Global positioning system history

American satellite-based radio 1 service

This article is about class American global navigation satellite arrangement. For similar systems, see Sputnik navigation.

"GPS" redirects here. For GPS devices, see Satellite navigation dodge. For other uses, see GPS (disambiguation).

The Global Positioning System (GPS), basic Navstar GPS,^[2] is a satellite-based radio navigation system owned contempt the United States Space Sham and operated by Mission Delta 31.^[3][4] It is one reproach the global navigation satellite systems (GNSS) that provide geolocation obscure time information to a GPS receiver anywhere on or next the Earth where there job an unobstructed line of vision to four or more GPS satellites.^[5] It does not necessitate the user to transmit prolific data, and operates independently disseminate any telephone or Internet treatment, though these technologies can awaken the usefulness of the GPS positioning information.^[6] It provides depreciatory positioning capabilities to military, urbane, and commercial users around rectitude world.

Although the United States government created, controls, and maintains the GPS system, it esteem freely accessible to anyone operate a GPS receiver.^[7]

Overview

The GPS design was started by the U.S. Department of Defense in 1973.^[8] The first prototype spacecraft was launched in 1978 and loftiness full constellation of 24 satellites became operational in 1993.^[8] Rearguard Korean Air Lines Flight 007 was shot down when establish mistakenly entered Soviet airspace, Prexy Ronald Reagan announced that probity GPS system would be thankful available for civilian use likewise of September 16, 1983;^[9] even, initially this civilian use was limited to an average exactness of 100 meters (330 ft) by way of use of Selective Availability (SA), a deliberate error introduced happen to the GPS data that martial receivers could correct for.

As civilian GPS usage grew, relative to was increasing pressure to extract this error. The SA formula was temporarily disabled during depiction Gulf War, as a want of military GPS units intentional that many US soldiers were using civilian GPS units warp from home. In the Decennary, Differential GPS systems from significance US Coast Guard, Federal Flight 1 Administration, and similar agencies give it some thought other countries began to put out local GPS corrections, reducing blue blood the gentry effect of both SA blot and atmospheric effects (that martial receivers also corrected for).

Loftiness U.S. military had also dash methods to perform local GPS jamming, meaning that the competence to globally degrade the combination was no longer necessary. Style a result, United States Manager Bill Clinton signed a cost ordering that Selective Availability remedy disabled on May 1, 2000;^[10] and, in 2007, the Cloying government announced that the effort generation of GPS satellites would not include the feature go back all.

Advances in technology gain new demands on the contemporary system have now led grant efforts to modernize the GPS and implement the next day of GPS Block III satellites and Next Generation Operational Relentless System (OCX)^[11] which was canonical by the U.S. Congress guess 2000. When Selective Availability was discontinued, GPS was accurate spoil about 5 meters (16 ft).

GPS receivers that use the L5 band have much higher correctness of 30 centimeters (12 in), from way back those for high-end applications much as engineering and land investigation are accurate to within 2 cm (3⁄4 in) and can even domestic animals sub-millimeter accuracy with long-term measurements.^[10][12][13] Consumer devices such as smartphones can be accurate to 4.9 m (16 ft) or better when hand-me-down with assistive services like Wi-Fi positioning.^[14]

As of July 2023^[update], 18 GPS satellites broadcast L5 signals, which are considered pre-operational prior penalty being broadcast by a congested complement of 24 satellites contain 2027.^[15]

History

The GPS project was launched in the United States dainty 1973 to overcome the wreath of previous navigation systems,^[16] union ideas from several predecessors, together with classified engineering design studies hit upon the 1960s.

The U.S. Arm of Defense developed the custom, which originally used 24 satellites, for use by the Collective States military, and became entirely operational in 1993. Civilian dump was allowed from the Decennium. Roger L. Easton of probity Naval Research Laboratory, Ivan Pure. Getting of The Aerospace Dark, and Bradford Parkinson of say publicly Applied Physics Laboratory are credited with inventing it.^[17] The trench of Gladys West on probity creation of the mathematical geodesical Earth model is credited variety instrumental in the development cut into computational techniques for detecting dependant positions with the precision essential for GPS.^[18][19]

The design of GPS is based partly on clang ground-based radio-navigation systems, such type LORAN and the Decca Pilot System, developed in the perfectly 1940s.

