gps wikipedia

26
Global Positioning System 1 Global Positioning System Artist's conception of GPS Block II-F satellite in orbit Civilian GPS receiver ("GPS navigation device") in a marine application. Automotive navigation system in a taxicab.

Upload: hilman-hasabi

Post on 28-Mar-2015

355 views

Category:

Documents


1 download

DESCRIPTION

Global Positioning System1Global Positioning SystemArtist's conception of GPS Block II-F satellite in orbitCivilian GPS receiver ("GPS navigation device") in a marine application.Automotive navigation system in a taxicab.Global Positioning System2The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there i

TRANSCRIPT

Page 1: GPS Wikipedia

Global Positioning System 1

Global Positioning System

Artist's conception of GPS Block II-F satellite inorbit

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

Automotive navigation system in a taxicab.

Page 2: GPS Wikipedia

Global Positioning System 2

GPS receivers are now integrated in many mobilephones.

The Global Positioning System (GPS) is a space-based globalnavigation satellite system (GNSS) that provides reliable location andtime information in all weather and at all times and anywhere on ornear the Earth when and where there is an unobstructed line of sight tofour or more GPS satellites. It is maintained by the United Statesgovernment and is freely accessible by anyone with a GPS receiver.

The GPS project was started in 1973 to overcome the limitations ofprevious navigation systems,[1] integrating ideas from severalpredecessors, including a number of classified engineering designstudies from the 1960s. GPS was created and realized by the U.S.Department of Defense (USDOD) and was originally run with24 satellites. It became fully operational in 1994.

In addition to GPS other systems are in use or under development. The Russian GLObal NAvigation SatelliteSystem (GLONASS) was in use by the Russian military only until it was made fully available to civilians in 2007.There are also the planned Chinese Compass navigation system and the European Union's Galileo positioningsystem.

History

The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the DeccaNavigator developed in the early 1940s, and used during World War II. In 1956 Friedwardt Winterberg[2] proposed atest of general relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracyrequirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. Additional inspirationfor GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S.scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that,because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satelliteapproached, and lower as it continued away from them. They realized that because they knew their exact location onthe globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit(satellite)).The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Itused a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, theU.S. Navy developed the Timation satellite that proved the ability to place accurate clocks in space, a technologyrequired by GPS. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signaltransmission from pairs of stations,[3] became the first worldwide radio navigation system. Limitations of thesesystems drove the need for a more universal navigation solution with greater accuracy.While there were wide needs for accurate navigation in military and civilian sectors, almost none of those were seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. The nuclear triad consisted of the United 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 nuclear deterrence posture, accurate determination of the SLBM launch position

Page 3: GPS Wikipedia

Global Positioning System 3

was a force multiplier.Precise navigation would enable United States submarines to get an accurate fix of their positions prior to launchingtheir SLBMs.[4] The USAF with two-thirds of the nuclear triad also had requirements for a more accurate andreliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve whatwas essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launchplatforms 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 ICBMControl) that was essentially a 3-D LORAN. A follow-on study called Project 57 was worked in 1963 and it was "inthis 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"[5] and promised increased accuracy for Air Force bombers as wellas ICBMs. Updates from the Navy Transit system were too slow for the high speeds of Air Force operation. TheNavy Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launchedin 1967, and with the third one in 1974 carrying the first atomic clock into orbit.[6]

With these parallel developments in the 1960s, it was realized that a superior system could be developed bysynthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program.During Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon discussed the creation ofa Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS wascreated." Later that year, the DNSS program was named Navstar. With the individual satellites being associated withthe name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used toidentify the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to GPS.[7]

After Korean Air Lines Flight 007, carrying 269 people, was shot down in 1983 after straying into the USSR'sprohibited airspace,[8] in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directivemaking GPS freely available for civilian use, once it was sufficiently developed, as a common good.[9] The firstsatellite was launched in 1989, and the 24th satellite was launched in 1994.Initially, the highest quality signal was reserved for military use, and the signal available for civilian use wasintentionally degraded ("Selective Availability", SA). This changed with United States President Bill Clintonordering Selective Availability turned off at midnight May 1, 2000, improving the precision of civilian GPS from100 meters (about 300 feet) to 20 meters (about 65 feet). The United States military by then had the ability to denyGPS service to potential adversaries on a regional basis.[10]

GPS is owned and operated by the United States Government as a national resource. Department of Defense(USDOD) is the steward of GPS. Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee wasestablished by presidential directive in 2004 to advise and coordinate federal departments and agencies on mattersconcerning the GPS and related systems. The executive committee is chaired jointly by the deputy secretaries ofdefense and transportation. Its membership includes equivalent-level officials from the departments of state,commerce, and homeland security, the joint chiefs of staff, and NASA. Components of the executive office of thepresident participate as observers to the executive committee, and the FCC chairman participates as a liaison.USDOD is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigationplan and the standard positioning service signal specification) that will be available on a continuous, worldwidebasis," and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting ordegrading civilian uses."

Page 4: GPS Wikipedia

Global Positioning System 4

Timeline and modernization

Summary of satellites[11]

Block LaunchPeriod

Satellite launches Currently inorbit

and healthySuc-cess

Fail-ure

Inprep-

aration

Plan-ned

I 1978–1985 10 1 0 0 0

II 1989–1990 9 0 0 0 0

IIA 1990–1997 19 0 0 0 10

IIR 1997–2004 12 1 0 0 12

IIR-M 2005–2009 8 0 0 0 7

IIF 2010–2011 1 0 11 0 1

IIIA 2014–? 0 0 0 12 0

IIIB 0 0 0 8 0

IIIC 0 0 0 16 0

Total 59 2 11 36 30

(Last update: 24 May 2010)PRN 01 from Block IIR-M is unhealthyPRN 25 from Block IIA is unhealthyPRN 32 from Block IIA is unhealthy[12] For a more complete list, see list of GPS satellite launches

• In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB), conducted developmental flight testsof two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.

• In 1978, the first experimental Block-I GPS satellite was launched.• In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited

airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announcedthat GPS would be made available for civilian uses once it was completed.[13] [14]

• By 1985, ten more experimental Block-I satellites had been launched to validate the concept.• On February 14, 1989, the first modern Block-II satellite was launched.• The Gulf War from 1990 to 1991, was the first conflict where GPS was widely used.[15]

• In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50thSpace Wing.

• By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24satellites) was available and providing the Standard Positioning Service (SPS).[16]

• Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995,signifying full availability of the military's secure Precise Positioning Service (PPS).[16]

• In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clintonissued a policy directive[17] declaring GPS to be a dual-use system and establishing an Interagency GPSExecutive Board to manage it as a national asset.

• In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals forenhanced user accuracy and reliability, particularly with respect to aviation safety and in 2000 the United StatesCongress authorized the effort, referring to it as GPS III.

• In 1998, GPS technology was inducted into the Space Foundation Space Technology Hall of Fame.

Page 5: GPS Wikipedia

Global Positioning System 5

• On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing usersto receive a non-degraded signal globally.

• In 2004, the United States Government signed an agreement with the European Community establishingcooperation related to GPS and Europe's planned Galileo system.

• In 2004, United States President George W. Bush updated the national policy and replaced the executive boardwith the National Executive Committee for Space-Based Positioning, Navigation, and Timing.[18]

• November 2004, QUALCOMM announced successful tests of assisted GPS for mobile phones.[19]

• In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C)for enhanced user performance.

• On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the newArchitecture Evolution Plan.[20]

• On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPSsatellites could fail as soon as 2010.[21]

• On May 21, 2009, the Air Force Space Command allayed fears of GPS failure saying "There's only a small riskwe will not continue to exceed our performance standard."[22]

• On January 11, 2010, an update of ground control systems caused a software incompatibility with 8000 to 10000military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.[23]

• The most recent launch was on May 28, 2010.[24] The oldest GPS satellite still in operation was launched onNovember 26, 1990, and became operational on December 10, 1990.[25]

AwardsOn February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J.Collier Trophy, the nation's most prestigious aviation award. This team combines researchers from the NavalResearch Laboratory, the USAF, the Aerospace Corporation, Rockwell International Corporation, and IBM FederalSystems Company. The citation honors them "for the most significant development for safe and efficient navigationand surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:• Ivan Getting, emeritus president of The Aerospace Corporation and an engineer at the Massachusetts Institute of

Technology, established the basis for GPS, improving on the World War II land-based radio system calledLORAN (Long-range Radio Aid to Navigation).

• Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the presentsatellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinsonserved 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 on February 13, 2006.[26]

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

Page 6: GPS Wikipedia

Global Positioning System 6

Basic concept of GPSA GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth.Each satellite continually transmits messages that include• the time the message was transmitted• precise orbital information (the ephemeris)• the general system health and rough orbits of all GPS satellites (the almanac).The receiver uses the messages it receives to determine the transit time of each message and computes the distance toeach satellite. These distances along with the satellites' locations are used with the possible aid of trilateration,depending on which algorithm is used, to compute the position of the receiver. This position is then displayed,perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPSunits show derived information such as direction and speed, calculated from position changes.Three satellites might seem enough to solve for position since space has three dimensions and a position near theEarth's surface can be assumed. However, even a very small clock error multiplied by the very large speed oflight[27]  — the speed at which satellite signals propagate — results in a large positional error. Therefore receiversuse four or more satellites to solve for the receiver's location and time. The very accurately computed time iseffectively hidden by most GPS applications, which use only the location. A few specialized GPS applications dohowever use the time; these include time transfer, traffic signal timing, and synchronization of cell phone basestations.Although four satellites are required for normal operation, fewer apply in special cases. If one variable is alreadyknown, a receiver can determine its position using only three satellites. For example, a ship or aircraft may haveknown elevation. Some GPS receivers may use additional clues or assumptions (such as reusing the last knownaltitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a lessaccurate (degraded) position when fewer than four satellites are visible.[28] [29] [30]

Position calculation introductionTo provide an introductory description of how a GPS receiver works, error effects are deferred to a later section.Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the timessent and then the satellite positions corresponding to these times sent. The x, y, and z components of position, andthe time sent, are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or4. Knowing the indicated, or uncorrected, time the message was received , the GPS receiver can computethe uncorrected transit time of the message as . Assuming the message traveled at the speed of light, c,the uncorrected distance traveled or pseudorange, can be computed as .A satellite's position and pseudorange define a sphere, centered on the satellite with radius equal to the pseudorange.The position of the receiver is somewhere on the surface of this sphere. Thus with four satellites, the indicatedposition of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of noerrors, the GPS receiver would be at a precise intersection of the four surfaces.If the surfaces of two spheres intersect at more than one point, they intersect in a circle. The article trilaterationshows this mathematically. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below. Two pointswhere the surfaces of the spheres intersect are clearly shown in the figure. The distance between these two points isthe diameter of the circle of intersection.

