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About GPS

Global Positioning SystemAnti-Spoofing Module (SAASM) in the
The Global Positioning System (GPS) isDefense Advanced GPS Receiver (DAGR). In
the only fully functional Globaldemonstration videos, the DAGR is able
Navigation Satellite System (GNSS).to detect jamming and maintain its lock
Utilizing a constellation of at least 24on the encrypted GPS signals during
medium Earth orbit satellites thatinterference which causes civilian
transmit precise microwave signals, thereceivers to lose lock.[26]
system enables a GPS receiver to[edit] Techniques to improve accuracy
determine its location, speed/direction,[edit] Augmentation
and time.Main article: GNSS Augmentation
Developed by the United StatesAugmentation methods of improving
Department of Defense, it is officiallyaccuracy rely on external information
named NAVSTAR GPS (Contrary to popularbeing integrated into the calculation
belief, NAVSTAR is not an acronym, butprocess. There are many such systems in
simply a name given by Mr. John Walsh, aplace and they are generally named or
key decision maker when it came to thedescribed based on how the GPS sensor
budget for the GPS program[1]). Thereceives the information. Some systems
satellite constellation is managed bytransmit additional information about
the United States Air Force 50th Spacesources of error (such as clock drift,
Wing. The cost of maintaining the systemephemeris, or ionospheric delay), others
is approximately US$750 million perprovide direct measurements of how much
year,[2] including the replacement ofthe signal was off in the past, while a
aging satellites, and research andthird group provide additional
development. Despite these costs, GPS isnavigational or vehicle information to
free for civilian use as a public good.be integrated in the calculation
GPS has become a widely used aid toprocess.
navigation worldwide, and a useful toolExamples of augmentation systems include
for map-making, land surveying,the Wide Area Augmentation System,
commerce, and scientific uses. GPS alsoDifferential GPS, Inertial Navigation
provides a precise time reference usedSystems and Assisted GPS.
in many applications including[edit] Precise monitoring
scientific study of earthquakes, andThe accuracy of a calculation can also
synchronization of telecommunicationsbe improved through precise monitoring
networks.and measuring of the existing GPS
Simplified method of operationsignals in additional or alternate ways.
A GPS receiver calculates its positionAfter SA, which has been turned off, the
by measuring the distance between itselflargest error in GPS is usually the
and three or more GPS satellites.unpredictable delay through the
Measuring the time delay betweenionosphere. The spacecraft broadcast
transmission and reception of each GPSionospheric model parameters, but errors
microwave signal gives the distance toremain. This is one reason the GPS
each satellite, since the signal travelsspacecraft transmit on at least two
at a known speed - the speed of light.frequencies, L1 and L2. Ionospheric
These signals also carry informationdelay is a well-defined function of
about the satellites' location andfrequency and the total electron content
general system health (known as almanac(TEC) along the path, so measuring the
and ephemeris data). By determining thearrival time difference between the
position of, and distance to, at leastfrequencies determines TEC and thus the
three satellites, the receiver canprecise ionospheric delay at each
compute its position usingfrequency.
trilateration.[3] Receivers typically doReceivers with decryption keys can
not have perfectly accurate clocks anddecode the P(Y)-code transmitted on both
therefore track one or more additionalL1 and L2. However, these keys are
satellites, using their atomic clocks toreserved for the military and
correct the receiver's own clock error."authorized" agencies and are not
[edit] Technical descriptionavailable to the public. Without keys,
Unlaunched GPS satellite on display atit is still possible to use a codeless
the San Diego Aerospace museumtechnique to compare the P(Y) codes on
Unlaunched GPS satellite on display atL1 and L2 to gain much of the same error
the San Diego Aerospace museuminformation. However, this technique is
[edit] System segmentationslow, so it is currently limited to
The current GPS consists of three majorspecialized surveying equipment. In the
segments. These are the space segmentfuture, additional civilian codes are
(SS), a control segment (CS), and a userexpected to be transmitted on the L2 and
segment (US).[4]L5 frequencies (see GPS modernization,
[edit] Space segmentbelow). Then all users will be able to
The space segment (SS) is composed ofperform dual-frequency measurements and
the orbiting GPS satellites, or Spacedirectly compute ionospheric delay
Vehicles (SV) in GPS parlance. The GPSerrors.
design calls for 24 SVs to beA second form of precise monitoring is
distributed equally among six circularcalled Carrier-Phase Enhancement
orbital planes.[5] The orbital planes(CPGPS). The error, which this corrects,
are centered on the Earth, not rotatingarises because the pulse transition of
with respect to the distant stars.[6]the PRN is not instantaneous, and thus
The six planes have approximately 55°the correlation (satellite-receiver
inclination (tilt relative to Earth'ssequence matching) operation is
equator) and are separated by 60°imperfect. The CPGPS approach utilizes
right ascension of the ascending nodethe L1 carrier wave, which has a period
(angle along the equator from a1000 times smaller than that of the C/A
reference point to the orbit'sbit period, to act as an additional
intersection).[2]clock signal and resolve the
Orbiting at an altitude of approximatelyuncertainty. The phase difference error
20,200 kilometers (12,600 miles orin the normal GPS amounts to between 2
10,900 nautical miles; orbital radius ofand 3 meters (6 to 10 ft) of ambiguity.
