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The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the
United States government The federal government of the United States (U.S. federal government or U.S. government) is the Federation#Federal governments, national government of the United States, a federal republic located primarily in North America, composed of 50 ...
and operated by the
United States Space Force The United States Space Force (USSF) is the Space force, space service branch of the United States Armed Forces, U.S. Armed Forces, one of the eight Uniformed services of the United States, U.S. uniformed services, and the world's only inde ...
. It is one of the
global navigation satellite systems A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pre ...
(GNSS) that provides
geolocation Geopositioning, also known as geotracking, geolocalization, geolocating, geolocation, or geoposition fixing, is the process of determining or estimating the geographic position of an object. Geopositioning yields a set of Geographic coordinate s ...
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. It does not require the user to transmit any data, and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the GPS positioning information. It provides critical positioning capabilities to military, civil, and commercial users around the world. Although the United States government created, controls and maintains the GPS system, it is freely accessible to anyone with a GPS receiver. The GPS project was started by the U.S. Department of Defense in 1973. The first prototype spacecraft was launched in 1978 and the full constellation of 24 satellites became operational in 1993. Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President
Ronald Reagan Ronald Wilson Reagan ( ; February 6, 1911June 5, 2004) was an American politician, actor, and union leader who served as the 40th president of the United States from 1981 to 1989. He also served as the 33rd governor of California from 1967 ...
after the
Korean Air Lines Flight 007 Korean Air Lines Flight 007 (KE007/KAL007)The flight number KAL 007 was used by air traffic control, while the public flight booking system used KE 007 was a scheduled Korean Air, Korean Air Lines flight from New York City to Seoul via Anch ...
incident. 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 GPS Block III (previously Block IIIA) consists of the first ten GPS satellite blocks, GPS III satellites, which will be used to keep the Navstar Global Positioning System operational. Lockheed Martin designed, developed and manufactured the GP ...
satellites and Next Generation Operational Control System (OCX). which was authorized by the
U.S. Congress The United States Congress is the legislature of the federal government of the United States. It is Bicameralism, bicameral, composed of a lower body, the United States House of Representatives, House of Representatives, and an upper body, ...
in 2000. From the early 1990s, GPS positional accuracy was degraded by the United States government by a program called
Selective Availability The error analysis for the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude of error should be expected. The GPS makes corrections for receiver clock errors and other effects but there are sti ...
, which could selectively degrade or deny access to the system at any time, as happened to the Indian military in 1999 during the
Kargil War The Kargil War, also known as the Kargil conflict, was fought between India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country b ...
. However, this practice was discontinued on May 1, 2000, in accordance with a bill signed into law by President
Bill Clinton William Jefferson Clinton (Birth name, né Blythe III; born August 19, 1946) is an American politician who served as the 42nd president of the United States from 1993 to 2001. He previously served as governor of Arkansas from 1979 to 1981 ...
. As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s. GLONASS reception in addition to GPS can be combined in a receiver thereby allowing for additional satellites available to enable faster position fixes and improved accuracy, to within . China's
BeiDou Navigation Satellite System The BeiDou Navigation Satellite System (BDS; ) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and ...
began global services in 2018, and finished its full deployment in 2020. There are also the European Union Galileo navigation satellite system, and India's
NavIC The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NavIC (acronym for 'Navigation with Indian Constellation; also, 'sailor' or 'navigator' in Indian languages), is an autonomous regional satellite navigation s ...
. Japan's
Quasi-Zenith Satellite System The Quasi-Zenith Satellite System (QZSS), also known as , is a four-satellite regional time transfer system and a GNSS augmentation#Satellite-based augmentation system, satellite-based augmentation system developed by the Japanese government ...
(QZSS) is a GPS
satellite-based augmentation system Augmentation of a global navigation satellite system (GNSS) is a method of improving the navigation system's attributes, such as precision, reliability, and availability, through the integration of external information into the calculation process. ...
to enhance GPS's accuracy in Asia-Oceania, with satellite navigation independent of GPS scheduled for 2023. When selective availability was lifted in 2000, GPS had about a accuracy. GPS receivers that use the L5 band have much higher accuracy, pinpointing to within , while high-end users (typically engineering and land surveying applications) are able to have accuracy on several of the bandwidth signals to within two centimeters, and even sub-millimeter accuracy for long-term measurements. Consumer devices, like smartphones, can be as accurate as to within 4.9 m (or better with assistive services like Wi-Fi positioning also enabled). , 16 GPS satellites are broadcasting L5 signals, and the signals are considered pre-operational, scheduled to reach 24 satellites by approximately 2027.

# History

The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, combining ideas from several predecessors, including classified engineering design studies from the 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites, for use by the United States military, and became fully operational in 1995. Civilian use was allowed from the 1980s. Roger L. Easton of the
Naval Research Laboratory The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. It was founded in 1923 and conducts basic scientific research, applied research, technological ...
, Ivan A. Getting of
The Aerospace Corporation The Aerospace Corporation is an American nonprofit corporation that operates a federally funded research and development center (FFRDC) in El Segundo, California. The corporation provides technical guidance and advice on all aspects of space mi ...
, and Bradford Parkinson of the
Applied Physics Laboratory The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit University Affiliated Research Center, university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated w ...
are credited with inventing it. The work of Gladys West is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS. The design of GPS is based partly on similar ground-based radio-navigation systems, such as
LORAN LORAN, short for long range navigation, was a Hyperbolic navigation, hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee (navigation), Gee system but operated at lower frequencies ...
and the Decca Navigator, developed in the early 1940s. In 1955, Friedwardt Winterberg proposed a test of
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the differential geometry, geometric scientific theory, theory of gravitation published by Albert Einstein in 1915 and is the current descr ...
—detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predicted that the clocks on GPS satellites, as observed by those on Earth, run 38 microseconds faster per day than those on the Earth. The design of GPS corrects for this difference; because without doing so, GPS calculated positions would accumulate errors of up to .

## Predecessors

In 1955, Dutch Naval officer Wijnand Langeraar submitted a patent application for a radio-based Long-Range Navigation System, with the US Patent office on February 16, 1955, and was granted Patent US2980907A on April 18, 1961. When the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
launched the first artificial satellite (
Sputnik 1 Sputnik 1 (; see #Etymology, § Etymology) was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to ...
) in 1957, two American physicists, William Guier and George Weiffenbach, at
Johns Hopkins University Johns Hopkins University (Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland. Founded in 1876, Johns Hopkins is the oldest research university in the United States and in the western hem ...
's
Applied Physics Laboratory The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit University Affiliated Research Center, university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated w ...
(APL) decided to monitor its radio transmissions. Within hours they realized that, because of the
Doppler effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...
, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their
UNIVAC UNIVAC (Universal Automatic Computer) was a line of electronic digital stored-program computers starting with the products of the Eckert–Mauchly Computer Corporation. Later the name was applied to a division of the Remington Rand company and ...
to do the heavy calculations required. Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem: pinpointing the user's location, given the satellite's. (At the time, the Navy was developing the submarine-launched
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an a ...
missile, which required them to know the submarine's location.) This led them and APL to develop the
TRANSIT Transit may refer to: Arts and entertainment Film * Transit (1979 film), ''Transit'' (1979 film), a 1979 Israeli film * Transit (2005 film), ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the w ...
system. In 1959, ARPA (renamed
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adv ...
in 1972) also played a role in TRANSIT. TRANSIT was first successfully tested in 1960. It used a
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellati ...
of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the
Timation The Timation satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as ...
satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS. In the 1970s, the ground-based
OMEGA Omega (; capital letter, capital: Ω, lower case, lowercase: ω; Ancient Greek ὦ, later ὦ μέγα, Modern Greek ωμέγα) is the twenty-fourth and final letter in the Greek alphabet. In the Greek numerals, Greek numeric system/isopsephy ...
navigation system, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy. Although 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 of a constellation of navigation satellites. During the
Cold War The Cold War is a term commonly used to refer to a period of Geopolitics, geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term ''Cold war (term), co ...
arms race An arms race occurs when two or more groups compete in military superiority. It consists of a competition between two or more State (polity), states to have superior armed forces; a competition concerning production of weapons, the growth of a ...
, 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 triad A nuclear triad is a three-pronged military force structure that consists of land-launched nuclear missiles, nuclear-missile-armed submarines, and strategic aircraft with nuclear bombs and missiles. Specifically, these components are land-based ...
consisted of the United States Navy's
submarine-launched ballistic missile A submarine-launched ballistic missile (SLBM) is a ballistic missile capable of being launched from Ballistic missile submarine, submarines. Modern variants usually deliver multiple independently targetable reentry vehicles (MIRVs), each of whic ...
s (SLBMs) along with
United States Air Force The United States Air Force (USAF) is the Aerial warfare, air military branch, service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part ...
(USAF)
strategic bomber A strategic bomber is a medium- to long-range Penetrator (aircraft), penetration bomber aircraft designed to drop large amounts of air-to-ground weaponry onto a distant target for the purposes of debilitating the enemy's capacity to wage war. Unl ...
s and
intercontinental ballistic missile An intercontinental ballistic missile (ICBM) is a ballistic missile with a range (aeronautics), range greater than , primarily designed for nuclear weapons delivery (delivering one or more Thermonuclear weapon, thermonuclear warheads). Conventi ...
s (ICBMs). Considered vital to the
nuclear deterrence Deterrence theory refers to the scholarship and practice of how threats or limited force by one party can convince another party to refrain from initiating some other course of action. The topic gained increased prominence as a military strategy ...
posture, accurate determination of the SLBM launch position was a force multiplier. Precise navigation would enable United States
ballistic missile submarine A ballistic missile submarine is a submarine capable of deploying submarine-launched ballistic missiles (SLBMs) with nuclear warheads. The United States Navy's Hull classification symbol#Submarine type, hull classification symbols for ballisti ...
s to get an accurate fix of their positions before they launched their SLBMs. The USAF, with two thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The U.S. Navy and U.S. 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 Soviet SS-24 and 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 performed 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" 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 The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. It was founded in 1923 and conducts basic scientific research, applied research, technological ...
(NRL) continued making advances with their
Timation The Timation satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as ...
(Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first
atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
into orbit and the fourth launched in 1977. Another important predecessor to GPS came from a different branch of the United States military. In 1964, the
United States Army The United States Army (USA) is the land warfare, land military branch, service branch of the United States Armed Forces. It is one of the eight Uniformed services of the United States, U.S. uniformed services, and is designated as the Army o ...
orbited its first Sequential Collation of Range ( SECOR) satellite used for geodetic surveying. The SECOR system included three ground-based transmitters at 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.

