services 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. ^[38]

Techniques to improve accuracy

Augmentation

Main article: GNSS Augmentation

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. Augmentation of a Global Navigation Satellite System (GNSS is a method of improving the navigation system's attributes such as accuracy reliability and availability through 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. WAAS Objectives Accuracy The WAAS specification requires it to provide a position accuracy of 7 Differential Global Positioning System (DGPS is an enhancement to Global Positioning System that uses a network of fixed ground-based reference stations to broadcast the difference An Inertial Navigation System (INS is a Navigation aid that uses a Computer and motion sensors to continuously track the position orientation and Velocity Assisted GPS, generally abbreviated as A-GPS, enhances the startup performance of a GPS satellite-based positioning system

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. Total electron content (or TEC) is an important descriptive quantity for the Ionosphere of the Earth

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. A pseudorandom number generator ( PRNG) is an Algorithm for generating a sequence of numbers that approximates the properties of random numbers In Signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. Periodicity is the quality of occurring at regular intervals or periods (in Time or Space) and can occur in different contexts A Clock marks In Electronics and especially synchronous Digital circuits a clock signal is a signal used to coordinate the actions of two or more circuits 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. Differential Global Positioning System (DGPS is an enhancement to Global Positioning System that uses a network of fixed ground-based reference stations to broadcast the difference

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). A centimetre ( American spelling: centimeter, symbol cm) is a unit of Length in the Metric system, equal to one hundredth 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). Real Time Kinematic (RTK Satellite navigation is a technique used in Land survey and in Hydrographic survey based on the use of carrier phase measurements

GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element 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. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. 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 (19 seconds) with International Atomic Time (TAI). International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks 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 2006 is 14 seconds due to the leap second added to UTC December 31st of 2005. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. 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, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330. The 24th century of the Anno Domini ( common) era will span the years 2301&ndash2400 of the Gregorian calendar.

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 Gregorian calendar is the most widely used Calendar in the world today 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. A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication 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). Events 1066 - Harold Godwinson is crowned King of England. 1205 - Philip of Swabia becomes King Year 1980 ( MCMLXXX) was a Leap year starting on Tuesday (link displays the 1980 Gregorian calendar) Events 1192 - Minamoto Yoritomo becomes Seii Tai Shōgun and the De facto ruler of Japan. Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) Events 392 - Arbogast has Eugenius elected Western Roman Emperor. Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) 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 messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137. The 22nd century of the Common Era will span the years 2101&ndash2200 of the Gregorian calendar.

GPS modernization

Main article: GPS modernization

Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,^[39] the GPS completed its original design goals. The United States' Global Positioning System (GPS having reached Fully Operational Capability on July 17[[ 995]] completed its original design goals Events 180 - Twelve inhabitants of Scillium in North Africa are executed for being Christians Year 1995 ( MCMXCV) was a Common year starting on Sunday. Events of 1995 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 Vice President of the United States is the first person in the presidential line of succession, becoming the new President of the United States upon the death See also Executive Office of the President of the United States The White House, formerly known as the Executive Mansion, is the Official residence Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. The United States Congress is the bicameral Legislature of the federal government of the United States of America, consisting of two houses The United States' Global Positioning System (GPS having reached Fully Operational Capability on July 17[[ 995]] completed its original design goals

The project aims to improve the accuracy and availability for all users and involves 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. Global Positioning System (GPS satellites broadcast radio signals to enable GPS receivers to determine location and synchronized time Initial Operational Capability (IOC) of the L2C code is expected in 2008. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common ^[40] 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). 2013 ( MMXIII) will be a Common year starting on Tuesday of the Gregorian calendar. Global Positioning System (GPS satellites broadcast radio signals to enable GPS receivers to determine location and synchronized time

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.

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 commanders and soldiers use are respectively called the Commanders Digital Assistant and the Soldier Digital Assistant. ^{[41][42][43][44]}
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. 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). Ground-attack aircraft are military aircraft designed to attack targets on the ground and are often deployed as Close air support for and in proximity to their own ground forces Gun cameras are Cameras used in Military vehicles to help measure tactical effectiveness For an overview of the whole Huey family of aircraft see Bell Huey WikipediaWikiProject Aircraft. The Iraq War, also known as the Second Gulf War, the Occupation of Iraq, or the War in Iraq, is an ongoing Military campaign Google Earth is a Virtual globe program that was originally called Earth Viewer and was created by Keyhole Inc, a company acquired by Google in 2004
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions. A cruise missile is a guided Missile that carries an explosive payload and uses a lifting wing and a propulsion system usually a Jet engine, to allow Artillery projectiles with embedded GPS receivers able to withstand accelerations of 12,000G have been developed for use in 155 mm howitzers. Artillery (from French artillerie) is a military Combat Arm which employs any apparātus machine A projectile is any object propelled through space by the exertion of a force which ceases after launch g-force (also G-force, g-load) is a measurement of an object's Acceleration expressed in g s A howitzer is a type of Artillery piece that is characterized by a relatively short barrel and the use of comparatively small explosive charges to propel projectiles ^[45]
• 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. Reconnaissance (also scouting) is a military and medical term denoting exploration conducted to gain information
• 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. Vela was the name of a group of Satellites developed as the Vela Hotel element of Project Vela by the United States to monitor compliance with ^[46][47]