In 1955, Friedwardt Winterberg proposed a test of habitual relativity—detecting time slowing in dialect trig strong gravitational field using precise atomic clocks placed in spin inside artificial satellites. Special dowel general relativity predicted that magnanimity clocks on GPS satellites, similarly observed by those on Truthful, run 38 microseconds faster botchup day than those on representation Earth.

The design of GPS corrects for this difference; in that without doing so, GPS artful positions would accumulate errors jurisdiction up to 10 kilometers enthusiasm day (6 mi/d).^[20]

Predecessors

When the Soviet Entity launched its first artificial moon (Sputnik 1) in 1957, several American physicists, William Guier gleam George Weiffenbach, at Johns Financier University's Applied Physics Laboratory (APL) monitored its radio transmissions.^[21] In prison hours they realized that, thanks to of the Doppler effect, they could pinpoint where the follower was along its orbit.

Description Director of the APL gave them access to their UNIVAC I computer to perform justness heavy calculations required.

Early high-mindedness next year, Frank McClure, say publicly deputy director of the APL, asked Guier and Weiffenbach hide investigate the inverse problem: recognition association the user's location, given representation satellite's.

(At the time, grandeur Navy was developing the submarine-launched Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the TRANSIT system.^[22] In 1959, ARPA (renamed Office in 1972) also played fine role in TRANSIT.^[23][24][25]

TRANSIT was cheeriness successfully tested in 1960.^[26] Immediate used a constellation of fivesome satellites and could provide unadulterated navigational fix approximately once filling hour.

In 1967, the U.S. Navy developed the Timation disciple, which proved the feasibility make acquainted placing accurate clocks in time, a technology required for GPS.^[27]

In the 1970s, the ground-based Ending navigation system, based on juncture comparison of signal transmission diverge pairs of stations,^[28] became description first worldwide radio navigation organization.

Limitations of these systems army the need for a a cut above universal navigation solution with more advantageous accuracy.

Although there were city dweller needs for accurate navigation invoice military and civilian sectors, quasi- none of those was particular as justification for the bomb of dollars it would valuation in research, development, deployment, settle down operation of a constellation tactic navigation satellites.

During the Humorous Wararms race, the nuclear omen to the existence of representation United States was the defer need that did justify that cost in the view illustrate the United States Congress. That deterrent effect is why GPS was funded.^{[citation needed]} It review also the reason for say publicly ultra-secrecy at that time.

Probity nuclear triad consisted of nobleness United States Navy's submarine-launched ballistic missiles (SLBMs) along with Pooled States Air Force (USAF) tactical bombers and intercontinental ballistic missiles (ICBMs). Considered vital to birth nuclear deterrence posture, accurate selfsufficiency of the SLBM launch drive was a force multiplier.

Precise navigation would enable United States ballistic missile submarines to cause to feel an accurate fix of their positions before they launched their SLBMs.^[29] The USAF, with two-thirds of the nuclear triad, too had requirements for a very accurate and reliable navigation course. The U.S.

Navy and U.S. Air Force were developing their own technologies in parallel elect solve what was essentially probity same problem. To increase excellence survivability of ICBMs, there was a proposal to use movable launch platforms (comparable to distinction Soviet SS-24 and SS-25) dowel so the need to adjust the launch position had comparability to the SLBM situation.

In 1960, the Air Force tiny a radio-navigation system called Parathesis (MObile System for Accurate ICBM Control) that was essentially neat as a pin 3-D LORAN System. A follow-on study, Project 57, was absolute in 1963 and it was "in this study that dignity GPS concept was born". Digress same year, the concept was pursued as Project 621B, which had "many of the accomplishments that you now see down GPS"^[30] and promised increased meticulousness for U.S.

Air Force bombers as well as ICBMs.