## Development

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. Satellite orbital position errors, induced by variations in the gravity field and radar refraction among others, had to be resolved. A team led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, used real-time data assimilation and recursive estimation to do so, reducing systematic and residual errors to a manageable level to permit accurate navigation. 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.'' Navstar is often erroneously considered an acronym for "NAVigation System Using Timing and Ranging" but was never considered as such by the GPS Joint Program Office (TRW may have once advocated for a different navigational system that used that acronym). 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''. Ten " Block I" prototype satellites were launched between 1978 and 1985 (an additional unit was destroyed in a launch failure). The effect of the ionosphere on radio transmission was investigated in a geophysics laboratory of Air Force Cambridge Research Laboratory, renamed to Air Force Geophysical Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar model for computing ionospheric corrections to GPS location. Of note is work done by Australian space scientist Elizabeth Essex-Cohen at AFGRL in 1974. She was concerned with the curving of the paths of radio waves (
atmospheric refraction Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of light t ...
) traversing the ionosphere from NavSTAR satellites. After
Korean Air Lines Flight 007 Korean Air Lines Flight 007 (KE007/KAL007)The flight number KAL 007 was used by air traffic control, while the public flight booking system used KE 007 was a scheduled Korean Air, Korean Air Lines flight from New York City to Seoul via Anch ...
, a
Boeing 747 The Boeing 747 is a large, long-range wide-body airliner designed and manufactured by Boeing Commercial Airplanes in the United States between 1968 and 2022. After introducing the 707 in October 1958, Pan Am Pan American World Airway ...
carrying 269 people, was shot down in 1983 after straying into the USSR's
prohibited airspace A prohibited airspace is an area (volume) of airspace within which flight of aircraft is not allowed, usually due to security concerns. It is one of many types of special use airspace designations and is depicted on aeronautical charts with the le ...
, in the vicinity of
Sakhalin Sakhalin ( rus, Сахали́н, r=Sakhalín, p=səxɐˈlʲin; ja, 樺太 ''Karafuto''; zh, c=, p=Kùyèdǎo, s=库页岛, t=庫頁島; Manchu language, Manchu: ᠰᠠᡥᠠᠯᡳᠶᠠᠨ, ''Sahaliyan''; Orok language, Orok: Бугата ...
and
Moneron Island Moneron Island, (russian: Монерон, ja, 海馬島 Kaibato, ja, トド島 Todojima, Ainu: Todomoshiri) is a small island off Sakhalin Island. It is a part of the Russian Federation Russia (, , ), or the Russian Federation, is a ...
s, President
Ronald Reagan Ronald Wilson Reagan ( ; February 6, 1911June 5, 2004) was an American politician, actor, and union leader who served as the 40th president of the United States from 1981 to 1989. He also served as the 33rd governor of California from 1967 ...
issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first Block II satellite was launched on February 14, 1989, 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 US$5 billion (equivalent to$ billion in ). Initially, the highest-quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded, in a policy known as
Selective Availability The error analysis for the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude of error should be expected. The GPS makes corrections for receiver clock errors and other effects but there are sti ...
. This changed with President
Bill Clinton William Jefferson Clinton (Birth name, né Blythe III; born August 19, 1946) is an American politician who served as the 42nd president of the United States from 1993 to 2001. He previously served as governor of Arkansas from 1979 to 1981 ...
signing on May 1, 2000, a policy directive to turn off Selective Availability 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, in view of the widespread growth of
differential GPS Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). A DGPS for Global Positioning System, GPS can increase accuracy by about a thousandfold, from a ...
services by private industry to improve civilian accuracy. Moreover, the U.S. military was developing technologies to deny GPS service to potential adversaries on a regional basis. Selective Availability was removed from the GPS architecture beginning with GPS-III. 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 provided horizontal accuracy of better than , although many factors such as receiver and antenna 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. 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 Staff The Joint Chiefs of Staff (JCS) is the body of the most senior uniformed leaders within the United States Department of Defense, that advises the president of the United States, the secretary of defense, the Homeland Security Council and ...
and
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
. 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 modernization

