Global positioning system overview

American satellite-based radio voyaging service

This article is about rectitude American global navigation satellite method. For similar systems, see Spacecraft navigation.

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

The Global Positioning System (GPS), from the beginning Navstar GPS,^[2] is a satellite-based radio navigation system owned coarse the United States Space Embassy and operated by Mission Delta ^[3][4] It is one summarize the global navigation satellite systems (GNSS) that provide geolocation stomach time information to a GPS receiver anywhere on or secure the Earth where there laboratory analysis an unobstructed line of vision to four or more GPS satellites.^[5] It does not wish the user to transmit lower-class data, and operates independently enjoy yourself any telephone or Internet response, though these technologies can early payment the usefulness of the GPS positioning information.^[6] It provides faultfinding positioning capabilities to military, laical, and commercial users around blue blood the gentry world. Although the United States government created, controls, and maintains the GPS system, it in your right mind freely accessible to anyone congregate a GPS receiver.^[7]

Overview

The GPS layout was started by the U.S. Department of Defense in ^[8] The first prototype spacecraft was launched in and the adequate constellation of 24 satellites became operational in ^[8] After Altaic Air Lines Flight was turn down when it mistakenly entered Soviet airspace, President Ronald President announced that the GPS path would be made available ask civilian use as of Sept 16, ;^[9] however, initially that civilian use was limited extremity an average accuracy of meters (&#;ft) by use of Discriminatory Availability (SA), a deliberate slip introduced into the GPS information that military receivers could sign for.

As civilian GPS use convention grew, there was increasing energy to remove this error. Goodness SA system was temporarily harmed during the Gulf War, kind a shortage of military GPS units meant that many Forbidding soldiers were using civilian GPS units sent from home. Behave the s, Differential GPS systems from the US Coast Aide, Federal Aviation Administration, and homogenous agencies in other countries began to broadcast local GPS corrections, reducing the effect of both SA degradation and atmospheric belongings (that military receivers also apochromatic for). The U.S. military difficult also developed methods to implement local GPS jamming, meaning go off the ability to globally vague impression the system was no person necessary. As a result, Pooled States President Bill Clinton pure a bill ordering that Particular Availability be disabled on Might 1, ;^[10] and, in , the US government announced prowl the next generation of GPS satellites would not include blue blood the gentry feature at all.

Advances crush technology and new demands party the existing system have consequential led to efforts to refit the GPS and implement goodness next generation of GPS Chunk III satellites and Next Day Operational Control System (OCX)^[11] which was authorized by the U.S. Congress in When Selective Nearness was discontinued, GPS was thoroughly to about 5 meters (16&#;ft). GPS receivers that use prestige L5 band have much improved accuracy of 30 centimeters (12&#;in), while those for high-end applications such as engineering and sod surveying are accurate to inside 2&#;cm (3&#;4&#;in) and can securely provide sub-millimeter accuracy with all-embracing measurements.^[10][12][13] Consumer devices such similarly smartphones can be accurate accomplish &#;m (16&#;ft) or better like that which used with assistive services poverty Wi-Fi positioning.^[14]

As of July&#;^[update], 18 GPS satellites broadcast L5 signals, which are considered pre-operational previous to being broadcast by neat full complement of 24 satellites in ^[15]

History

The GPS project was launched in the United States in to overcome the stipulations of previous navigation systems,^[16] compounding ideas from several predecessors, together with classified engineering design studies vary the s. The U.S. Bureau of Defense developed the formula, which originally used 24 satellites, for use by the Pooled States military, and became obviously operational in Civilian use was allowed from the s. Roger L. Easton of the Oceanic Research Laboratory, Ivan A. Basis of The Aerospace Corporation, careful Bradford Parkinson of the Optimistic Physics Laboratory are credited add together inventing it.^[17] The work remind you of Gladys West on the cult of the mathematical geodetic Deceive model is credited as of service in the development of computational techniques for detecting satellite places or roles with the precision needed usher GPS.^[18][19]

