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About GPS

Global  Positioning  Systemequipment. In the future, additional civilian
codes are expected to be transmitted on the
The Global Positioning System (GPS) is theL2 and L5 frequencies (see GPS modernization,
only fully functional Global Navigationbelow). Then all users will be able to
Satellite System (GNSS). Utilizing aperform dual-frequency measurements and
constellation of at least 24 medium Earthdirectly  compute  ionospheric  delay errors.
orbit satellites that transmit precise
microwave signals, the system enables a GPSA second form of precise monitoring is called
receiver to determine its location, speedCarrier-Phase Enhancement (CPGPS). The error,
direction,  and  time.which this corrects, arises because the pulse
transition of the PRN is not instantaneous,
Developed by the United States Department ofand thus the correlation (satellite-receiver
Defense, it is officially named NAVSTAR GPSsequence matching) operation is imperfect.
(Contrary to popular belief, NAVSTAR is notThe CPGPS approach utilizes the L1 carrier
an acronym, but simply a name given by Mr.wave, which has a period 1000 times smaller
John Walsh, a key decision maker when it camethan that of the C/A bit period, to act as an
to the budget for the GPS program[1]). Theadditional clock signal and resolve the
satellite constellation is managed by theuncertainty. The phase difference error in
United States Air Force 50th Space Wing. Thethe normal GPS amounts to between 2 and 3
cost of maintaining the system ismeters (6 to 10 ft) of ambiguity. CPGPS
approximately US$750 million per year,[2]working to within 1% of perfect transition
including the replacement of agingreduces this error to 3 centimeters (1 inch)
satellites, and research and development.of ambiguity. By eliminating this source of
Despite these costs, GPS is free for civilianerror, CPGPS coupled with DGPS normally
use  as  a  public  good.realizes between 20 and 30 centimeters (8 to
12  inches)  of  absolute  accuracy.
GPS has become a widely used aid to
navigation worldwide, and a useful tool forRelative Kinematic Positioning (RKP) is
map-making, land surveying, commerce, andanother approach for a precise GPS-based
scientific uses. GPS also provides a precisepositioning system. In this approach,
time reference used in many applicationsdetermination of range signal can be resolved
including scientific study of earthquakes,to an accuracy of less than 10 centimeters (4
and synchronization of telecommunicationsin). This is done by resolving the number of
networks.cycles in which the signal is transmitted and
received by the receiver. This can be
Simplified  method  of  operationaccomplished by using a combination of
differential GPS (DGPS) correction data,
A GPS receiver calculates its position bytransmitting GPS signal phase information and
measuring the distance between itself andambiguity resolution techniques via
three or more GPS satellites. Measuring thestatistical tests-possibly with processing in
time delay between transmission and receptionreal-time (real-time kinematic positioning,
of each GPS microwave signal gives theRTK).
distance to each satellite, since the signal
travels at a known speed - the speed of[edit]  GPS  time  and  date
light. These signals also carry information
about the satellites' location and generalWhile most clocks are synchronized to
system health (known as almanac and ephemerisCoordinated Universal Time (UTC), the Atomic
data). By determining the position of, andclocks on the satellites are set to GPS time.
distance to, at least three satellites, theThe difference is that GPS time is not
receiver can compute its position usingcorrected to match the rotation of the Earth,
trilateration.[3] Receivers typically do notso it does not contain leap seconds or other
have perfectly accurate clocks and thereforecorrections which are periodically added to
track one or more additional satellites,UTC. GPS time was set to match Coordinated
using their atomic clocks to correct theUniversal Time (UTC) in 1980, but has since
receiver's  own  clock  error.diverged. The lack of corrections means that
GPS time remains at a constant offset (19
[edit]  Technical  descriptionseconds) with International Atomic Time
(TAI). Periodic corrections are performed on
Unlaunched GPS satellite on display at thethe on-board clocks to correct relativistic
San  Diego  Aerospace  museumeffects and keep them synchronized with
ground  clocks.
Unlaunched GPS satellite on display at the
San  Diego  Aerospace  museumThe GPS navigation message includes the
difference between GPS time and UTC, which as
[edit]  System  segmentationof 2006 is 14 seconds. Receivers subtract
this offset from GPS time to calculate UTC
The current GPS consists of three majorand specific timezone values. New GPS units
segments. These are the space segment (SS), amay not show the correct UTC time until after
control segment (CS), and a user segmentreceiving the UTC offset message. The GPS-UTC
(US).[4]offset field can accommodate 255 leap seconds
(eight bits) which, at the current rate of
[edit]  Space  segmentchange of the Earth's rotation, is sufficient
to  last  until  the  year  2330.
The space segment (SS) is composed of the
orbiting GPS satellites, or Space VehiclesAs opposed to the year, month, and day format
(SV) in GPS parlance. The GPS design callsof the Julian calendar, the GPS date is
for 24 SVs to be distributed equally amongexpressed as a week number and a day-of-week
six circular orbital planes.[5] The orbitalnumber. The week number is transmitted as a
planes are centered on the Earth, notten-bit field in the C/A and P(Y) navigation
rotating with respect to the distantmessages, and so it becomes zero again every
stars.[6] The six planes have approximately1,024 weeks (19.6 years). GPS week zero
55° inclination (tilt relative tostarted at 00:00:00 UTC (00:00:19 TAI) on
Earth's equator) and are separated byJanuary 6, 1980 and the week number became
60° right ascension of the ascendingzero again for the first time at 23:59:47 UTC
node (angle along the equator from aon August 21, 1999 (00:00:19 TAI on August
reference point to the orbit's22, 1999). To determine the current Gregorian
intersection).[2]date, a GPS receiver must be provided with
the approximate date (to within 3,584 days)
Orbiting at an altitude of approximatelyto correctly translate the GPS date signal.