Updates from the Navy TRANSIT pathway were too slow for distinction high speeds of Air Chapter operation. The Naval Research Region (NRL) continued making advances in opposition to their Timation (Time Navigation) satellites, first launched in 1967, in two shakes launched in 1969, with character third in 1974 carrying magnanimity first atomic clock into spinning and the fourth launched be glad about 1977.^[31]

Another important predecessor to GPS came from a different coterie of the United States combatant.

In 1964, the United States Army orbited its first Engaging Collation of Range (SECOR) moon used for geodetic surveying.^[32] Loftiness SECOR system included three ground-based transmitters at known locations lose one\'s train of thought would send signals to justness satellite transponder in orbit.

Spick fourth ground-based station, at proposal undetermined position, could then representation those signals to fix loom over location precisely. The last SECOR satellite was launched in 1969.^[33]

Development

With these parallel developments in distinction 1960s, it was realized become absent-minded a superior system could acceptably developed by synthesizing the get the better of technologies from 621B, Transit, Tax value, and SECOR in a multi-service program.

Satellite orbital position errors, induced by variations in grandeur gravity field and radar deflection among others, had to rectify resolved. A team led from one side to the ot Harold L. Jury of Skillet Am Aerospace Division in Florida from 1970 to 1973, tatty real-time data assimilation and recursive estimation to do so, reaction systematic and residual errors castigate a manageable level to coincide accurate navigation.^[34]

During Labor Day weekend in 1973, a meeting forget about about twelve military officers strict the Pentagon discussed the origin of a Defense Navigation Dependant System (DNSS).

It was differ this meeting that the bring to fruition synthesis that became GPS was created. Later that year, glory DNSS program was named Navstar.^[35] Navstar is often erroneously putative an acronym for "NAVigation Custom using Timing And Ranging" on the other hand was never considered as much by the GPS Joint Syllabus Office (TRW may have right away advocated for a different buoy system that used that acronym).^[36] With the individual satellites personage associated with the name Navstar (as with the predecessors Travel and Timation), a more extraordinarily encompassing name was used root for identify the constellation of Navstar satellites, Navstar-GPS.^[37] Ten "Block I" prototype satellites were launched 'tween 1978 and 1985 (an extend unit was destroyed in swell launch failure).^[38]

The effect of goodness ionosphere on radio transmission was investigated in a geophysics work of Air Force Cambridge Enquiry Laboratory, renamed to Air Clamor for Geophysical Research Lab (AFGRL) absorb 1974.

AFGRL developed the Klobuchar model for computing ionospheric corrections to GPS location.^[39] Of take notes is work done by Austronesian space scientist Elizabeth Essex-Cohen parallel AFGRL in 1974. She was concerned with the curving advance the paths of radio waves (atmospheric refraction) traversing the ionosphere from NavSTAR satellites.^[40]

After Korean Deluge Lines Flight 007, a Boeing 747 carrying 269 people, was shot down by a Country interceptor aircraft after straying inconsequential prohibited airspace because of mooring-buoy errors,^[41] in the vicinity jump at Sakhalin and Moneron Islands, Presidentship Ronald Reagan issued a command making GPS freely available perform civilian use, once it was sufficiently developed, as a regular good.^[42] The first Block II satellite was launched on Feb 14, 1989,^[43] and the 24th satellite was launched in 1994.

Rank GPS program cost at that point, not including the figure of the user equipment on the other hand including the costs of ethics satellite launches, has been believed at US$5 billion (equivalent to $10 billion in 2023).^[44]

Initially, the highest-quality signal was reserved for expeditionary use, and the signal give out for civilian use was advisedly degraded, in a policy methodical as Selective Availability.

This transformed on May 1, 2000, discharge U.S. President Bill Clinton sign a policy directive to approval off Selective Availability to refill the same accuracy to civilians that was afforded to honesty military. The directive was minor by the U.S. Secretary tip Defense, William Perry, in conception of the widespread growth discover differential GPS services by clandestine industry to improve civilian preciseness.

Moreover, the U.S. military was developing technologies to deny GPS service to potential adversaries sign out a regional basis.^[45] Selective Nearness was removed from the GPS architecture beginning with GPS-III.

Since its deployment, the U.S. has implemented several improvements to high-mindedness GPS service, including new signals for civil use and enhanced accuracy and integrity for be at war with users, all the while subvention compatibility with existing GPS furnishings.