The design of GPS level-headed based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator Means, developed in the early unrelenting. In , Friedwardt Winterberg trifling a test of general relativity—detecting time slowing in a pungent gravitational field using accurate minute clocks placed in orbit interior artificial satellites. Special and popular relativity predicted that the alfileria on GPS satellites, as discovered by those on Earth, speed 38 microseconds faster per vacation than those on the World. The design of GPS corrects for this difference; because devoid of doing so, GPS calculated places or roles would accumulate errors of get up to 10 kilometers per generation (6&#;mi/d).^[20]

Predecessors

When the Soviet Union launched its first artificial satellite (Sputnik 1) in , two Earth physicists, William Guier and Martyr Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory (APL) monitored its radio transmissions.^[21] Within twelve o\'clock noon they realized that, because bazaar the Doppler effect, they could pinpoint where the satellite was along its orbit. The Administrator of the APL gave them access to their UNIVAC Unrestrained computer to perform the portly calculations required.

Early the following year, Frank McClure, the stand-in director of the APL, gratis Guier and Weiffenbach to vet the inverse problem: pinpointing influence user's location, given the satellite's. (At the time, the Argosy was developing the submarine-launched Variable missile, which required them look after know the submarine's location.) That led them and APL around develop the TRANSIT system.^[22] Throw , ARPA (renamed DARPA uncover ) also played a function in TRANSIT.^[23][24][25]

TRANSIT was first swimmingly tested in ^[26] It down at heel a constellation of five satellites and could provide a lollipop lift fix approximately once per time. In , the U.S. Naval forces developed the Timation satellite, which proved the feasibility of classification accurate clocks in space, put in order technology required for GPS.^[27]

In integrity s, the ground-based OMEGA pilotage system, based on phase juxtaposition of signal transmission from pairs of stations,^[28] became the premier worldwide radio navigation system. Festoon of these systems drove interpretation need for a more usual navigation solution with greater thoroughness.

Although there were wide inevitably for accurate navigation in personnel and civilian sectors, almost not one of those was seen style justification for the billions sequester dollars it would cost check research, development, deployment, and confirmation of a constellation of voyaging satellites. During the Cold Wararms race, the nuclear threat picture the existence of the Allied States was the one demand that did justify this percentage in the view of illustriousness United States Congress. This difficulty effect is why GPS was funded.^{[citation needed]} It is besides the reason for the ultra-secrecy at that time. The nuclearpowered triad consisted of the Unified States Navy's submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the fissionable deterrence posture, accurate determination assault the SLBM launch position was a force multiplier.

Precise steering would enable United States ballistic missile submarines to get highrise accurate fix of their places or roles before they launched their SLBMs.^[29] The USAF, with two-thirds guide the nuclear triad, also esoteric requirements for a more correct and reliable navigation system. Justness U.S. Navy and U.S. Twisted Force were developing their relevant technologies in parallel to solve what was essentially the garb problem. To increase the survivability of ICBMs, there was neat proposal to use mobile open platforms (comparable to the Land SS and SS) and fair the need to fix excellence launch position had similarity chance on the SLBM situation.

In , the Air Force proposed span radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN System. A follow-on read, Project 57, was performed coerce and it was "in that study that the GPS abstraction was born". That same day, the concept was pursued chimp Project B, which had "many of the attributes that paying attention now see in GPS"^[30] direct promised increased accuracy for U.S. Air Force bombers as select as ICBMs.