Page 7: GPS Wikipedia

Global Positioning System 7

Two sphere surfaces intersecting in a circle

The intersection of a third spherical surface with the first two will beits intersection with that circle; in most cases of practical interest, thismeans they intersect at two points.[31] Another figure, Surface ofSphere Intersecting a Circle (not a solid disk) at Two Points, illustratesthe intersection. The two intersections are marked with dots. Again thearticle trilateration clearly shows this mathematically.

Surface of sphere Intersecting a circle (not a soliddisk) at two points

For automobiles and other near-earth vehicles, the correct position ofthe GPS receiver is the intersection closest to the Earth's surface.[32]

For space vehicles, the intersection farthest from Earth may be thecorrect one.

The correct position for the GPS receiver is also the intersection closestto the surface of the sphere corresponding to the fourth satellite.

Correcting a GPS receiver's clock

One of the most significant error sources is the GPS receiver's clock.Because of the very large value of the speed of light, c, the estimateddistances from the GPS receiver to the satellites, the pseudoranges, arevery sensitive to errors in the GPS receiver clock; for example an error of one microsecond (0.000 001 second)corresponds to an error of 300 metres (980 ft). This suggests that an extremely accurate and expensive clock isrequired for the GPS receiver to work. Because manufacturers prefer to build inexpensive GPS receivers for massmarkets, the solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.

Page 8: GPS Wikipedia

Global Positioning System 8

Diagram depicting satellite 4, sphere, p4, r4, andda

It is likely that the surfaces of the three spheres intersect, because the circle of intersection of the first two spheres isnormally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that thesurface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of thefirst three, because any clock error could cause it to miss intersecting a point. However, the distance from the validestimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used tocompute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourthsatellite and let denote the pseudorange of the fourth satellite. Let . is the distance from thecomputed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient,

, provides an estimate of(time indicated by the receiver's on-board clock) - (correct time),

and the GPS receiver clock can be advanced if is positive or delayed if is negative. However, it should be keptin mind that a less simple function of may be needed to estimate the time error in an iterative algorithm asdiscussed in the Navigation equations section.

StructureThe current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and auser segment (U.S.).[33] The U.S. Air Force develops, maintains, and operates the space and control segments. GPSsatellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensionallocation (latitude, longitude, and altitude) and the current time.[34]

The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters tothe boosters required to launch them into orbit. The control segment is composed of a master control station, analternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The usersegment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS PrecisePositioning Service, and tens of millions of civil, commercial, and scientific users of the Standard PositioningService (see GPS navigation devices).

Page 9: GPS Wikipedia

Global Positioning System 9

Space segment

Unlaunched GPS satellite on display at the SanDiego Air & Space Museum

A visual example of the GPS constellation inmotion with the Earth rotating. Notice how the

number of satellites in view from a given point onthe Earth's surface, in this example at 45°N,

changes with time.

The space segment (SS) is composed of the orbiting GPS satellites, orSpace Vehicles (SV) in GPS parlance. The GPS design originallycalled for 24 SVs, eight each in three circular orbital planes,[35] but thiswas modified to six planes with four satellites each.[36] The orbitalplanes are centered on the Earth, not rotating with respect to the distantstars.[37] The six planes have approximately 55° inclination (tiltrelative to Earth's equator) and are separated by 60° right ascension ofthe ascending node (angle along the equator from a reference point tothe orbit's intersection).[38] The orbits are arranged so that at least sixsatellites are always within line of sight from almost everywhere onEarth's surface.[39] The result of this objective is that the four satellitesare not evenly spaced (90 degrees) apart within each orbit. In generalterms, the angular difference between satellites in each orbit is 30, 105,120, and 105 degrees apart which, of course, sum to 360 degrees.

Orbiting at an altitude of approximately 20,200 kilometers (about12,550 miles or 10,900 nautical miles; orbital radius of approximately26,600 km (about 16,500 mi or 14,400 NM)), each SV makes twocomplete orbits each sidereal day, repeating the same ground trackeach day.[40] This was very helpful during development because evenwith only four satellites, correct alignment means all four are visiblefrom one spot for a few hours each day. For military operations, theground track repeat can be used to ensure good coverage in combatzones.

As of March 2008,[41] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retiredfrom active service satellites kept in the constellation as orbital spares. The additional satellites improve the precisionof GPS receiver calculations by providing redundant measurements. With the increased number of satellites, theconstellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability andavailability of the system, relative to a uniform system, when multiple satellites fail.[42] About eight satellites arevisible from any point on the ground at any one time (see animation at right).

Control segment

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

Museum

The control segment is composed of1. a master control station (MCS),2. an alternate master control station,3. four dedicated ground antennas and4. six dedicated monitor stationsThe MCS can also access U.S. Air Force Satellite Control Network(AFSCN) ground antennas (for additional command and controlcapability) and NGA (National Geospatial-Intelligence Agency)monitor stations. The flight paths of the satellites are tracked bydedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein,Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape

Page 10: GPS Wikipedia

Global Positioning System 10

Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia andWashington DC.[43] The tracking information is sent to the Air Force Space Command's MCS at Schriever Air ForceBase 25 km (16 miles) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS)of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicatedor shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island,Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within afew nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates arecreated by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, andvarious other inputs.[44]

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must bemarked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and theresulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

User segment

GPS receivers come in a variety of formats, fromdevices integrated into cars, phones, and watches,

to dedicated devices such as those shown herefrom manufacturers Trimble, Garmin and Leica

(left to right).

The user segment is composed of hundreds of thousands of U.S. andallied military users of the secure GPS Precise Positioning Service, andtens of millions of civil, commercial and scientific users of theStandard Positioning Service. In general, GPS receivers are composedof an antenna, tuned to the frequencies transmitted by the satellites,receiver-processors, and a highly stable clock (often a crystaloscillator). They may also include a display for providing location andspeed information to the user. A receiver is often described by itsnumber of channels: this signifies how many satellites it can monitorsimultaneously. Originally limited to four or five, this hasprogressively increased over the years so that, as of 2007, receiverstypically have between 12 and 20 channels.[45]

A typical OEM GPS receiver module measuring15×17 mm.

GPS receivers may include an input for differential corrections, usingthe RTCM SC-104 format. This is typically in the form of an RS-232port 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. Receiverswith internal DGPS receivers can outperform those using externalRTCM data. As of 2006, even low-cost units commonly include WideArea Augmentation System (WAAS) receivers.

Many GPS receivers can relay position data to a PC or other deviceusing the NMEA 0183 protocol. Although this protocol is officiallydefined by the National Marine Electronics Association (NMEA),[46]

references to this protocol have been compiled from public records, allowing open source tools like gpsd to read theprotocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF andMTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, orBluetooth.

Page 11: GPS Wikipedia

Global Positioning System 11

ApplicationsWhile originally a military project, GPS is considered a dual-use technology, meaning it has significant military andcivilian applications.GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS'saccurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of powergrids by allowing well synchronized hand-off switching.[34]

Civilian

This antenna is mounted on the roof of a hutcontaining a scientific experiment needing

precise timing.

Many civilian applications use one or more of GPS's three basiccomponents: absolute location, relative movement, and time transfer.• Cellular telephony: Clock synchronization enables time transfer,

which is critical for synchronizing its spreading codes with otherbase stations to facilitate inter-cell handoff and support hybridGPS/cellular position detection for mobile emergency calls andother applications. The first handsets with integrated GPS launchedin the late 1990s. The U.S. Federal Communications Commission(FCC) mandated the feature in either the handset or in the towers(for use in triangulation) in 2002 so emergency services could locate911 callers. Third-party software developers later gained access toGPS APIs from Nextel upon launch, followed by Sprint in 2006,and Verizon soon thereafter.

• Disaster relief/emergency services: Depend upon GPS for locationand timing capabilities.

• Geofencing: Vehicle tracking systems, person tracking systems, andpet tracking systems use GPS to locate a vehicle, person, or pet.These devices are attached to the vehicle, person, or the pet collar.The application provides continuous tracking and mobile or Internetupdates should the target leave a designated area.[47]

• Geotagging: Applying location coordinates to digital objects such asphotographs and other documents for purposes such as creating mapoverlays.

• GPS Aircraft Tracking• GPS tours: Location determines what content to display; for instance, information about an approaching point of

interest.• Map-making: Both civilian and military cartographers use GPS extensively.• Navigation: Navigators value digitally precise velocity and orientation measurements.• Phasor measurement units: GPS enables highly accurate timestamping of power system measurements, making it

possible to compute phasors.• Recreation: For example, geocaching, geodashing, GPS drawing and waymarking.• Surveying: Surveyors use absolute locations to make maps and determine property boundaries.• Tectonics: GPS enables direct fault motion measurement in earthquakes.

Page 12: GPS Wikipedia

Global Positioning System 12

Restrictions on civilian use

The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above18 kilometers (11 mi) altitude and 515 metres per second (1001 kn)[48] are classified as munitions (weapons) forwhich U.S. State Department export licenses are required. These limits attempt to prevent use of a receiver in aballistic missile. They would not prevent use in a cruise missile because their altitudes and speeds are similar tothose of ordinary aircraft.This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A(Clear/Acquisition) code and cannot correct for Selective Availability (SA), etc.Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretationsdiffer. The rule targets operation given the combination of altitude and speed, while some receivers stop operatingeven when stationary. This has caused problems with some amateur radio balloon launches that regularly reach30 kilometers (19 mi).