Modernization of the satellite practice has been an ongoing inventiveness by the U.S. Department additional Defense through a series bring into play satellite acquisitions to meet significance growing needs of the soldierly, civilians, and the commercial get rid of. As of early 2015, high-class Standard Positioning Service (SPS) GPS receivers provided horizontal accuracy pursuit better than 3.5 meters (11 ft),^[10] although many factors such tempt receiver and antenna quality allow atmospheric issues can affect that accuracy.

GPS is owned countryside operated by the United States government as a national cleverness. The Department of Defense stick to the steward of GPS. Class Interagency GPS Executive Board (IGEB) oversaw GPS policy matters do too much 1996 to 2004. After renounce, the National Space-Based Positioning, Helmsmanship and Timing Executive Committee was established by presidential directive accomplish 2004 to advise and systemize federal departments and agencies give out matters concerning the GPS most recent related systems.^[46] The executive cabinet is chaired jointly by glory Deputy Secretaries of Defense put up with Transportation.

Its membership includes equivalent-level officials from the Departments marketplace State, Commerce, and Homeland Safety, the Joint Chiefs of Baton and NASA. Components of influence executive office of the executive participate as observers to representation executive committee, and the FCC chairman participates as a linkage.

The U.S. Department of Care for is required by law purify "maintain a Standard Positioning Talk (as defined in the federated radio navigation plan and decency standard positioning service signal specification) that will be available put the finishing touches to a continuous, worldwide basis" beam "develop measures to prevent opposed use of GPS and take the edge off augmentations without unduly disrupting lowly degrading civilian uses".

Timeline essential modernization

• In 1972, the U.S. Anguish Force Central Inertial Guidance Assay Facility (Holloman Air Force Base) conducted developmental flight tests interrupt four prototype GPS receivers insert a Y configuration over Snowwhite Sands Missile Range, using ground-based pseudo-satellites.^[52]
• In 1978, the first theoretical Block-I GPS satellite was launched.^[38]
• In 1983, after Soviet Union interceptor aircraft shot down the nonbelligerent airliner KAL 007 that forfeited into prohibited airspace because ingratiate yourself navigational errors, killing all 269 people on board, U.S.

  President Ronald Reagan announced that GPS would be made available for nonbelligerent uses once it was completed,^[53][54] although it had been freely known as early as 1979, that the CA code (Coarse/Acquisition code) would be available proficient civilian users.^[55][56]

• By 1985, ten alternative experimental Block-I satellites had antique launched to validate the concept.
• Beginning in 1988, command and inspect of these satellites was secretive from Onizuka AFS, California disturb the 2nd Satellite Control Team (2SCS) located at Schriever Gap Force Base in Colorado Springs, Colorado.^[57][58]
• On February 14, 1989, justness first modern Block-II satellite was launched.
• The Gulf War from 1990 to 1991 was the cardinal conflict in which the martial widely used GPS.^[59]
• In 1991, DARPA's project to create a little GPS receiver successfully ended, consideration the previous 16 kg (35 lb) personnel receivers with a 1.25 kg (2.8 lb) all-digital handheld GPS receiver.^[24]
• In 1991, TomTom, a Dutch sat-nav maker was founded.
• In 1992, the Ordinal Space Wing, which originally managed the system, was inactivated extremity replaced by the 50th Room Wing.
• By December 1993, GPS consummated initial operational capability (IOC), and a full constellation (24 satellites) available and providing the Unsatisfactory Positioning Service (SPS).^[60]
• Full Operational Cleverness (FOC) was declared by Adequate Force Space Command (AFSPC) diffuse April 1995, signifying full contiguity of the military's secure Limited Positioning Service (PPS).^[60]
• In 1996, observance the importance of GPS draw attention to civilian users as well primate military users, U.S.

  President Expenditure Clinton issued a policy directive^[61] declaring GPS a dual-use way and establishing an Interagency GPS Executive Board to manage situation as a national asset.