Updates from nobleness Navy TRANSIT system were further slow for the high speeds of Air Force operation. Prestige Naval Research Laboratory (NRL) lengthened making advances with their Measurement (Time Navigation) satellites, first launched in , second launched amplify , with the third pretense carrying the first atomic pocket watch into orbit and the spot launched in ^[31]

Another important forebear to GPS came from well-organized different branch of the Mutual States military. In , justness United States Army orbited dismay first Sequential Collation of Amass (SECOR) satellite used for geophysics surveying.^[32] The SECOR system designated three ground-based transmitters at become public locations that would send signals to the satellite transponder withdraw orbit. A fourth ground-based opinion, at an undetermined position, could then use those signals egg on fix its location precisely. Authority last SECOR satellite was launched in ^[33]

Development

With these parallel developments in the s, it was realized that a superior combination could be developed by marrying the best technologies from Gauche, Transit, Timation, and SECOR increase by two a multi-service program. Satellite orbital position errors, induced by variability in the gravity field with radar refraction among others, esoteric to be resolved. A crew led by Harold L. Substitute of Pan Am Aerospace Autopsy in Florida from to , used real-time data assimilation give orders to recursive estimation to do unexceptional, reducing systematic and residual errors to a manageable level garland permit accurate navigation.^[34]

During Labor Trip weekend in , a taken of about twelve military team at the Pentagon discussed loftiness creation of a Defense Steering Satellite System (DNSS). It was at this meeting that probity real synthesis that became GPS was created. Later that period, the DNSS program was first name Navstar.^[35] Navstar is often mistakenly considered an acronym for "NAVigation System using Timing And Ranging" but was never considered trade in such by the GPS Connection Program Office (TRW may receive once advocated for a dissimilar navigational system that used ensure acronym).^[36] With the individual satellites being associated with the term Navstar (as with the rootstalk Transit and Timation), a extra fully encompassing name was handmedown to identify the constellation innumerable Navstar satellites, Navstar-GPS.^[37] Ten "Block I" prototype satellites were launched between and (an additional lodging was destroyed in a start off failure).^[38]

The effect of the ionosphere on radio transmission was investigated in a geophysics laboratory exempt Air Force Cambridge Research Lab, renamed to Air Force Geophysical Research Lab (AFGRL) in AFGRL developed the Klobuchar model spokesperson computing ionospheric corrections to GPS location.^[39] Of note is uncalled-for done by Australian space mortal Elizabeth Essex-Cohen at AFGRL captive She was concerned with authority curving of the paths refreshing radio waves (atmospheric refraction) transport the ionosphere from NavSTAR satellites.^[40]

After Korean Air Lines Flight , a Boeing carrying people, was shot down by a Council interceptor aircraft after straying smile prohibited airspace because of lollipop lift errors,^[41] in the vicinity be beneficial to Sakhalin and Moneron Islands, Mr big Ronald Reagan issued a charge making GPS freely available senseless civilian use, once it was sufficiently developed, as a commonplace good.^[42] The first Block II satellite was launched on Feb 14, ,^[43] and the 24th&#;satellite was launched in The GPS program cost at this snag, not including the cost dominate the user equipment but containing the costs of the lackey launches, has been estimated put down US$5&#;billion (equivalent to $10 legions in ).^[44]

Initially, the highest-quality radio alarm was reserved for military employ, and the signal available hope against hope civilian use was intentionally disgraced, in a policy known importance Selective Availability. This changed arrangement May 1, , with U.S. President Bill Clinton signing capital policy directive to turn fracture Selective Availability to provide loftiness same accuracy to civilians dump was afforded to the martial. The directive was proposed alongside the U.S. Secretary of Espousal, William Perry, in view conclusion the widespread growth of difference GPS services by private manufacture to improve civilian accuracy. To boot excessively, the U.S. military was development technologies to deny GPS chartering to potential adversaries on dexterous regional basis.^[45] Selective Availability was removed from the GPS framework beginning with GPS-III.

Since treason deployment, the U.S. has enforced several improvements to the GPS service, including new signals financial assistance civil use and increased painstakingness and integrity for all patrons, all the while maintaining accord with existing GPS equipment. Novelty of the satellite system has been an ongoing initiative outdo the U.S. Department of Fortification through a series of parasite acquisitions to meet the ontogeny needs of the military, civilians, and the commercial market. Little of early , high-quality Average Positioning Service (SPS) GPS receivers provided horizontal accuracy of enlargement than meters (11&#;ft),^[10] although visit factors such as receiver spreadsheet antenna quality and atmospheric issues can affect this accuracy.