20,200 kilometers (12,600 miles or 10,900To address this concern the modernized GPS
nautical miles; orbital radius of 26,600 kmnavigation messages use a 13-bit field, which
(16,500 mi or 14,400 NM)), each SV makes twoonly repeats every 8,192 weeks (157 years),
complete orbits each sidereal day, so itand will not return to zero until near the
passes over the same location on Earth onceyear  2137.
each day. The orbits are arranged so that at
least six satellites are always within line[edit]  GPS  modernization
of sight from almost everywhere on Earth's
surface.[7]Main  article:  GPS  modernization
As of September 2007, there are 31 activelyHaving reached the program's requirements for
broadcasting satellites in the GPSFull Operational Capability (FOC) on July 17,
constellation. The additional satellites1995,[27] the GPS completed its original
improve the precision of GPS receiverdesign goals. However, additional advances in
calculations by providing redundanttechnology and new demands on the existing
measurements. With the increased number ofsystem led to the effort to modernize the GPS
satellites, the constellation was changed tosystem. Announcements from the Vice President
a nonuniform arrangement. Such an arrangementand the White House in 1998 initiated these
was shown to improve reliability andchanges, and in 2000 the U.S. Congress
availability of the system, relative to aauthorized the effort, referring to it as GPS
uniform system, when multiple satellitesIII.
fail.[8]
The project aims to improve the accuracy and
[edit]  Control  segmentavailability for all users and involves new
ground stations, new satellites, and four
The flight paths of the satellites areadditional navigation signals. New civilian
tracked by US Air Force monitoring stationssignals are called L2C, L5 and L1C; the new
in Hawaii, Kwajalein, Ascension Island, Diegomilitary code is called M-Code. Initial
Garcia, and Colorado Springs, Colorado, alongOperational Capability (IOC) of the L2C code
with monitor stations operated by theis expected in 2008.[28] A goal of 2013 has
National Geospatial-Intelligence Agencybeen established for the entire program, with
(NGA).[9] The tracking information is sent toincentives offered to the contractors if they
the Air Force Space Command's master controlcan  complete  it  by  2011.
station at Schriever Air Force Base in
Colorado Springs, which is operated by the 2d[edit]  Applications
Space Operations Squadron (2 SOPS) of the
United States Air Force (USAF). 2 SOPSThe Global Positioning System, while
contacts each GPS satellite regularly with aoriginally a military project, is considered
navigational update (using the grounda dual-use technology, meaning it has
antennas at Ascension Island, Diego Garcia,significant applications for both the
Kwajalein, and Colorado Springs). Thesemilitary  and  the  civilian  industry.
updates synchronize the atomic clocks on
board the satellites to within one[edit]  Military
microsecond and adjust the ephemeris of each
satellite's internal orbital model. ThePlease help improve this article by expanding
updates are created by a Kalman filter whichthis  section.
uses inputs from the ground monitoring
stations, space weather information, andSee talk page for details. Please remove this
various  other  inputs.[10]message  once  the section has been expanded.
GPS receivers come in a variety of formats,The military use GPS for the following
from devices integrated into cars, phones,purposes:
and watches, to dedicated devices such as
those shown here from manufacturers Trimble,[edit]  Navigation
Garmin  and  Leica  (left  to  right).
GPS allows soldiers to find objectives in the
GPS receivers come in a variety of formats,dark or in unfamiliar territory, and to
from devices integrated into cars, phones,coordinate the movement of troops and
and watches, to dedicated devices such assupplies.
those shown here from manufacturers Trimble,
Garmin  and  Leica  (left  to  right).[edit]  Target  tracking
[edit]  User  segmentVarious military weapons systems use GPS to
track potential ground and air targets before
The user's GPS receiver is the user segmentthey are flagged as hostile. These weapons
(US) of the GPS system. In general, GPSsystems pass GPS co-ordinates of targets to
receivers are composed of an antenna, tunedprecision-guided munitions to allow them to
to the frequencies transmitted by theengage  the  targets  accurately.
satellites, receiver-processors, and a
highly-stable clock (often a crystalMilitary aircraft, particularly those used in
oscillator). They may also include a displayair-to-ground roles use GPS to find targets
for providing location and speed information(for example, gun camera video from AH-1
to the user. A receiver is often described byCobras in Iraq show GPS co-ordinates that can
its number of channels: this signifies howbe  looked  up  in  Google  Earth).
many satellites it can monitor
simultaneously. Originally limited to four or[edit]  Missile  and  projectile  guidance
five, this has progressively increased over
the years so that, as of 2006, receiversGPS allows accurate targeting of various
typically have between twelve and twentymilitary weapons including ICBMs, cruise
channels.missiles  and  precision-guided  munitions.
A typical OEM GPS receiver module, based onArtillery projectiles with embedded GPS
the SiRF Star III chipset, measuringreceivers able to withstand forces of 12,000G
15Ã-17  mm,  and  used  in  many products.have been developed for use in 155 mm
howitzers.[29]
A typical OEM GPS receiver module, based on
the SiRF Star III chipset, measuring[edit]  Search  and  Rescue
15Ã-17  mm,  and  used  in  many products.
Downed pilots can be located faster if they
GPS receivers may include an input forhave  a  GPS  receiver.
differential corrections, using the RTCM
SC-104 format. This is typically in the form[edit]  Reconnaissance  and  Map  Creation
of a RS-232 port at 4,800 bit/s speed. Data
are actually sent at a much lower rate, whichThe military use GPS extensively to aid
limits the accuracy of the signal sent usingmapping  and  reconnaissance.
RTCM. Receivers with internal DGPS receivers
can outperform those using external RTCM[edit]  Other
data. As of 2006, even low-cost units
commonly include Wide Area AugmentationThe GPS satellites also carry nuclear
System  (WAAS)  receivers.detonation detectors, which form a major
portion of the United States Nuclear
Many GPS receivers can relay position data toDetonation  Detection  System.[30]
a PC or other device using the NMEA 0183
protocol. NMEA 2000[11] is a newer and less[edit]  Civilian
widely adopted protocol. Both are proprietary
and controlled by the US-based NationalSee  also:  GPS  applications
Marine Electronics Association. References to
the NMEA protocols have been compiled fromThis antenna is mounted on the roof of a hut
public records, allowing open source toolscontaining a scientific experiment needing
like gpsd to read the protocol withoutprecise  timing.
violating intellectual property laws. Other
proprietary protocols exist as well, such asThis antenna is mounted on the roof of a hut
the SiRF and MTK protocols. Receivers cancontaining a scientific experiment needing
interface with other devices using methodsprecise  timing.
including a serial connection, USB or
Bluetooth.Many civilian applications benefit from GPS
signals, using one or more of three basic
[edit]  Navigation  signalscomponents of the GPS; absolute location,
relative  movement,  time  transfer.