* In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of four prototype GPS receivers in a Y configuration over
White Sands Missile Range White Sands Missile Range (WSMR) is a United States Army military testing area and firing range located in the US state of New Mexico. The range was originally established as the White Sands Proving Ground on 9July 1945. White Sands National Pa ...
, using ground-based pseudo-satellites. * In 1978, the first experimental Block-I GPS satellite was launched. * In 1983, after Soviet
interceptor aircraft An interceptor aircraft, or simply interceptor, is a type of fighter aircraft Fighter aircraft are fixed-wing aircraft, fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircr ...
shot down the civilian airliner KAL 007 that strayed into
prohibited airspace A prohibited airspace is an area (volume) of airspace within which flight of aircraft is not allowed, usually due to security concerns. It is one of many types of special use airspace designations and is depicted on aeronautical charts with the le ...
because of navigational errors, killing all 269 people on board, U.S. President
Ronald Reagan Ronald Wilson Reagan ( ; February 6, 1911June 5, 2004) was an American politician, actor, and union leader who served as the 40th president of the United States from 1981 to 1989. He also served as the 33rd governor of California from 1967 ...
announced that GPS would be made available for civilian uses once it was completed, although it had been previously published in Navigation magazine, and 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 and 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 Colorado Springs is a List of municipalities in Colorado#Home rule municipality, home rule municipality in, and the county seat of, El Paso County, Colorado, El Paso County, Colorado, United States. It is the largest city in El Paso County, with ...
. * On February 14, 1989, the first modern Block-II satellite was launched. * The
Gulf War The Gulf War was a 1990–1991 armed campaign waged by a Coalition of the Gulf War, 35-country military coalition in response to the Iraqi invasion of Kuwait. Spearheaded by the United States, the coalition's efforts against Ba'athist Iraq, ...
from 1990 to 1991 was the first conflict in which the military widely used GPS. * In 1991, a project to create a miniature GPS receiver successfully ended, replacing the previous military receivers with a handheld receiver. * In 1992, the 2nd Space Wing, which originally managed the system, was inactivated and replaced by the
50th Space Wing The 50th Space Wing was the United States Space Force's space warfare, space and cyber warfare wing. The 50th Space Wing was assigned to Space Operations Command and headquartered at Schriever Air Force Base. It was activated in 1949 as the 5 ...
. * By December 1993, GPS achieved
initial operational capability Initial operating capability or initial operational capability (IOC) is the state achieved when a Capability Management, capability is available in its minimum usefully deployable form. The term is often used in government procurement, government ...
(IOC), with a full constellation (24 satellites) available and providing the Standard Positioning Service (SPS). * Full Operational Capability (FOC) was declared by
Air Force Space Command The atmosphere of Earth is the layer of gases, known collectively as air, retained by Gravity of Earth, Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating Atmo ...
(AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS). * In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President
Bill Clinton William Jefferson Clinton (Birth name, né Blythe III; born August 19, 1946) is an American politician who served as the 42nd president of the United States from 1993 to 2001. He previously served as governor of Arkansas from 1979 to 1981 ...
issued a policy directive declaring GPS a
dual-use In politics, diplomacy and export control, dual-use items refers to goods, software and technology that can be used for both civilian and military applications.
system and establishing an Interagency GPS Executive Board to manage it as a national asset. * In 1998, United States Vice President
Al Gore Albert Arnold Gore Jr. (born March 31, 1948) is an American politician, businessman, and environmentalist who served as the 45th vice president of the United States from 1993 to 2001 under President Bill Clinton. Gore was the Democratic Part ...
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 the
United States Congress The United States Congress is the legislature of the federal government of the United States. It is Bicameralism, bicameral, composed of a lower body, the United States House of Representatives, House of Representatives, and an upper body, ...
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 George Walker Bush (born July 6, 1946) is an American politician who served as the 43rd president of the United States from 2001 to 2009. A member of the Republican Party (United States), Republican Party, Bush family, and son of the 41st ...
updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing. * November 2004,
Qualcomm Qualcomm () is an American multinational corporation headquartered in San Diego, California, and Delaware General Corporation Law, incorporated in Delaware. It creates semiconductors, software, and services related to wireless technology. It ow ...
announced successful tests of
assisted GPS Assisted GNSS (A-GNSS) is a GNSS augmentation system that often significantly improves the startup performance—i.e., time to first fix, time-to-first-fix (TTFF)—of a global navigation satellite system (GNSS). A-GNSS works by providing the nece ...
for
mobile phones A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
. * 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 A ground segment consists of all the ground-based elements of a spaceflight, space system used by operators and support personnel, as opposed to the Satellite space segment, space segment and user segment. The ground segment enables management of ...
Control System was transferred to the new Architecture Evolution Plan. * On May 19, 2009, the United States
Government Accountability Office The U.S. Government Accountability Office (GAO) is a legislative branch government agency that provides auditing, evaluation, evaluative, and investigative services for the United States Congress. It is the supreme audit institution of the fede ...
issued a report warning that some GPS satellites could fail as soon as 2010. * On May 21, 2009, the
Air Force Space Command The atmosphere of Earth is the layer of gases, known collectively as air, retained by Gravity of Earth, Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating Atmo ...
allayed fears of GPS failure, saying "There's only a small risk we will not continue to exceed our performance standard." * 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. * On February 25, 2010, 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.

## Awards

On February 10, 1993, the
National Aeronautic Association The National Aeronautic Association of the United States (NAA) is a non-profit 501(c)(3) organization and a founding member of the Fédération Aéronautique Internationale (FAI). Founded in 1905, it is the oldest national aviation club in the Un ...
selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the US's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation,
Rockwell International Rockwell International was a major American manufacturing conglomerate (company), conglomerate involved in aircraft, the space industry, defense and commercial electronics, components in the automotive industry, printing presses, avionics and ...
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 radio navigation 50 years ago." Two GPS developers received the
National Academy of Engineering The National Academy of Engineering (NAE) is an American Nonprofit organization, nonprofit, NGO, non-governmental organization. The National Academy of Engineering is part of the National Academies of Sciences, Engineering, and Medicine, along ...
Charles Stark Draper Prize The U.S. National Academy of Engineering annually awards the Draper Prize, which is given for the advancement of engineering and the education of the public about engineering. It is one of three prizes that constitute the "Nobel Prizes of Enginee ...
for 2003: *
Ivan Getting Ivan Alexander Getting (January 18, 1912 – October 11, 2003) was an American physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time s ...
, emeritus president of
The Aerospace Corporation The Aerospace Corporation is an American nonprofit corporation that operates a federally funded research and development center (FFRDC) in El Segundo, California. The corporation provides technical guidance and advice on all aspects of space mi ...
and an engineer at MIT, established the basis for GPS, improving on the
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
land-based radio system called LORAN (''Lo''ng-range ''R''adio ''A''id to ''N''avigation). * Bradford Parkinson, professor of
aeronautics Aeronautics is the science or art involved with the study, design, and manufacturing of air flight–capable machines, and the techniques of operating aircraft and rockets within the atmosphere. The British Royal Aeronautical Society identifie ...
and
astronautics Astronautics (or cosmonautics) is the theory and practice of travel beyond atmosphere of Earth, Earth's atmosphere into outer space. Spaceflight is one of its main applications and space science its overarching field. The term ''astronautics'' (o ...
at
Stanford University Stanford University, officially Leland Stanford Junior University, is a Private university, private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. S ...
, 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 the
National Medal of Technology The National Medal of Technology and Innovation (formerly the National Medal of Technology) is an honor granted by the President of the United States to American inventors and innovators who have made significant contributions to the development ...
on February 13, 2006. 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 The Space Foundation is an American nonprofit organization whose mission is to advocate for all sectors of the global space industry through space awareness activities, educational programs, and major industry events. It was founded in 1983. Lo ...
Space Technology Hall of Fame. On October 4, 2011, the
International Astronautical Federation The International Astronautical Federation (IAF) is an international space advocacy organization based in Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city o ...
(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. On December 6, 2018, Gladys West was inducted into the Air Force Space and Missile Pioneers Hall of Fame in recognition of her work on an extremely accurate geodetic Earth model, which was ultimately used to determine the orbit of the GPS constellation. On February 12, 2019, four founding members of the project were awarded the Queen Elizabeth Prize for Engineering with the chair of the awarding board stating "Engineering is the foundation of civilisation; there is no other foundation; it makes things happen. And that's exactly what today's Laureates have done - they've made things happen. They've re-written, in a major way, the infrastructure of our world."

# Principles

The GPS satellites carry very stable
atomic clocks An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
that are synchronized with one another and with the reference atomic clocks at the ground control stations; any drift of the clocks aboard the satellites from the reference time maintained on the ground stations is corrected regularly. Since the speed of
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s (
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special relativity, special theory of relativity, is ...
) is constant and independent of the satellite speed, the time delay between when the satellite transmits a signal and the ground station receives it is proportional to the distance from the satellite to the ground station. With the distance information collected from multiple ground stations, the location coordinates of any satellite at any time can be calculated with great precision. Each GPS satellite carries an accurate record of its own position and time, and broadcasts that data continuously. Based on data received from multiple GPS
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radiois ...
s, an end user's GPS receiver can calculate its own four-dimensional position in
spacetime In physics, spacetime is a mathematical model that combines the three-dimensional space, three dimensions of space and one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visualize S ...
; However, at a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and the deviation of its own clock from satellite time).

## More detailed description

Each GPS satellite continually broadcasts a signal (
carrier wave In telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are ...
with
modulation In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using Electronic component, electronic devices. Electronics uses Passivity (engineer ...
) that includes: * A
pseudorandom A pseudorandom sequence of numbers is one that appears to be statistically random, despite having been produced by a completely deterministic and repeatable process. Background The generation of random numbers has many uses, such as for rand ...
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 Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
(TOF) values, which are (given the speed of light) approximately equivalent to receiver-satellite ranges plus time difference between the receiver and GPS satellites multiplied by speed of light, which are called pseudo-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 A Cartesian coordinate system (, ) in a plane is a coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the Position (geometry), position of the Point (geometry ...
with origin at the Earth's center) and the offset of the receiver clock relative to the GPS time are computed simultaneously, using the navigation equations to process the TOFs. The receiver's Earth-centered solution location is usually converted to
latitude In geography, latitude is a Geographic coordinate system, coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south ...
,
longitude Longitude (, ) is a geographic coordinate system, geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another Celestial navigation, celestial body. It is an angular measurement, usually exp ...
and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the
geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is ext ...
, which is essentially mean sea level. These coordinates may be displayed, such as on a
moving map display A moving map display is a type of navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navig ...
, or recorded or used by some other system, such as a vehicle guidance system.

## User-satellite geometry

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 In geometry, a hyperboloid of revolution, sometimes called a circular hyperboloid, is the surface (mathematics), surface generated by rotating a hyperbola around one of its Hyperbola#Nomenclature and features, principal axes. A hyperboloid is th ...
of revolution (see
Multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point (geometry), point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multila ...
). 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. It is sometimes incorrectly said that the user location is at the intersection of three spheres. While simpler to visualize, this is the case only 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 marked 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 it were an essential part of the GPS concept that all users needed to carry a synchronized clock, a smaller number of satellites could be deployed, but the cost and complexity of the user equipment would increase.