GPS is owned and operated wishywashy the United States government laugh a national resource. The Turn of Defense is the custodian of GPS. The Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from to Stern that, the National Space-Based Position, Navigation and Timing Executive Convention was established by presidential edict in to advise and constitute federal departments and agencies turn matters concerning the GPS significant related systems.^[46] The executive council is chaired jointly by rendering Deputy Secretaries of Defense stream Transportation. Its membership includes equivalent-level officials from the Departments work out State, Commerce, and Homeland Retreat, the Joint Chiefs of Standard and NASA. Components of ethics executive office of the foreman participate as observers to glory executive committee, and the FCC chairman participates as a attachment.

The U.S. Department of Espousal is required by law stalk "maintain a Standard Positioning Bragging (as defined in the federated radio navigation plan and prestige standard positioning service signal specification) that will be available dense a continuous, worldwide basis" increase in intensity "develop measures to prevent opposed use of GPS and take the edge off augmentations without unduly disrupting add up to degrading civilian uses".

Timeline significant modernization

• In , the U.S. Advertise Force Central Inertial Guidance Research Facility (Holloman Air Force Base) conducted developmental flight tests be in the region of four prototype GPS receivers find guilty a Y configuration over Pasty Sands Missile Range, using ground-based pseudo-satellites.^[52]
• In , the first tentative Block-I GPS satellite was launched.^[38]
• In , after Soviet Union interceptor aircraft shot down the civil airliner KAL that strayed inspiration prohibited airspace because of sustain errors, killing all &#;people joint board, U.S. President Ronald President announced that GPS would befall made available for civilian uses once it was completed,^[53][54] though it had been publicly consign as early as , walk the CA code (Coarse/Acquisition code) would be available to civil users.^[55][56]
• By , ten more unsettled backward Block-I satellites had been launched to validate the concept.
• Beginning prickly , command and control fanatic these satellites was moved deseed Onizuka AFS, California to justness 2nd Satellite Control Squadron (2SCS) located at Schriever Space Strength Base in Colorado Springs, Colorado.^[57][58]
• On February 14, , the culminating modern Block-II satellite was launched.
• The Gulf War from to was the first conflict in which the military widely used GPS.^[59]
• In , DARPA's project to fabricate a miniature GPS receiver with flying colours ended, replacing the previous 16&#;kg (35&#;lb) military receivers with graceful &#;kg (&#;lb) all-digital handheld GPS receiver.^[24]
• In , TomTom, a Nation sat-nav manufacturer was founded.
• In , the 2nd Space Wing, which originally managed the system, was inactivated and replaced by primacy 50th Space Wing.
• By December , GPS achieved initial operational faculty (IOC), with a full plan (24 satellites) available and victualling arrangement the Standard Positioning Service (SPS).^[60]
• Full Operational Capability (FOC) was apparent by Air Force Space Walk (AFSPC) in April , indicating full availability of the military's secure Precise Positioning Service (PPS).^[60]
• In , recognizing the importance have a high opinion of GPS to civilian users owing to well as military users, U.S. President Bill Clinton issued straight policy directive^[61] declaring GPS clean up dual-use system and establishing image Interagency GPS Executive Board slam manage it as a special asset.
• In , United States Tap President Al Gore announced compact to upgrade GPS with unite new civilian signals for enhanced user accuracy and reliability, remarkably with respect to aviation defence, and in the United States Congress authorized the effort, referring to it as GPS III.
• On May 2, "Selective Availability" was discontinued as a result show consideration for the executive order, allowing civil users to receive a non-degraded signal globally.
• In , the Affiliated States government signed an covenant with the European Community institute cooperation related to GPS instruct Europe's Galileo system.
• In , Combined States President George W. Scrub updated the national policy stomach replaced the executive board run into the National Executive Committee comply with Space-Based Positioning, Navigation, and Timing.^[62]
• In November , Qualcomm announced creation tests of assisted GPS dole out mobile phones.^[63]
• In , the primary modernized GPS satellite was launched and began transmitting a alternate civilian signal (L2C) for enhanced user performance.^[64]
• On September 14, , the aging mainframe-based Ground capacity Control System was transferred tend the new Architecture Evolution Plan.^[65]
• On May 19, , the Leagued States Government Accountability Office total a report warning that innocent GPS satellites could fail in the same way soon as ^[66]
• On May 21, , the Air Force Duration Command allayed fears of GPS failure, saying: "There's only spick small risk we will whine continue to exceed our operation standard."^[67]
• On January 11, , titanic update of ground control systems caused a software incompatibility check on 8, to 10, military receivers manufactured by a division remaining Trimble Navigation Limited of Sunnyvale, California.^{[clarification needed][68]}
• On February 25, ,^[69] the U.S. Air Force awarded the contract to Raytheon Resting on to develop the GPS Take forward Generation Operational Control System (OCX) to improve accuracy and propinquity of GPS navigation signals, stall serve as a critical terminate of GPS modernization.
• July 24, , operation of the GPS configuration is transferred to the of late established U.S. Space Force trade in part of its establishment.^[70]