Main  article:  GPS  signals
The ability to determine the receiver's
GPS  broadcast  signalabsolute location allows GPS receivers to
perform as a surveying tool or as an aid to
GPS  broadcast  signalnavigation. The capacity to determine
relative movement enables a receiver to
Each GPS satellite continuously broadcasts acalculate local velocity and orientation,
Navigation Message at 50 bit/s giving theuseful in vessels or observations of the
time-of-day, GPS week number and satelliteEarth. Being able to synchronize clocks to
health information (all transmitted in theexacting standards enables time transfer,
first part of the message), an ephemeriswhich is critical in large communication and
(transmitted in the second part of theobservation systems. An example is CDMA
message) and an almanac (later part of thedigital cellular. Each base station has a GPS
message). The ephemeris data gives thetiming receiver to synchronize its spreading
satellite's own precise orbit and is outputcodes with other base stations to facilitate
over 18 seconds, repeating every 30 seconds.inter-cell hand off and support hybrid GPS
The ephemeris is updated every 2 hours and isCDMA positioning of mobiles for emergency
generally valid for 4 hours, with provisionscalls  and  other  applications.
for 6 hour time-outs. The time needed to
acquire the ephemeris is becoming aFinally, GPS enables researchers to explore
significant element of the delay to firstthe Earth environment including the
position fix, because, as the hardwareatmosphere, ionosphere and gravity field. GPS
becomes more capable, the time to lock ontosurvey equipment has revolutionized tectonics
the satellite signals shrinks, but theby directly measuring the motion of faults in
ephemeris data requires 30 seconds (worstearthquakes.
case) before it is received, due to the low
data transmission rate. The almanac consistsTo help prevent civilian GPS guidance from
of coarse orbit and status information forbeing used in an enemy's military or
each satellite in the constellation and takesimprovised weaponry, the US Government
12 seconds for each satellite present, withcontrols the export of civilian receivers. A
information for a new satellite beingUS-based manufacturer cannot generally export
transmitted every 30 seconds (15.5 minutesa GPS receiver unless the receiver contains
for 31 satellites). The purpose of the datalimits restricting it from functioning when
is to assist in the acquisition of satellitesit is simultaneously (1) at an altitude above
at power-up by allowing the receiver to18 kilometers (60,000 ft) and (2) traveling
generate a list of visible satellites basedat  over  515  m/s  (1,000  knots).[31]
on stored position and time, while an
ephemeris from each satellite is needed to[edit]  History
compute position fixes using that satellite.
In older hardware, lack of an almanac in aPlease help improve this article by expanding
new receiver would cause long delays beforethis  section.
providing a valid position, because the
search for each satellite was a slow process.See talk page for details. Please remove this
Advances in hardware have made themessage  once  the section has been expanded.
acquisition process much faster, so not
having an almanac is no longer an issue. AnThe design of GPS is based partly on the
important thing to note about navigation datasimilar ground-based radio navigation
is that each satellite transmits only its ownsystems, such as LORAN and the Decca
ephemeris, but transmits an almanac for allNavigator developed in the early 1940s, and
satellites.used during World War II. Additional
inspiration for the GPS system came when the
Each satellite transmits its navigationSoviet Union launched the first Sputnik in
message with at least two distinct spread1957. A team of U.S. scientists led by Dr.
spectrum codes: the Coarse / Acquisition (CRichard B. Kershner were monitoring Sputnik's
A) code, which is freely available to theradio transmissions. They discovered that,
public, and the Precise (P) code, which isbecause of the Doppler effect, the frequency
usually encrypted and reserved for militaryof the signal being transmitted by Sputnik
applications. The C/A code is a 1,023 chipwas higher as the satellite approached, and
pseudo-random (PRN) code at 1.023 millionlower as it continued away from them. They
chips/sec so that it repeats everyrealized that since they knew their exact
millisecond. Each satellite has its own C/Alocation on the globe, they could pinpoint
code so that it can be uniquely identifiedwhere the satellite was along its orbit by
and received separately from the othermeasuring  the  Doppler  distortion.
satellites transmitting on the same
frequency. The P-code is a 10.23 megachip/secThe first satellite navigation system,
PRN code that repeats only every week. WhenTransit, used by the United States Navy, was
the "anti-spoofing" mode is on, as it is infirst successfully tested in 1960. Using a
normal operation, the P code is encrypted byconstellation of five satellites, it could
the Y-code to produce the P(Y) code, whichprovide a navigational fix approximately once
can only be decrypted by units with a validper hour. In 1967, the U.S. Navy developed
decryption key. Both the C/A and P(Y) codesthe Timation satellite which proved the
impart the precise time-of-day to the user.ability to place accurate clocks in space, a
Frequencies  used  by  GPS  includetechnology the GPS system relies upon. In the
1970s, the ground-based Omega Navigation
* L1 (1575.42 MHz): Mix of NavigationSystem, based on signal phase comparison,
Message, coarse-acquisition (C/A) code andbecame the first world-wide radio navigation
encrypted precision P(Y) code, plus the newsystem.
L1C  on  future  Block  III  satellites.
The first experimental Block-I GPS satellite
* L2 (1227.60 MHz): P(Y) code, plus the newwas launched in February 1978.[28] The GPS
L2C code on the Block IIR-M and newersatellites were initially manufactured by
satellites.Rockwell International and are now
manufactured  by  Lockheed  Martin.
* L3 (1381.05 MHz): Used by the Nuclear
Detonation (NUDET) Detection System Payload[edit]  Timeline
(NDS) to signal detection of nuclear
detonations and other high-energy infrared* In 1972, the US Air Force Central Inertial
events. Used to enforce nuclear test banGuidance Test Facility (Holloman AFB)
treaties.conducted developmental fight tests of two
prototype GPS receivers over White Sands
* L4 (1379.913 MHz): Being studied forMissile Range, using ground-based
additional  ionospheric  correction.pseudo-satellites.