26,600 km (16,500 mi or 14,400 NM)),CPGPS working to within 1% of perfect
each SV makes two complete orbits eachtransition reduces this error to 3
sidereal day, so it passes over the samecentimeters (1 inch) of ambiguity. By
location on Earth once each day. Theeliminating this source of error, CPGPS
orbits are arranged so that at least sixcoupled with DGPS normally realizes
satellites are always within line ofbetween 20 and 30 centimeters (8 to 12
sight from almost everywhere on Earth'sinches) of absolute accuracy.
surface.[7]Relative Kinematic Positioning (RKP) is
As of September 2007, there are 31another approach for a precise GPS-based
actively broadcasting satellites in thepositioning system. In this approach,
GPS constellation. The additionaldetermination of range signal can be
satellites improve the precision of GPSresolved to an accuracy of less than 10
receiver calculations by providingcentimeters (4 in). This is done by
redundant measurements. With theresolving the number of cycles in which
increased number of satellites, thethe signal is transmitted and received
constellation was changed to aby the receiver. This can be
nonuniform arrangement. Such anaccomplished by using a combination of
arrangement was shown to improvedifferential GPS (DGPS) correction data,
reliability and availability of thetransmitting GPS signal phase
system, relative to a uniform system,information and ambiguity resolution
when multiple satellites fail.[8]techniques via statistical
[edit] Control segmenttests-possibly with processing in
The flight paths of the satellites arereal-time (real-time kinematic
tracked by US Air Force monitoringpositioning, RTK).
stations in Hawaii, Kwajalein, Ascension[edit] GPS time and date
Island, Diego Garcia, and ColoradoWhile most clocks are synchronized to
Springs, Colorado, along with monitorCoordinated Universal Time (UTC), the
stations operated by the NationalAtomic clocks on the satellites are set
Geospatial-Intelligence Agency (NGA).[9]to GPS time. The difference is that GPS
The tracking information is sent to thetime is not corrected to match the
Air Force Space Command's master controlrotation of the Earth, so it does not
station at Schriever Air Force Base incontain leap seconds or other
Colorado Springs, which is operated bycorrections which are periodically added
the 2d Space Operations Squadron (2to UTC. GPS time was set to match
SOPS) of the United States Air ForceCoordinated Universal Time (UTC) in
(USAF). 2 SOPS contacts each GPS1980, but has since diverged. The lack
satellite regularly with a navigationalof corrections means that GPS time
update (using the ground antennas atremains at a constant offset (19
Ascension Island, Diego Garcia,seconds) with International Atomic Time
Kwajalein, and Colorado Springs). These(TAI). Periodic corrections are
updates synchronize the atomic clocks onperformed on the on-board clocks to
board the satellites to within onecorrect relativistic effects and keep
microsecond and adjust the ephemeris ofthem synchronized with ground clocks.
each satellite's internal orbital model.The GPS navigation message includes the
The updates are created by a Kalmandifference between GPS time and UTC,
filter which uses inputs from the groundwhich as of 2006 is 14 seconds.
monitoring stations, space weatherReceivers subtract this offset from GPS
information, and various othertime to calculate UTC and specific
inputs.[10]timezone values. New GPS units may not
GPS receivers come in a variety ofshow the correct UTC time until after
formats, from devices integrated intoreceiving the UTC offset message. The
cars, phones, and watches, to dedicatedGPS-UTC offset field can accommodate 255
devices such as those shown here fromleap seconds (eight bits) which, at the
manufacturers Trimble, Garmin and Leicacurrent rate of change of the Earth's
(left to right).rotation, is sufficient to last until
GPS receivers come in a variety ofthe year 2330.
formats, from devices integrated intoAs opposed to the year, month, and day
cars, phones, and watches, to dedicatedformat of the Julian calendar, the GPS
devices such as those shown here fromdate is expressed as a week number and a
manufacturers Trimble, Garmin and Leicaday-of-week number. The week number is
(left to right).transmitted as a ten-bit field in the C
[edit] User segmentA and P(Y) navigation messages, and so
The user's GPS receiver is the userit becomes zero again every 1,024 weeks
segment (US) of the GPS system. In(19.6 years). GPS week zero started at
general, GPS receivers are composed of00:00:00 UTC (00:00:19 TAI) on January
an antenna, tuned to the frequencies6, 1980 and the week number became zero
transmitted by the satellites,again for the first time at 23:59:47 UTC
receiver-processors, and a highly-stableon August 21, 1999 (00:00:19 TAI on
clock (often a crystal oscillator). TheyAugust 22, 1999). To determine the
may also include a display for providingcurrent Gregorian date, a GPS receiver
location and speed information to themust be provided with the approximate
user. A receiver is often described bydate (to within 3,584 days) to correctly
its number of channels: this signifiestranslate the GPS date signal. To
how many satellites it can monitoraddress this concern the modernized GPS
simultaneously. Originally limited tonavigation messages use a 13-bit field,
four or five, this has progressivelywhich only repeats every 8,192 weeks
increased over the years so that, as of(157 years), and will not return to zero
2006, receivers typically have betweenuntil near the year 2137.
twelve and twenty channels.[edit] GPS modernization
A typical OEM GPS receiver module, basedMain article: GPS modernization
on the SiRF Star III chipset, measuringHaving reached the program's
15Ã-17 mm, and used in many products.requirements for Full Operational
A typical OEM GPS receiver module, basedCapability (FOC) on July 17, 1995,[27]
on the SiRF Star III chipset, measuringthe GPS completed its original design
15Ã-17 mm, and used in many products.goals. However, additional advances in
GPS receivers may include an input fortechnology and new demands on the
differential corrections, using the RTCMexisting system led to the effort to
SC-104 format. This is typically in themodernize the GPS system. Announcements
form of a RS-232 port at 4,800 bit/sfrom the Vice President and the White
speed. Data are actually sent at a muchHouse in 1998 initiated these changes,
lower rate, which limits the accuracy ofand in 2000 the U.S. Congress authorized
the signal sent using RTCM. Receiversthe effort, referring to it as GPS III.
with internal DGPS receivers canThe project aims to improve the accuracy
outperform those using external RTCMand availability for all users and
data. As of 2006, even low-cost unitsinvolves new ground stations, new
commonly include Wide Area Augmentationsatellites, and four additional
System (WAAS) receivers.navigation signals. New civilian signals
Many GPS receivers can relay positionare called L2C, L5 and L1C; the new
data to a PC or other device using themilitary code is called M-Code. Initial
NMEA 0183 protocol. NMEA 2000[11] is aOperational Capability (IOC) of the L2C
newer and less widely adopted protocol.code is expected in 2008.[28] A goal of
Both are proprietary and controlled by2013 has been established for the entire
the US-based National Marine Electronicsprogram, with incentives offered to the
Association. References to the NMEAcontractors if they can complete it by
protocols have been compiled from public2011.
records, allowing open source tools like[edit] Applications
gpsd to read the protocol withoutThe Global Positioning System, while
violating intellectual property laws.originally a military project, is
Other proprietary protocols exist asconsidered a dual-use technology,
well, such as the SiRF and MTKmeaning it has significant applications
protocols. Receivers can interface withfor both the military and the civilian
other devices using methods including aindustry.
serial connection, USB or Bluetooth.[edit] Military
[edit] Navigation signalsPlease help improve this article by
Main article: GPS signalsexpanding this section.
