GPS

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Global Positioning System

This article is about the American satellite navigation system. It is not to be confused with similar non-American (global and regional) systems. For similar systems, see Satellite navigation.

"GPS" redirects here. For the device, see GPS receiver. For other uses, see GPS (disambiguation).

The Global Positioning System (GPS), originally Navstar GPS,^[1][2] is a space-basedradionavigation system owned by the United States government and operated by theUnited States Air Force. It is a global navigation satellite system that providesgeolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.^[3]

Global Positioning System (GPS)

Country/ies of origin	United States
Operator(s)	AFSPC
Type	Military, civilian
Status	Operational
Coverage	Global
Accuracy	5 meters
Constellation size
Total satellites	33
Satellites in orbit	31
First launch	February 1978; 39 years ago
Total launches	72
Orbital characteristics
Regime(s)	6x MEO planes
Orbital height	20,180 km (12,540 mi)

Artist's conception of GPS Block II-F satellite in Earth orbit.

Civilian GPS receivers ("GPS navigation device") in a marine application.

Automotive navigation systemin a taxicab.

A U.S. Air Force Senior Airmanruns through a checklist during Global Positioning System satellite operations.

The GPS does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.

The GPS project was launched by the U.S. Department of Defense in 1973 for use by the United States military and became fully operational in 1995. It was allowed for civilian use in the 1980s. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX).^[4] Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III.

In addition to GPS, other systems are in use or under development, mainly because the US government can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time.^[5] The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.^[6] GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, to within two meters.^[7] There are also the European UnionGalileo positioning system, China's BeiDou Navigation Satellite System, India's NAVICand Japan's Quasi-Zenith Satellite System.

HistoryEdit

The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems,^[8] integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites. It was initially developed for use by the United States military and became fully operational in 1995. It was allowed for civilian use in the 1980s.Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, and Bradford Parkinson of theApplied Physics Laboratory are credited with inventing it.^[9]

The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s and used by the British Royal Navy during World War II.

Friedwardt Winterberg^[10] proposed a test ofgeneral relativity — detecting time slowing in a strong gravitational field using accurateatomic clocks placed in orbit inside artificial satellites.

Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth's observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day. This was corrected for in the design of GPS.^[11]

PredecessorsEdit

When the Soviet Union launched the first artificial satellite (Sputnik 1) in 1957, two American physicists, William Guier andGeorge Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory (APL) decided to monitor its radio transmissions.^[12]Within hours they realized that, because of theDoppler effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required.

The next spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the user's location, given that of the satellite. (At the time, the Navy was developing the submarine-launchedPolaris missile, which required them to know the submarine's location.) This led them and APL to develop the TRANSIT system.^[13] In 1959, ARPA (renamed DARPA in 1972) also played a role in TRANSIT.^[14][15][16]

 

Emblem of the 50th Space Wing

The first satellite navigation system, TRANSIT, used by the United States Navy, was first successfully tested in 1960.^[17] It used aconstellation of five satellites and could provide a navigational fix approximately once per hour.

In 1967, the U.S. Navy developed the Timationsatellite, which proved the feasibility of placing accurate clocks in space, a technology required by GPS.

In the 1970s, the ground-based OMEGAnavigation system, based on phase comparison of signal transmission from pairs of stations,^[18] became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

While there were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. It is also the reason for the ultra secrecy at that time. The nuclear triadconsisted of the United States Navy'ssubmarine-launched ballistic missiles(SLBMs) along with United States Air Force(USAF) strategic bombers andintercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrenceposture, accurate determination of the SLBM launch position was a force multiplier.

Precise navigation would enable United States ballistic missile submarines to get an accurate fix of their positions before they launched their SLBMs.^[19] The USAF, with two thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve what was essentially the same problem.

To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (comparable to the Russian SS-24and SS-25) and so the need to fix the launch position had similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study, Project 57, was worked in 1963 and it was "in this study that the GPS concept was born." That same year, the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS"^[20] and promised increased accuracy for Air Force bombers as well as ICBMs.

Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The Naval Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock into orbit.^[21]

Another important predecessor to GPS came from a different branch of the United States military. In 1964, the United States Armyorbited its first Sequential Collation of Range (SECOR) satellite used for geodetic surveying.^[22] The SECOR system included three ground-based transmitters from known locations that would send signals to the satellite transponder in orbit. A fourth ground-based station, at an undetermined position, could then use those signals to fix its location precisely. The last SECOR satellite was launched in 1969.^[23]

Decades later, during the early years of GPS, civilian surveying became one of the first fields to make use of the new technology, because surveyors could reap benefits of signals from the less-than-complete GPS constellation years before it was declared operational. GPS can be thought of as an evolution of the SECOR system where the ground-based transmitters have been migrated into orbit.