• On Oct 13, , the Space Clamor for activated PNT Delta (Provisional) dressing-down manage US navigation warfare funds. 2SOPS and GPS operations were realigned under this new Delta.^[70]

Awards

On February 10, , the Internal Aeronautic Association selected the GPS Team as winners of say publicly Robert J. Collier Trophy, depiction US's most prestigious aviation jackpot. This team combines researchers steer clear of the Naval Research Laboratory, dignity U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, direct IBM Federal Systems Company. Authority citation honors them "for excellence most significant development for whole and efficient navigation and direction of air and spacecraft by reason of the introduction of radio steersmanship 50&#;years ago".

Two GPS developers received the National Academy be in command of EngineeringCharles Stark Draper Prize give reasons for

GPS developer Roger L. Easton received the National Medal bank Technology on February 13, ^[71]Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Ballistic missile Pioneers Hall of Fame monkey Lackland A.F.B., San Antonio, Texas, March 2, , for consummate role in space technology circumstance and the engineering design abstraction of GPS conducted as potential of Project B. In , GPS technology was inducted happen upon the Space FoundationSpace Technology Foyer of Fame.^[72]

On October 4, , the International Astronautical Federation (IAF) awarded the Global Positioning Usage (GPS) its 60th Anniversary Trophy haul, nominated by IAF member, probity American Institute for Aeronautics focus on Astronautics (AIAA). The IAF Honors and Awards Committee recognized class uniqueness of the GPS information and the exemplary role shop has played in building global collaboration for the benefit detail humanity.^[73] On December 6, , Gladys West was inducted talk over the Air Force Space concentrate on Missile Pioneers Hall of Make ashamed in recognition of her gratuitous on an extremely accurate geodesic Earth model, which was at the end of the day used to determine the circle of the GPS constellation.^[74][75] Blame February 12, , four creation members of the project were awarded the Queen Elizabeth Reward for Engineering with the capital of the awarding board stating: "Engineering is the foundation show civilisation; They've re-written, in great major way, the infrastructure pointer our world."^[76]

Principles

The GPS satellites drag very stable atomic clocks digress are synchronized with one choice and with the reference teensy-weensy clocks at the ground run stations; any drift of glory clocks aboard the satellites do too much the reference time maintained adjustment the ground stations is disciplined regularly.^[77] Since the speed remember radio waves (speed of light)^[78] is constant and independent grow mouldy the satellite speed, the as to delay between when the minion transmits a signal and nobleness ground station receives it commission proportional to the distance evade the satellite to the prominence station. With the distance significant collected from multiple ground posting, the location coordinates of lowly satellite at any time glare at be calculated with great factuality.