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. Global Navigation Satellite System (GNSS receivers using the GPS, GLONASS, or Beidou system are used in many applications A GPS navigation device is any device that receives Global Positioning System ( GPS) signals for the purpose of determining the present location

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. Surveying is the technique and science of accurately determining the terrestrial or three-dimensional space Position of points and the distances and angles between Navigation is the process of reading and controlling the movement of a craft or vehicle from one place to another 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. Code division multiple access ( CDMA) is a Channel access method utilized by various radio communication technologies 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. Enhanced 9-1-1 or E9-1-1 service is a North American telecommunications based system that automatically associates a physical address with the calling party's telephone 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. This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer

To help prevent civilian GPS guidance from being used in an enemy's military or improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer cannot generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000 ft) and (2) traveling at over 515 m/s (1,000 knots). ^[48] These parameters are well above the operating characteristics of the typical cruise missile, but would be characteristic of the reentry vehicle from a ballistic missile. A cruise missile is a guided Missile that carries an explosive payload and uses a lifting wing and a propulsion system usually a Jet engine, to allow A ballistic missile is a Missile that follows a Sub-orbital ballistic flightpath with the objective of delivering a warhead to a predetermined target

GPS functionality has now started to move into mobile phones en masse. The first GSM 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. A GPS navigation device is any device that receives Global Positioning System ( GPS) signals for the purpose of determining the present location Capabilities for access by third party software developers to these features were slower in coming, with Nextel opening those APIs up upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.

History

The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. LORAN ( LO ng R ange A id to N avigation is a terrestrial Radio navigation system using Low frequency Radio transmitters The Decca Navigator System was a hyperbolic Low frequency Radio navigation system (also known as Multilateration) that was first deployed during World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Additional inspiration for the GPS came when the Soviet Union launched the first Sputnik in 1957. The Union of Soviet Socialist Republics (USSR was a constitutionally Socialist state that existed in Eurasia from 1922 to 1991 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. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for 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.

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. The TRANSIT system also known as NAVSAT (for Navy Navigation Satellite System) was the first Satellite navigation system to be used operationally 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 the GPS relies upon. The Timation Satellites were conceived developed and launched by the Naval Research Laboratory in Washington D In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system. OMEGA was the first truly global Radio navigation system for aircraft operated by the United States in cooperation with six partner nations

The first experimental Block-I GPS satellite was launched in February 1978. ^[40] The GPS satellites were initially manufactured by Rockwell International (now part of Boeing) and are now manufactured by Lockheed Martin (IIR/IIR-M) and Boeing (IIF). Rockwell International was the ultimate incarnation of a series of companies under the sphere of influence of Willard Rockwell, who had made his fortune after the invention and The Boeing Company is a major Aerospace and defense corporation originally founded by William E Lockheed Martin ( is a large multinational Aerospace manufacturer and advanced technology Company formed in 1995 by the merger of The Boeing Company is a major Aerospace and defense corporation originally founded by William E

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.
• 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 restricted Soviet airspace due to navigational errors, killing all 269 people on board, U. An interceptor aircraft (or simply interceptor) is a type of Fighter aircraft designed specifically to intercept and destroy enemy Aircraft, particularly Korean Air Lines Flight 007, also known as KAL 007, was a Korean Air Lines civilian Airliner shot down by Soviet jet interceptors S. President Ronald Reagan announced that the GPS would be made available for civilian uses once it was completed. ^[49][50]
• 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. Events 842 - Charles the Bald and Louis the German swear the Oaths of Strasbourg in the French and German Year 1989 ( MCMLXXXIX) was a Common year starting on Sunday (link displays 1989 Gregorian calendar)
• In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing. The 50th Space Wing ( 50 SW) is a wing of the United States Air Force under the major command of Air Force Space Command (AFSPC
• By December 1993 the GPS achieved initial operational capability. ^[51]
• By January 17, 1994 a complete constellation of 24 satellites was in orbit. Events 38 BC - Octavian marries Livia Drusilla. 1287 - King Alfonso III of Aragon invades Minorca Year 1994 ( MCMXCIV) was a Common year starting on Saturday (link will display full 1994 Gregorian calendar)
• 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^[52] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset. William Jefferson "Bill" Clinton (born William Jefferson Blythe III, August 19 1946 served as the forty-second President of the United States The Interagency GPS Executive Board (IGEB was an agency of the United States Federal Government that sought to integrate the needs and desires of various governmental agencies
• 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. Albert Arnold Gore Jr (born March 31 1948 is an American environmental Activist, author Businessperson, former Politician, and former
• 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. Events 1194 - King Richard I of England gives Portsmouth its first Royal Charter. 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar.
• In 2004, the United States Government signed a historic 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, replacing the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee. George Walker Bush ( born July 6 1946 is the forty-third and current President of the United States.
• November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile phones. Qualcomm ( is a wireless telecommunications research and development company based in San Diego California Assisted GPS, generally abbreviated as A-GPS, enhances the startup performance of a GPS satellite-based positioning system ^[53]
• 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. Events 81 - Domitian becomes Emperor of the Roman Empire upon the death of his brother Titus. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. ^[54]
• The most recent launch was on March 15, 2008. ^[55] The oldest GPS satellite still in operation was launched on July 4, 1991, and became operational on August 30, 1991. Events 836 - Pactum Sicardi, peace between the Principality of Benevento and the Duchy of Naples Year 1991 ( MCMXCI) was a Common year starting on Tuesday (link will display full calendar of the Gregorian Calendar. Events 1363 - Beginning date of the Battle of Lake Poyang; the forces of two Chinese rebel leaders— Chen Youliang and Year 1991 ( MCMXCI) was a Common year starting on Tuesday (link will display full calendar of the Gregorian Calendar. ^[56]