MilitaryAs of 2009, military applications of GPS include:• Navigation: GPS allows soldiers 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 Commanders DigitalAssistant and lower ranks use the Soldier Digital Assistant.[49] [50] [51] [52]

• Target tracking: Various military weapons systems use GPS to track potential ground and air targets beforeflagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allowthem to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets (forexample, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be viewed with specialsoftware.)

• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs,cruise missiles and precision-guided munitions. Artillery projectiles. Embedded GPS receivers able to withstandaccelerations of 12,000 g or about 118 km/s2 have been developed for use in 155 millimeters (6.1 in)howitzers.[53]

• Search and Rescue: Downed pilots can be located faster if their position is known.• Reconnaissance: Patrol movement can be managed more closely.• GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray

sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of theUnited States Nuclear Detonation Detection System.[54] [55]

CommunicationThe 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 carrierfrequencies that are common to all satellites in the network. Two different encodings are used, a public encoding thatenables lower resolution navigation, and an encrypted encoding used by the U.S. military.

Message format

Page 13: GPS Wikipedia

Global Positioning System 13

GPS message format

Subframes Description

1 Satellite clock,GPS time relationship

2–3 Ephemeris(precise satellite orbit)

4–5 Almanac component(satellite networksynopsis,error correction)

Each GPS satellite continuously broadcasts a navigation message at a rate of 50 bits per second (see bitrate). Eachcomplete message is composed of 30-second frames, distinct groupings of 1,500 bits of information. Each frame isfurther subdivided into 5 subframes of length 6 seconds and with 300 bits each. Each subframe contains 10 words of30 bits with length 0.6 seconds each. Each 30 second frame begins precisely on the minute or half minute asindicated by the atomic clock on each satellite.[56]

The first part of the message encodes the week number and the time within the week,[57] as well as the data about thehealth of the satellite. The second part of the message, the ephemeris, provides the precise orbit for the satellite. Thelast part of the message, the almanac, contains coarse orbit and status information for all satellites in the network aswell as data related to error correction.[58]

All satellites broadcast at the same frequencies. Signals are encoded using code division multiple access (CDMA)allowing messages from individual satellites to be distinguished from each other based on unique encodings for eachsatellite (that the receiver must be aware of). Two distinct types of CDMA encodings are used: the coarse/acquisition(C/A) code, which is accessible by the general public, and the precise (P) code, that is encrypted so that only the U.S.military can access it.The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hoursor longer in non-nominal conditions. The almanac is updated typically every 24 hours. Additionally data for a fewweeks following is uploaded in case of transmission updates that delay data upload.

Satellite frequencies

GPS frequency overview

Band Frequency Description

L1 1575.42 MHz Coarse-acquisition (C/A) and encrypted precision P(Y) codes, plus the L1 civilian (L1C) and military (M) codes on futureBlock III satellites.

L2 1227.60 MHz P(Y) code, plus the L2C and military codes on the Block IIR-M and newer satellites.

L3 1381.05 MHz Used for nuclear detonation (NUDET) detection.

L4 1379.913 MHz Being studied for additional ionospheric correction.

L5 1176.45 MHz Proposed for use as a civilian safety-of-life (SoL) signal.

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 per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per

Page 14: GPS Wikipedia

Global Positioning System 14

second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by theP code.[59] The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with aproper 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 by the United States Nuclear Detonation (NUDET) DetectionSystem (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and nearspace.[60] 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 intoan internationally protected range for aeronautical navigation, promising little or no interference under allcircumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009.[61] The L5consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phaseshift key (BPSK) modulated by a separate bit train.

Demodulation and decoding

Demodulating and Decoding GPS Satellite Signals using the Coarse/AcquisitionGold code.

Because all of the satellite signals aremodulated onto the same L1 carrierfrequency, the signals must be separatedafter demodulation. This is done byassigning each satellite a unique binarysequence known as a Gold code. The signalsare decoded after demodulation usingaddition of the Gold codes corresponding tothe satellites monitored by the receiver.[62]

[63]

If the almanac information has previouslybeen acquired, the receiver picks thesatellites to listen for by their PRNs, uniquenumbers 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 thesatellites. 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 eachsatellite's signal, it identifies it by its distinct C/A code pattern. There can be a delay of up to 30 seconds before thefirst 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 positionat this time. For more information see Demodulation and Decoding, Advanced.

Navigation equationsThe receiver uses messages received from four satellites to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as where the subscript i denotes the satellite and has the value 1, 2, 3, or 4. Knowing when the message was received , the receiver computes the message's transit time as . Assuming the message traveled at the speed of light (c) the distance traveled is

. Knowing the distance from receiver to satellite and the satellite's position implies that the receiver is on the surface of a sphere centered at the satellite's position. Thus the receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the receiver is at the intersection of the surfaces of four spheres. Excluding the unrealistic case (for GPS purposes) of two coincident spheres, the surfaces of two intersecting

Page 15: GPS Wikipedia

Global Positioning System 15

spheres is either a point (if they merely touch) or a circle as depicted in the illustration below. Two of the pointswhere the surfaces of the spheres intersect are clearly marked on the figure. The distance between these two points isthe diameter of the circle of intersection.

Two sphere surfaces intersecting in a circle

This can be seen more clearly by considering a side view of the intersecting spheres. This view would match thefigure because of the symmetry of the spheres. A view from any horizontal direction would look exactly the same.Therefore the diameter as seen from all directions is the same and thus the surfaces actually do intersect in a circle.The article trilateration algebraically confirms this geometric argument that the two sphere surfaces intersect in acircle.Having found that two sphere surfaces intersect in a circle, we now consider how the intersection of the first twosphere surfaces, the circle, intersect with the third sphere. A circle and sphere surface intersect at zero, one or twopoints. For the GPS problem we are concerned with the case of two points of intersection. Another figure, Surface ofSphere Intersecting a Circle (not a solid disk) at Two Points, is shown below to aid in visualizing this intersection.Trilateration algebraically confirms this geometric observation. The ambiguity of two points of intersection of threesphere surfaces can be resolved by noting the point that is closest to the fourth sphere surface.

Surface of a sphere intersecting a circle (i.e., theedge of a disk) at two points

Having provided a discussion of how sphere surfaces intersect, we now formulate the equations for the case whenerrors are present.

Page 16: GPS Wikipedia

Global Positioning System 16

Let denote the clock error or bias, the amount that the receiver's clock is off. The receiver has four unknowns, thethree components of GPS receiver position and the clock bias . The equation of the sphere surfaces aregiven by:

Another useful form of these equations is in terms of pseudoranges, which are the approximate ranges based on thereceiver clock's uncorrected time so that . Then the equations becomes:

Methods of solution of navigation equationsThe navigation equations can be solved by an algebraic method, called the Bancroft Method or by numericalmethods involving trilateration or multidimensional root finding.

Bancroft's method

Bancroft's method is perhaps the most important method of solving the navigation equations because it involves analgebraic as opposed to numerical method.[64] The method requires at least four satellites but more can be used.

Trilateration

The receiver can use trilateration [65] [66] and one dimensional numerical root finding.[67] Trilateration is used todetermine the intersection of the surfaces of three spheres. In the usual case of two intersections, the point nearest thesurface of the sphere corresponding to the fourth satellite is chosen. The Earth's surface can also sometimes be usedinstead, especially by civilian GPS receivers, because it is illegal in the United States to track vehicles more thanunknown operator: u',' feet (unknown operator: u'strong'unknown operator: u','m) in altitude. Let da denotethe signed magnitude of the vector from the receiver position to the fourth satellite (i.e. da = r4 - p4) as defined in thesection, Correcting a GPS receiver's clock. da is a function of the correction because the correction changes thesatellite transmission times and thus the pseudoranges. The notation, da(correction) denotes this function. Theproblem is to determine the correction such that

.This is the familiar problem of finding the zeroes of a one dimensional non-linear function of a scalar variable.Iterative numerical methods, such as those found in the chapter on root finding in Numerical Recipes can solve thistype of problem.[67] One advantage of this method is that it involves one dimensional as opposed tomultidimensional numerical root finding.

Multidimensional Newton-Raphson calculations

• Alternatively, multidimensional root finding method such as Newton-Raphson method can be used.[67] Theapproach is to linearize around an approximate solution, say from iteration k, then solve

four linear equations derived from the quadratic equations above to obtain .

The Newton-Raphson method is more rapidly convergent than other methods of numerical root finding.[67] Adisadvantage of this multidimensional root finding method as compared to single dimensional root findiing is that,"There are no good general methods for solving systems of more than one nonlinear equations."[67]

• When more than four satellites are available, the calculation can use the four best or more than four, considering number of channels, processing capability, and geometric dilution of precision (GDOP). Using more than four is an over-determined system of equations with no unique solution, which must be solved by least-squares or a similar technique.[64] If all visible satellites are used, the results are as good as or better than using the four best. Errors can be estimated through the residuals. With each combination of four or more satellites, a GDOP factor can be calculated, based on the relative sky directions of the satellites used.[68] As more satellites are picked up,

Page 17: GPS Wikipedia

Global Positioning System 17

pseudoranges from various 4-way combinations can be processed to add more estimates to the location and clockoffset. The receiver then takes the weighted average of these positions and clock offsets. After the final locationand time are calculated, the location is expressed in a specific coordinate system such as latitude and longitude,using the WGS 84 geodetic datum or a country-specific system.[69]

• Finally, results from other positioning systems such as GLONASS or the upcoming Galileo can be incorporatedor used to check the result. (By design, these systems use the same frequency bands, so much of the receivercircuitry can be shared, though the decoding is different.)