GPS broadcast signalSee talk page for details. Please remove
GPS broadcast signalthis message once the section has been
Each GPS satellite continuouslyexpanded.
broadcasts a Navigation Message at 50The military use GPS for the following
bit/s giving the time-of-day, GPS weekpurposes:
number and satellite health information[edit] Navigation
(all transmitted in the first part ofGPS allows soldiers to find objectives
the message), an ephemeris (transmittedin the dark or in unfamiliar territory,
in the second part of the message) andand to coordinate the movement of troops
an almanac (later part of the message).and supplies.
The ephemeris data gives the satellite's[edit] Target tracking
own precise orbit and is output over 18Various military weapons systems use GPS
seconds, repeating every 30 seconds. Theto track potential ground and air
ephemeris is updated every 2 hours andtargets before they are flagged as
is generally valid for 4 hours, withhostile. These weapons systems pass GPS
provisions for 6 hour time-outs. Theco-ordinates of targets to
time needed to acquire the ephemeris isprecision-guided munitions to allow them
becoming a significant element of theto engage the targets accurately.
delay to first position fix, because, asMilitary aircraft, particularly those
the hardware becomes more capable, theused in air-to-ground roles use GPS to
time to lock onto the satellite signalsfind targets (for example, gun camera
shrinks, but the ephemeris data requiresvideo from AH-1 Cobras in Iraq show GPS
30 seconds (worst case) before it isco-ordinates that can be looked up in
received, due to the low dataGoogle Earth).
transmission rate. The almanac consists[edit] Missile and projectile guidance
of coarse orbit and status informationGPS allows accurate targeting of various
for each satellite in the constellationmilitary weapons including ICBMs, cruise
and takes 12 seconds for each satellitemissiles and precision-guided munitions.
present, with information for a newArtillery projectiles with embedded GPS
satellite being transmitted every 30receivers able to withstand forces of
seconds (15.5 minutes for 3112,000G have been developed for use in
satellites). The purpose of the data is155 mm howitzers.[29]
to assist in the acquisition of[edit] Search and Rescue
satellites at power-up by allowing theDowned pilots can be located faster if
receiver to generate a list of visiblethey have a GPS receiver.
satellites based on stored position and[edit] Reconnaissance and Map Creation
time, while an ephemeris from eachThe military use GPS extensively to aid
satellite is needed to compute positionmapping and reconnaissance.
fixes using that satellite. In older[edit] Other
hardware, lack of an almanac in a newThe GPS satellites also carry nuclear
receiver would cause long delays beforedetonation detectors, which form a major
providing a valid position, because theportion of the United States Nuclear
search for each satellite was a slowDetonation Detection System.[30]
process. Advances in hardware have made[edit] Civilian
the acquisition process much faster, soSee also: GPS applications
not having an almanac is no longer anThis antenna is mounted on the roof of a
issue. An important thing to note abouthut containing a scientific experiment
navigation data is that each satelliteneeding precise timing.
transmits only its own ephemeris, butThis antenna is mounted on the roof of a
transmits an almanac for all satellites.hut containing a scientific experiment
Each satellite transmits its navigationneeding precise timing.
message with at least two distinctMany civilian applications benefit from
spread spectrum codes: the Coarse /GPS signals, using one or more of three
Acquisition (C/A) code, which is freelybasic components of the GPS; absolute
available to the public, and the Preciselocation, relative movement, time
(P) code, which is usually encrypted andtransfer.
reserved for military applications. TheThe ability to determine the receiver's
C/A code is a 1,023 chip pseudo-randomabsolute location allows GPS receivers
(PRN) code at 1.023 million chips/sec soto perform as a surveying tool or as an
that it repeats every millisecond. Eachaid to navigation. The capacity to
satellite has its own C/A code so thatdetermine relative movement enables a
it can be uniquely identified andreceiver to calculate local velocity and
received separately from the otherorientation, useful in vessels or
satellites transmitting on the sameobservations of the Earth. Being able to
frequency. The P-code is a 10.23synchronize clocks to exacting standards
megachip/sec PRN code that repeats onlyenables time transfer, which is critical
every week. When the "anti-spoofing"in large communication and observation
mode is on, as it is in normalsystems. An example is CDMA digital
operation, the P code is encrypted bycellular. Each base station has a GPS
the Y-code to produce the P(Y) code,timing receiver to synchronize its
which can only be decrypted by unitsspreading codes with other base stations
with a valid decryption key. Both the Cto facilitate inter-cell hand off and
A and P(Y) codes impart the precisesupport hybrid GPS/CDMA positioning of
time-of-day to the user. Frequenciesmobiles for emergency calls and other
used by GPS includeapplications.