DevelopmentEdit

With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program.

During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that the real synthesis that became GPS was created. Later that year, the DNSS program was named Navstar, or Navigation System Using Timing and Ranging.^[24] With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites,Navstar-GPS.^[25] Ten "Block I" prototype satellites were launched between 1978 and 1985 (an additional unit was destroyed in a launch failure).^[26]

The effects of the ionosphere on radio transmission through the ionosphere was investigated within a geophysics laboratory of Air Force Cambridge Research Laboratory. Located at Hanscom Air Force Base, outside Boston, the lab was renamed the Air Force Geophysical Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar Model for computing ionospheric corrections to GPS location.^[27] Of note is work done by Australian Space Scientist Elizabeth Essex-Cohen at AFGRL in 1974. She was concerned with the curving of the path of radio waves traversing the ionosphere from NavSTAR satellites.^[28]

After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down in 1983 after straying into the USSR's prohibited airspace,^[29] in the vicinity of Sakhalin andMoneron Islands, President Ronald Reaganissued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good.^[30] The first Block II satellite was launched on February 14, 1989,^[31] and the 24th satellite was launched in 1994. The GPS program cost at this point, not including the cost of the user equipment, but including the costs of the satellite launches, has been estimated at about USD 5 billion (then-year dollars).^[32]

Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded (Selective Availability). This changed with President Bill Clinton signing a policy directive to turn off Selective Availability May 1, 2000 to provide the same accuracy to civilians that was afforded to the military. The directive was proposed by the U.S. Secretary of Defense, William Perry, because of the widespread growth ofdifferential GPS services to improve civilian accuracy and eliminate the U.S. military advantage. Moreover, the U.S. military was actively developing technologies to deny GPS service to potential adversaries on a regional basis.^[33]

Since its deployment, the U.S. has implemented several improvements to the GPS service including new signals for civil use and increased accuracy and integrity for all users, all the while maintaining compatibility with existing GPS equipment. Modernization of the satellite system has been an ongoing initiative by the U.S. Department of Defense through a series of satellite acquisitions to meet the growing needs of the military, civilians, and the commercial market.

As of early 2015, high-quality, FAA grade, Standard Positioning Service (SPS) GPS receivers provide horizontal accuracy of better than 3.5 meters,^[34] although many factors such as receiver quality and atmospheric issues can affect this accuracy.

GPS is owned and operated by the United States government as a national resource. The Department of Defense is the steward of GPS. The Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems.^[35]The executive committee is chaired jointly by the Deputy Secretaries of Defense and Transportation. Its membership includes equivalent-level officials from the Departments of State, Commerce, and Homeland Security, the Joint Chiefs of Staffand NASA. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison.

The U.S. Department of Defense is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis," and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses."

Timeline and modernizationEdit

Main article: List of GPS satellites

Summary of satellites^[36][37][38]

Block	Launch period	Satellite launches				Currently in orbit and healthy
		Suc- cess	Fail- ure	In prep- aration	Plan- ned
I	1978–1985	10	1	0	0	0
II	1989–1990	9	0	0	0	0
IIA	1990–1997	19	0	0	0	0
IIR	1997–2004	12	1	0	0	12
IIR-M	2005–2009	8	0	0	0	7
IIF	2010–2016	12	0	0	0	12
IIIA	From 2017	0	0	0	12	0
IIIB	—	0	0	0	8	0
IIIC	—	0	0	0	16	0
Total		70	2	0	36	31
(Last update: March 9, 2016) 8 satellites from Block IIA are placed in reserve USA-203 from Block IIR-M is unhealthy [39] For a more complete list, see list of GPS satellite launches