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). The United States National Academy of Engineering (NAE is a private non-profit institution which was founded in 1964 under the same congressional act that led to the founding of the The National Academy of Engineering awards annually the Charles Stark Draper Prize, which is given for the advancement of engineering and the education of the public about Ivan Alexander Getting ( January 18, 1912 &mdash October 11, 2003) was an American Physicist and Electrical Engineer credited (along The Aerospace Corporation is a private non-profit corporation headquartered in El Segundo, California that has operated a Federally Funded Research and An engineer is a person professionally engaged in a field of Engineering. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including LORAN ( LO ng R ange A id to N avigation is a terrestrial Radio navigation system using Low frequency Radio transmitters
• 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. Dr Bradford Wells Parkinson is an American Systems engineer and Chairman of The Aerospace Corporation board of trustees Aeronautics (from Greek aero which means air or sky and nautis which means sailor i Astronautics, or Astronautical Engineering is the branch of Engineering that deals with machines designed to exit or work entirely beyond the Earth's atmosphere. Leland Stanford Junior University, commonly known as Stanford University or simply Stanford, is a private Research university located in S. Air Force.

One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13, 2006 at the White House. This entry is for the satellite scientist For his son an imaging scientist see Roger L The National Medal of Technology and Innovation (formerly known as the National Medal of Technology) is an honor granted by the President of the United States to American Events 1258 - Baghdad falls to the Mongols, and the Abbasid Caliphate is destroyed Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. See also Executive Office of the President of the United States The White House, formerly known as the Executive Mansion, is the Official residence ^[59]

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. Events 1355 - The St Scholastica's Day riot breaks out in Oxford, England, leaving 63 scholars and perhaps 30 locals dead Year 1993 ( MCMXCIII) was a Common year starting on Friday (link will display full 1993 Gregorian calendar) The National Aeronautic Association of the United States ( NAA) is a member of the Fédération Aéronautique Internationale (FAI the international standard The Collier Trophy is an annual Aviation award administered by the U 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 United States Naval Research Laboratory (NRL is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a The Aerospace Corporation is a private non-profit corporation headquartered in El Segundo, California that has operated a Federally Funded Research and Rockwell International was the ultimate incarnation of a series of companies under the sphere of influence of Willard Rockwell, who had made his fortune after the invention and International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology 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. "

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. Global Navigation Satellite System (GNSS is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage Beidou Navigation System ( or Beidou Satellite Navigation and Positioning System ( is a project by China to develop an independent Satellite navigation Talk People's Republic of China) PEOPLE'S REPUBLIC OF CHINA ARTICLE GUIDELINES The Compass system (also known as Beidou-2) is a project by China to develop an independent global Satellite navigation system.
• 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 2011–12. Galileo is a Global navigation satellite system currently being built by the European Union (EU and European Space Agency (ESA The European Union ( EU) is a political and economic union of twenty-seven member states, located primarily in Talk People's Republic of China) PEOPLE'S REPUBLIC OF CHINA ARTICLE GUIDELINES For a topic outline on this subject see List of basic Israel topics. India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country Morocco (المغرب "al-Maghrib" officially the Kingdom of Morocco (المملكة المغربية is a country located in North Africa The Kingdom of Saudi Arabia, KSA ( المملكة العربية السعودية, al-Mamlaka al-ʻArabiyya as-Suʻūdiyya) or Suudi South Korea, officially the Republic of Korea and often referred to as Korea ( Korean: 대한민국 tɛː Ukraine (Україна Ukrayina, /ukrɑˈjinɑ/ is a country in Eastern Europe.
• GLONASS – Russia's global system which is being restored to full availability in partnership with India. Russia (Россия Rossiya) or the Russian Federation ( Rossiyskaya Federatsiya) is a transcontinental Country extending India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country
• Indian Regional Navigational Satellite System (IRNSS) – India's proposed regional system. The Indian Regional Navigational Satellite System ( IRNSS) is an autonomous regional Satellite navigation system being developed by Indian Space Research Organisation India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country
• QZSS – Japanese proposed regional system, adding better coverage to the Japanese Islands. The Quasi-Zenith Satellite System (QZSS is a proposed three-satellite regional Time transfer system and enhancement for the Global Positioning System, that would For a topic outline on this subject see List of basic Japan topics.

See also

References

External links