* L1 (1575.42 MHz): Mix of NavigationFinally, GPS enables researchers to
Message, coarse-acquisition (C/A) codeexplore the Earth environment including
and encrypted precision P(Y) code, plusthe atmosphere, ionosphere and gravity
the new L1C on future Block IIIfield. GPS survey equipment has
satellites.revolutionized tectonics by directly
* L2 (1227.60 MHz): P(Y) code, plus themeasuring the motion of faults in
new L2C code on the Block IIR-M andearthquakes.
newer satellites.To help prevent civilian GPS guidance
* L3 (1381.05 MHz): Used by the Nuclearfrom being used in an enemy's military
Detonation (NUDET) Detection Systemor improvised weaponry, the US
Payload (NDS) to signal detection ofGovernment controls the export of
nuclear detonations and othercivilian receivers. A US-based
high-energy infrared events. Used tomanufacturer cannot generally export a
enforce nuclear test ban treaties.GPS receiver unless the receiver
* L4 (1379.913 MHz): Being studied forcontains limits restricting it from
additional ionospheric correction.functioning when it is simultaneously
* L5 (1176.45 MHz): Proposed for use as(1) at an altitude above 18 kilometers
a civilian safety-of-life (SoL) signal(60,000 ft) and (2) traveling at over
(see GPS modernization). This frequency515 m/s (1,000 knots).[31]
falls into an internationally protected[edit] History
range for aeronautical navigation,Please help improve this article by
promising little or no interferenceexpanding this section.
under all circumstances. The first BlockSee talk page for details. Please remove
IIF satellite that would provide thisthis message once the section has been
signal is set to be launched in 2008.expanded.
[edit] Calculating positionsThe design of GPS is based partly on the
[edit] Using the C/A codesimilar ground-based radio navigation
To start off, the receiver picks which Csystems, such as LORAN and the Decca
A codes to listen for by PRN number,Navigator developed in the early 1940s,
based on the almanac information it hasand used during World War II. Additional
previously acquired. As it detects eachinspiration for the GPS system came when
satellite's signal, it identifies it bythe Soviet Union launched the first
its distinct C/A code pattern, thenSputnik in 1957. A team of U.S.
measures the time delay for eachscientists led by Dr. Richard B.
satellite. To do this, the receiverKershner were monitoring Sputnik's radio
produces an identical C/A sequence usingtransmissions. They discovered that,
the same seed number as the satellite.because of the Doppler effect, the
By lining up the two sequences, thefrequency of the signal being
receiver can measure the delay andtransmitted by Sputnik was higher as the
calculate the distance to the satellite,satellite approached, and lower as it
called the pseudorange[12].continued away from them. They realized
Overlapping pseudoranges, represented asthat since they knew their exact
curves, are modified to yield thelocation on the globe, they could
probable positionpinpoint where the satellite was along
Overlapping pseudoranges, represented asits orbit by measuring the Doppler
curves, are modified to yield thedistortion.
probable positionThe first satellite navigation system,
Next, the orbital position data, orTransit, used by the United States Navy,
ephemeris, from the Navigation Messagewas first successfully tested in 1960.
is then downloaded to calculate theUsing a constellation of five
satellite's precise position. Asatellites, it could provide a
more-sensitive receiver will potentiallynavigational fix approximately once per
acquire the ephemeris data quicker thanhour. In 1967, the U.S. Navy developed
a less-sensitive receiver, especially inthe Timation satellite which proved the
a noisy environment.[13] Knowing theability to place accurate clocks in
position and the distance of a satellitespace, a technology the GPS system
indicates that the receiver is locatedrelies upon. In the 1970s, the
somewhere on the surface of an imaginaryground-based Omega Navigation System,
sphere centered on that satellite andbased on signal phase comparison, became
whose radius is the distance to it.the first world-wide radio navigation
Receivers can substitute altitude forsystem.
one satellite, which the GPS receiverThe first experimental Block-I GPS
translates to a pseudorange measuredsatellite was launched in February
from the center of the earth.1978.[28] The GPS satellites were
Locations are calculated not ininitially manufactured by Rockwell
three-dimensional space, but inInternational and are now manufactured
four-dimensional spacetime, meaning aby Lockheed Martin.
measure of the precise time-of-day is[edit] Timeline
very important. The measured* In 1972, the US Air Force Central
pseudoranges from four satellites haveInertial Guidance Test Facility
already been determined with the(Holloman AFB) conducted developmental
receiver's internal clock, and thus havefight tests of two prototype GPS
an unknown amount of clock error. (Thereceivers over White Sands Missile
clock error or actual time does notRange, using ground-based
matter in the initial pseudorangepseudo-satellites.
calculation, because that is based on* In 1978 the first experimental Block-I
how much time has passed betweenGPS satellite was launched.
reception of each of the* In 1983, after Soviet interceptor
signals.[clarify][citation needed]) Theaircraft shot down the civilian airliner
four-dimensional point that isKAL 007 in restricted Soviet airspace,
equidistant from the pseudoranges iskilling all 269 people on board, U.S.
calculated as a guess as to thePresident Ronald Reagan announced that
receiver's location, and the factor usedthe GPS system would be made available
to adjust those pseudoranges tofor civilian uses once it was completed.
intersect at that four-dimensional point* By 1985, ten more experimental Block-I
gives a guess as to the receiver's clocksatellites had been launched to validate
offset. With each guess, a geometricthe concept.
dilution of precision (GDOP) vector is* On February 14, 1989, the first modern
calculated, based on the relative skyBlock-II satellite was launched.
positions of the satellites used. As* In 1992, the 2nd Space Wing, which
more satellites are picked up,originally managed the system, was
pseudoranges from more combinations ofde-activated and replaced by the 50th
four satellites can be processed to addSpace Wing.
more guesses to the location and clock* By December 1993 the GPS system
offset. The receiver then determinesachieved initial operational
which combinations to use and how tocapability[32]
calculate the estimated position by* By January 17, 1994 a complete
determining the weighted average ofconstellation of 24 satellites was in
these positions and clock offsets. Afterorbit.
the final location and time are* Full Operational Capability was
calculated, the location is expressed indeclared by NAVSTAR in April 1995.
a specific coordinate system, e.g.* In 1996, recognizing the importance of
latitude/longitude, using the WGS 84GPS to civilian users as well as
geodetic datum or a local systemmilitary users, U.S. President Bill
specific to a country.Clinton issued a policy directive[33]
[edit] Using the P(Y) codedeclaring GPS to be a dual-use system
Calculating a position with the P(Y)and establishing an Interagency GPS
signal is generally similar in concept,Executive Board to manage it as a
assuming one can decrypt it. Thenational asset.