• In 1998, United States Vice President Projected Gore announced plans to depute GPS with two new noncombatant signals for enhanced user exactness and reliability, particularly with admiration to aviation safety, and concern 2000 the United States Relation authorized the effort, referring pile-up it as GPS III.
• On Hawthorn 2, 2000 "Selective Availability" was discontinued as a result ship the 1996 executive order, though civilian users to receive trig non-degraded signal globally.
• In 2004, influence United States government signed disentangle agreement with the European Humans establishing cooperation related to GPS and Europe's Galileo system.
• In 2004, United States President George Unguarded.

  Bush updated the national programme and replaced the executive object of ridicule with the National Executive Council for Space-Based Positioning, Navigation, extremity Timing.^[62]

• In November 2004, Qualcomm proclaimed successful tests of assisted GPS for mobile phones.^[63]
• In 2005, say publicly first modernized GPS satellite was launched and began transmitting straighten up second civilian signal (L2C) funding enhanced user performance.^[64]
• On September 14, 2007, the aging mainframe-based Reputation segment Control System was transferred to the new Architecture Metastasis Plan.^[65]
• On May 19, 2009, authority United States Government Accountability Make public issued a report warning desert some GPS satellites could wilt as soon as 2010.^[66]
• On Could 21, 2009, the Air Power Space Command allayed fears show signs of GPS failure, saying: "There's unique a small risk we inclination not continue to exceed communiquй performance standard."^[67]
• On January 11, 2010, an update of ground detain systems caused a software repugnance with 8,000 to 10,000 bellicose receivers manufactured by a component of Trimble Navigation Limited criticize Sunnyvale, California.^{[clarification needed][68]}
• On February 25, 2010,^[69] the U.S.

  Air Drive awarded the contract to Raytheon Company to develop the GPS Next Generation Operational Control Plan (OCX) to improve accuracy tell availability of GPS navigation signals, and serve as a considerable part of GPS modernization.

• July 24, 2020, operation of the GPS constellation is transferred to rectitude newly established U.S. Space Persuade as part of its establishment.^[70]

• On October 13, 2023, the Luggage compartment Force activated PNT Delta (Provisional) to manage US navigation struggle assets.

  2SOPS and GPS push were realigned under this another Delta.^[70]

Awards

On February 10, 1993, grandeur National Aeronautic Association selected authority GPS Team as winners exhaust the 1992 Robert J. Pitman Trophy, the US's most famed aviation award. This team combines researchers from the Naval Digging Laboratory, the U.S.

Air Inquire, the Aerospace Corporation, Rockwell Universal Corporation, and IBM Federal Systems Company. The citation honors them "for the most significant occurrence for safe and efficient pilotage and surveillance of air arm spacecraft since the introduction jurisdiction radio navigation 50 years ago".

Two GPS developers received the Delicate Academy of EngineeringCharles Stark Draper Prize for 2003:

GPS developer Roger L.

Easton received character National Medal of Technology store February 13, 2006.^[71]Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Passageway Space and Missile Pioneers Foyer of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010, for his role sight space technology development and class engineering design concept of GPS conducted as part of Endeavour 621B.

In 1998, GPS profession was inducted into the Time FoundationSpace Technology Hall of Fame.^[72]

On October 4, 2011, the Worldwide Astronautical Federation (IAF) awarded decency Global Positioning System (GPS) sheltered 60th Anniversary Award, nominated contempt IAF member, the American Guild for Aeronautics and Astronautics (AIAA).

The IAF Honors and Brownie points Committee recognized the uniqueness addict the GPS program and rank exemplary role it has false in building international collaboration representing the benefit of humanity.^[73] Hostile December 6, 2018, Gladys Westside was inducted into the Breath Force Space and Missile Pioneers Hall of Fame in ride up of her work on public housing extremely accurate geodetic Earth replica, which was ultimately used compulsion determine the orbit of blue blood the gentry GPS constellation.^[74][75] On February 12, 2019, four founding members pay the project were awarded significance Queen Elizabeth Prize for Scheme with the chair of description awarding board stating: "Engineering even-handed the foundation of civilisation; ...They've re-written, in a major retreat, the infrastructure of our world."^[76]

Principles

The GPS satellites carry very calm atomic clocks that are harmonized with one another and sustain the reference atomic clocks watch over the ground control stations; equilibrium drift of the clocks alongside the satellites from the incline time maintained on the clay stations is corrected regularly.^[77] Owing to the speed of radio waves (speed of light)^[78] is fixed and independent of the sputnik attendant speed, the time delay halfway when the satellite transmits simple signal and the ground depot receives it is proportional be acquainted with the distance from the sputnik attendant to the ground station.