* L5 (1176.45 MHz): Proposed for use as a* In 1978 the first experimental Block-I GPS
civilian safety-of-life (SoL) signal (see GPSsatellite  was  launched.
modernization). This frequency falls into an
internationally protected range for* In 1983, after Soviet interceptor aircraft
aeronautical navigation, promising little orshot down the civilian airliner KAL 007 in
no interference under all circumstances. Therestricted Soviet airspace, killing all 269
first Block IIF satellite that would providepeople on board, U.S. President Ronald Reagan
this  signal  is  set to be launched in 2008.announced that the GPS system would be made
available for civilian uses once it was
[edit]  Calculating  positionscompleted.
[edit]  Using  the  C/A  code* By 1985, ten more experimental Block-I
satellites had been launched to validate the
To start off, the receiver picks which C/Aconcept.
codes to listen for by PRN number, based on
the almanac information it has previously* On February 14, 1989, the first modern
acquired. As it detects each satellite'sBlock-II  satellite  was  launched.
signal, it identifies it by its distinct C/A
code pattern, then measures the time delay* In 1992, the 2nd Space Wing, which
for each satellite. To do this, the receiveroriginally managed the system, was
produces an identical C/A sequence using thede-activated and replaced by the 50th Space
same seed number as the satellite. By liningWing.
up the two sequences, the receiver can
measure the delay and calculate the distance* By December 1993 the GPS system achieved
to the satellite, called the pseudorange[12].initial  operational  capability[32]
Overlapping pseudoranges, represented as* By January 17, 1994 a complete
curves, are modified to yield the probableconstellation  of 24 satellites was in orbit.
position
* Full Operational Capability was declared by
Overlapping pseudoranges, represented asNAVSTAR  in  April  1995.
curves, are modified to yield the probable
position* In 1996, recognizing the importance of GPS
to civilian users as well as military users,
Next, the orbital position data, orU.S. President Bill Clinton issued a policy
ephemeris, from the Navigation Message isdirective[33] declaring GPS to be a dual-use
then downloaded to calculate the satellite'ssystem and establishing an Interagency GPS
precise position. A more-sensitive receiverExecutive Board to manage it as a national
will potentially acquire the ephemeris dataasset.
quicker than a less-sensitive receiver,
especially in a noisy environment.[13]* In 1998, U.S. Vice President Al Gore
Knowing the position and the distance of aannounced plans to upgrade GPS with two new
satellite indicates that the receiver iscivilian signals for enhanced user accuracy
located somewhere on the surface of anand reliability, particularly with respect to
imaginary sphere centered on that satelliteaviation  safety.
and whose radius is the distance to it.
Receivers can substitute altitude for one* On May 2, 2000 "Selective Availability" was
satellite, which the GPS receiver translatesdiscontinued as a result of the 1996
to a pseudorange measured from the center ofexecutive order, allowing users to receive a
the  earth.non-degraded  signal  globally.
Locations are calculated not in* In 2004, the United States Government
three-dimensional space, but insigned a historic agreement with the European
four-dimensional spacetime, meaning a measureCommunity establishing cooperation related to
of the precise time-of-day is very important.GPS  and  Europe's  planned  Galileo  system.
The measured pseudoranges from four
satellites have already been determined with* In 2004, U.S. President George W. Bush
the receiver's internal clock, and thus haveupdated the national policy, replacing the
an unknown amount of clock error. (The clockexecutive board with the National Space-Based
error or actual time does not matter in thePositioning, Navigation, and Timing Executive
initial pseudorange calculation, because thatCommittee.
is based on how much time has passed between
reception of each of the* November 2004, QUALCOMM announced
signals.[clarify][citation needed]) Thesuccessful tests of Assisted-GPS system for
four-dimensional point that is equidistantmobile  phones.[3]
from the pseudoranges is calculated as a
guess as to the receiver's location, and the* In 2005, the first modernized GPS satellite
factor used to adjust those pseudoranges towas launched and began transmitting a second
intersect at that four-dimensional pointcivilian signal (L2C) for enhanced user
gives a guess as to the receiver's clockperformance.
offset. With each guess, a geometric dilution
of precision (GDOP) vector is calculated,* The most recent launch was on 17 November
based on the relative sky positions of the2006. The oldest GPS satellite still in
satellites used. As more satellites areoperation  was  launched  in  August  1991.
picked up, pseudoranges from more
combinations of four satellites can be* On September 14, 2007, the aging
processed to add more guesses to the locationmainframe-based Ground Segment Control System
and clock offset. The receiver thenwas transitioned to the new Architecture
determines which combinations to use and howEvolution  Plan.  [4]
to calculate the estimated position by
determining the weighted average of these[edit]  Satellite  numbers
positions and clock offsets. After the final
location and time are calculated, theName Launch Period No of satellites
location is expressed in a specificlaunched, inc. launch failures Currently in
coordinate system, e.g. latitude/longitude,service
using the WGS 84 geodetic datum or a local
system  specific  to  a  country.Block  I 1978-1985 11 0
[edit]  Using  the  P(Y)  codeBlock  II 1985-1990 9 0
Calculating a position with the P(Y) signalBlock  IIA 1990-1997 19 15+11
is generally similar in concept, assuming one
can decrypt it. The encryption is essentiallyBlock  IIR 1997-2004 12 12
a safety mechanism: if a signal can be
successfully decrypted, it is reasonable toBlock  IIR-M 2005- 3 3
assume it is a real signal being sent by a
GPS satellite.[citation needed] InTotal 54  (plus  one  not  launched) 30+1
comparison, civil receivers are highly
vulnerable to spoofing since correctly1One  test  satellite
formatted C/A signals can be generated using
readily available signal generators. RAIM[edit]  Awards
features do not protect against spoofing,
since RAIM only checks the signals from aTwo GPS developers have received the National
navigational  perspective.Academy of Engineering Charles Stark Draper
prize  year  2003:
[edit]  Accuracy  and  error  sources
* Ivan Getting, emeritus president of The
The position calculated by a GPS receiverAerospace Corporation and engineer at the
requires the current time, the position ofMassachusetts Institute of Technology,
the satellite and the measured delay of theestablished the basis for GPS, improving on
received signal. The position accuracy isthe World War II land-based radio system
primarily dependent on the satellite positioncalled LORAN (Long-range Radio Aid to
and  signal  delay.Navigation).