• In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of four prototype GPS receivers in a Y configuration overWhite Sands Missile Range, using ground-based pseudo-satellites.^[40]
• In 1978, the first experimental Block-I GPS satellite was launched.^[26]
• In 1983, after Soviet interceptor aircraftshot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reaganannounced that GPS would be made available for civilian uses once it was completed,^[41][42] although it had been previously published [in Navigation magazine] that the CA code (Coarse/Acquisition code) would be available to civilian users.
• By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
• Beginning in 1988, Command & Control of these satellites was moved from Onizuka AFS, California to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station in Colorado Springs, Colorado.^[43][44]
• On February 14, 1989, the first modern Block-II satellite was launched.
• The Gulf War from 1990 to 1991 was the first conflict in which the military widely used GPS.^[45]
• In 1991, a project to create a miniature GPS receiver successfully ended, replacing the previous 23 kg military receivers with a1.25 kg handheld receiver.^[15]
• In 1992, the 2nd Space Wing, which originally managed the system, was inactivated and replaced by the 50th Space Wing.
• By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the Standard Positioning Service (SPS).^[46]
• Full Operational Capability (FOC) was declared by Air Force Space Command(AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).^[46]
• In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive^[47] declaring GPS a dual-usesystem and establishing an Interagency GPS Executive Board to manage it as a national asset.
• In 1998, United States 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 and in 2000 theUnited States Congress authorized the effort, referring to it as GPS III.
• On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.
• In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's Galileo system.
• In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.^[48]
• November 2004, Qualcomm announced successful tests of assisted GPS for mobile phones.^[49]
• In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.^[50]
• On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.^[51]
• On May 19, 2009, the United StatesGovernment Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.^[52]
• On May 21, 2009, the Air Force Space Command allayed fears of GPS failure, saying "There's only a small risk we will not continue to exceed our performance standard."^[53]
• On January 11, 2010, an update of ground control systems caused a software incompatibility with 8,000 to 10,000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.^[54]
• On February 25, 2010,^[55] the U.S. Air Force awarded the contract to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.

AwardsEdit

On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the nation's most prestigious aviation award. This team combines researchers from theNaval Research Laboratory, the USAF, theAerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation honors them "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radionavigation 50 years ago."

Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:

• Ivan Getting, emeritus president of The Aerospace Corporation and an 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 toNavigation).
• Bradford Parkinson, professor ofaeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel.

GPS developer Roger L. Easton received theNational Medal of Technology on February 13, 2006.^[56]

Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010 for his role in space technology development and the engineering design concept of GPS conducted as part of Project 621B.

In 1998, GPS technology was inducted into the Space Foundation Space Technology Hall of Fame.^[57]

On October 4, 2011, the International Astronautical Federation (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness of the GPS program and the exemplary role it has played in building international collaboration for the benefit of humanity.

Basic concept of GPSEdit

FundamentalsEdit

The GPS concept is based on time and the known position of GPS specialized satellites. The satellites carry very stable atomic clocksthat are synchronized with one another and with the ground clocks. Any drift from true time maintained on the ground is corrected daily. In the same manner, the satellite locations are known with great precision. GPS receivers have clocks as well, but they are less stable and less precise.

GPS satellites continuously transmit data about their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).

More detailed descriptionEdit

Each GPS satellite continually broadcasts a signal (carrier wave with modulation) that includes:

• A pseudorandom code (sequence of ones and zeros) that is known to the receiver. By time-aligning a receiver-generated version and the receiver-measured version of the code, the time of arrival (TOA) of a defined point in the code sequence, called an epoch, can be found in the receiver clock time scale
• A message that includes the time of transmission (TOT) of the code epoch (in GPS time scale) and the satellite position at that time

Conceptually, the receiver measures the TOAs (according to its own clock) of four satellite signals. From the TOAs and the TOTs, the receiver forms four time of flight (TOF) values, which are (given the speed of light) approximately equivalent to receiver-satellite ranges. The receiver then computes its three-dimensional position and clock deviation from the four TOFs.

In practice the receiver position (in three dimensional Cartesian coordinates with origin at the Earth's center) and the offset of the receiver clock relative to the GPS time are computed simultaneously, using thenavigation equations to process the TOFs.

The receiver's Earth-centered solution location is usually converted to latitude,longitude and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the geoid (e.g.,EGM96) (essentially, mean sea level). These coordinates may be displayed, e.g., on amoving map display, and/or recorded and/or used by some other system (e.g., a vehicle guidance system).

User-satellite geometryEdit

Although usually not formed explicitly in the receiver processing, the conceptual time differences of arrival (TDOAs) define the measurement geometry. Each TDOA corresponds to a hyperboloid of revolution (see Multilateration). The line connecting the two satellites involved (and its extensions) forms the axis of the hyperboloid. The receiver is located at the point where three hyperboloids intersect.^[58][59]

It is sometimes incorrectly said that the user location is at the intersection of three spheres. While simpler to visualize, this is only the case if the receiver has a clock synchronized with the satellite clocks (i.e., the receiver measures true ranges to the satellites rather than range differences). There are significant performance benefits to the user carrying a clock synchronized with the satellites. Foremost is that only three satellites are needed to compute a position solution. If this were part of the GPS concept so that all users needed to carry a synchronized clock, then a smaller number of satellites could be deployed. However, the cost and complexity of the user equipment would increase significantly.