encryption is essentially a safety* In 1998, U.S. Vice President Al Gore
mechanism: if a signal can beannounced plans to upgrade GPS with two
successfully decrypted, it is reasonablenew civilian signals for enhanced user
to assume it is a real signal being sentaccuracy and reliability, particularly
by a GPS satellite.[citation needed] Inwith respect to aviation safety.
comparison, civil receivers are highly* On May 2, 2000 "Selective
vulnerable to spoofing since correctlyAvailability" was discontinued as a
formatted C/A signals can be generatedresult of the 1996 executive order,
using readily available signalallowing users to receive a non-degraded
generators. RAIM features do not protectsignal globally.
against spoofing, since RAIM only checks* In 2004, the United States Government
the signals from a navigationalsigned a historic agreement with the
perspective.European Community establishing
[edit] Accuracy and error sourcescooperation related to GPS and Europe's
The position calculated by a GPSplanned Galileo system.
receiver requires the current time, the* In 2004, U.S. President George W. Bush
position of the satellite and theupdated the national policy, replacing
measured delay of the received signal.the executive board with the National
The position accuracy is primarilySpace-Based Positioning, Navigation, and
dependent on the satellite position andTiming Executive Committee.
signal delay.* November 2004, QUALCOMM announced
To measure the delay, the receiversuccessful tests of Assisted-GPS system
compares the bit sequence received fromfor mobile phones.[3]
the satellite with an internally* In 2005, the first modernized GPS
generated version. By comparing thesatellite was launched and began
rising and trailing edges of the bittransmitting a second civilian signal
transitions, modern electronics can(L2C) for enhanced user performance.
measure signal offset to within about 1%* The most recent launch was on 17
of a bit time, or approximately 10November 2006. The oldest GPS satellite
nanoseconds for the C/A code. Since GPSstill in operation was launched in
signals propagate nearly at the speed ofAugust 1991.
light, this represents an error of about* On September 14, 2007, the aging
3 meters. This is the minimum errormainframe-based Ground Segment Control
possible using only the GPS C/A signal.System was transitioned to the new
Position accuracy can be improved byArchitecture Evolution Plan. [4]
using the higher-chiprate P(Y) signal.[edit] Satellite numbers
Assuming the same 1% bit time accuracy,Name Launch Period No of satellites
the high frequency P(Y) signal resultslaunched, inc. launch failures
in an accuracy of about 30 centimeters.Currently in service
Electronics errors are one of severalBlock I 1978-1985 11 0
accuracy-degrading effects outlined inBlock II 1985-1990 9 0
the table below. When taken together,Block IIA 1990-1997 19 15+11
autonomous civilian GPS horizontalBlock IIR 1997-2004 12 12
position fixes are typically accurate toBlock IIR-M 2005- 3 3
about 15 meters (50 ft). These effectsTotal 54 (plus one not launched) 30+1
also reduce the more precise P(Y) code's1One test satellite
accuracy.[edit] Awards
Sources of User Equivalent Range ErrorsTwo GPS developers have received the
(UERE) Source EffectNational Academy of Engineering Charles
Ionospheric effects ± 5 meterStark Draper prize year 2003:
Ephemeris errors ± 2.5 meter* Ivan Getting, emeritus president of
Satellite clock errors ± 2 meterThe Aerospace Corporation and engineer
Multipath distortion ± 1 meterat the Massachusetts Institute of
Tropospheric effects ± 0.5 meterTechnology, established the basis for
Numerical errors ± 1 meterGPS, improving on the World War II
[edit] Atmospheric effectsland-based radio system called LORAN
Inconsistencies of atmospheric(Long-range Radio Aid to Navigation).
conditions affect the speed of the GPS* Bradford Parkinson, professor of
signals as they pass through the Earth'saeronautics and astronautics at Stanford
atmosphere and ionosphere. CorrectingUniversity, conceived the present
these errors is a significant challengesatellite-based system in the early
to improving GPS position accuracy.1960s and developed it in conjunction
These effects are smallest when thewith the U.S. Air Force.
satellite is directly overhead andOne GPS developer, Roger L. Easton,
become greater for satellites nearer thereceived the National Medal of
horizon since the signal is affected forTechnology on February 13, 2006 at the
a longer time. Once the receiver'sWhite House.[34]
approximate location is known, aOn February 10, 1993, the National
mathematical model can be used toAeronautic Association selected the
estimate and compensate for theseGlobal Positioning System Team as
errors.winners of the 1992 Robert J. Collier
Because ionospheric delay affects theTrophy, the most prestigious aviation
speed of microwave signals differentlyaward in the United States. This team
based on frequency-a characteristicconsists of researchers from the Naval
known as dispersion-both frequency bandsResearch Laboratory, the U.S. Air Force,
can be used to help reduce this error.the Aerospace Corporation, Rockwell
Some military and expensive survey-gradeInternational Corporation, and IBM
civilian receivers compare the differentFederal Systems Company. The citation
delays in the L1 and L2 frequencies toaccompanying the presentation of the
measure atmospheric dispersion, andtrophy honors the GPS Team "for the most
apply a more precise correction. Thissignificant development for safe and
can be done in civilian receiversefficient navigation and surveillance of
without decrypting the P(Y) signalair and spacecraft since the
carried on L2, by tracking the carrierintroduction of radio navigation 50
wave instead of the modulated code. Toyears ago."
facilitate this on lower cost receivers,[edit] Other systems
a new civilian code signal on L2, calledMain article: Global Navigation
L2C, was added to the Block IIR-MSatellite System
satellites, which was first launched inOther satellite navigation systems in
2005. It allows a direct comparison ofuse or various states of development
the L1 and L2 signals using the codedinclude:
signal instead of the carrier wave.* Beidou - China's regional system that
The effects of the ionosphere generallyChina has proposed to expand into a
change slowly, and can be averaged overglobal system named COMPASS.
time. The effects for any particular* Galileo - a proposed global system
geographical area can be easilybeing developed by the European Union,
calculated by comparing the GPS-measuredjoined by China, Israel, India, Morocco,
position to a known surveyed location.Saudi Arabia and South Korea, Ukraine
This correction is also valid for otherplanned to be operational by 2011-12.
receivers in the same general location.* GLONASS - Russia's global system which
Several systems send this informationis being restored to full availability
over radio or other links to allow L1in partnership with India.
only receivers to make ionospheric* Indian Regional Navigational Satellite
corrections. The ionospheric data areSystem (IRNSS) - India's proposed
transmitted via satellite in Satelliteregional system.