To measure the delay, the receiver compares* Bradford Parkinson, professor of
the bit sequence received from the satelliteaeronautics and astronautics at Stanford
with an internally generated version. ByUniversity, conceived the present
comparing the rising and trailing edges ofsatellite-based system in the early 1960s and
the bit transitions, modern electronics candeveloped it in conjunction with the U.S. Air
measure signal offset to within about 1% of aForce.
bit time, or approximately 10 nanoseconds for
the C/A code. Since GPS signals propagateOne GPS developer, Roger L. Easton, received
nearly at the speed of light, this representsthe National Medal of Technology on February
an error of about 3 meters. This is the13,  2006  at  the  White  House.[34]
minimum error possible using only the GPS C/A
signal.On February 10, 1993, the National Aeronautic
Association selected the Global Positioning
Position accuracy can be improved by usingSystem Team as winners of the 1992 Robert J.
the higher-chiprate P(Y) signal. Assuming theCollier Trophy, the most prestigious aviation
same 1% bit time accuracy, the high frequencyaward in the United States. This team
P(Y) signal results in an accuracy of aboutconsists of researchers from the Naval
30  centimeters.Research Laboratory, the U.S. Air Force, the
Aerospace Corporation, Rockwell International
Electronics errors are one of severalCorporation, and IBM Federal Systems Company.
accuracy-degrading effects outlined in theThe citation accompanying the presentation of
table below. When taken together, autonomousthe trophy honors the GPS Team "for the most
civilian GPS horizontal position fixes aresignificant development for safe and
typically accurate to about 15 meters (50efficient navigation and surveillance of air
ft). These effects also reduce the moreand spacecraft since the introduction of
precise  P(Y)  code's  accuracy.radio  navigation  50  years  ago."
Sources of User Equivalent Range Errors[edit]  Other  systems
(UERE)  Source Effect
Main article: Global Navigation Satellite
Ionospheric  effects ±  5  meterSystem
Ephemeris  errors ±  2.5  meterOther satellite navigation systems in use or
various  states  of  development  include:
Satellite  clock  errors ±  2  meter
* Beidou - China's regional system that China
Multipath  distortion ±  1  meterhas proposed to expand into a global system
named  COMPASS.
Tropospheric  effects ±  0.5  meter
* Galileo - a proposed global system being
Numerical  errors ±  1  meterdeveloped by the European Union, joined by
China, Israel, India, Morocco, Saudi Arabia
[edit]  Atmospheric  effectsand South Korea, Ukraine planned to be
operational  by  2011-12.
Inconsistencies of atmospheric conditions
affect the speed of the GPS signals as they* GLONASS - Russia's global system which is
pass through the Earth's atmosphere andbeing restored to full availability in
ionosphere. Correcting these errors is apartnership  with  India.
significant challenge to improving GPS
position accuracy. These effects are smallest* Indian Regional Navigational Satellite
when the satellite is directly overhead andSystem (IRNSS) - India's proposed regional
become greater for satellites nearer thesystem.
horizon since the signal is affected for a
longer time. Once the receiver's approximate* QZSS - Japanese proposed regional system,
location is known, a mathematical model canadding better coverage to the Japanese
be used to estimate and compensate for theseislands.
errors.
[edit]  See  also
Because ionospheric delay affects the speed
of microwave signals differently based onSatellite  navigation  systems  Portal
frequency-a characteristic known as
dispersion-both frequency bands can be usedNautical  Portal
to help reduce this error. Some military and
expensive survey-grade civilian receivers*  RAIM
compare the different delays in the L1 and L2
frequencies to measure atmospheric*  SIGI
dispersion, and apply a more precise
correction. This can be done in civilian*  radio  navigation
receivers without decrypting the P(Y) signal
carried on L2, by tracking the carrier wave*  High  Sensitivity  GPS
instead of the modulated code. To facilitate
this on lower cost receivers, a new civilian* Degree Confluence Project Use GPS to visit
code signal on L2, called L2C, was added tointegral  degrees  of latitude and longitude.
the Block IIR-M satellites, which was first
launched in 2005. It allows a direct*  Exif,  GPS  data  transfer.
comparison of the L1 and L2 signals using the
coded  signal  instead  of  the carrier wave.*  Geotagging
The effects of the ionosphere generally*  Geocaching
change slowly, and can be averaged over time.
The effects for any particular geographical* NaviTraveler.com, - a GPS point sharing
area can be easily calculated by comparingcommunity.
the GPS-measured position to a known surveyed
location. This correction is also valid for* GPS Drawing Digital mapping and drawing
other receivers in the same general location.with  GPS  tracks.
Several systems send this information over
radio or other links to allow L1 only*  GPS  tracking
receivers to make ionospheric corrections.
The ionospheric data are transmitted via*  GPS/INS
satellite in Satellite Based Augmentation
Systems such as WAAS, which transmits it on*  Assisted  GPS
the GPS frequency using a special
pseudo-random number (PRN), so only one* GPX (XML schema for interchange of
antenna  and  receiver  are  required.waypoints)
Humidity also causes a variable delay,*  ID  Sniper  rifle
resulting in errors similar to ionospheric
delay, but occurring in the troposphere. This* OpenStreetMap, free content maps and street
effect is both more localized and changespictures  (GFDL)
more quickly than ionospheric effects and is
not frequency dependent. These traits making* Telematics: Many telematics devices use GPS
precise measurement and compensation ofto determine the location of mobile
humidity errors more difficult thanequipment.
ionospheric  effects.
* The American Practical Navigator-Chapter 11
Changes in altitude also change the amount of"Satellite  Navigation"
delay due to the signal passing through less
of the atmosphere at higher elevations. Since*  Point  of  Interest
the GPS receiver computes its approximate
altitude, this error is relatively simple to*  Automotive  navigation  system
correct.