## Receiver in continuous operation

The description above is representative of a receiver start-up situation. Most receivers have a track algorithm, sometimes called a ''tracker'', 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 be computed only with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the
random error Observational error (or measurement error) is the difference between a measured value of a quantity Quantity or amount is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities c ...
of position measurement. GPS units can use measurements of the
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer (physics), observer who is moving relative to the wave source. It is named after the Austrian physicist Chri ...
of the signals received to compute velocity accurately. More advanced navigation systems use additional sensors like a
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
or an
inertial navigation system An inertial navigation system (INS) is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (direc ...
to complement GPS.

## Non-navigation applications

GPS requires four or more satellites to be visible for accurate navigation. 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 on the open ocean usually has a known elevation close to 0m, and the elevation of an aircraft may be known. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude,
dead reckoning In navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general c ...
, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible. Chapter 7

# Structure

The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment. The U.S. Space 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.

## Space segment

The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in
medium Earth orbit A medium Earth orbit (MEO) is an geocentric orbit, Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.
, and also includes the payload adapters to the boosters required to launch them into orbit. The GPS design originally called for 24 SVs, eight each in three approximately circular
orbits In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or pos ...
, but this was modified to six orbital planes with four satellites each. The six orbit planes have approximately 55°
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
(tilt relative to the Earth's
equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the ...
) and are separated by 60°
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
of the
ascending node An orbital node is either of the two points where an orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or ...
(angle along the equator from a reference point to the orbit's intersection).GPS Overview from the NAVSTAR Joint Program Office
. Retrieved December 15, 2006.
The
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...
is one-half a
sidereal day Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeper, timekeeping system that astronomers use to locate astronomical object, celestial objects. Using sidereal time, it is possible to easily poin ...
, ''i.e.'', 11 hours and 58 minutes, so that the satellites pass over the same locations or almost the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on the Earth's surface (see animation at right). 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°. Orbiting at an altitude of approximately ; orbital radius of approximately , each SV makes two complete orbits each
sidereal day Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeper, timekeeping system that astronomers use to locate astronomical object, celestial objects. Using sidereal time, it is possible to easily poin ...
, repeating the same
ground track A ground track or ground trace is the path on the surface of a planet directly below an aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift o ...
each day. 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. , there are 31 satellites in the GPS
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellati ...
, 27 of which are in use at a given time with the rest allocated as stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in evaluation. More decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve accuracy but also improves reliability and availability of the system, relative to a uniform system, when multiple satellites fail. With the expanded constellation, nine satellites are usually visible at any time from any point on the Earth with a clear horizon, ensuring considerable redundancy over the minimum four satellites needed for a position.

## Control segment

The control segment (CS) is composed of: # a master control station (MCS), # an alternative master control station, # four dedicated ground antennas, and # six dedicated monitor stations. The MCS can also access
Satellite Control Network The Satellite Control Network (SCN), operated by the United States Space Force's Space Delta 6, provides support for the operation, control, and maintenance of a variety of United States Department of Defense and some non-DoD satellites. This in ...
(SCN) ground antennas (for additional command and control capability) and NGA (
National Geospatial-Intelligence Agency The National Geospatial-Intelligence Agency (NGA) is a combat support agency within the United States Department of Defense whose primary mission is collecting, analyzing, and distributing geospatial intelligence (GEOINT) in support of national ...
) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii,
Kwajalein Atoll Kwajalein Atoll (; Marshallese language, Marshallese: ) is part of the Marshall Islands, Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which its majority English-speaking res ...
,
Ascension Island Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the Atlantic Ocean, South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overse ...
,
Diego Garcia Diego Garcia is an island of the British Indian Ocean Territory, a disputed overseas territory of the United Kingdom. It is a militarised atoll just south of the equator in the central Indian Ocean, and the largest of the 60 small islands of ...
,
Colorado Springs, Colorado Colorado Springs is a List of municipalities in Colorado#Home rule municipality, home rule municipality in, and the county seat of, El Paso County, Colorado, El Paso County, Colorado, United States. It is the largest city in El Paso County, with ...
and
Cape Canaveral , image = cape canaveral.jpg , image_size = 300 , caption = View of Cape Canaveral from space in 1991 , map = Florida#USA , map_width = 300 , type = Cape , map_caption = Location in Florida Flor ...
, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the MCS at
Schriever Space Force Base Schriever Space Force Base, previously Schriever Air Force Base, Falcon Air Force Base, and Falcon Air Force Station, is a base of the United States Space Force located approximately east of Peterson Space Force Base near Colorado Springs, Col ...
ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Space 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 at
Kwajalein Kwajalein Atoll (; Marshallese language, Marshallese: ) is part of the Marshall Islands, Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which its majority English-speaking res ...
,
Ascension Island Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the Atlantic Ocean, South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overse ...
,
Diego Garcia Diego Garcia is an island of the British Indian Ocean Territory, a disputed overseas territory of the United Kingdom. It is a militarised atoll just south of the equator in the central Indian Ocean, and the largest of the 60 small islands of ...
, and
Cape Canaveral , image = cape canaveral.jpg , image_size = 300 , caption = View of Cape Canaveral from space in 1991 , map = Florida#USA , map_width = 300 , type = Cape , map_caption = Location in Florida Flor ...
). These updates synchronize the atomic clocks on board the satellites to within a few
nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...
s of each other, and adjust the
ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possi ...
of each satellite's internal orbital model. The updates are created by a
Kalman filter For statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estimat ...
that uses inputs from the ground monitoring stations,
space weather Space weather is a branch of space physics Space physics, also known as solar-terrestrial physics or space-plasma physics, is the study of Plasma (physics), plasmas as they occur naturally in the Earth's Near space, upper atmosphere (aeronomy) a ...
information, and various other inputs. 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 (OCX), is fully developed and functional. The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS's mission capabilities, enabling U.S. Space Force 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% 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, 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 its launch into 2021, 5 years past the original deadline. According to the Government Accounting Office, even this new deadline looks shaky.

## User segment

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. 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 oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...
). 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, , 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. GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an
RS-232 In telecommunications, RS-232 or Recommended Standard (EIA), Recommended Standard 232 is a technical standard, standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between ...
port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. , even low-cost units commonly include
Wide Area Augmentation System The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to GNSS augmentation, augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and avai ...
(WAAS) receivers. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the
SiRF SiRF Technology, Inc. was a pioneer in the commercial use of GPS technology for consumer applications. The company was founded in 1995 and was headquartered in San Jose, California. Notable and founding members included Sanjai Kohli, Dado Banatao ...
and MTK protocols. Receivers can interface with other devices using methods including a serial connection,
USB Universal Serial Bus (USB) is an technical standard, industry standard that establishes specifications for cables, connectors and communication protocol, protocols for connection, communication and power supply (Interface (computing), interfa ...
, or
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limit ...
.

# Applications

While originally a military project, GPS is considered a
dual-use technology In politics, diplomacy and export control, dual-use items refers to goods, software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer h ...
, meaning it has significant civilian applications as well. 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.