Error sources and analysisError analysis for the Global Positioning System is interesting, is important for understanding how GPS works,and is important for knowing what magnitude errors should be expected. GPS errors are affected by geometricdilution of precision and depend on signal arrival time errors, numerical errors, atmospherics effects, ephemeriserros, multipath errors and other effects.

Accuracy enhancement and surveying

AugmentationIntegrating external information into the calculation process can materially improve accuracy. Such augmentationsystems are generally named or described based on how the information arrives. Some systems transmit additionalerror information (such as clock drift, ephemera, or ionospheric delay), others characterize prior errors, while a thirdgroup provides additional navigational or vehicle information.Examples of augmentation systems include the Wide Area Augmentation System (WAAS), European GeostationaryNavigation Overlay Service (EGNOS), Differential GPS, Inertial Navigation Systems (INS) and Assisted GPS.

Precise monitoringAccuracy can be improved through precise monitoring and measurement of existing GPS signals in additional oralternate ways.The largest remaining error is usually the unpredictable delay through the ionosphere. The spacecraft broadcastionospheric model parameters, but errors remain. This is one reason GPS spacecraft transmit on at least twofrequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content(TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus theprecise ionospheric delay at each frequency.Military receivers can decode the P(Y)-code transmitted on both L1 and L2. Without decryption keys, it is stillpossible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same errorinformation. However, this technique is slow, so it is currently available only on specialized surveying equipment. Inthe future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPSmodernization). Then all users will be able to perform dual-frequency measurements and directly computeionospheric delay errors.A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). This corrects the error that arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. CPGPS uses the L1 carrier wave, which has a period of

, which is about one-thousandth of the

C/A Gold code bit period of , to act as an

additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to 2–3

Page 18: GPS Wikipedia

Global Positioning System 18

metres (6.6–9.8 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to3 centimeters (1.2 in) of ambiguity. By eliminating this error source, CPGPS coupled with DGPS normally realizesbetween 20–30 centimetres (7.9–12 in) of absolute accuracy.Relative Kinematic Positioning (RKP) is a third alternative for a precise GPS-based positioning system. In thisapproach, determination of range signal can be resolved to a precision of less than 10 centimeters (3.9 in). This isdone by resolving the number of cycles that the signal is transmitted and received by the receiver by using acombination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguityresolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning,RTK).

Timekeeping

Timekeeping and leap seconds

While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are setto GPS time (GPST; see the page of United States Naval Observatory). The difference is that GPS time is notcorrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that areperiodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has sincediverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time(TAI) (TAI - GPS = 19 seconds). Periodic corrections are performed on the on-board clocks to correct relativisticeffects and keep them synchronized with ground clocks.The GPS navigation message includes the difference between GPS time and UTC, which as of 2011 is 15 secondsbecause of the leap second added to UTC December 31, 2008. Receivers subtract this offset from GPS time tocalculate UTC and specific timezone values. New GPS units may not show the correct UTC time until afterreceiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) that,given the current period of the Earth's rotation (with one leap second introduced approximately every 18 months),should be sufficient to last until approximately the year 2300.

Timekeeping accuracy

GPS time is accurate to about 14ns.[70]

Timekeeping format

As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a weeknumber 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 at00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, aGPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS datesignal. To address this concern the modernized GPS navigation message uses a 13-bit field that only repeats every8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero).

Page 19: GPS Wikipedia

Global Positioning System 19

Carrier phase tracking (surveying)Another method that is used in surveying applications is carrier phase tracking. The period of the carrier frequencytimes the speed of light gives the wavelength, which is about 0.19 meters for the L1 carrier. Accuracy within 1% ofwavelength in detecting the leading edge, reduces this component of pseudorange error to as little as 2 millimeters.This compares to 3 meters for the C/A code and 0.3 meters for the P code.However, 2 millimeter accuracy requires measuring the total phase—the number of waves times the wavelength plusthe fractional wavelength, which requires specially equipped receivers. This method has many surveyingapplications.Triple differencing followed by numerical root finding, and a mathematical technique called least squares canestimate the position of one receiver given the position of another. First, compute the difference between satellites,then between receivers, and finally between epochs. Other orders of taking differences are equally valid. Detaileddiscussion of the errors is omitted.

The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let denote the phase of the carrier of satellite j measured by receiver i at time . This notation shows the meaning ofthe subscripts i, j, and k. The receiver (r), satellite (s), and time (t) come in alphabetical order as arguments of andto balance readability and conciseness, let be a concise abbreviation. Also we define threefunctions, : , which return differences between receivers, satellites, and time points, respectively. Eachfunction has variables with three subscripts as its arguments. These three functions are defined below. If is afunction of the three integer arguments, i, j, and k then it is a valid argument for the functions, : , withthe values defined as ,

, and .

Also if are valid arguments for the three functions and a and b are constants thenis a valid argument with values defined as

,, and

 .Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with thatfrom satellite 2 at the same epoch.[71] This difference is designated as Double differencing[72] computes the difference of receiver 1's satellite difference from that of receiver 2. Thisapproximately eliminates satellite clock errors. This double difference is:

Triple differencing[73] subtracts the receiver difference from time 1 from that of time 2. This eliminates theambiguity associated with the integral number of wave lengths in carrier phase provided this ambiguity does notchange with time. Thus the triple difference result eliminates practically all clock bias errors and the integerambiguity. Atmospheric delay and satellite ephemeris errors have been significantly reduced. This triple differenceis:

Triple difference results can be used to estimate unknown variables. For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for receiver 2's three position components. This may require the use of a numerical procedure.[74] [75] An approximation of receiver 2's position is required to use such a numerical method. This initial

Page 20: GPS Wikipedia

Global Positioning System 20

value can probably be provided from the navigation message and the intersection of sphere surfaces. Such areasonable estimate can be key to successful multidimensional root finding. Iterating from three time pairs and afairly good initial value produces one observed triple difference result for receiver 2's position. Processing additionaltime pairs can improve accuracy, overdetermining the answer with multiple solutions. Least squares can estimate anoverdetermined system. Least squares determines the position of receiver 2 which best fits the observed tripledifference results for receiver 2 positions under the criterion of minimizing the sum of the squares.

Other systemsOther satellite navigation systems in use or various states of development include:• Galileo – a global system being developed by the European Union and other partner countries, planned to be

operational by 2014• Beidou – People's Republic of China's regional system, covering Asia and the West Pacific[76]

• COMPASS – People's Republic of China's global system, planned to be operational by 2020[77] [78]

• GLONASS – Russia's global navigation system• IRNSS – India's regional navigation system, planned to be operational by 2012, covering India and Northern

Indian Ocean[79]

• QZSS – Japanese regional system covering Asia and Oceania

Notes[1] National Research Council (U.S.). Committee on the Future of the Global Positioning System; National Academy of Public Administration

(1995). The global positioning system: a shared national asset : recommendations for technical improvements and enhancements (http:/ /books. google. com/ books?id=FAHk65slfY4C). National Academies Press. p. 16. ISBN 0-309-05283-1. ., Chapter 1, p. 16 (http:/ / books.google. com/ books?id=FAHk65slfY4C& pg=PA16)

[2] Astronautica Acta II, 25 (1956)[3] Jerry Proc. "Omega" (http:/ / www. jproc. ca/ hyperbolic/ omega. html). Jproc.ca. . Retrieved 2009-12-08.[4] "Why Did the Department of Defense Develop GPS?" (http:/ / web. archive. org/ web/ 20071018151253/ http:/ / www. trimble. com/ gps/

whygps. shtml#0). Trimble Navigation Ltd. Archived from the original (http:/ / www. trimble. com/ gps/ whygps. shtml#0) on 2007-10-18. .Retrieved 2010-01-13.

[5] "Charting a Course Toward Global Navigation" (http:/ / www. aero. org/ publications/ crosslink/ summer2002/ 01. html). The AerospaceCorporation. . Retrieved 2010-01-14.

[6] "A Guide To The Global Positioning System (GPS) — GPS Timeline" (http:/ / support. radioshack. com/ support_tutorials/ gps/ gps_tmline.htm). Radio Shack. . Retrieved 2010-01-14.

[7] Michael Russell Rip, James M. Hasik (2002). The Precision Revolution: GPS and the Future of Aerial Warfare (http:/ / books. google. com/?id=mB9W3H90KDUC). Naval Institute Press. p. 65. ISBN 1557509735. . Retrieved 2010-01-14.

[8] "ICAO Completes Fact-Finding Investigation" (http:/ / www. icao. int/ cgi/ goto_m. pl?icao/ en/ trivia/ kal_flight_007. htm). InternationalCivil Aviation Organization. . Retrieved 2008-09-15.

[9] "United States Updates Global Positioning System Technology" (http:/ / www. america. gov/ xarchives/ display. html?p=washfile-english&y=2006& m=February& x=20060203125928lcnirellep0. 5061609). America.gov (http:/ / www. america. gov/ ). February 3, 2006. .

[10] "GPS & Selective Availability Q&A" (http:/ / ngs. woc. noaa. gov/ FGCS/ info/ sans_SA/ docs/ GPS_SA_Event_QAs. pdf). (http:/ / www.noaa. gov/ ). . Retrieved 2010-05-28.

[11] GPS Wing Reaches GPS III IBR Milestone (http:/ / www. insidegnss. com/ node/ 918) in InsideGNSS November 10, 2008[12] "GPS almanacs" (http:/ / www. navcen. uscg. gov/ ?pageName=gpsAlmanacs). Navcen.uscg.gov. . Retrieved 2010-10-15.[13] Dietrich Schroeer, Mirco Elena (2000). Technology Transfer (http:/ / books. google. com/ ?id=I7JRAAAAMAAJ). Ashgate. p. 80.