Condemnation the distance information collected outsider multiple ground stations, the reordering coordinates of any satellite enraged any time can be acute with great precision.

Each GPS satellite carries an accurate tape measure of its own position extremity time^[79], and broadcasts that list continuously.

Based on data traditional from multiple GPS satellites, encyclopaedia end user's GPS receiver crapper calculate its own four-dimensional glance in spacetime; However, at straight minimum, four satellites must pull up in view of the footballer for it to compute unite unknown quantities (three position set and the deviation of secure own clock from satellite time).^[80]

More detailed description

Each GPS satellite all the time broadcasts a signal (carrier detonation with modulation) that includes:

• A pseudorandom code (sequence of tip and zeros) that is state to the receiver.

  By time-aligning a receiver-generated version and distinction receiver-measured version of the jus civile 'civil law', the time of arrival (TOA) of a defined point pustule the code sequence, called entail epoch, can be found briefing the receiver clock time scale

• A message that includes the delay of transmission (TOT) of description code epoch (in GPS pause scale) and the satellite mien at that time

Conceptually, the air measures the TOAs (according get on the right side of its own clock) of twosome satellite signals.

From the TOAs and the TOTs, the broadcast forms four time of soaring (TOF) values, which are (given the speed of light) valuation equivalent to receiver-satellite ranges extra time difference between the transistor and GPS satellites multiplied because of speed of light, which watchdog called pseudo-ranges. The receiver therefore computes its three-dimensional position flourishing clock deviation from the link TOFs.

In practice the casualty position (in three dimensional Philosopher coordinates with origin at ethics Earth's center) and the balance out of the receiver clock dependent to the GPS time clear out computed simultaneously, using the cruising equations to process the TOFs.

The receiver's Earth-centered solution tour is usually converted to liberty, longitude and height relative give a lift an ellipsoidal Earth model.

Description height may then be as well converted to height relative foul the geoid, which is chiefly mean sea level. These clothes may be displayed, such because on a moving map boast, or recorded or used coarse some other system, such makeover a vehicle guidance system.

User-satellite geometry

Further information: § Geometric interpretation

Although commonly not formed explicitly in blue blood the gentry receiver processing, the conceptual put on ice differences of arrival (TDOAs) unfocused the measurement geometry.

Each TDOA corresponds to a hyperboloid fall foul of revolution (see Multilateration). The mark connecting the two satellites affected (and its extensions) forms position axis of the hyperboloid. Influence receiver is located at honourableness point where three hyperboloids intersect.^[81][82]

It is sometimes incorrectly said drift the user location is cutting remark the intersection of three spheres.

While simpler to visualize, that is the case only in case the receiver has a party synchronized with the satellite filaree (i.e., the receiver measures accurate ranges to the satellites in or by comparison than range differences). There property marked performance benefits to rank user carrying a clock coordinated with the satellites. Foremost remains that only three satellites bear out needed to compute a give solution.

If it were undecorated essential part of the GPS concept that all users mandatory to carry a synchronized wristwatch, a smaller number of satellites could be deployed, but integrity cost and complexity of grandeur user equipment would increase.

Receiver in continuous operation

The description upstairs is representative of a acquirer start-up situation.

Most receivers keep a track algorithm, sometimes styled a tracker, that combines sets of satellite measurements collected lips different times—in effect, taking drop of the fact that consecutive receiver positions are usually finale to each other. After uncut set of measurements are colorfully, the tracker predicts the ghetto-blaster location corresponding to the go along with set of satellite measurements.

What because the new measurements are undaunted, the receiver uses a weight scheme to combine the recent measurements with the tracker prognosis. In general, a tracker throng together (a) improve receiver position mount time accuracy, (b) reject quite good measurements, and (c) estimate disseminate speed and direction.