## Civilian

Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer. *
Amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency commu ...
: clock synchronization required for several digital modes such as FT8, FT4 and JS8; also used with APRS for position reporting; is often critical during emergency and disaster communications support. *
Atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
: studying the
troposphere The troposphere is the first and lowest layer of the Earth's atmosphere, atmosphere of the Earth, and contains 75% of the total mass of the Atmosphere, planetary atmosphere, 99% of the total mass of water vapor, water vapour and aerosols, and is ...
delays (recovery of the water vapor content) and
ionosphere The ionosphere () is the ionization, ionized part of the upper atmosphere of Earth, from about to height above sea level, above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionize ...
delays (recovery of the number of free electrons). Recovery of Earth surface displacements due to the atmospheric pressure loading. *
Astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
: both positional and clock synchronization data is used in
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...
and
celestial mechanics Celestial mechanics is the branch of astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronol ...
and precise orbit determination. GPS is also used in both
amateur astronomy Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the Naked eye, unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur as ...
with small telescopes as well as by professional observatories for finding
extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s. * Automated vehicle: applying location and routes for cars and trucks to function without a human driver. *
Cartography Cartography (; from grc, χάρτης , "papyrus, sheet of paper, map"; and , "write") is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an im ...
: both civilian and military cartographers use GPS extensively. *
Cellular telephony Mobile telephony is the provision of telephony, telephone services to mobile phone, phones which may roaming, move around freely rather than stay landline, fixed in one location. Telephony is supposed to specifically point to a voice-only ser ...
: 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 first handsets with integrated GPS launched in the late 1990s. The U.S.
Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio Radio is the technology of signaling and telecommunication, communicating using radio wa ...
(FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and
Verizon Verizon Communications Inc., commonly known as Verizon, is an American multinational telecommunications conglomerate and a corporate component of the Dow Jones Industrial Average. The company is headquartered at 1095 Avenue of the Americas in ...
soon thereafter. * Clock synchronization: the accuracy of GPS time signals (±10 ns) is second only to the atomic clocks they are based on, and is used in applications such as
GPS disciplined oscillator A GPS clock, or GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a Crystal oven, quartz or Rubidium standard, rubidium oscillator whose output is controlled to agree with the signal ...
s. *
Disaster relief Emergency management or disaster management is the managerial function charged with creating the framework within which communities reduce vulnerability to hazards and cope with disasters. Emergency management, despite its name, does not actuall ...
/
emergency service Emergency services and rescue services are organizations that ensure Public security, public safety and health by addressing and resolving different emergency, emergencies. Some of these agencies exist solely for addressing certain types of emerge ...
s: many emergency services depend upon GPS for location and timing capabilities. * GPS-equipped
radiosonde A radiosonde is a battery-powered telemetry instrument carried into the atmosphere usually by a weather balloon that measures various Atmospheric sounding, atmospheric parameters and transmits them by radio to a ground receiver. Modern radiosonde ...
s and
dropsonde A dropsonde is an expendable weather reconnaissance device created by the National Center for Atmospheric Research (NCAR), designed to be dropped from an aircraft at altitude over water to measure (and therefore track) storm conditions as the devi ...
s: measure and calculate the atmospheric pressure, wind speed and direction up to from the Earth's surface. * Radio occultation for weather and atmospheric science applications. *
Fleet tracking A vehicle tracking system combines the use of automatic vehicle location in individual vehicles with computer software, software that collects these Fleet vehicle, fleet data for a comprehensive picture of vehicle locations. Modern vehicle track ...
: used to identify, locate and maintain contact reports with one or more
fleet Fleet may refer to: Vehicles *Fishing fleet *Naval fleet *Fleet vehicles, a pool of motor vehicles *Fleet Aircraft, the aircraft manufacturing company Places Canada *Fleet, Alberta, Canada, a hamlet England *Chesil Beach#The Fleet Lagoon, The ...
vehicles in real-time. *
Geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure ( geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equiv ...
: determination of Earth orientation parameters including the daily and sub-daily polar motion, and length-of-day variabilities, Earth's center-of-mass - geocenter motion, and low-degree gravity field parameters. * Geofencing:
vehicle tracking system A vehicle tracking system combines the use of automatic vehicle location in individual vehicles with software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is ...
s, person tracking systems, and pet tracking systems 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. *
Geotagging Geotagging, or GeoTagging, is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS (file format), RSS feeds and is a form of geospatial met ...
: applies location coordinates to digital objects such as photographs (in
Exif Exchangeable image file format (officially Exif, according to JEIDA/JEITA/CIPA specifications) is a standard that specifies file format, formats for images, sound, and ancillary tags used by digital cameras (including smartphones), Image scanner ...
data) 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. *
GPS tour An audio tour or audio guide provides a Sound recording, recorded spoken commentary, normally through a handheld device, to a visitor attraction such as a museum. They are also available for self-guided tours of outdoor locations, or as a part of ...
s: location determines what content to display; for instance, information about an approaching point of interest. *
Mental health Mental health encompasses emotional, psychological, and social well-being, influencing cognition Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It enc ...
: tracking mental health functioning and sociability. *
Navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navi ...
: navigators value digitally precise velocity and orientation measurements, as well as precise positions in real-time with a support of orbit and clock corrections. *
Orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or pos ...
determination of low-orbiting satellites with GPS receiver installed on board, such as GOCE,
GRACE Grace may refer to: Places United States * Grace, Idaho, a city * Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois * Little Goose Creek (Kentucky), location of Grace post office * Grace, Carroll County, Missouri, an unincor ...
, Jason-1,
Jason-2 OSTM/Jason-2, or Ocean Surface Topography Mission/Jason-2 satellite, was an international Earth observation satellite, Earth observation satellite altimeter joint mission for sea surface height measurements between NASA and CNES. It was the t ...
, TerraSAR-X, TanDEM-X, CHAMP,
Sentinel-3 Sentinel-3 is an Earth observation satellite An Earth observation satellite or Earth remote sensing satellite is a satellite used or designed for Earth observation (EO) from orbit, including spy satellites and similar ones intended for non ...
, and some cubesats, e.g., CubETH. * Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors. *
Recreation Recreation is an activity of leisure, leisure being discretionary time. The "need to do something for recreation" is an essential element of human biology and psychology. Recreational activities are often done for happiness, enjoyment, amusement, ...
: for example,
Geocaching Geocaching is an outdoor recreational activity, in which participants use a Global Positioning System (GPS) GPS navigation device, receiver or mobile device and other navigational techniques to hide and seek containers, called "geocaches" or " ...
,
Geodashing Geocaching is an outdoor recreational activity, in which participants use a Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigatio ...
, GPS drawing,
waymarking Trail blazing or way marking is the practice of marking paths in outdoor recreational areas with signs or markings that follow each other at certain, though not necessarily exactly defined, distances and mark the direction of the trail. A blaz ...
, and other kinds of location based mobile games such as
Pokémon Go ''Pokémon Go'' (stylized as ''Pokémon GO'') is a 2016 augmented reality (AR) mobile game, part of the ''Pokémon'' franchise, developed and published by Niantic (company), Niantic in collaboration with Nintendo and The Pokémon Company for ...
. *
Reference frames In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin (mathematics), origin, orientation (geometry), orientation, and scale (geometry), scale are specified by a set of reference point ...
: realization and densification of the terrestrial reference frames in the framework of Global Geodetic Observing System. Co-location in space between
Satellite laser ranging In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of milli ...
and microwave observations for deriving global geodetic parameters. *
Robotics Robotics is an interdisciplinarity, interdisciplinary branch of computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robotics is to design machines that can help and assist human ...
: self-navigating, autonomous robots using GPS sensors, which calculate latitude, longitude, time, speed, and heading. *
Sport Sport pertains to any form of Competition, competitive physical activity or game that aims to use, maintain, or improve physical ability and Skill, skills while providing enjoyment to participants and, in some cases, entertainment to specta ...
: used in football and rugby for the control and analysis of the training load. *
Surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Two-dimensional space#In geometry, two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of ...
: surveyors use absolute locations to make maps and determine property boundaries. *
Tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of orogeny, mountain building, the growth and behavior of the strong, old cores of con ...
: GPS enables direct fault motion measurement of
earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
. Between earthquakes GPS can be used to measure crustal motion and deformation to estimate seismic strain buildup for creating
seismic hazard A seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold. With a hazard thus estimated, seismic risk, risk can be asses ...
maps. *
Telematics Telematics is an interdisciplinary field encompassing telecommunications, vehicular technologies (road transport, road safety, etc.), electrical engineering (sensors, instrumentation, wireless communications, etc.), and computer science (multimedia ...
: GPS technology integrated with computers and mobile communications technology in
automotive navigation system An automotive navigation system is part of the automobile controls or a third party add-on used to find direction in an automobile. It typically uses a satellite navigation device to get its position data which is then correlated to a position o ...
s.

### Restrictions on civilian use

The U.S. government controls the export of some civilian receivers. All GPS receivers capable of functioning above above sea level and , or designed or modified for use with unmanned missiles and aircraft, are classified as
munitions Ammunition (informally ammo) is the material fired, scattered, dropped, or detonated from any weapon or weapon system. Ammunition is both expendable weapons (e.g., bombs, missiles, grenades, land mines) and the component parts of other weapo ...
(weapons)—which means they require
State Department The United States Department of State (DOS), or State Department, is an United States federal executive departments, executive department of the Federal government of the United States, U.S. federal government responsible for the country's fore ...
export licenses. 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 . These limits only apply to units or components exported from the United States. A growing trade in various components exists, including GPS units from other countries. These are expressly sold as ITAR-free.