Based Augmentation Systems such as WAAS,* QZSS - Japanese proposed regional
which transmits it on the GPS frequencysystem, adding better coverage to the
using a special pseudo-random numberJapanese islands.
(PRN), so only one antenna and receiver[edit] See also
are required.Satellite navigation systems Portal
Humidity also causes a variable delay,Nautical Portal
resulting in errors similar to* RAIM
ionospheric delay, but occurring in the* SIGI
troposphere. This effect is both more* radio navigation
localized and changes more quickly than* High Sensitivity GPS
ionospheric effects and is not frequency* Degree Confluence Project Use GPS to
dependent. These traits making precisevisit integral degrees of latitude and
measurement and compensation of humiditylongitude.
errors more difficult than ionospheric* Exif, GPS data transfer.
effects.* Geotagging
Changes in altitude also change the* Geocaching
amount of delay due to the signal* NaviTraveler.com, - a GPS point
passing through less of the atmospheresharing community.
at higher elevations. Since the GPS* GPS Drawing Digital mapping and
receiver computes its approximatedrawing with GPS tracks.
altitude, this error is relatively* GPS tracking
simple to correct.* GPS/INS
[edit] Multipath effects* Assisted GPS
GPS signals can also be affected by* GPX (XML schema for interchange of
multipath issues, where the radiowaypoints)
signals reflect off surrounding terrain;* ID Sniper rifle
buildings, canyon walls, hard ground,* OpenStreetMap, free content maps and
etc. These delayed signals can causestreet pictures (GFDL)
inaccuracy. A variety of techniques,* Telematics: Many telematics devices
most notably narrow correlator spacing,use GPS to determine the location of
have been developed to mitigatemobile equipment.
multipath errors. For long delay* The American Practical
multipath, the receiver itself canNavigator-Chapter 11 "Satellite
recognize the wayward signal and discardNavigation"
it. To address shorter delay multipath* Point of Interest
from the signal reflecting off the* Automotive navigation system
ground, specialized antennas may be used* NextGen
to reduce the signal power as received[edit] Notes
by the antenna. Short delay reflections1. ^ Parkinson, B.W. (1996), Global
are harder to filter out because theyPositioning System: Theory and
interfere with the true signal, causingApplications, chap. 1: Introduction and
effects almost indistinguishable fromHeritage of NAVSTAR, the Global
routine fluctuations in atmosphericPositioning System. pp. 3-28, American
delay.Institute of Aeronautics and
Multipath effects are much less severeAstronautics, Washington, D.C.
in moving vehicles. When the GPS antenna2. ^ a b GPS Overview from the NAVSTAR
is moving, the false solutions usingJoint Program Office. Accessed December
reflected signals quickly fail to15, 2006.
converge and only the direct signals3. ^ HowStuffWorks. How GPS Receivers
result in stable solutions.Work. Accessed May 14, 2006.
[edit] Ephemeris and clock errors4. ^ globalsecurity.org [1].
The navigation message from a satellite5. ^ Dana, Peter H. GPS Orbital Planes.
is sent out only every 30 seconds. InAugust 8, 1996.
reality, the data contained in these6. ^ What the Global Positioning System
messages tend to be "out of date" by anTells Us about Relativity. Accessed
even larger amount. Consider the caseJanuary 2, 2007.
when a GPS satellite is boosted back7. ^ USCG Navcen: GPS Frequently Asked
into a proper orbit; for some timeQuestions. Accessed January 3, 2007.
following the maneuver, the receiver's8. ^ Massatt, Paul and Brady, Wayne.
calculation of the satellite's position"Optimizing performance through
will be incorrect until it receivesconstellation management", Crosslink,
another ephemeris update. The onboardSummer 2002, pages 17-21.
clocks are extremely accurate, but they9. ^ US Coast Guard General GPS News
do suffer from some clock drift. This9-9-05
problem tends to be very small, but may10. ^ USNO. NAVSTAR Global Positioning
add up to 2 meters (6 ft) of inaccuracy.System. Accessed May 14, 2006.
This class of error is more "stable"11. ^ NMEA NMEA 2000
than ionospheric problems and tends to12. ^
change over days or weeks rather than13. ^ AN02 Network Assistance (HTML).
minutes. This makes correction fairlyRetrieved on 2007-09-10.
simple by sending out a more accurate14. ^ a b Office of Science and
almanac on a separate channel.Technology Policy. Presidential
[edit] Selective availabilitystatement to stop degrading GPS. May 1,
The GPS includes a feature called2000.
Selective Availability (SA) that15. ^ FAA, Selective Availability.
introduces intentional, slowly changingRetrieved Jan. 6, 2007.
random errors of up to a hundred meters16. ^
(328 ft) into the publicly available17. ^ Rizos, Chris. University of New
navigation signals to confound, forSouth Wales. GPS Satellite Signals.
example, guiding long range missiles to1999.
precise targets. Additional accuracy was18. ^ The Global Positioning System by
available in the signal, but in anRobert A. Nelson Via Satellite, November
encrypted form that was only available1999
to the United States military, its19. ^ Ashby, Neil Relativity and GPS.
allies and a few others, mostlyPhysics Today, May 2002.
government users.20. ^ Space Environment Center. SEC
SA typically added signal errors of upNavigation Systems GPS Page. August 26,
to about 10 meters (32 ft) horizontally1996.
and 30 meters (98 ft) vertically. The21. ^ The hunt for an unintentional GPS
inaccuracy of the civilian signal wasjammer. GPS World. January 1, 2003.
deliberately encoded so as not to change22. ^ Low Cost and Portable GPS Jammer.
very quickly, for instance the entirePhrack issue 0x3c (60), article 13].
eastern U.S. area might read 30 m off,Published December 28, 2002.
but 30 m off everywhere and in the same23. ^ American Forces Press Service.
direction. To improve the usefulness ofCENTCOM charts progress. March 25, 2003.