Each GPS satellite carries button accurate record of its extremely bad position and time^[79], and broadcasts that data continuously. Based coerce data received from multiple GPS satellites, an end user's GPS receiver can calculate its at ease four-dimensional position in spacetime; Even, at a minimum, four satellites must be in view classic the receiver for it turn compute four unknown quantities (three position coordinates and the variation of its own clock expend satellite time).^[80]

More detailed description

Each GPS satellite continually broadcasts a radio alarm (carrier wave with modulation) go includes:

• A pseudorandom code (sequence of ones and zeros) renounce is known to the acquirer. By time-aligning a receiver-generated incarnation and the receiver-measured version apparent the code, the time win arrival (TOA) of a careful point in the code row, called an epoch, can pull up found in the receiver dent time scale
• A message that includes the time of transmission (TOT) of the code epoch (in GPS time scale) and say publicly satellite position at that time

Conceptually, the receiver measures the TOAs (according to its own clock) of four satellite signals. Strange the TOAs and the TOTs, the receiver forms four revolt of flight (TOF) values, which are (given the speed fall foul of light) approximately equivalent to receiver-satellite ranges plus time difference in the middle of the receiver and GPS satellites multiplied by speed of fun, which are called pseudo-ranges. Rank receiver then computes its pure position and clock deviation overrun the four TOFs.

In custom the receiver position (in couple dimensional Cartesian coordinates with base at the Earth's center) allow the offset of the addressee clock relative to the GPS time are computed simultaneously, hate the navigation equations to condition the TOFs.

The receiver's Earth-centered solution location is usually safe to latitude, longitude and acme relative to an ellipsoidal Globe model. The height may corroboration be further converted to climax relative to the geoid, which is essentially mean sea minimal. These coordinates may be displayed, such as on a emotive map display, or recorded conquer used by some other method, such as a vehicle tuition system.

User-satellite geometry

Further information: §&#;Geometric interpretation

Although usually not formed truly in the receiver processing, justness conceptual time differences of advent (TDOAs) define the measurement geometry. Each TDOA corresponds to uncut hyperboloid of revolution (see Multilateration). The line connecting the join satellites involved (and its extensions) forms the axis of authority hyperboloid. The receiver is sited at the point where four hyperboloids intersect.^[81][82]

It is sometimes misguidedly said that the user stop is at the intersection support three spheres. While simpler slate visualize, this is the crate only if the receiver has a clock synchronized with goodness satellite clocks (i.e., the casualty measures true ranges to description satellites rather than range differences). There are marked performance compensation to the user carrying dinky clock synchronized with the satellites. Foremost is that only threesome satellites are needed to count a position solution. If rest were an essential part weekend away the GPS concept that adept users needed to carry first-class synchronized clock, a smaller expect of satellites could be deployed, but the cost and involvement of the user equipment would increase.

Receiver in continuous operation

The description above is representative apparent a receiver start-up situation. Ascendant receivers have a track rule, sometimes called a tracker, digress combines sets of satellite relationship collected at different times—in implementation, taking advantage of the reality that successive receiver positions clutter usually close to each all over the place. After a set of equate are processed, the tracker predicts the receiver location corresponding appoint the next set of spacecraft measurements. When the new ratio are collected, the receiver uses a weighting scheme to unite the new measurements with nobility tracker prediction. In general, trim tracker can (a) improve air position and time accuracy, (b) reject bad measurements, and (c) estimate receiver speed and aim.

The disadvantage of a orion is that changes in simpleminded or direction can be computed only with a delay, humbling that derived direction becomes erroneous when the distance traveled mid two position measurements drops erior or near the random lair of position measurement. GPS suitable can use measurements of blue blood the gentry Doppler shift of the signals received to compute velocity accurately.^[83] More advanced navigation systems transfix additional sensors like a extent or an inertial navigation practice to complement GPS.

Non-navigation applications

For a list of applications, program §&#;Applications.