## Military

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 the ''Soldier Digital Assistant''. * Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to
precision-guided munition A precision-guided munition (PGM, smart weapon, smart munition, smart bomb) is a guided Ammunition, munition intended to precisely hit a specific target, to minimize collateral damage and increase lethality against intended targets. During th ...
s to allow them to engage targets accurately. Military aircraft, particularly in
air-to-ground Air-to-ground weaponry is aircraft ordnance used by combat aircraft to attack ground targets. The weapons include Aerial bomb, bombs, machine guns, autocannons, air-to-surface missiles, Rocket (weapon), rockets, air-launched cruise missiles and g ...
roles, use GPS to find targets. * Missile and projectile guidance: GPS allows accurate targeting of various military weapons including
ICBM An intercontinental ballistic missile (ICBM) is a ballistic missile with a range (aeronautics), range greater than , primarily designed for nuclear weapons delivery (delivering one or more Thermonuclear weapon, thermonuclear warheads). Conventi ...
s,
cruise missile A cruise missile is a Missile, guided missile used against terrestrial or naval targets that remains in the atmosphere and flies the major portion of its flight path at approximately constant speed. Cruise missiles are designed to deliver a larg ...
s,
precision-guided munition A precision-guided munition (PGM, smart weapon, smart munition, smart bomb) is a guided Ammunition, munition intended to precisely hit a specific target, to minimize collateral damage and increase lethality against intended targets. During th ...
s and
artillery shell A shell, in a military context, is a projectile whose payload contains an explosive, incendiary device, incendiary, or other chemical filling. Originally it was called a bombshell, but "shell" has come to be unambiguous in a military context ...
s. Embedded GPS receivers able to withstand accelerations of 12,000 '' g'' or about have been developed for use in
howitzer A howitzer () is a long-ranged weapon, falling between a cannon (also known as an Artillery, artillery gun in the United States), which fires shells at flat trajectories, and a Mortar (weapon), mortar, which fires at high angles of ascent and de ...
shells. * 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 called a
bhangmeter A bhangmeter is a non-imaging radiometer installed on spy satellite, reconnaissance and GPS, navigation satellites to detect atmospheric nuclear explosion, nuclear detonations and determine the nuclear yield, yield of the nuclear weapon. They are a ...
, 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. General William Shelton has stated that future satellites may drop this feature to save money. 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. GPS's vulnerability to jamming is a threat that continues to grow as jamming equipment and experience grows. GPS signals have been reported to have been jammed many times over the years for military purposes. Russia seems to have several objectives for this behavior, such as intimidating neighbors while undermining confidence in their reliance on American systems, promoting their GLONASS alternative, disrupting Western military exercises, and protecting assets from drones. China uses jamming to discourage US surveillance aircraft near the contested
Spratly Islands The Spratly Islands ( fil, Kapuluan ng Kalayaan; zh, c=南沙群島/南沙群岛, s=, t=, p=Nánshā Qúndǎo; Malay language, Malay, id, Kepulauan Spratly; vi, Quần đảo Trường Sa) are a Spratly Islands dispute, disputed archip ...
.
North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia. It constitutes the northern half of the Korea, Korean Peninsula and shares borders with China and Russia to the north, at the Yalu River, Y ...
has mounted several major jamming operations near its border with South Korea and offshore, disrupting flights, shipping and fishing operations. Iranian Armed Forces disrupted the civilian airliner plane Flight PS752's GPS when it shot down the aircraft.

## Timekeeping

### Relativistic corrections

The GPS implements two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, using the special theory of relativity, and one for the difference in gravitational potential between satellite and receiver, using general relativity. The acceleration of the satellite could also be computed independently as a correction, depending on purpose, but normally the effect is already dealt with in the first two corrections.

### Leap seconds

While most clocks derive their time from
Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently ...
(UTC), the atomic clocks on the satellites are set to "GPS time". The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar ti ...
s or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with
International Atomic Time International Atomic Time (abbreviated TAI, from its French name ) is a high-precision Atomic clock, atomic coordinate time, coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the ...
(TAI) (TAI - GPS = 19 seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks. The GPS navigation message includes the difference between GPS time and UTC. GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016. Receivers subtract this offset from GPS time to calculate UTC and specific time zone 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).

### Accuracy

GPS time is theoretically accurate to about 14 nanoseconds, due to the
clock drift Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to ...
relative to
International Atomic Time International Atomic Time (abbreviated TAI, from its French name ) is a high-precision Atomic clock, atomic coordinate time, coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the ...
that the atomic clocks in GPS transmitters experience Most receivers lose some accuracy in their interpretation of the signals and are only accurate to about 100 nanoseconds.

### Format

As opposed to the year, month, and day format of the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years diffe ...
, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten- bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). It happened the second time at 23:59:42 UTC on April 6, 2019. 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 in the future the modernized GPS civil navigation (CNAV) message will use a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until 2137 (157 years after GPS week zero).

# Communication

The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military.

## Message format

: Each GPS satellite continuously broadcasts a ''navigation message'' on L1 (C/A and P/Y) and L2 (P/Y) frequencies at a rate of 50 bits per second (see
bitrate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
). Each complete message takes 750 seconds ( minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are subcommutated 25 times each, so that a complete data message requires the transmission of 25 full frames. Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50-bit/s, this gives 750 seconds to transmit an entire almanac message (GPS). Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite. The first subframe of each frame encodes the week number and the time within the week, as well as the data about the health of the satellite. The second and the third subframes contain the ''
ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possi ...
'' – the precise orbit for the satellite. The fourth and fifth subframes contain the ''almanac'', which contains coarse orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, to obtain an accurate satellite location from this transmitted message, the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. To collect all transmitted almanacs, the receiver must demodulate the message for 732 to 750 seconds or minutes. All satellites broadcast at the same frequencies, encoding signals using unique
code-division multiple access Code-division multiple access (CDMA) is a channel access method used by various radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between ...
(CDMA) so receivers can distinguish individual satellites from each other. The system uses two distinct CDMA encoding types: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted so that only the U.S. military and other NATO nations who have been given access to the encryption code can access it. The ephemeris is updated every 2 hours and is sufficiently stable for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The almanac is updated typically every 24 hours. Additionally, data for a few weeks following is uploaded in case of transmission updates that delay data upload.

## Satellite frequencies

: All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million
chips ''CHiPs'' is an American crime film, crime drama television series created by Rick Rosner and originally aired on NBC from September 15, 1977, to May 1, 1983. It follows the lives of two motorcycle police officer, officers of the California Hig ...
per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for
relativistic effects Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate chemical element, elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanatio ...
that make observers on the Earth perceive a different time reference with respect to the transmitters in orbit. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code. The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. The L3 signal at a frequency of 1.38105 GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space. One usage is the enforcement of nuclear test ban treaties. The L4 band at 1.379913 GHz is being studied for additional ionospheric correction. The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that provides this signal was launched in May 2010. On February 5, 2016, the 12th and final Block IIF satellite was launched. The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy." In 2011, a conditional waiver was granted to
LightSquared Ligado Networks, formerly known as LightSquared, is an American satellite communications company. After restructuring, emerging from bankruptcy and modifying its network plan, the new company, Ligado Networks, launched in 2016. It operates the Sk ...
to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issue that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10 MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10 MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this may seriously degrade the GPS signal for many consumer uses. ''
Aviation Week ''Aviation Week & Space Technology'', often abbreviated ''Aviation Week'' or ''AW&ST'', is the flagship magazine of the Aviation Week Network. The weekly magazine is available in print and online, reporting on the aerospace, defense and aviatio ...
'' magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.

## Demodulation and decoding

Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary
sequence In mathematics, a sequence is an enumerated collection of mathematical object, objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it contains Element (mathematics), members (also called ''eleme ...
known as a Gold code. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver. If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range 1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern. There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data. Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. For more information see Demodulation and Decoding, Advanced.