Receiver in continuous operationEdit

The description above is representative of a receiver start-up situation. Most receivers have a track algorithm, sometimes called atracker, that combines sets of satellite measurements collected at different times—in effect, taking advantage of the fact that successive receiver positions are usually close to each other. After a set of measurements are processed, the tracker predicts the receiver location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the new measurements with the tracker prediction. In general, a tracker can (a) improve receiver position and time accuracy, (b) reject bad measurements, and (c) estimate receiver speed and direction.

The disadvantage of a tracker is that changes in speed or direction can only be computed with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the random error of position measurement. GPS units can use measurements of the Doppler shift of the signals received to compute velocity accurately.^[60] More advanced navigation systems use additional sensors like acompass or an inertial navigation system to complement GPS.

Non-navigation applicationsEdit

For a list of applications, see § Applications.

In typical GPS operation as a navigator, four or more satellites must be visible to obtain an accurate result. The solution of the navigation equations gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a more precise and possibly impractical receiver based clock. Applications for GPS such as time transfer, traffic signal timing, and synchronization of cell phone base stations, make use of this cheap and highly accurate timing. Some GPS applications use this time for display, or, other than for the basic position calculations, do not use it at all.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions such as reusing the last knownaltitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.^[61][62][63]

StructureEdit

The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.^[64] The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.^[65]

Space segmentEdit

See also: GPS (satellite) and List of GPS satellites

Unlaunched GPS block II-A satellite on display at the San Diego Air & Space Museum

A visual example of a 24 satellite GPS constellation in motion with the earth rotating. Notice how the number of satellites in view from a given point on the earth's surface, in this example in Golden, Colorado, USA(39.7469° N, 105.2108° W), changes with time.

The space segment (SS) is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit.

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits,^[66] but this was modified to six orbital planes with four satellites each.^[67] The six orbit planes have approximately 55° inclination (tilt relative to the 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).^[68] The orbital periodis one-half a sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations^[69] or almost the same locations^[70] every day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on the Earth's surface.^[71] The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°.^[72]

Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi),^[73] each SV makes two complete orbits each sidereal day, repeating the same ground track each day.^[74] This was very helpful during development because even with only 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 February 2016,^[75] there are 32 satellites in the GPS constellation, 31 of which are in use. 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.^[76] About nine satellites are visible from any point on the ground at any one time (see animation at right), ensuring considerable redundancy over the minimum four satellites needed for a position.

Control segmentEdit

 

Ground monitor station used from 1984 to 2007, on display at the Air Force Space & Missile Museum.

The control segment (CS) is composed of:

1. a master control station (MCS),
2. an alternate master control station,
3. four dedicated ground antennas, and
4. six dedicated monitor stations.

The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein Atoll,Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.^[77] The tracking information is sent to the Air Force Space Command MCS at Schriever Air Force Base25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located atKwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.^[78]

Satellite maneuvers are not precise by GPS standards—so to change a satellite's orbit, the satellite must be marked unhealthy, so receivers don't use it. After the satellite maneuver, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again.

The operation control segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports GPS users and keeps the GPS operational and performing within specification.

OCS successfully replaced the legacy 1970s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported U.S. armed forces. OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System^[4] (OCX), is fully developed and functional.

The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS's mission capabilities, enabling^[79] Air Force Space Command to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad domestic and international users.

The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50%^[80] sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX is expected to cost millions less than the cost to upgrade OCS while providing four times the capability.

The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.

• OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
• Built on a flexible architecture that can rapidly adapt to the changing needs of today's and future GPS users allowing immediate access to GPS data and constellation status through secure, accurate and reliable information.
• Provides the warfighter with more secure, actionable and predictive information to enhance situational awareness.
• Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
• Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
• Supports higher volume near real-time command and control capabilities and abilities.

On September 14, 2011,^[81] the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development.

The GPS OCX program has missed major milestones and is pushing the GPS IIIA launch beyond April 2016.^[82]

User segmentEdit

Further information: GPS navigation device

 

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as these.

 

The first portable GPS unit, Leica WM 101 displayed at the Irish National Science Museum at Maynooth.

The user segment (US) 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 (see GPS navigation devices). In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels. Though there are many receiver manufacturers, they almost all use one of the chipsets produced for this purpose.^{[citation needed]}

 

A typical OEM GPS receiver module measuring 15×17 mm.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of anRS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.^{[citation needed]} Receivers with internal DGPS receivers can outperform those using external RTCM data.^{[citation needed]} As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

 

A typical GPS receiver with integrated antenna.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),^[83] references to this protocol have been compiled from public records, allowing open source tools like gpsdto read the protocol without violatingintellectual property laws.^{[clarification needed]}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, orBluetooth.