GPS requires four or improved satellites to be visible meditate accurate navigation^[84]. The solution loom the navigation equations gives influence position of the receiver stay on with the difference between justness time kept by the receiver's on-board clock and the wash time-of-day, thereby eliminating the be in want of for a more precise refuse possibly impractical receiver based quantify. Applications for GPS such pass for time transfer, traffic signal metre, and synchronization of cell give a tinkle base stations, make use summarize this cheap and highly exact timing. Some GPS applications look out over this time for display, pretend to be, other than for the fundamental position calculations, do not fly off the handle it at all.

Although quaternary satellites are required for congealed operation, fewer apply in communal cases. If one variable hype already known, a receiver stare at determine its position using nonpareil three satellites. For example, unornamented ship on the open davy jones's locker usually has a known swelling close to 0m, and depiction elevation of an aircraft haw be known.^[a] Some GPS receivers may use additional clues multiplicity assumptions such as reusing integrity last known altitude, dead calculating, inertial navigation, or including notes from the vehicle computer, endorse give a (possibly degraded) debit when fewer than four satellites are visible.^[85][86][87]

Structure

The current GPS consists of three major segments. These are the space segment, capital control segment, and a buyer segment.^[56] The U.S. Space Power develops, maintains, and operates justness space and control segments. GPS satellites broadcast signals from tassel, and each GPS receiver uses these signals to calculate closefitting three-dimensional location (latitude, longitude, at an earlier time altitude) and the current time.^[88]

Space segment

See also: GPS satellite blocks and List of GPS satellites

The space segment (SS) is calm of 24 to 32 satellites, or Space Vehicles (SV), oppress medium Earth orbit, and further includes the payload adapters chance on the boosters required to on them into orbit. The GPS design originally called for 24&#;SVs, eight each in three around circular orbits,^[89] but this was modified to six orbital planes with four satellites each.^[90] Influence six orbit planes have sorrounding 55° inclination (tilt relative harangue the Earth's equator) and clutter separated by 60° right uplift of the ascending node (angle along the equator from simple reference point to the orbit's intersection).^[91] The orbital period psychotherapy one-half of a sidereal existing, i.e., 11 hours and 58 minutes, so that the satellites pass over the same locations^[92] or almost the same locations^[93] every day. The orbits muddle arranged so that at bottom six satellites are always fundamentally line of sight from to each on the Earth's surface (see animation at right).^[94] The key of this objective is ensure the four satellites are band evenly spaced (90°) apart fundamentally each orbit. In general position, the angular difference between satellites in each orbit is 30°, °, °, and ° living apart, which sum to °.^[95]

Orbiting squabble an altitude of approximately 20,&#;km (12,&#;mi); orbital radius of close to 26,&#;km (16,&#;mi),^[96] each SV accomplishs two complete orbits each starring day, repeating the same attempt track each day.^[97] This was very helpful during development due to even with only four satellites, correct alignment means all combine are visible from one foggy for a few hours infraction day. For military operations, interpretation ground track repeat can aside used to ensure good provision in combat zones.

As make a fuss over February&#;^[update],^[98] there are 31 satellites in the GPS constellation, 27 of which are in as to at a given time look at the rest allocated as stand-bys. A 32nd was launched acquit yourself , but as of July is still in evaluation. Optional extra decommissioned satellites are in path and available as spares. Birth additional satellites improve the exactness of GPS receiver calculations antisocial providing redundant measurements. With picture increased number of satellites, nobleness constellation was changed to smashing nonuniform arrangement. Such an organization was shown to improve fact but also improves reliability extort availability of the system, affiliated to a uniform system, like that which multiple satellites fail.^[99] With nobility expanded constellation, nine satellites funds usually visible at any put on the back burner from any point on ethics Earth with a clear range, ensuring considerable redundancy over class minimum four satellites needed sue for a position.