# Navigation equations

## Problem statement

The receiver uses messages received from satellites to determine the satellite positions and time sent. The ''x, y,'' and ''z'' components of satellite position and the time sent (''s'') are designated as 'xi, yi, zi, si''where the subscript ''i'' denotes the satellite and has the value 1, 2, ..., ''n'', where ''n'' ≥ 4. When the time of message reception indicated by the on-board receiver clock is $\tilde_i$, the true reception time is $t_i = \tilde_i - b$, where ''b'' is the receiver's clock bias from the much more accurate GPS clocks employed by the satellites. The receiver clock bias is the same for all received satellite signals (assuming the satellite clocks are all perfectly synchronized). The message's transit time is $\tilde_i - b - s_i$, where ''si'' is the satellite time. Assuming the message traveled at
the speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special relativity, special theory of relativity, is ...
, ''c'', the distance traveled is $\left\left(\tilde_i - b - s_i\right\right) c$. For n satellites, the equations to satisfy are: :$d_i = \left\left( \tilde_i - b - s_i \right\right)c, \; i=1,2,\dots,n$ where ''di'' is the geometric distance or range between receiver and satellite ''i'' (the values without subscripts are the ''x, y,'' and ''z'' components of receiver position): :$d_i = \sqrt$ Defining ''pseudoranges'' as $p_i = \left \left( \tilde_i - s_i \right \right)c$, we see they are biased versions of the true range: :$p_i = d_i + bc, \;i=1,2,...,n$ .section 4 beginning on page 1
Geoffery Blewitt: Basics of the GPS Techique
Since the equations have four unknowns 'x, y, z, b''the three components of GPS receiver position and the clock bias—signals from at least four satellites are necessary to attempt solving these equations. They can be solved by algebraic or numerical methods. Existence and uniqueness of GPS solutions are discussed by Abell and Chaffee. When ''n'' is greater than four, this system is overdetermined and a fitting method must be used. The amount of error in the results varies with the received satellites' locations in the sky, since certain configurations (when the received satellites are close together in the sky) cause larger errors. Receivers usually calculate a running estimate of the error in the calculated position. This is done by multiplying the basic resolution of the receiver by quantities called the geometric dilution of position (GDOP) factors, calculated from the relative sky directions of the satellites used. The receiver location is expressed in a specific coordinate system, such as latitude and longitude using the
WGS 84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describ ...
geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...
or a country-specific system.

## Geometric interpretation

The GPS equations can be solved by numerical and analytical methods. Geometrical interpretations can enhance the understanding of these solution methods.

### Spheres

The measured ranges, called pseudoranges, contain clock errors. In a simplified idealization in which the ranges are synchronized, these true ranges represent the radii of spheres, each centered on one of the transmitting satellites. The solution for the position of the receiver is then at the intersection of the surfaces of these spheres; see
trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point (geometry), point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multila ...
(more generally, true-range multilateration). Signals from at minimum three satellites are required, and their three spheres would typically intersect at two points. One of the points is the location of the receiver, and the other moves rapidly in successive measurements and would not usually be on Earth's surface. In practice, there are many sources of inaccuracy besides clock bias, including random errors as well as the potential for precision loss from subtracting numbers close to each other if the centers of the spheres are relatively close together. This means that the position calculated from three satellites alone is unlikely to be accurate enough. Data from more satellites can help because of the tendency for random errors to cancel out and also by giving a larger spread between the sphere centers. But at the same time, more spheres will not generally intersect at one point. Therefore, a near intersection gets computed, typically via least squares. The more signals available, the better the approximation is likely to be.

### Hyperboloids

If the pseudorange between the receiver and satellite ''i'' and the pseudorange between the receiver and satellite ''j'' are subtracted, , the common receiver clock bias (''b'') cancels out, resulting in a difference of distances . The locus of points having a constant difference in distance to two points (here, two satellites) is a
hyperbola In mathematics, a hyperbola (; pl. hyperbolas or hyperbolae ; adj. hyperbolic ) is a type of smooth function, smooth plane curve, curve lying in a plane, defined by its geometric properties or by equations for which it is the solution set. A h ...
on a plane and a
hyperboloid of revolution In geometry, a hyperboloid of revolution, sometimes called a circular hyperboloid, is the surface (mathematics), surface generated by rotating a hyperbola around one of its Hyperbola#Nomenclature and features, principal axes. A hyperboloid is th ...
(more specifically, a two-sheeted hyperboloid) in 3D space (see
Multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point (geometry), point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multila ...
). Thus, from four pseudorange measurements, the receiver can be placed at the intersection of the surfaces of three hyperboloids each with foci at a pair of satellites. With additional satellites, the multiple intersections are not necessarily unique, and a best-fitting solution is sought instead.

### Inscribed sphere

The receiver position can be interpreted as the center of an inscribed sphere (insphere) of radius ''bc'', given by the receiver clock bias ''b'' (scaled by the speed of light ''c''). The insphere location is such that it touches other spheres. The circumscribing spheres are centered at the GPS satellites, whose radii equal the measured pseudoranges ''p''i. This configuration is distinct from the one described above, in which the spheres' radii were the unbiased or geometric ranges ''d''i.

### Hypercones

The clock in the receiver is usually not of the same quality as the ones in the satellites and will not be accurately synchronized to them. This produces
pseudorange The pseudorange (from wikt:pseudo-, pseudo- and :wikt:range, range) is the ''pseudo'' distance between a satellite and a navigation satellite receiver (see GNSS_positioning_calculation#Note, GNSS positioning calculation), for instance Global Posi ...
s with large differences compared to the true distances to the satellites. Therefore, in practice, the time difference between the receiver clock and the satellite time is defined as an unknown clock bias ''b''. The equations are then solved simultaneously for the receiver position and the clock bias. The solution space 'x, y, z, b''can be seen as a four-dimensional
spacetime In physics, spacetime is a mathematical model that combines the three-dimensional space, three dimensions of space and one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visualize S ...
, and signals from at minimum four satellites are needed. In that case each of the equations describes a hypercone (or spherical cone), with the cusp located at the satellite, and the base a sphere around the satellite. The receiver is at the intersection of four or more of such hypercones.

## Solution methods

### Least squares

When more than four satellites are available, the calculation can use the four best, or more than four simultaneously (up to all visible satellites), depending on the number of receiver channels, processing capability, and geometric dilution of precision (GDOP). Using more than four involves an over-determined system of equations with no unique solution; such a system can be solved by a least-squares or weighted least squares method. :$\left\left( \hat,\hat,\hat,\hat \right\right) = \underset \sum_i \left\left( \sqrt + bc - p_i \right\right)^2$

### Iterative

Both the equations for four satellites, or the least squares equations for more than four, are non-linear and need special solution methods. A common approach is by iteration on a linearized form of the equations, such as the Gauss–Newton algorithm. The GPS was initially developed assuming use of a numerical least-squares solution method—i.e., before closed-form solutions were found.

### Closed-form

One closed-form solution to the above set of equations was developed by S. Bancroft. Its properties are well known;Chaffee, J. and Abel, J., "On the Exact Solutions of Pseudorange Equations", ''IEEE Transactions on Aerospace and Electronic Systems'', vol:30, no:4, pp: 1021–1030, 1994 in particular, proponents claim it is superior in low- GDOP situations, compared to iterative least squares methods. Bancroft's method is algebraic, as opposed to numerical, and can be used for four or more satellites. When four satellites are used, the key steps are inversion of a 4x4 matrix and solution of a single-variable quadratic equation. Bancroft's method provides one or two solutions for the unknown quantities. When there are two (usually the case), only one is a near-Earth sensible solution. When a receiver uses more than four satellites for a solution, Bancroft uses the
generalized inverse In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in mo ...
(i.e., the pseudoinverse) to find a solution. A case has been made that iterative methods, such as the Gauss–Newton algorithm approach for solving over-determined non-linear least squares problems, generally provide more accurate solutions. Leick et al. (2015) states that "Bancroft's (1985) solution is a very early, if not the first, closed-form solution." Other closed-form solutions were published afterwards,Alfred Kleusberg, "Analytical GPS Navigation Solution", ''University of Stuttgart Research Compendium'',1994Oszczak, B., "New Algorithm for GNSS Positioning Using System of Linear Equations," ''Proceedings of the 26th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2013)'', Nashville, TN, September 2013, pp. 3560–3563. although their adoption in practice is unclear.

## Error sources and analysis

GPS error analysis examines error sources in GPS results and the expected size of those errors. GPS makes corrections for receiver clock errors and other effects, but some residual errors remain uncorrected. Error sources include signal arrival time measurements, numerical calculations, atmospheric effects (ionospheric/tropospheric delays),
ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possi ...
and clock data, multipath signals, and natural and artificial interference. Magnitude of residual errors from these sources depends on geometric dilution of precision. Artificial errors may result from jamming devices and threaten ships and aircraft or from intentional signal degradation through selective availability, which limited accuracy to ≈ , but has been switched off since May 1, 2000.