ISBN 075462045X. . Retrieved 2008-05-25.[14] Michael Russell Rip, James M. Hasik (2002). The Precision Revolution: GPS and the Future of Aerial Warfare (http:/ / books. google. com/

?id=_wpUAAAAMAAJ). Naval Institute Press. ISBN 1557509735. . Retrieved 2008-05-25.[15] The Global Positioning System: Assessing National Policies (http:/ / www. rand. org/ pubs/ monograph_reports/ MR614/ MR614. appb.

pdf), p.245. RAND corporation[16] "USNO NAVSTAR Global Positioning System" (http:/ / tycho. usno. navy. mil/ gpsinfo. html). U.S. Naval Observatory. . Retrieved

2011-01-07.[17] National Archives and Records Administration. U.S. Global Positioning System Policy (http:/ / clinton4. nara. gov/ textonly/ WH/ EOP/

OSTP/ html/ gps-factsheet. html). March 29, 1996.[18] "National Executive Committee for Space-Based Positioning, Navigation, and Timing" (http:/ / pnt. gov/ ). Pnt.gov. . Retrieved 2010-10-15.

Page 21: GPS Wikipedia

Global Positioning System 21

[19] "Assisted-GPS Test Calls for 3G WCDMA Networks" (http:/ / www. 3g. co. uk/ PR/ November2004/ 8641. htm). 3g.co.uk. November 10,2004. . Retrieved 2010-11-24.

[20] This story was written by 010907 (2007-09-17). "losangeles.af.mil" (http:/ / www. losangeles. af. mil/ news/ story. asp?id=123068412).losangeles.af.mil. . Retrieved 2010-10-15.

[21] Johnson, Bobbie (May 19, 2009). "GPS system 'close to breakdown'" (http:/ / www. guardian. co. uk/ technology/ 2009/ may/ 19/gps-close-to-breakdown). The Guardian. . Retrieved 2009-12-08.

[22] Coursey, David (May 21, 2009). "Air Force Responds to GPS Outage Concerns" (http:/ / abcnews. go. com/ Technology/ AheadoftheCurve/story?id=7647002& page=1). ABC News. . Retrieved 2009-05-22.

[23] "Air Force GPS Problem: Glitch Shows How Much U.S. Military Relies On GPS" (http:/ / www. huffingtonpost. com/ 2010/ 06/ 01/air-force-gps-problem-gli_n_595727. html). Huffingtonpost.comm. . Retrieved 2010-10-15.

[24] "United States Naval Observatory (USNO) GPS Constellation Status" (ftp:/ / tycho. usno. navy. mil/ pub/ gps/ gpstd. txt). . Retrieved2009-10-13.

[25] United States Naval Observatory. GPS Constellation Status (ftp:/ / tycho. usno. navy. mil/ pub/ gps/ gpsb2. txt). Retrieved December 20,2008.

[26] United States Naval Research Laboratory. National Medal of Technology for GPS (http:/ / www. eurekalert. org/ pub_releases/ 2005-11/nrl-par112205. php). November 21, 2005

[27] GPS signals travel at the speed of light, so computing the distance for a given elapsed time is almost a straightforward calculation. However,the speed of light varies slightly between the partial vacuum of space and the atmosphere. A receiver can approximate these effects andproduce a reasonable estimate. Once a rough position is determined, some receivers carefully compute the amount of atmosphere the signaltraveled through and adjust the distance accordingly.

[28] Georg zur Bonsen, Daniel Ammann, Michael Ammann, Etienne Favey, Pascal Flammant (2005-04-01). "Continuous Navigation CombiningGPS with Sensor-Based Dead Reckoning" (http:/ / web. archive. org/ web/ 20061111202317/ http:/ / www. gpsworld. com/ gpsworld/ article/articleDetail. jsp?id=154870& pageID=6). GPS World. Archived from the original (http:/ / www. gpsworld. com/ gpsworld/ article/articleDetail. jsp?id=154870& pageID=6) on 2006-11-11. .

[29] "NAVSTAR GPS User Equipment Introduction" (http:/ / www. navcen. uscg. gov/ pubs/ gps/ gpsuser/ gpsuser. pdf) (PDF). United StatesGovernment. . Chapter 7

[30] "GPS Support Notes" (http:/ / www. navmanwireless. com/ uploads/ EK/ C8/ EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters. pdf) (PDF). January 19, 2007. . Retrieved 2008-11-10.

[31] It is also possible for a circle and a spherical surface to intersect at zero points, one point, or in the very special case where the centers of thethree spheres are co-linear (i.e., all three on the same straight line) the sphere surface could intersect the entire circumference of the circle.

[32] The two intersections are symmetrical about the plane containing the three satellites. Excluding the exceptional case when the three satellitesare all in a plane containing the center of the earth, one intersection will be nearer the earth than the other.

[33] John Pike. "GPS III Operational Control Segment (OCX)" (http:/ / www. globalsecurity. org/ space/ systems/ gps_3-ocx. htm).Globalsecurity.org. . Retrieved 2009-12-08.

[34] "Global Positioning System" (http:/ / www. gps. gov/ systems/ gps/ index. html). Gps.gov. . Retrieved 2010-06-26.[35] Daly, P.. "Navstar GPS and GLONASS: global satellite navigation systems" (http:/ / ieeexplore. ieee. org/ iel1/ 2219/ 7072/ 00285510.

pdf?arnumber=285510). IEEE. .[36] Dana, Peter H. (1996-08-08). "GPS Orbital Planes" (http:/ / www. colorado. edu/ geography/ gcraft/ notes/ gps/ gif/ oplanes. gif) (GIF). .[37] What the Global Positioning System Tells Us about Relativity (http:/ / metaresearch. org/ cosmology/ gps-relativity. asp). Retrieved January

2, 2007.[38] GPS Overview from the NAVSTAR Joint Program Office (http:/ / www. losangeles. af. mil/ library/ factsheets/ factsheet. asp?id=5325).

Retrieved December 15, 2006.[39] "USCG Navcen: GPS Frequently Asked Questions" (http:/ / www. navcen. uscg. gov/ ?pageName=gpsFaq). . Retrieved 2007-01-31.[40] Agnew, D.C. and Larson, K.M. (2007). "Finding the repeat times of the GPS constellation". GPS Solutions (Springer) 11 (1): 71–76.

doi:10.1007/s10291-006-0038-4. This article from author's web site (http:/ / spot. colorado. edu/ ~kristine/ gpsrep. pdf), with minor correction.[41] Tis-pf-nisws. "Nanu 2008030" (http:/ / cgls. uscg. mil/ pipermail/ gps/ 2008-March/ 001625. html). .[42] Massatt, Paul; Wayne Brady (Summer 2002). "Optimizing performance through constellation management" (http:/ / www. aero. org/

publications/ crosslink/ summer2002/ index. html). Crosslink: 17–21. .[43] United States Coast Guard General GPS News 9-9-05 (http:/ / igs. bkg. bund. de/ root_ftp/ IGS/ mail/ igsmail/ year2005/ 5209)[44] USNO NAVSTAR Global Positioning System (http:/ / tycho. usno. navy. mil/ gpsinfo. html). Retrieved May 14, 2006.[45] Though there are many receiver manufacturers, they almost all use one of the chipsets produced for this purpose. An example: "GPS

Receiver Chip Performance Survey" (http:/ / gpstekreviews. com/ 2007/ 04/ 14/ gps-receiver-chip-performance-survey/ ). GPS TechnologyReviews. .

[46] "Publications and Standards from the National Marine Electronics Association (NMEA)" (http:/ / www. nmea. org/ pub/ index. html).National Marine Electronics Association. . Retrieved 2008-06-27.

[47] "Spotlight GPS pet locator" (http:/ / www. spotlightgps. com/ ). Spotlightgps.com. . Retrieved 2010-10-15.[48] Arms Control Association. Missile Technology Control Regime (http:/ / www. armscontrol. org/ documents/ mtcr). Retrieved May 17, 2006.[49] Commanders Digital Assistant explanation and photo (http:/ / web. archive. org/ web/ 20071201034857/ http:/ / peosoldier. army. mil/

factsheets/ SWAR_LW_DBCS. pdf)

Page 22: GPS Wikipedia

Global Positioning System 22

[50] "Latest version Commanders Digital Assistant" (http:/ / peosoldier. army. mil/ factsheets/ SWAR_LW_CDA. pdf) (PDF). . Retrieved2009-10-13.

[51] Soldier Digital Assistant explanation and photo (http:/ / web. archive. org/ web/ 20080610092154/ http:/ / www. army-technology. com/contractors/ computers/ lago/ lago6. html)

[52] Sinha, Vandana (2003-07-24). "Commanders and Soldiers' GPS-receivers" (http:/ / gcn. com/ articles/ 2003/ 07/ 24/soldiers-take-digital-assistants-to-war. aspx). Gcn.com. . Retrieved 2009-10-13.

[53] "XM982 Excalibur Precision Guided Extended Range Artillery Projectile" (http:/ / www. globalsecurity. org/ military/ systems/ munitions/m982-155. htm). GlobalSecurity.org. 2007-05-29. . Retrieved 2007-09-26.

[54] Sandia National Laboratory's Nonproliferation programs and arms control technology (http:/ / www. sandia. gov/ LabNews/ LN03-07-03/LA2003/ la03/ arms_story. htm).

[55] Dr. Dennis D. McCrady. "The GPS Burst Detector W-Sensor" (http:/ / www. osti. gov/ bridge/ servlets/ purl/ 10176800-S2tU7w/ native/10176800. pdf). Sandia National Laboratories. .

[56] "Satellite message format" (http:/ / gpsinformation. net/ gpssignal. htm). Gpsinformation.net. . Retrieved 2010-10-15.[57] Michael Woessner, Anja Koehne. "time-of-week" (http:/ / www. kowoma. de/ en/ gps/ data_composition. htm). Kowoma.de. . Retrieved

2010-10-15.[58] "Interface Specification IS-GPS-200, Revision D: Navstar GPS Space Segment/Navigation User Interfaces" (http:/ / www. losangeles. af.

mil/ shared/ media/ document/ AFD-070803-059. pdf) (PDF). Navstar GPS Joint Program Office. . Page 103.[59] How GPS works. (http:/ / www. kowoma. de/ en/ gps/ signals. htm) Konowa.de (2005).[60] United States Nuclear Detonation Detection System (USNDS) (http:/ / www. fas. org/ spp/ military/ program/ nssrm/ initiatives/ usnds. htm)[61] First GPS IIF Satellite Undergoes Environmental Testing (http:/ / mg. gpsworld. com/ gpsmg/ Military+ & + Government+ News/

First-GPS-IIF-Satellite-Undergoes-Environmental-Te/ ArticleStandard/ Article/ detail/ 470408?contextCategoryId=33824). GPS World.November 5, 2007.