GPS for civilian navigation,24. ^ [2]
Differential GPS was used by many25. ^ Ruley, John. AVweb. GPS jamming.
civilian GPS receivers to greatlyFebruary 12, 2003.
improve accuracy.26. ^ Commercial GPS Receivers: Facts
During the Gulf War, the shortage offor the Warfighter. Hosted at the Joint
military GPS units and the wideChiefs website, linked by the USAF's GPS
availability of civilian ones amongWing DAGR program website. Accessed on
personnel resulted in a decision to10 April, 2007
disable Selective Availability. This was27. ^ US Coast Guard news release.
ironic, as SA had been introducedGlobal Positioning System Fully
specifically for these situations,Operational
allowing friendly troops to use the28. ^ a b Hydrographic Society Journal.
signal for accurate navigation, while atDevelopments in Global Navigation
the same time denying it to the enemy.Satellite Systems. Issue #104, April
But since SA was also denying the same2002. Accessed April 5, 2007.
accuracy to thousands of friendly29. ^ XM982 Excalibur Precision Guided
troops, turning it off or setting it toExtended Range Artillery Projectile.
an error of zero meters (effectively theGlobalSecurity.org (2007-05-29).
same thing) presented a clear benefit.Retrieved on 2007-09-26.
In the 1990s, the FAA started pressuring30. ^ Sandia National Laboratory's
the military to turn off SA permanently.Nonproliferation programs and arms
This would save the FAA millions ofcontrol technology.
dollars every year in maintenance of31. ^ Arms Control Association. Missile
their own radio navigation systems. TheTechnology Control Regime. Accessed May
military resisted for most of the 1990s,17, 2006.
and it ultimately took an executive32. ^ United States Department of
order to have SA removed from the GPSDefense. Announcement of Initial
signal. The amount of error added wasOperational Capability. December 8,
"set to zero"[14] at midnight on May 1,1993.
2000 following an announcement by U.S.33. ^ National Archives and Records
President Bill Clinton, allowing usersAdministration. U.S. GLOBAL POSITIONING
access to the error-free L1 signal. PerSYSTEM POLICY. March 29, 1996.
the directive, the induced error of SA34. ^ United States Naval Research
was changed to add no error to theLaboratory. National Medal of Technology
public signals (C/A code). Selectivefor GPS. November 21, 2005
Availability is still a system[edit] External links
capability of GPS, and error could, inWikimedia Commons has media related to:
theory, be reintroduced at any time. InGlobal Positioning System
practice, in view of the hazards andGovernment links
costs this would induce for US and* GPS.gov-General public education
foreign shipping, it is unlikely to bewebsite created by the U.S. Government
reintroduced, and various government* National Space-Based PNT Executive
agencies, including the FAA,[15] haveCommittee-Established in 2004 to oversee
stated that it is not intended to bemanagement of GPS and GPS augmentations
reintroduced.at a national level.
The US military has developed the* USCG Navigation Center-Status of the
ability to locally deny GPS (and otherGPS constellation, government policy,
navigation services) to hostile forcesand links to other references. Also
in a specific area of crisis withoutincludes satellite almanac data.
affecting the rest of the world or its* The GPS Joint Program Office (GPS
own military systems.[14]JPO)-Responsible for designing and
One interesting side effect of theacquiring the system on behalf of the US
Selective Availability hardware is theGovernment.
capability to correct the frequency of* U.S. Naval Observatory's GPS
the GPS caesium and rubidium atomicconstellation status
clocks to an accuracy of approximately 2* U.S. Army Corps of Engineers manual:
Ã- 10-13 (one in five trillion). ThisNAVSTAR HTML and PDF (22.6 MB, 328
represented a significant improvementpages)
over the raw accuracy of the* PNT Selective Availability
clocks.[citation needed]Announcements
On 19 September 2007, the United States* GPS SPS Signal Specification, 2nd
Department of Defense announced thatEdition-The official Standard
they would not procure any morePositioning Signal specification.
satellites capable of implementing SA.* Federal Aviation Administration's GPS
[16]FAQ
[edit] RelativityIntroductory / tutorial links
According to the theory of relativity,* How does GPS work? TomTom explains
due to their constant movement andGPS, navigation, and digital maps
height relative to the Earth-centered* GPS Academy Garmin interactive video
inertial reference frame, the clocks onweb site explaing what exactly GPS is
the satellites are affected by theirand what it can do for you
speed (special relativity) as well as* HowStuffWorks' Simplified explanation
their gravitational potential (generalof GPS and video about how GPS works.
relativity). For the GPS satellites,* Trimble's Online GPS Tutorial Tutorial
general relativity predicts that thedesigned to introduce you to the
atomic clocks at GPS orbital altitudesprinciples behind GPS
will tick more rapidly, by about 45,900* GPS and GLONASS Simulation(Java
nanoseconds (ns) per day, because theyapplet) Simulation and graphical
are in a weaker gravitational field thandepiction of space vehicle motion
atomic clocks on Earth's surface.including computation of dilution of
Special relativity predicts that atomicprecision (DOP)
clocks moving at GPS orbital speeds willTechnical, historical, and ancillary
tick more slowly than stationary groundtopics links
clocks by about 7,200 ns per day. When* Dana, Peter H. "Global Positioning
combined, the discrepancy is 38System Overview"
microseconds per day; a difference of* Satellite Navigation: GPS & Galileo
4.465 parts in 1010.[17]. To account for(PDF)-16-page paper about the history
this, the frequency standard onboardand working of GPS, touching on the
each satellite is given a rate offsetupcoming Galileo
prior to launch, making it run slightly* History of GPS, including information
slower than the desired frequency onabout each satellite's configuration and
Earth; specifically, at 10.22999999543launch.