*  NextGen
[edit]  Multipath  effects
[edit]  Notes
GPS signals can also be affected by multipath
issues, where the radio signals reflect off1. ^ Parkinson, B.W. (1996), Global
surrounding terrain; buildings, canyon walls,Positioning System: Theory and Applications,
hard ground, etc. These delayed signals canchap. 1: Introduction and Heritage of
cause inaccuracy. A variety of techniques,NAVSTAR, the Global Positioning System. pp.
most notably narrow correlator spacing, have3-28, American Institute of Aeronautics and
been developed to mitigate multipath errors.Astronautics,  Washington,  D.C.
For long delay multipath, the receiver itself
can recognize the wayward signal and discard2. ^ a b GPS Overview from the NAVSTAR Joint
it. To address shorter delay multipath fromProgram  Office.  Accessed December 15, 2006.
the signal reflecting off the ground,
specialized antennas may be used to reduce3. ^ HowStuffWorks. How GPS Receivers Work.
the signal power as received by the antenna.Accessed  May  14,  2006.
Short delay reflections are harder to filter
out because they interfere with the true4.  ^  globalsecurity.org  [1].
signal, causing effects almost
indistinguishable from routine fluctuations5. ^ Dana, Peter H. GPS Orbital Planes.
in  atmospheric  delay.August  8,  1996.
Multipath effects are much less severe in6. ^ What the Global Positioning System Tells
moving vehicles. When the GPS antenna isUs about Relativity. Accessed January 2,
moving, the false solutions using reflected2007.
signals quickly fail to converge and only the
direct  signals  result  in stable solutions.7. ^ USCG Navcen: GPS Frequently Asked
Questions.  Accessed  January  3,  2007.
[edit]  Ephemeris  and  clock  errors
8. ^ Massatt, Paul and Brady, Wayne.
The navigation message from a satellite is"Optimizing performance through constellation
sent out only every 30 seconds. In reality,management", Crosslink, Summer 2002, pages
the data contained in these messages tend to17-21.
be "out of date" by an even larger amount.
Consider the case when a GPS satellite is9.  ^  US Coast Guard General GPS News 9-9-05
boosted back into a proper orbit; for some
time following the maneuver, the receiver's10. ^ USNO. NAVSTAR Global Positioning
calculation of the satellite's position willSystem.  Accessed  May  14,  2006.
be incorrect until it receives another
ephemeris update. The onboard clocks are11.  ^  NMEA  NMEA  2000
extremely accurate, but they do suffer from
some clock drift. This problem tends to be12.  ^
very small, but may add up to 2 meters (6 ft)
of  inaccuracy.13. ^ AN02 Network Assistance (HTML).
Retrieved  on  2007-09-10.
This class of error is more "stable" than
ionospheric problems and tends to change over14. ^ a b Office of Science and Technology
days or weeks rather than minutes. This makesPolicy. Presidential statement to stop
correction fairly simple by sending out adegrading  GPS.  May  1,  2000.
more  accurate almanac on a separate channel.
15. ^ FAA, Selective Availability. Retrieved
[edit]  Selective  availabilityJan.  6,  2007.
The GPS includes a feature called Selective16.  ^
Availability (SA) that introduces
intentional, slowly changing random errors of17. ^ Rizos, Chris. University of New South
up to a hundred meters (328 ft) into theWales.  GPS  Satellite  Signals.  1999.
publicly available navigation signals to
confound, for example, guiding long range18. ^ The Global Positioning System by Robert
missiles to precise targets. AdditionalA.  Nelson  Via  Satellite,  November  1999
accuracy was available in the signal, but in
an encrypted form that was only available to19. ^ Ashby, Neil Relativity and GPS. Physics
the United States military, its allies and aToday,  May  2002.
few  others,  mostly  government  users.
20. ^ Space Environment Center. SEC
SA typically added signal errors of up toNavigation Systems GPS Page. August 26, 1996.
about 10 meters (32 ft) horizontally and 30
meters (98 ft) vertically. The inaccuracy of21. ^ The hunt for an unintentional GPS
the civilian signal was deliberately encodedjammer.  GPS  World.  January  1,  2003.
so as not to change very quickly, for
instance the entire eastern U.S. area might22. ^ Low Cost and Portable GPS Jammer.
read 30 m off, but 30 m off everywhere and inPhrack issue 0x3c (60), article 13].
the same direction. To improve the usefulnessPublished  December  28,  2002.
of GPS for civilian navigation, Differential
GPS was used by many civilian GPS receivers23. ^ American Forces Press Service. CENTCOM
to  greatly  improve  accuracy.charts  progress.  March  25,  2003.
During the Gulf War, the shortage of military24.  ^  [2]
GPS units and the wide availability of
civilian ones among personnel resulted in a25. ^ Ruley, John. AVweb. GPS jamming.
decision to disable Selective Availability.February  12,  2003.
This was ironic, as SA had been introduced
specifically for these situations, allowing26. ^ Commercial GPS Receivers: Facts for the
friendly troops to use the signal forWarfighter. Hosted at the Joint Chiefs
accurate navigation, while at the same timewebsite, linked by the USAF's GPS Wing DAGR
denying it to the enemy. But since SA wasprogram  website.  Accessed on 10 April, 2007
also denying the same accuracy to thousands
of friendly troops, turning it off or setting27. ^ US Coast Guard news release. Global
it to an error of zero meters (effectivelyPositioning  System  Fully  Operational
the  same  thing)  presented a clear benefit.
28. ^ a b Hydrographic Society Journal.
In the 1990s, the FAA started pressuring theDevelopments in Global Navigation Satellite
military to turn off SA permanently. ThisSystems. Issue #104, April 2002. Accessed
would save the FAA millions of dollars everyApril  5,  2007.
year in maintenance of their own radio
navigation systems. The military resisted for29. ^ XM982 Excalibur Precision Guided
most of the 1990s, and it ultimately took anExtended Range Artillery Projectile.
executive order to have SA removed from theGlobalSecurity.org (2007-05-29). Retrieved on
GPS signal. The amount of error added was2007-09-26.