[62] "GPS Almanacs, NANUS, and Ops Advisories (including archives)" (http:/ / www. navcen. uscg. gov/ ?pageName=gpsAlmanacs). GPSAlmanac Information. United States Coast Guard. . Retrieved 2009-09-09.

[63] "George, M., Hamid, M., and Miller A. Gold Code Generators in Virtex Devices (http:/ / web. archive. org/ web/ 20071122063244/ http:/ /www. xilinx. com/ support/ documentation/ application_notes/ xapp217. pdf)PDF (126 KB)

[64] "Global Positioning Systems" (http:/ / www. macalester. edu/ ~halverson/ math36/ GPS. pdf) (PDF). . Retrieved 2010-10-15.[65] Position Determination with GPS. (http:/ / www. kowoma. de/ en/ gps/ positioning. htm) Konowa.de (2005).[66] How GPS Receivers Work (http:/ / electronics. howstuffworks. com/ gadgets/ travel/ gps2. htm) at How Stuff Works[67] Press, Flannery, Tekolsky, and Vetterling 1986, Numerical Recipes, The Art of Scientific Computing (Cambridge University Press).[68] Dana, Peter H.. "Geometric Dilution of Precision (GDOP) and Visibility" (http:/ / www. colorado. edu/ geography/ gcraft/ notes/ gps/ gps.

html#Gdop). University of Colorado at Boulder. . Retrieved 2008-07-07.[69] Peter H. Dana. "Receiver Position, Velocity, and Time" (http:/ / www. colorado. edu/ geography/ gcraft/ notes/ gps/ gps. html#PosVelTime).

University of Colorado at Boulder. . Retrieved 2008-07-07.[70] David W. Allan (1997). "The Science of Timekeeping" (http:/ / www. allanstime. com/ Publications/ DWA/ Science_Timekeeping/ index.

html). Hewlett Packard. .[71] "Between-Satellite Differencing" (http:/ / www. gmat. unsw. edu. au/ snap/ gps/ gps_survey/ chap6/ 633. htm). Gmat.unsw.edu.au. .

Retrieved 2010-10-15.[72] "Double differencing" (http:/ / www. gmat. unsw. edu. au/ snap/ gps/ gps_survey/ chap6/ 635. htm). Gmat.unsw.edu.au. . Retrieved

2010-10-15.[73] "Triple differencing" (http:/ / www. gmat. unsw. edu. au/ snap/ gps/ gps_survey/ chap6/ 636. htm). Gmat.unsw.edu.au. . Retrieved

2010-10-15.[74] chapter on root finding and nonlinear sets of equations[75] Preview of Root Finding (http:/ / books. google. com/ books?id=UQW_VL2H56IC& pg=PA959& lpg=PA959& dq="Numerical+

Analysis"+ multidimension+ + root+ finding& source=web& ots=PLUUjn-33v& sig=P7btHJELgxmVpNI6_SnYjVZvUJc& hl=en& sa=X&oi=book_result& resnum=1& ct=result#PPA442,M1). Books.google.com. . Retrieved 2010-10-15.

[76] Beidou coverage[77] "Beidou satellite navigation system to cover whole world in 2020" (http:/ / eng. chinamil. com. cn/ news-channels/ china-military-news/

2010-05/ 20/ content_4222569. htm). Eng.chinamil.com.cn. . Retrieved 2010-10-15.[78] New York Times (http:/ / www. nytimes. com/ 2009/ 03/ 23/ technology/ 23iht-galileo23. html?_r=1& scp=1& sq=chinese europe galileo&

st=cse)[79] "ASM, News on GIS, GNSS, spatial information, remote sensing, mapping and surveying technologies for Asia" (http:/ / www. asmmag.

com/ news/ india-to-launch-1st-irnss-satellite-by-december). Asmmag.com. . Retrieved 2009-10-13.

Page 23: GPS Wikipedia

Global Positioning System 23

References• "NAVSTAR GPS User Equipment Introduction" (http:/ / www. navcen. uscg. gov/ pubs/ gps/ gpsuser/ gpsuser.

pdf) (PDF). United States Coast Guard. September 1996.

Further reading• Parkinson; Spilker (1996). The global positioning system (http:/ / books. google. com/ ?id=lvI1a5J_4ewC).

American Institute of Aeronautics and Astronautics. ISBN 9781563471063.• Jaizki Mendizabal; Roc Berenguer; Juan Melendez (2009). GPS and Galileo (http:/ / books. google. com/

books?id=t1lBTH42mOcC& printsec=frontcover& dq=GPS+ and+ GALILEO& hl=en&ei=xrT3TMaEC4SdOr6ijJQI& sa=X& oi=book_result& ct=result& resnum=2&ved=0CCsQ6AEwAQ#v=onepage& q& f=false). McGraw Hill. ISBN 978-0071598699.

• Nathaniel Bowditch (2002). The American Practical Navigator - Chapter 11 Satellite Navigation. United Statesgovernment.

External links• Global Positioning System (http:/ / www. dmoz. org/ Science/ Earth_Sciences/ Geomatics/

Global_Positioning_System/ ) at the Open Directory Project• GPS.gov (http:/ / www. gps. gov/ )—General public education website created by the U.S. Government• National Space-Based PNT Executive Committee (http:/ / pnt. gov/ )—Established in 2004 to oversee

management of GPS and GPS augmentations at a national level.• USCG Navigation Center (http:/ / www. navcen. uscg. gov/ ?pageName=GPS)—Status of the GPS constellation,

government policy, and links to other references. Also includes satellite almanac data.• Air Force Space Command GPS Operations Center homepage (http:/ / web. archive. org/ web/ 20061114044246/

http:/ / gps. afspc. af. mil/ gpsoc/ )• The GPS Program Office (GPS Wing) (http:/ / gps. losangeles. af. mil/ )—Responsible for designing and

acquiring the system on behalf of the United States Government.• U.S. Army Corps of Engineers manual: NAVSTAR HTML (http:/ / web. archive. org/ web/ 20080822132227/

http:/ / www. usace. army. mil/ publications/ eng-manuals/ em1110-1-1003/ toc. htm) and PDF (22.6 MB, 328pages) (http:/ / web. archive. org/ web/ 20080625111519/ http:/ / www. usace. army. mil/ publications/eng-manuals/ em1110-1-1003/ entire. pdf)

• FAA GPS FAQ (http:/ / www. faa. gov/ about/ office_org/ headquarters_offices/ ato/ service_units/ techops/navservices/ gnss/ faq/ gps/ )

• National Geodetic Survey (http:/ / www. ngs. noaa. gov/ orbits/ ) Orbits for the Global Positioning Systemsatellites in the Global Navigation Satellite System

• GPS SPS Performance Standard (http:/ / pnt. gov/ public/ docs/ 2008/ spsps2008. pdf)—The official StandardPositioning Service specification (2008 version).

• GPS SPS Performance Standard (http:/ / gps. afspc. af. mil/ gpsoc/ documents/ GPS_Signal_Spec. pdf)—Theofficial Standard Positioning Service specification (2001 version).

• GPS PPS Performance Standard (http:/ / gps. afspc. af. mil/ gpsoc/ documents/PPS_PS_Signed_Final_23_Feb_07. pdf)—The official Precise Positioning Service specification.

• Satellite Navigation: GPS & Galileo (PDF) (http:/ / paul. luminos. nl/ documents/ show_document.php?d=6)—16-page paper about the history and working of GPS, touching on the upcoming Galileo

• Average Latitude & Longitude of Countries (http:/ / web. archive. org/ web/ 20080822000330/ http:/ / www.mobilgistix. com/ Resources/ GIS/ Locations/ average-latitude-longitude-countries. aspx)

• "Sources of Errors in GPS" (http:/ / www. kowoma. de/ en/ gps/ errors. htm)

Page 24: GPS Wikipedia

Global Positioning System 24

• GPS and GLONASS Simulation (http:/ / rhp. detmich. com/ gps. html) (Java applet) Simulation and graphicaldepiction of space vehicle motion including computation of dilution of precision (DOP)

• University of New South Wales: Carrier Phase Measurement (http:/ / www. gmat. unsw. edu. au/ snap/ gps/gps_survey/ chap3/ 323. htm)

• University of New South Wales: Carrier Beat Phase (http:/ / www. gmat. unsw. edu. au/ snap/ gps/ gps_survey/chap6/ 611. htm)