MHz instead of 10.23 MHz.[18]* Chadha, Kanwar. "The Global
GPS observation processing must alsoPositioning System: Challenges in
compensate for another relativisticBringing GPS to Mainstream Consumers"
effect, the Sagnac effect. The GPS timeTechnical Article (1998)
scale is defined in an inertial system* GPS Weapon Guidance Techniques
but observations are processed in an* RAND history of the GPS system (PDF)
Earth-centered, Earth-fixed* GPS Anti-Jam Protection Techniques
(co-rotating) system, a system in which* Crosslink Summer 2002 issue by The
simultaneity is not uniquely defined.Aerospace Corporation on satellite
The Lorentz transformation between thenavigation.
two systems modifies the signal run* Improved weather predictions from
time, a correction having oppositeCOSMIC GPS satellite signal occultation
algebraic signs for satellites in thedata.
Eastern and Western celestial* David L. Wilson's GPS Accuracy Web
hemispheres. Ignoring this effect willPage A thorough analysis of the accuracy
produce an east-west error on the orderof GPS.
of hundreds of nanoseconds, or tens of* Innovation: Spacecraft Navigator,
meters in position.[19]Autonomous GPS Positioning at High Earth
The atomic clocks on board the GPSOrbits Example of GPS receiver designed
satellites are precisely tuned, makingfor high altitude spaceflight.
the system a practical engineering* The Navigator GPS Receiver GSFC's
application of the scientific theory ofNavigator spaceflight receiver.
relativity in a real-world environment.* Neil Ashby's Relativity in the Global
[edit] GPS interference and jammingPositioning System
Since GPS signals at terrestrial[show]v • d • e
receivers tend to be relatively weak, itSatellite navigation systems
is easy for other sources ofHistorical Flag of the United States
electromagnetic radiation to desensitizeTransit
the receiver, making acquiring andOperational Flag of the Soviet Union /
tracking the satellite signals difficultFlag of Russia GLONASS · Flag of the
or impossible.United States GPS
Solar flares are one such naturallyDevelopmental Flag of the People's
occurring emission with the potential toRepublic of China Beidou/COMPASS ·
degrade GPS reception, and their impactFlag of Europe Galileo · Flag of
can affect reception over the half ofIndia IRNSS · Flag of Japan QZSS
the Earth facing the sun. GPS signalsRelated topics EGNOS · GAGAN ·
can also be interfered with by naturallyGPS·C · LAAS · MSAS · WAAS
occurring geomagnetic storms,[show]v • d • e
predominantly found near the poles ofTime signal stations
the Earth's magnetic field.[20] AnotherLongwave DCF77 · HBG · JJY ·
source of problems is the metal embeddedMSF · TDF · WWVB
in some car windscreens to preventShortwave BPM · CHU · RWM ·
icing, degrading reception just insideWWV · WWVH · YVTO
the car.GNSS time transfer Beidou · Galileo
Man-made interference can also disrupt,· GLONASS · GPS · IRNSS
or jam, GPS signals. In one wellDefunct time stations OMA · VNG
documented case, an entire harbor was[show]v • d • e
unable to receive GPS signals due toGlobal structure in Systems, Systems
unintentional jamming caused by asciences and Systems scientists
malfunctioning TV antennaCategories Category:Conceptual systems
preamplifier.[21] Intentional jamming is· Category:Physical systems ·
also possible. Generally, strongerCategory:Social systems ·
signals can interfere with GPS receiversCategory:Systems · Category:Systems
when they are within radio range, orscience · Category:Systems scientists
line of sight. In 2002, a detailed· Category:Systems theory
description of how to build a shortSystems Biological system · Complex
range GPS L1 C/A jammer was published insystem · Complex adaptive system ·
the online magazine Phrack.[22]Conceptual system · Cultural system
The U.S. government believes that such· Dynamical system · Economic
jammers were used occasionally duringsystem · Ecosystem · Formal
the 2001 war in Afghanistan and the U.S.system · Global Positioning System
military claimed to destroy a GPS jammer· Human organ systems ·
with a GPS-guided bomb during the IraqInformation systems · Legal system
War.[23] Such a jammer is relatively· Metric system · Nervous system
easy to detect and locate, making it an· Non-linear system · Operating
attractive target for anti-radiationsystem · Physical system ·
missiles. The UK Ministry of DefencePolitical system · Sensory system
tested a jamming system in the UK's West· Social system · Solar System
Country on 7 and 8 June 2007. [24]· System · Systems of measurement
Some countries allow the use of GPSFields of theory Chaos theory ·
repeaters to allow for the reception ofComplex systems · Control theory ·
GPS signals indoors and in obscuredCybernetics · Holism in science ·
locations, however, under EU and UKSociotechnical systems theory ·
laws, the use of these is prohibited asSystems biology · System dynamics
the signals can cause interference to· Systems ecology · Systems
other GPS receivers that may receiveengineering · Systems theory ·
data from both GPS satellites and theSystems science
repeater.Systems scientists Russell L. Ackoff
Due to the potential for both natural· William Ross Ashby · Gregory
and man-made noise, numerous techniquesBateson · Ludwig von Bertalanffy
continue to be developed to deal with· Kenneth E. Boulding · Peter
the interference. The first is to notCheckland · C. West Churchman ·
rely on GPS as a sole source. AccordingHeinz von Foerster · Charles
to John Ruley, "IFR pilots should have aFrançois · Jay Wright Forrester
fallback plan in case of a GPS· Ralph W. Gerard · Debora Hammond
malfunction".[25] Receiver Autonomous· George Klir · Niklas Luhmann
Integrity Monitoring (RAIM) is a feature· Humberto Maturana · Donella
now included in some receivers, which isMeadows · Mihajlo D. Mesarovic ·
designed to provide a warning to theHoward T. Odum · Talcott Parsons ·
user if jamming or another problem isIlya Prigogine · Anatol Rapoport
detected. The U.S. military has also· Francisco Varela · John N.
deployed their Selective Availability /



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