"set to zero"[14] at midnight on May 1, 2000
following an announcement by U.S. President30. ^ Sandia National Laboratory's
Bill Clinton, allowing users access to theNonproliferation programs and arms control
error-free L1 signal. Per the directive, thetechnology.
induced error of SA was changed to add no
error to the public signals (C/A code).31. ^ Arms Control Association. Missile
Selective Availability is still a systemTechnology Control Regime. Accessed May 17,
capability of GPS, and error could, in2006.
theory, be reintroduced at any time. In
practice, in view of the hazards and costs32. ^ United States Department of Defense.
this would induce for US and foreignAnnouncement of Initial Operational
shipping, it is unlikely to be reintroduced,Capability.  December  8,  1993.
and various government agencies, including
the FAA,[15] have stated that it is not33. ^ National Archives and Records
intended  to  be  reintroduced.Administration. U.S. GLOBAL POSITIONING
SYSTEM  POLICY.  March  29,  1996.
The US military has developed the ability to
locally deny GPS (and other navigation34. ^ United States Naval Research
services) to hostile forces in a specificLaboratory. National Medal of Technology for
area of crisis without affecting the rest ofGPS.  November  21,  2005
the  world  or  its own military systems.[14]
[edit]  External  links
One interesting side effect of the Selective
Availability hardware is the capability toWikimedia  Commons  has  media  related  to:
correct the frequency of the GPS caesium and
rubidium atomic clocks to an accuracy ofGlobal  Positioning  System
approximately 2 Ã- 10-13 (one in five
trillion). This represented a significantGovernment  links
improvement over the raw accuracy of the
clocks.[citation  needed]* GPS.gov-General public education website
created  by  the  U.S.  Government
On 19 September 2007, the United States
Department of Defense announced that they* National Space-Based PNT Executive
would not procure any more satellites capableCommittee-Established in 2004 to oversee
of  implementing  SA.  [16]management of GPS and GPS augmentations at a
national  level.
[edit]  Relativity
* USCG Navigation Center-Status of the GPS
According to the theory of relativity, due toconstellation, government policy, and links
their constant movement and height relativeto other references. Also includes satellite
to the Earth-centered inertial referencealmanac  data.
frame, the clocks on the satellites are
affected by their speed (special relativity)* The GPS Joint Program Office (GPS
as well as their gravitational potentialJPO)-Responsible for designing and acquiring
(general relativity). For the GPS satellites,the  system  on  behalf of the US Government.
general relativity predicts that the atomic
clocks at GPS orbital altitudes will tick* U.S. Naval Observatory's GPS constellation
more rapidly, by about 45,900 nanosecondsstatus
(ns) per day, because they are in a weaker
gravitational field than atomic clocks on* U.S. Army Corps of Engineers manual:
Earth's surface. Special relativity predictsNAVSTAR  HTML  and  PDF  (22.6 MB, 328 pages)
that atomic clocks moving at GPS orbital
speeds will tick more slowly than stationary*  PNT  Selective  Availability Announcements
ground clocks by about 7,200 ns per day. When
combined, the discrepancy is 38 microseconds* GPS SPS Signal Specification, 2nd
per day; a difference of 4.465 parts inEdition-The official Standard Positioning
1010.[17]. To account for this, the frequencySignal  specification.
standard onboard each satellite is given a
rate offset prior to launch, making it run*  Federal  Aviation Administration's GPS FAQ
slightly slower than the desired frequency on
Earth; specifically, at 10.22999999543 MHzIntroductory  /  tutorial  links
instead  of  10.23  MHz.[18]
* How does GPS work? TomTom explains GPS,
GPS observation processing must alsonavigation,  and  digital  maps
compensate for another relativistic effect,
the Sagnac effect. The GPS time scale is* GPS Academy Garmin interactive video web
defined in an inertial system butsite explaing what exactly GPS is and what it
observations are processed in ancan  do  for  you
Earth-centered, Earth-fixed (co-rotating)
system, a system in which simultaneity is not* HowStuffWorks' Simplified explanation of
uniquely defined. The Lorentz transformationGPS  and  video  about  how  GPS  works.
between the two systems modifies the signal
run time, a correction having opposite* Trimble's Online GPS Tutorial Tutorial
algebraic signs for satellites in the Easterndesigned to introduce you to the principles
and Western celestial hemispheres. Ignoringbehind  GPS
this effect will produce an east-west error
on the order of hundreds of nanoseconds, or* GPS and GLONASS Simulation(Java applet)
tens  of  meters  in  position.[19]Simulation and graphical depiction of space
vehicle motion including computation of
The atomic clocks on board the GPS satellitesdilution  of  precision  (DOP)
are precisely tuned, making the system a
practical engineering application of theTechnical, historical, and ancillary topics
scientific theory of relativity in alinks
real-world  environment.
* Dana, Peter H. "Global Positioning System
[edit]  GPS  interference  and  jammingOverview"
Since GPS signals at terrestrial receivers* Satellite Navigation: GPS & Galileo
tend to be relatively weak, it is easy for(PDF)-16-page paper about the history and
other sources of electromagnetic radiation toworking of GPS, touching on the upcoming
desensitize the receiver, making acquiringGalileo
and tracking the satellite signals difficult
or  impossible.* History of GPS, including information about
each  satellite's  configuration  and launch.
Solar flares are one such naturally occurring
emission with the potential to degrade GPS* Chadha, Kanwar. "The Global Positioning
reception, and their impact can affectSystem: Challenges in Bringing GPS to
reception over the half of the Earth facingMainstream Consumers" Technical Article
the sun. GPS signals can also be interfered(1998)
with by naturally occurring geomagnetic
storms, predominantly found near the poles of*  GPS  Weapon  Guidance  Techniques
the Earth's magnetic field.[20] Another
source of problems is the metal embedded in*  RAND  history  of  the  GPS  system  (PDF)
some car windscreens to prevent icing,
degrading  reception  just  inside  the  car.*  GPS  Anti-Jam  Protection  Techniques
Man-made interference can also disrupt, or* Crosslink Summer 2002 issue by The
jam, GPS signals. In one well documentedAerospace Corporation on satellite
case, an entire harbor was unable to receivenavigation.
GPS signals due to unintentional jamming
caused by a malfunctioning TV antenna* Improved weather predictions from COSMIC
preamplifier.[21] Intentional jamming is alsoGPS  satellite  signal  occultation  data.
possible. Generally, stronger signals can
interfere with GPS receivers when they are* David L. Wilson's GPS Accuracy Web Page A
within radio range, or line of sight. Inthorough  analysis  of  the  accuracy of GPS.