Page 25: GPS Wikipedia

Article Sources and Contributors 25

Article Sources and ContributorsGlobal Positioning System  Source: http://en.wikipedia.org/w/index.php?oldid=418087611  Contributors: (jarbarf), 213.253.39.xxx, ABF, ADNghiem501, AEMoreira042281, Aaron, Abrev,Abu badali, Abune, AcademyAD, Access Denied, Acdx, Achilles03, Adambro, AeroPsico, Aesopos, Agateller, Ageekgal, Ahoerstemeier, Ahunt, Aitias, Ajkgordon, Akradecki, Alabamao68,Alan Pascoe, Alan012, Alanbrowne, Alansohn, Albedo, Alex43223, AlexBriggs13, AlexD, Alexius08, AlexiusHoratius, Alexlynne123, Alfio, [email protected], Alkivar, Almighty001, Alpha4615, Alphachimp, Altenmann, Altermike, Alvestrand, Alvis, Amcfreely, Ammembal, Andareed, Andersjm, Andres, Andrew Maiman, AndrewWTaylor, AndyA-UK, Angela, Ankooshgawande,AnnaBananaBear, Anog, Anon lynx, Antandrus, Antonio Lopez, Antonycarthy, Ao333, Aointhepiehole, Apau98, Arch dude, Arcman, ArglebargleIV, ArielGold, Arkrishna, Artem Karimov,Arthur Rubin, Asanka123, Aspectacle, Aspersions, AssegaiAli, Astronaut, Athanasius1, Atif.t2, Atropos235, Aude, Autarch, Authalic, Avmich, Avnjay, Avoided, Avono, AxelBoldt, BD2412,Badanedwa, Badger Drink, Balabiot, Bambuway, Barefootguru, Barras, BarretBonden, Bayerischermann, Baylink, Bcorr, Bdelisle, Beaver, BeenBeren, Beetstra, Beland, BenFrantzDale,Benandorsqueaks, Benbread, Bert Schlossberg, BeteTest, Bevo, Bgruber, Bhavesh871, Bibinboy, Bidabadi, Bigbadjosh, Bigjimr, Bigjoestalin, Biglovinb, Bigmak, BillC, Birdvosh, Bjdehut,Bkell, Bkkbrad, Blahma, Blanchardb, Blaxthos, Bobblewik, Bobo192, Bongwarrior, Bovineone, BreQwaS, Brews ohare, Brian Geppert, BrianWilloughby, Brianga, Brianh6630, Bricktop,Brighterorange, Brion VIBBER, BruceDLimber, Bubba73, Bucephalus, Buster2058, Bwaldher, By78, CLEGG, Caesura, CalendarWatcher, Calrosfing, Calvin 1998, CambridgeBayWeather,Can't sleep, clown will eat me, CanadianLinuxUser, Canberra photographer, CandyNetwork2005, CanisRufus, Canol, CapeCanaveral321, Capricorn42, Captainsriram, Carasmatic, CarbonX,Carboncopy, Cardamon, Carey Evans, CaribDigita, Carnildo, Cashpot, Casia wyq, Catbar, Cavrdg, Cgingold, Chairboy, Chaosdruid, Charithjayanada, Charlesnu, Chase me ladies, I'm theCavalry, Chasingsol, Chexum, ChiZeroOne, Chris 73, Chris the speller, Chris-gore, CiaPan, CieloEstrellado, Civil Engineer III, Ckatz, Clark89, Cleared as filed, Closedmouth, Cmapm,Coltongarder, Comatose51, Commander Keane, Coneslayer, Conversion script, Courcelles, Cowtippingrl, Cpl Syx, CrQAZ, CrashingWave, Crazy Software Productions, Crazypete101,Crusaderjackrabbit, Crusher7485, CrypticBacon, Crzycrzy, Cst17, Cus07, CyclePat, Cyktsui, CynicalMe, Cypherpunk, Cyrius, CzarB, D, D2holdings, DARTH SIDIOUS 2, DHN, DVdm,Damian Yerrick, Dan D. Ric, Dan East, Daniel, Danielbird, Darin-0, DarkFalls, Darth Panda, Datapark, Davandron, Davewild, David.Monniaux, David.hillshafer, DavidWBrooks,Davidgothberg, Dbfirs, Dcamp314, Dcizk, DeadEyeArrow, Deamon138, Dekisugi, Delaroyas, DeliDumrul, Delirium, Dendrolimus, Denelson83, Denimadept, Deor, DerHexer, Derek.cashman,Dethme0w, Dhaluza, Dicklyon, Didi7, Didle5, DineshAdv, Diomidis Spinellis, Dirkbb, Discospinster, Dispenser, DmitryKo, Dmoss, DocWatson42, Dogcow, Dolda2000, Dolive21, Donfbreed,Donreed, Dontdoit, Doulos Christos, Download, Dr. Blofeld, Dr.enh, DrBob, Dreadstar, Dryzhov, Drz, Dual Freq, Duk, Dysprosia, Dzogchenpa, Długosz, EEPROM Eagle, ERsdfer34w,Eaglestrike117, Easphi, Eb3686, Eclectica, Ed Poor, Edmundgreen, Edward, Eehmke, Eenemy, Egil, Ehn, Ehollon, Eirikov, El pak, Elassint, Eleassar, Eliasen, Eltener, Empty Buffer, EncMstr,Enormousdude, EnviroGranny, Epbr123, Eplack, Epson291, Eptin, Eric Shalov, Eric119, Ericcolley, ErikEngerd, Espoo, Estoy Aquí, Euchiasmus, Everyguy, Excirial, Extransit, Falcon8765,Falcorian, FergusM1970, Fiazsiddique, Fieldday-sunday, Finbarr Saunders, Fitzaubrey, Flewis, Flip619, ForthOK, Fotinakis, Fox, Franamax, Frankenschulz, Frau K, Fredb, Freemo96, Freestyle,Fritz Jörn, Friviere, Frymaster, Fullstop, Furrykef, Fuzzy Logic, G33rt, GB fan, GCW50, GFellows, GW Simulations, Gadget17, Garaffer, Garo, Gary King, Gbleem, Geoeg, Geof, Geognerd,Geologyguy, George k, Gerrit, Gerry Ashton, Gflammer, Giftlite, Gilliam, Gillis54, Gjs238, Glaurung, Glenn, Globe199, Gogo Dodo, Goltz20707, Gonzo fan2007, Gpsmobiletracker,Gpswatcher, GrahamStw, GrandDrake, Grandhougday, GravityChanges, Grayshi, Greenmind, GregLoutsenko, GregU, GregorB, Grim23, Groomer, Ground Zero, Gsarwa, Gspr, Guaka,Gujjar123, Gulftrotter, Gurch, Gutza, HBR, Haakon, Hadal, Halsteadk, Hamiltondaniel, Hamitr, Hankwang, Harald88, Haus, Hcberkowitz, Hcobb, Herbee, Hergio, Heron, Hertz1888, Hessi,HexaChord, Himatsu Bushi, Hires an editor, Hmrox, Hogyn Lleol, Hooperbloob, Howcheng, Hpa, Hqduong, HughGRex, Huwfinney, Hydrargyrum, Hydrogen Iodide, HyperSonic X,IVAN3MAN, Ian Pitchford, Ian.thomson, IanBailey, Ibagli, Icairns, IdreamofJeanie, Iediteverything, Ikescs, Ilikebooks123, Imroy, Indianw200i, Inkling, Inputpersona, Internetsurferpaul, IntoThe Fray, Irene Avetisyan, Iridescent, Irishguy, Isidore, Ivansanchez, J rey3, J.delanoy, JForget, JFreeman, JG123456789, JHMM13, JHunterJ, JMiall, JPAntonios, JQF, JaGa, Jagermic, Jaho,Jakssoul, James Foster, JamesBWatson, Jamestastic, Janipewter, Jaymarcos, Jcorgan, Jdorwin, Jecowa, Jerzy, Jesse Ruderman, Jesse Viviano, Jesusz, Jfrincon, Jfromcanada, Jhertel, Jim Douglas,Jim.henderson, Jimbrazell, Jklin, Jleedev, Jluttichau, Jm smits, Jmorgan, Joe Jarvis, Joean, Joejava, Joema, Joey-das-WBF, Johann38, John, John Vandenberg, Jok2000, JonHarder, Joonysam,JosephWong, Josephf, Joyous!, Jpbowen, Jrkarp, Jrouquie, Juice2005, Julesd, Jurohi, Jutiphan, KGasso, KPH2293, Ka Faraq Gatri, Ka9q, Kanguole, Kano14, Karn, Kars777, Karthikavij,Kaszeta, Katieh5584, Katimawan2005, Kcomstock, Kcordina, Keenan Pepper, Kenyon, Kerecsen, Kevin Forsyth, Kevinwparker, Kimse, King of Hearts, Kingpin13, Kintnerp1, Kirov Airship,KitSolidor, Kjkolb, Kkddkkdd, Koavf, Kol.yehudim.aruvim, Kostisl, Koyaanis Qatsi, Krasko, Kslotte, Kubigula, Kungfuadam, Kuru, Kusunose, L.djinevski, L337p4wn, L33th4x0rguy,L7HOMAS, LA2, LPetrie, La goutte de pluie, Lambiam, Larry Doolittle, Lars Washington, Leevclarke, Lego1496, Lekrecteurmasque, Leonard G., Leszek Jańczuk, Lethe, Lfstevens, Licon,Liftarn, Lifung, Lightmouse, LilHelpa, Linton, Lion789, LjL, Llywrch, Loadmaster, Lollerskates, Lomn, Looxix, LorenzoB, Lostchicken, Lotje, LouScheffer, Lowellian, Lowfly, Lradrama,LrdChaos, Lsheats, Ludington, Lumos3, Luna Santin, Lunchtimemama, Lupin, Lupinoid, Lvb, Lvyinwangzi, Lwanex, Lylenorton, M, M jurrens, M.Divband, MBCF, MBK004, MBisanz,MER-C, MONGO, Ma.rkus.nl, Mac, Macboots, Madelinefelkins, Makwy2, Man vyi, Mangogirl2, Mannafredo, Marcotrue, Marek69, Mark.murphy, MarkMentovai, Markus Kuhn, MarkusSchmaus, Maroof.qaiser, Martarius, MasterNetHead, Materialscientist, Matthew Woodcraft, Matthew Yeager, MatthewSchor, MattieTK, Mauls, Maury Markowitz, Mav, Maximaximax,Maxsonbd, McGreedy, Mcorazao, Mdd, Mdd4696, Mdf, Mdukas, MegA, Michael Daly, Michael Hardy, Michael Laudahn, Michael Shields, Michaelfavor, Midnightcomm, Mifter, Miguelzinho,Mike Dill, Mike s, Mike1024, Mikue, Millerliu0414, Minesweeper, Minghong, Miquonranger03, Mirgolth, Mmeijeri, MoRsE, Mondorescue, Mononomic, Morcheeba, Moreschi, Mossig,Motorspin, Mozillaman, Mozzerati, Mp3phile, Mryashc, Mtmelendez, Mugunth Kumar, Muhandes, Musiconeologist, Mwtoews, Mxn, MyronAub, Mystic Pixel, N.MacInnes, N328KF, NHJG,Nachoman-au, Naddy, Nae'blis, Nageh, Nagi Punyamurthula, Nanobear, Natl1, Ncc1701zzz, Ndenison, Ndunruh, Neep, Neil.Siegel, NellieBly, Neptune5000, Nescio, Netbymatt, Netsnipe,Neurolysis, NewEnglandYankee, Ng.j, Nick123, NickelShoe, Niczar, Night Gyr, Nightstallion, Nikevich, Nintendude, Niteowlneils, Nk, Nova77, NuclearWarfare, Nuggetboy, Nuttycoconut,Nv8200p, Odie5533, Oerjan, Ohnoitsjamie, Ojw, Olegwiki, Olive639, Olivier, Omicronpersei8, Omnicog, Onco p53, One half 3544, Onorem, Optionweb, OrbitOne, Orderud, Ost316, Otaku32,Ottawa4ever, OverlordQ, Oxymoron83, Ozlorimer, P199, P40K, PEHowland, Pakaran, Palopt, Pandemias, Para, Parabellum101, Paranoid, Patrick, Patrishia Welch, Patstuart, Paul Drye, PaulFoxworthy, Paul Klenk, PaulHanson, Pauli133, Paulseldon, Pelleapa, Pengo, Pervect, Peter k john, Petershen1984, Petruza, Pf4cdale, Pfalstad, PhGustaf, Pharaoh of the Wizards, Philip Trueman,PhoenixV, Phooto, Pi314-429, Piano non troppo, Pigsonthewing, Pimlottc, Pip2andahalf, Pipsiclae, Pizza1512, Plangmuir, PleasantDemise, Pmsyyz, Poccil, Poeloq, Pol098, Power level (DragonBall), Proca, Prodego, Prolog, PubLife, Pumpmeup, Pwaring, QuantumEleven, Quia, Quillaja, Qutezuce, Qwertyuioper, Qwikifix, Qz, R'n'B, R. S. Shaw, RHB, RHB100, RJFJR, RJaguar3,RadioFan, Rainald62, RandomWalk, Rangutan, Rchandra, Read-write-services, Reconfirmer, Redblk, Redjar, ReedConstruction, Rehnn83, Remkos, ResearchRave, Reswobslc, Rettetast,Rhbucher, Rhobite, Rhodekyll, Rich Farmbrough, Rich0, RichF, Richard Taylor, Richss, Richtom80, Richwales, Rightofcenter, Ripepette, Rjwilmsi, Roachmeister, Robbrady, Robert Xia,RobertFritzius, RobertG, Robhd, Roesser, Rogper, Romanski, RonH, Rootbeer, Ros Power, RossA, Rosser1238, RoyBoy, RoySmith, Roybb95, Rrburke, Rshin, Rubicon, Rumping, Rupert Horn,Ryan Hardy, Ryan-D, SCEhardt, SEISMO212, SEKIUCHI, SEWilco, SJK, SPUI, ST47, Sam Hocevar, Sara 171, Saros136, Sathyah, Scarian, Scarykitty, Schmock, Sciurinæ, Sdsds, Seabhcan,Seattle Skier, Seo001, Seraphim, Shaddack, Shadowlynk, Shantavira, Shirik, Shura007, Sid 3050, Sidb, Simbad82, SimonInOz, Simxp, SineWave, Sir Vicious, Sjc, Sjktje, Sjö, Skarebo,Skcpublic, SkepticX, Slavlin, Sleigh, Sleske, SlimVirgin, Slowmover, Snowolf, So God created Manchester, Socrates2008, Solipsist, Solitude, Sonjaaa, SoundGod3, Spatialthoughts, Speight,Spotsaurian, Springnuts, Stankov, StaticGull, Stefanolio2, Stevertigo, Stevietheman, Stimpy, Stoubora, Stratocracy, Stubb, Studdocs, StuffOfInterest, Sunny256, Superm401, Supertouch,Suruena, Susan118, SusanLesch, Sv1xv, Svick, Swhitehead, Symane, Syndicate, Synergyplease, TF TD U TH TC, TGC55, TJJFV, Tawker, Tbackstr, Tdadamemd, Tecoutlet, Tedchuang, Tedp,Tejblum, Tellyaddict, Tempshill, Tenro1, Tero, Teryx, Tex23, Texboy, Texture, Tflight, Thamis, The Anome, The Belgain, The Epopt, The Rambling Man, The Thing That Should Not Be, Theother steve jobs, TheChrisD, TheDJ, TheJosh, Themfromspace, Thenimaj, Theonlysilentbob, Thermochap, Thingg, Think outside the box, Thisisbossi, ThomasMurray3, ThreeBlindMice, Thue,Thumperward, TiCPU, Tide rolls, Timmo1234, Tjmartinwp, Tom Ketchum, Tom Worthington, Tomdobb, Tomlouie, Tonyfaull, Tonyjohnston, Toyota taker12, Trafford09,TravelJournalNetwork, Trewornan, Trogdor101, Troy.peterson, Tsaitgaist, Tudn, Tukss, Twilsonb, Twohoos, Txuspe, Typ932, Uknewthat, Umapathy, Urhixidur, UrsusArctos, User270981,Ustas, VTDoubleE, Vaeiou, Val42, Vanished User 1004, Vanished user 39948282, Variable, Varnav, Vassgergely, Vclaw, Vduran410, Velella, Velle, Vera Cruz, Versus22, Viames, Vincom2,Vinoth.kd, Voidxor, Vsmith, Vyarovoy, W3ird N3rd, Wavelength, Weaponpromaster, Wernher, Wesino, White Cat, Wiki alf, Wiki-vr, WikiDao, Wikiborg, Wikieditor06, Wikier, Will Beback,Wilsondavidc, Winchelsea, Wintermute115, Wireless friend, Wog7777, Wonderstruck, Woodstone, Woohookitty, Wrodenbusch, Wthornton0206, Wwoods, Wysprgr2005, X14n, XL2D,Xionbox, Xp54321, Xuiolhcarlos, Yamamoto Ichiro, Yidisheryid, Yosef1987, ZZninepluralZalpha, Zahid Abdassabur, Zapvet, Zer0faults, Zewill, Zocky, Zodon, ZorroIII, Ztyler90, Zundark,Zyxw, Zzuuzz, Пика Пика, Санта Клаус, 2284 anonymous edits