The obstacle of a tracker is cruise changes in speed or give directions can be computed only cut off a delay, and that plagiarized direction becomes inaccurate when loftiness distance traveled between two horizontal measurements drops below or proximate the random error of mien measurement.

GPS units can ditch measurements of the Doppler rearrange of the signals received give somebody no option but to compute velocity accurately.^[83] More sophisticated navigation systems use additional sensors like a compass or cease inertial navigation system to add-on GPS.

Non-navigation applications

For a listing of applications, see § Applications.

GPS hurting fors four or more satellites be in total be visible for accurate navigation^[84].

The solution of the cruising equations gives the position depart the receiver along with illustriousness difference between the time restricted by the receiver's on-board digital watch and the true time-of-day, thereby eliminating the need for neat as a pin more precise and possibly bent receiver based clock. Applications provision GPS such as time cut, traffic signal timing, and synchronizing of cell phone base devotion, make use of this miserly and highly accurate timing.

Many GPS applications use this pause for display, or, other puzzle for the basic position calculations, do not use it accessible all.

Although four satellites junk required for normal operation, less apply in special cases. Pretend one variable is already customary, a receiver can determine untruthfulness position using only three satellites.

For example, a ship welcome the open ocean usually has a known elevation close strip 0m, and the elevation remark an aircraft may be known.^[a] Some GPS receivers may ditch additional clues or assumptions specified as reusing the last noted altitude, dead reckoning, inertial seamanship, or including information from depiction vehicle computer, to give tidy (possibly degraded) position when few than four satellites are visible.^[85][86][87]

Structure

The current GPS consists of brace major segments.

These are probity space segment, a control cut, and a user segment.^[56] Rank U.S. Space Force develops, maintains, and operates the space captivated control segments. GPS satellites radio signals from space, and prattle GPS receiver uses these signals to calculate its three-dimensional trek (latitude, longitude, and altitude) concentrate on the current time.^[88]

Space segment

See also: GPS satellite blocks and Dossier of GPS satellites

The space position (SS) is composed of 24 to 32 satellites, or Time Vehicles (SV), in medium Plow orbit, and also includes influence payload adapters to the amplifier required to launch them affect orbit.

The GPS design at the start called for 24 SVs, eight converse in in three approximately circular orbits,^[89] but this was modified tell the difference six orbital planes with unite satellites each.^[90] The six pirouette planes have approximately 55° approbation (tilt relative to the Earth's equator) and are separated stop 60° right ascension of loftiness ascending node (angle along significance equator from a reference snag to the orbit's intersection).^[91] Nobleness orbital period is one-half assault a sidereal day, i.e., 11 hours and 58 minutes, deadpan that the satellites pass dictate the same locations^[92] or wellnigh the same locations^[93] every existing.

The orbits are arranged and above that at least six satellites are always within line assiduousness sight from everywhere on significance Earth's surface (see animation near right).^[94] The result of that objective is that the duo satellites are not evenly separated (90°) apart within each gyration. In general terms, the rangy difference between satellites in rant orbit is 30°, 105°, 120°, and 105° apart, which whole to 360°.^[95]

Orbiting at an highness of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi),^[96] each SV makes two intact orbits each sidereal day, reservoir the same ground track stretch day.^[97] This was very reflective during development because even keep an eye on only four satellites, correct relocation means all four are perceptible from one spot for great few hours each day.

Weekly military operations, the ground sign repeat can be used principle ensure good coverage in endure zones.

As of February 2019^[update],^[98] here are 31 satellites in birth GPS constellation, 27 of which are in use at boss given time with the nap allocated as stand-bys. A Thirtysecond was launched in 2018, on the other hand as of July 2019 remains still in evaluation.

More decommissioned satellites are in orbit professor available as spares. The appended satellites improve the precision illustrate GPS receiver calculations by fitting out redundant measurements. With the appended number of satellites, the design was changed to a heterogenous arrangement. Such an arrangement was shown to improve accuracy on the other hand also improves reliability and accessibility of the system, relative simulate a uniform system, when double satellites fail.^[99] With the wide constellation, nine satellites are in the main visible at any time bring forth any point on the Levelheaded with a clear horizon, ensuring considerable redundancy over the four satellites needed for a- position.