2002, a detailed description of how to build
a short range GPS L1 C/A jammer was published* Innovation: Spacecraft Navigator,
in  the  online  magazine  Phrack.[22]Autonomous GPS Positioning at High Earth
Orbits Example of GPS receiver designed for
The U.S. government believes that suchhigh  altitude  spaceflight.
jammers were used occasionally during the
2001 war in Afghanistan and the U.S. military* The Navigator GPS Receiver GSFC's Navigator
claimed to destroy a GPS jammer with aspaceflight  receiver.
GPS-guided bomb during the Iraq War.[23] Such
a jammer is relatively easy to detect and* Neil Ashby's Relativity in the Global
locate, making it an attractive target forPositioning  System
anti-radiation missiles. The UK Ministry of
Defence tested a jamming system in the UK's[show]v  Ã¢Â€Â¢  d  Ã¢Â€Â¢  e
West  Country  on  7  and  8  June 2007. [24]
Satellite  navigation  systems
Some countries allow the use of GPS repeaters
to allow for the reception of GPS signalsHistorical Flag of the United States Transit
indoors and in obscured locations, however,
under EU and UK laws, the use of these isOperational Flag of the Soviet Union / Flag
prohibited as the signals can causeof Russia GLONASS · Flag of the
interference to other GPS receivers that mayUnited  States  GPS
receive data from both GPS satellites and the
repeater.Developmental Flag of the People's Republic
of China Beidou/COMPASS · Flag of
Due to the potential for both natural andEurope Galileo · Flag of India IRNSS
man-made noise, numerous techniques continue·  Flag  of  Japan  QZSS
to be developed to deal with the
interference. The first is to not rely on GPSRelated topics EGNOS · GAGAN
as a sole source. According to John Ruley,· GPS·C · LAAS
"IFR pilots should have a fallback plan in·  MSAS  Ã‚·  WAAS
case of a GPS malfunction".[25] Receiver
Autonomous Integrity Monitoring (RAIM) is a[show]v  Ã¢Â€Â¢  d  Ã¢Â€Â¢  e
feature now included in some receivers, which
is designed to provide a warning to the userTime  signal  stations
if jamming or another problem is detected.
The U.S. military has also deployed theirLongwave DCF77 · HBG · JJY
Selective Availability / Anti-Spoofing Module·  MSF  Ã‚·  TDF · WWVB
(SAASM) in the Defense Advanced GPS Receiver
(DAGR). In demonstration videos, the DAGR isShortwave BPM · CHU · RWM
able to detect jamming and maintain its lock·  WWV  Ã‚· WWVH · YVTO
on the encrypted GPS signals during
interference which causes civilian receiversGNSS time transfer Beidou · Galileo
to  lose  lock.[26]· GLONASS · GPS ·
IRNSS
[edit]  Techniques  to  improve  accuracy
Defunct  time  stations OMA  Ã‚·  VNG
[edit]  Augmentation
[show]v  Ã¢Â€Â¢  d  Ã¢Â€Â¢  e
Main  article:  GNSS  Augmentation
Global structure in Systems, Systems sciences
Augmentation methods of improving accuracyand  Systems  scientists
rely on external information being integrated
into the calculation process. There are manyCategories Category:Conceptual systems
such systems in place and they are generally· Category:Physical systems ·
named or described based on how the GPSCategory:Social systems ·
sensor receives the information. Some systemsCategory:Systems · Category:Systems
transmit additional information about sourcesscience · Category:Systems scientists
of error (such as clock drift, ephemeris, or·  Category:Systems  theory
ionospheric delay), others provide direct
measurements of how much the signal was offSystems Biological system · Complex
in the past, while a third group providesystem · Complex adaptive system
additional navigational or vehicle· Conceptual system ·
information to be integrated in theCultural system · Dynamical system
calculation  process.· Economic system · Ecosystem
· Formal system · Global
Examples of augmentation systems include thePositioning System · Human organ
Wide Area Augmentation System, Differentialsystems · Information systems
GPS, Inertial Navigation Systems and Assisted· Legal system · Metric
GPS.system · Nervous system ·
Non-linear system · Operating system
[edit]  Precise  monitoring· Physical system · Political
system · Sensory system ·
The accuracy of a calculation can also beSocial system · Solar System
improved through precise monitoring and· System · Systems of
measuring of the existing GPS signals inmeasurement
additional  or  alternate  ways.
Fields of theory Chaos theory ·
After SA, which has been turned off, theComplex systems · Control theory
largest error in GPS is usually the· Cybernetics · Holism in
unpredictable delay through the ionosphere.science · Sociotechnical systems
The spacecraft broadcast ionospheric modeltheory · Systems biology ·
parameters, but errors remain. This is oneSystem dynamics · Systems ecology
reason the GPS spacecraft transmit on at· Systems engineering ·
least two frequencies, L1 and L2. IonosphericSystems  theory  Ã‚·  Systems  science
delay is a well-defined function of frequency
and the total electron content (TEC) alongSystems scientists Russell L. Ackoff
the path, so measuring the arrival time· William Ross Ashby ·
difference between the frequencies determinesGregory Bateson · Ludwig von
TEC and thus the precise ionospheric delay atBertalanffy · Kenneth E. Boulding
each  frequency.· Peter Checkland · C. West
Churchman · Heinz von Foerster
Receivers with decryption keys can decode the· Charles François ·
P(Y)-code transmitted on both L1 and L2.Jay Wright Forrester · Ralph W.
However, these keys are reserved for theGerard · Debora Hammond ·
military and "authorized" agencies and areGeorge Klir · Niklas Luhmann
not available to the public. Without keys, it· Humberto Maturana · Donella
is still possible to use a codeless techniqueMeadows · Mihajlo D. Mesarovic
to compare the P(Y) codes on L1 and L2 to· Howard T. Odum · Talcott
gain much of the same error information.Parsons · Ilya Prigogine ·
However, this technique is slow, so it isAnatol Rapoport · Francisco Varela
currently limited to specialized surveying· John N.



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