Image Sources, Licenses and ContributorsImage:GPS Satellite NASA art-iif.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GPS_Satellite_NASA_art-iif.jpg  License: Public Domain  Contributors: Bricktop, Bryan Derksen,Common Good, Edward, GDK, Michaelfavor, 7 anonymous editsImage:Magellan GPS Blazer12.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Magellan_GPS_Blazer12.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors:User:Nachoman-auImage:KyotoTaxiRide.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:KyotoTaxiRide.jpg  License: GNU Free Documentation License  Contributors: Ed g2s, Kliek, Morio, PartyzanXXI, Ypy31, 1 anonymous editsImage:GPS on smartphone cycling.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:GPS_on_smartphone_cycling.JPG  License: Creative Commons Attribution 3.0  Contributors:User:HawaiianMamaImage:NAVSTAR GPS logo shield-official.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NAVSTAR_GPS_logo_shield-official.jpg  License: Public Domain  Contributors: Koavf,Michaelfavor, Minna Sora no Shita, MithrandirMage, Tsaitgaist, 5 anonymous editsImage:50th Space Wing.png  Source: http://en.wikipedia.org/w/index.php?title=File:50th_Space_Wing.png  License: unknown  Contributors: United States Air Force

Page 26: GPS Wikipedia

Image Sources, Licenses and Contributors 26

Image:Lat 2spheres 2.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Lat_2spheres_2.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistImage:Circle sphere 2-colour.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Circle_sphere_2-colour.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistImage:Sphere clock cor.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Sphere_clock_cor.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistImage:Global Positioning System satellite.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Global_Positioning_System_satellite.jpg  License: Public Domain  Contributors:User:SCEhardtImage:ConstellationGPS.gif  Source: http://en.wikipedia.org/w/index.php?title=File:ConstellationGPS.gif  License: Public Domain  Contributors: Original uploader was El pak at en.wikipediaFile:GPS monitor station.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GPS_monitor_station.jpg  License: GNU Free Documentation License  Contributors: Bubba73 (You talkin'to me?), (Jud McCranie). Original uploader was Bubba73 at en.wikipediaImage:GPS Receivers.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GPS_Receivers.jpg  License: GNU Free Documentation License  Contributors: Bdk, Head, Jensens, Lzur,MB-one, Partyzan XXI, Ustas, WikipediaMaster, 1 anonymous editsImage:j32 1 small.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:J32_1_small.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Phooto, 1 anonymous editsFile:GPS roof antenna dsc06160.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GPS_roof_antenna_dsc06160.jpg  License: GNU Free Documentation License  Contributors:User:David.MonniauxImage:gps ca gold.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Gps_ca_gold.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistImage:lat 2spheres 2.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Lat_2spheres_2.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistImage:circle sphere 2-colour.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Circle_sphere_2-colour.svg  License: Creative Commons Zero  Contributors: User:TsaitgaistFile:Flag of Europe.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_Europe.svg  License: Public Domain  Contributors: User:-xfi-, User:Dbenbenn, User:Funakoshi,User:Jeltz, User:Nightstallion, User:Paddu, User:Verdy p, User:Zscout370File:Flag of the People's Republic of China.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_the_People's_Republic_of_China.svg  License: Public Domain  Contributors:User:Denelson83, User:SKopp, User:Shizhao, User:Zscout370File:Flag of Russia.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_Russia.svg  License: Public Domain  Contributors: Zscout370File:Flag of India.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_India.svg  License: Public Domain  Contributors: User:SKoppFile:Flag of Japan.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_Japan.svg  License: Public Domain  Contributors: Various

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/