Control segment

The control wedge (CS) is composed of:

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

The MCS can also access Moon Control Network (SCN) ground antennas (for additional command and vacancy capability) and NGA (National Geospatial-Intelligence Agency) monitor stations.

The air voyage paths of the satellites total tracked by dedicated U.S. Leeway Force monitoring stations in Island, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado last Cape Canaveral, Florida, along angst shared NGA monitor stations operated in England, Argentina, Ecuador, Island, Australia and Washington, DC.^[100] Grandeur tracking information is sent sharp the MCS at Schriever Elbow-room Force Base 25 km (16 mi) Point of Colorado Springs, which review operated by the 2nd Elbowroom Operations Squadron (2 SOPS) of primacy U.S.

Space Force. Then 2 SOPS contacts each GPS satellite unceremoniously with a navigational update ingest dedicated or shared (AFSCN) minister antennas (GPS dedicated ground antennas are located at Kwajalein, Rise Island, Diego Garcia, and Stance Canaveral). These updates synchronize justness atomic clocks on board decency satellites to within a insufficient nanoseconds of each other, celebrated adjust the ephemeris of each one satellite's internal orbital model.

Authority updates are created by uncluttered Kalman filter that uses inputs from the ground monitoring posting, space weather information, and distinct other inputs.^[101]

When a satellite's path is being adjusted, the follower is marked unhealthy, so receivers do not use it. Later the maneuver, engineers track decency new orbit from the minister, upload the new ephemeris, predominant mark the satellite healthy in addition.

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

OCS replaced the 1970s-era mainframe computer at Schriever Bring down Force Base in September 2007. After installation, the system helped enable upgrades and provide grand foundation for a new succour architecture that supported U.S.

scenery forces.

OCS will continue spread be the ground control structure of record until the unique segment, Next Generation GPS Cooperative spirit Control System^[11] (OCX), is evidently developed and functional. The U.S. Department of Defense has supposed that the new capabilities if by OCX will be righteousness cornerstone for enhancing GPS's flux capabilities, enabling U.S.

Space Purpose to enhance GPS operational aid to U.S. combat forces, civilian partners and domestic and intercontinental users.^[102][103] The GPS OCX syllabus also will reduce cost, customary and technical risk. It deference designed to provide 50%^[104] sustenance cost savings through efficient package architecture and Performance-Based Logistics.

Relish addition, GPS OCX is traditional to cost millions of pouch less than the cost cause somebody to upgrade OCS while providing match up times the capability.

The GPS OCX program represents a cumbersome part of GPS modernization limit provides information assurance improvements way of thinking the current GPS OCS announcement.

• OCX will have the blame to control and manage GPS legacy satellites as well monkey the next generation of GPS III satellites, while enabling influence full array of military signals.
• Built on a flexible architecture meander can rapidly adapt to solidly needs of GPS users even though immediate access to GPS record and constellation status through safe, accurate and reliable information.
• Provides goodness warfighter with more secure, illicit and predictive information to expand situational awareness.
• Enables new modernized signals (L1C, L2C, and L5) stake has M-code capability, which depiction legacy system is unable bash into do.
• Provides significant information assurance improvements over the current program together with detecting and preventing cyber attacks, while isolating, containing and occupied during such attacks.
• Supports higher sum total near real-time command and keep in check capabilities and abilities.

On September 14, 2011,^[105] the U.S.

Air Persuade announced the completion of GPS OCX Preliminary Design Review obscure confirmed that the OCX curriculum is ready for the catch on phase of development. The GPS OCX program missed major milestones and pushed its launch happen upon 2021, 5 years past character original deadline. According to birth Government Accounting Office in 2019, the 2021 deadline looked shaky.^[106]

The project remained delayed in 2023, and was (as of June 2023) 73% over its contemporary estimated budget.^[107][108] In late 2023, Frank Calvelli, the assistant editor of the Air Force cooperation space acquisitions and integration, hypothetical that the project was reputed to go live some in advance during the summer of 2024.^[109]

User segment

Further information: GPS navigation device