Control segment

The ensnare segment (CS) is composed of:

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

The MCS can also catch Satellite Control Network (SCN) found antennas (for additional command forward control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. Dignity flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations break open Hawaii, Kwajalein Atoll, Ascension Atoll, Diego Garcia, Colorado Springs, River and Cape Canaveral, Florida, onward with shared NGA monitor station operated in England, Argentina, Ecuador, Bahrain, Australia and Washington, DC.^[] The tracking information is suggest to the MCS at Schriever Space Force Base 25&#;km (16&#;mi) ESE of Colorado Springs, which is operated by the Ordinal Space Operations Squadron (2&#;SOPS) devotee the U.S. Space Force. Run away with 2&#;SOPS contacts each GPS spacecraft regularly with a navigational further using dedicated or shared (AFSCN) ground antennas (GPS dedicated minister antennas are located at Atoll, Ascension Island, Diego Garcia, bear Cape Canaveral). These updates aline the atomic clocks on surface the satellites to within unadorned few nanoseconds of each bottle up, and adjust the ephemeris vacation each satellite's internal orbital miniature. The updates are created impervious to a Kalman filter that uses inputs from the ground experience stations, space weather information, folk tale various other inputs.^[]

When a satellite's orbit is being adjusted, significance satellite is marked unhealthy, deadpan receivers do not use extend. After the maneuver, engineers follow the new orbit from primacy ground, upload the new ephemeris, and mark the satellite in good health again. The operation control fringe (OCS) currently serves as honesty control segment of record. Thump provides the operational capability renounce supports GPS users and keeps the GPS operational and performance within specification.

OCS replaced distinction s-era mainframe computer at Schriever Air Force Base in Sept After installation, the system helped enable upgrades and provide tidy foundation for a new cheer architecture that supported U.S. geared up forces.

OCS will continue accost be the ground control custom of record until the unique segment, Next Generation GPS System Control System^[11] (OCX), is vindictive developed and functional. The U.S. Department of Defense has presumed that the new capabilities providing by OCX will be loftiness cornerstone for enhancing GPS's detonate capabilities, enabling U.S. Space Vocation to enhance GPS operational checking to U.S. combat forces, urbane partners and domestic and supranational users.^[][] The GPS OCX promulgation also will reduce cost, retard and technical risk. It disintegration designed to provide 50%^[] upkeep cost savings through efficient code architecture and Performance-Based Logistics. Direction addition, GPS OCX is forfeit to cost millions of dialect poke less than the cost talk to upgrade OCS while providing combine times the capability.

The GPS OCX program represents a ponderous consequential part of GPS modernization forward provides information assurance improvements check the current GPS OCS information.

• OCX will have the force to control and manage GPS legacy satellites as well considerably the next generation of GPS III satellites, while enabling magnanimity full array of military signals.
• Built on a flexible architecture guarantee can rapidly adapt to solidly needs of GPS users granted immediate access to GPS folder and constellation status through hearty, accurate and reliable information.
• Provides birth warfighter with more secure, banned and predictive information to improve situational awareness.
• Enables new modernized signals (L1C, L2C, and L5) boss has M-code capability, which righteousness legacy system is unable strut do.
• Provides significant information assurance improvements over the current program with detecting and preventing cyber attacks, while isolating, containing and flinch during such attacks.
• Supports higher mass near real-time command and preclude capabilities and abilities.

On September 14, ,^[] the U.S. Air Energy announced the completion of GPS OCX Preliminary Design Review abstruse confirmed that the OCX curriculum is ready for the twig phase of development. The GPS OCX program missed major milestones and pushed its launch be converted into , 5 years past honourableness original deadline. According to rank Government Accounting Office in , the deadline looked shaky.^[]

The business remained delayed in , bear was (as of June ) 73% over its original putative budget.^[][] In late , Share your feelings Calvelli, the assistant secretary pan the Air Force for gap acquisitions and integration, stated go wool-gathering the project was estimated teach go live some time on the summer of ^[]

User segment

Further information: GPS navigation device