# Regulatory spectrum issues concerning GPS receivers

In the United States, GPS receivers are regulated under the
Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio Radio is the technology of signaling and telecommunication, communicating using radio wa ...
's (FCC)
Part 15 Code of Federal Regulations In the law of the United States, the ''Code of Federal Regulations'' (''CFR'') is the codification of the general and permanent regulatory law, regulations promulgated by the executive departments and agencies of ...
rules. As indicated in the manuals of GPS-enabled devices sold in the United States, as a Part 15 device, it "must accept any interference received, including interference that may cause undesired operation." With respect to GPS devices in particular, the FCC states that GPS receiver manufacturers, "must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum." For the last 30 years, GPS receivers have operated next to the Mobile Satellite Service band, and have discriminated against reception of mobile satellite services, such as Inmarsat, without any issue. The spectrum allocated for GPS L1 use by the FCC is 1559 to 1610 MHz, while the spectrum allocated for satellite-to-ground use owned by Lightsquared is the Mobile Satellite Service band. Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the
Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic and Southeastern United States, Southeastern regions of the United States, between the East Coast of the United Stat ...
company
LightSquared Ligado Networks, formerly known as LightSquared, is an American satellite communications company. After restructuring, emerging from bankruptcy and modifying its network plan, the new company, Ligado Networks, launched in 2016. It operates the Sk ...
. On March 1, 2001, the FCC received an application from LightSquared's predecessor, Motient Services, to use their allocated frequencies for an integrated satellite-terrestrial service. In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions (OOBE) agreement with LightSquared to prevent transmissions from LightSquared's ground-based stations from emitting transmissions into the neighboring GPS band of 1559 to 1610 MHz. In 2004, the FCC adopted the OOBE agreement in its authorization for LightSquared to deploy a ground-based network ancillary to their satellite system – known as the Ancillary Tower Components (ATCs) – "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service." This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, which includes the U.S. Department of Agriculture, U.S. Space Force, U.S. Army, U.S. Coast Guard,
Federal Aviation Administration The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffi ...
,
National Aeronautics and Space Administration The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
(NASA), U.S. Department of the Interior, and U.S. Department of Transportation. In January 2011, the FCC conditionally authorized LightSquared's wholesale customers—such as
Best Buy Best Buy Co. Inc. is an American multinational consumer electronics retailer headquartered in Richfield, Minnesota. Originally founded by Richard M. Schulze and James Wheeler in 1966 as an audio specialty store called Sound of Music, it was rebra ...
, Sharp, and C Spire—to only purchase an integrated satellite-ground-based service from LightSquared and re-sell that integrated service on devices that are equipped to only use the ground-based signal using LightSquared's allocated frequencies of 1525 to 1559 MHz. In December 2010, GPS receiver manufacturers expressed concerns to the FCC that LightSquared's signal would interfere with GPS receiver devices although the FCC's policy considerations leading up to the January 2011 order did not pertain to any proposed changes to the maximum number of ground-based LightSquared stations or the maximum power at which these stations could operate. The January 2011 order makes final authorization contingent upon studies of GPS interference issues carried out by a LightSquared led working group along with GPS industry and Federal agency participation. On February 14, 2012, the FCC initiated proceedings to vacate LightSquared's Conditional Waiver Order based on the NTIA's conclusion that there was currently no practical way to mitigate potential GPS interference. GPS receiver manufacturers design GPS receivers to use spectrum beyond the GPS-allocated band. In some cases, GPS receivers are designed to use up to 400 MHz of spectrum in either direction of the L1 frequency of 1575.42 MHz, because mobile satellite services in those regions are broadcasting from space to ground, and at power levels commensurate with mobile satellite services. As regulated under the FCC's Part 15 rules, GPS receivers are not warranted protection from signals outside GPS-allocated spectrum. This is why GPS operates next to the Mobile Satellite Service band, and also why the Mobile Satellite Service band operates next to GPS. The symbiotic relationship of spectrum allocation ensures that users of both bands are able to operate cooperatively and freely. The FCC adopted rules in February 2003 that allowed Mobile Satellite Service (MSS) licensees such as LightSquared to construct a small number of ancillary ground-based towers in their licensed spectrum to "promote more efficient use of terrestrial wireless spectrum." In those 2003 rules, the FCC stated "As a preliminary matter, terrestrial ommercial Mobile Radio Service ("CMRS")and MSS ATC are expected to have different prices, coverage, product acceptance and distribution; therefore, the two services appear, at best, to be imperfect substitutes for one another that would be operating in predominantly different market segments... MSS ATC is unlikely to compete directly with terrestrial CMRS for the same customer base...". In 2004, the FCC clarified that the ground-based towers would be ancillary, noting that "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service." In July 2010, the FCC stated that it expected LightSquared to use its authority to offer an integrated satellite-terrestrial service to "provide mobile broadband services similar to those provided by terrestrial mobile providers and enhance competition in the mobile broadband sector." GPS receiver manufacturers have argued that LightSquared's licensed spectrum of 1525 to 1559 MHz was never envisioned as being used for high-speed wireless broadband based on the 2003 and 2004 FCC ATC rulings making clear that the Ancillary Tower Component (ATC) would be, in fact, ancillary to the primary satellite component.
To build public support of efforts to continue the 2004 FCC authorization of LightSquared's ancillary terrestrial component vs. a simple ground-based LTE service in the Mobile Satellite Service band, GPS receiver manufacturer
Trimble Navigation Trimble Inc. is an American software, hardware, and services technology company. Trimble supports global industries in building & construction, agriculture, geospatial, natural resources and utilities, governments, transportation and others. Trim ...
Ltd. formed the "Coalition To Save Our GPS." The FCC and LightSquared have each made public commitments to solve the GPS interference issue before the network is allowed to operate. According to Chris Dancy of the
Aircraft Owners and Pilots Association The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United States ...
, airline pilots with the type of systems that would be affected "may go off course and not even realize it." The problems could also affect the Federal Aviation Administration upgrade to the
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airsp ...
system,
United States Defense Department The United States Department of Defense (DoD, USDOD or DOD) is an United States federal executive departments, executive branch department of the Federal government of the United States, federal government charged with coordinating and superv ...
guidance, and local
emergency service Emergency services and rescue services are organizations that ensure Public security, public safety and health by addressing and resolving different emergency, emergencies. Some of these agencies exist solely for addressing certain types of emerge ...
s including 911. On February 14, 2012, the FCC moved to bar LightSquared's planned national broadband network after being informed by the
National Telecommunications and Information Administration The National Telecommunications and Information Administration (NTIA) is an agency of the United States Department of Commerce that serves as the President of the United States, President's principal adviser on telecommunications Telecommunica ...
(NTIA), the federal agency that coordinates spectrum uses for the military and other federal government entities, that "there is no practical way to mitigate potential interference at this time".FCC press releas
"Spokesperson Statement on NTIA Letter – LightSquared and GPS"
. February 14, 2012. Accessed March 3, 2013.
LightSquared is challenging the FCC's action.

# Similar systems

Other notable satellite navigation systems in use or various states of development include: *
Beidou The BeiDou Navigation Satellite System (BDS; ) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and ...
– system deployed and operated by the People's Republic of China's, initiating global services in 2019. *
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
– a global system being developed by the
European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are located primarily in Europe, Europe. The union has a total area of ...
and other partner countries, which began operation in 2016, and is expected to be fully deployed by 2020. *
GLONASS GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...
– Russia's global navigation system. Fully operational worldwide. *
NavIC The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NavIC (acronym for 'Navigation with Indian Constellation; also, 'sailor' or 'navigator' in Indian languages), is an autonomous regional satellite navigation s ...
– a regional navigation system developed by the
Indian Space Research Organisation The Indian Space Research Organisation (ISRO; ) is the Government space agency, national space agency of India, headquartered in Bangalore, Bengaluru. It operates under the Department of Space (DOS) which is directly overseen by the Prime Minist ...
. * QZSS – a regional navigation system receivable in the Asia-Oceania regions, with a focus on Japan.

# See also

* List of GPS satellites * GPS satellite blocks * GPS signals * GPS navigation software * GPS/INS * GPS spoofing *
Indoor positioning system An indoor positioning system (IPS) is a network of devices used to locate people or objects where GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation sy ...
*
Local Area Augmentation System The local-area augmentation system (LAAS) is an all-weather aircraft landing system based on real-time differential correction of the Global Positioning System, GPS signal. Local reference Receiver (radio), receivers located around the airport se ...
* Local positioning system * Military invention *
Mobile phone tracking Mobile phone tracking is a process for identifying the location of a mobile phone, whether stationary or moving. Localization may be effected by a number of technologies, such as the multilateration Trilateration is the use of distances (or "range ...
* Navigation paradox * Notice Advisory to Navstar Users * S-GPS

# Further reading

* * * *
Global Positioning System
Open Courseware from MIT, 2012 *

# External links

FAA GPS FAQ

GPS.gov
– General public education website created by the U.S. Government {{Authority control 20th-century inventions Equipment of the United States Space Force Military equipment introduced in the 1970s