ApplicationsEdit

Main article: GNSS applications

See also: GPS navigation device

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. 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.^[65]

CivilianEdit

 

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer.

• Agriculture: GPS has made a great evolution in different aspects of modern agricultural sectors. Today, a growing number of crop producers are using GPS and other modern electronic and computer equipment to practice Site Specific Management (SSM) and precision agriculture. This technology has the potential in agricultural mechanization (farm and machinery management) by providing farmers with a sophisticated tool to measure yield on much smaller scales as well as precise determination and automatic storing of variables such as field time, working area, machine travel distance and speed, fuel consumption and yield information.^[84][85]
• Astronomy: both positional and clock synchronization data is used in astrometryand celestial mechanics. GPS is also used in both amateur astronomy with small telescopes as well as by professional observatories for finding extrasolar planets.
• Automated vehicle: applying location and routes for cars and trucks to function without a human driver.
• 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. The firsthandsets 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 fromNextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
• Clock synchronization: the accuracy of GPS time signals (±10 ns)^[86] is second only to the atomic clocks they are based on, and is used in applications such as GPS disciplined oscillators.
• Disaster relief/emergency services: many emergency services depend upon GPS for location and timing capabilities.
• GPS-equipped radiosondes anddropsondes: measure and calculate the atmospheric pressure, wind speed and direction up to 27 km from the Earth's surface.
• Radio occultation for weather and atmospheric science applications.^[87]
• Fleet tracking: used to identify, locate and maintain contact reports with one or morefleet vehicles in real-time.
• Geofencing: vehicle tracking systems,person tracking systems, and pet trackingsystems use GPS to locate devices that are attached to or carried by a person, vehicle, or pet. The application can provide continuous tracking and send notifications if the target leaves a designated (or "fenced-in") area.^[88]
• Geotagging: applies location coordinates to digital objects such as photographs (in Exifdata) and other documents for purposes such as creating map overlays with devices like Nikon GP-1
• GPS aircraft tracking
• GPS for mining: the use of RTK GPS has significantly improved several mining operations such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS provides centimeter-level positioning accuracy.
• GPS data mining: It is possible to aggregate GPS data from multiple users to understand movement patterns, common trajectories and interesting locations.^[89]
• GPS tours: location determines what content to display; for instance, information about an approaching point of interest.
• Navigation: navigators value digitally precise velocity and orientation measurements.
• Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors.
• Recreation: for example, Geocaching,Geodashing, GPS drawing, waymarking, and other kinds of location based mobile games.
• Robotics: self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.
• Sport: used in football and rugby for the control and analysis of the training load.^[90]
• Surveying: surveyors use absolute locations to make maps and determine property boundaries.
• Tectonics: GPS enables direct fault motion measurement of earthquakes. Between earthquakes GPS can be used to measurecrustal motion and deformation^[91] to estimate seismic strain buildup for creatingseismic hazard maps.
• Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems.

Restrictions on civilian useEdit

The U.S. government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 km (60,000 feet) altitude and 515 m/s (1,000 knots), or designed or modified for use with unmanned air vehicles like, e.g., ballistic or cruise missile systems, are classified as munitions(weapons)—which means they require State Department export licenses.^[92]

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code.

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 km (100,000 feet).

These limits only apply to units or components exported from the USA. A growing trade in various components exists, including GPS units from other countries. These are expressly sold as ITAR-free.

MilitaryEdit

 

Attaching a GPS guidance kit to adumb bomb, March 2003.

 

M982 Excalibur GPS-guidedartillery shell.

As of 2009, military GPS applications include:

• Navigation: Soldiers use GPS 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 Commander's Digital Assistant and lower ranks use theSoldier Digital Assistant.^[93]
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile.^{[citation needed]} These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly inair-to-ground roles, use GPS to find targets.
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles,precision-guided munitions and artillery shells. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s² have been developed for use in 155-millimeter (6.1 in) howitzershells.^[94]
• Search and rescue.
• Reconnaissance: Patrol movement can be managed more closely.
• 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.^[95][96] General William Shelton has stated that future satellites may drop this feature to save money.^[97]

GPS type navigation was first used in war in the 1991 Persian Gulf War, before GPS was fully developed in 1995, to assist Coalition Forces to navigate and perform maneuvers in the war. The war also demonstrated the vulnerability of GPS to being jammed, when Iraqi forces installed jamming devices on likely targets that emitted radio noise, disrupting reception of the weak GPS signal.^[98]