About GPS

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About GPS

Global Positioning System				Anti-Spoofing Module (SAASM) in the
The Global Positioning System (GPS) is				Defense Advanced GPS Receiver (DAGR). In
the only fully functional Global				demonstration videos, the DAGR is able
Navigation Satellite System (GNSS).				to detect jamming and maintain its lock
Utilizing a constellation of at least 24				on the encrypted GPS signals during
medium Earth orbit satellites that				interference which causes civilian
transmit precise microwave signals, the				receivers 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 States				Augmentation methods of improving
Department of Defense, it is officially				accuracy rely on external information
named NAVSTAR GPS (Contrary to popular				being integrated into the calculation
belief, NAVSTAR is not an acronym, but				process. There are many such systems in
simply a name given by Mr. John Walsh, a				place and they are generally named or
key decision maker when it came to the				described based on how the GPS sensor
budget for the GPS program[1]). The				receives the information. Some systems
satellite constellation is managed by				transmit additional information about
the United States Air Force 50th Space				sources of error (such as clock drift,
Wing. The cost of maintaining the system				ephemeris, or ionospheric delay), others
is approximately US$750 million per				provide direct measurements of how much
year,[2] including the replacement of				the signal was off in the past, while a
aging satellites, and research and				third group provide additional
development. Despite these costs, GPS is				navigational or vehicle information to
free for civilian use as a public good.				be integrated in the calculation
GPS has become a widely used aid to				process.
navigation worldwide, and a useful tool				Examples of augmentation systems include
for map-making, land surveying,				the Wide Area Augmentation System,
commerce, and scientific uses. GPS also				Differential GPS, Inertial Navigation
provides a precise time reference used				Systems and Assisted GPS.
in many applications including				[edit] Precise monitoring
scientific study of earthquakes, and				The accuracy of a calculation can also
synchronization of telecommunications				be improved through precise monitoring
networks.				and measuring of the existing GPS
Simplified method of operation				signals in additional or alternate ways.
A GPS receiver calculates its position				After SA, which has been turned off, the
by measuring the distance between itself				largest error in GPS is usually the
and three or more GPS satellites.				unpredictable delay through the
Measuring the time delay between				ionosphere. The spacecraft broadcast
transmission and reception of each GPS				ionospheric model parameters, but errors
microwave signal gives the distance to				remain. This is one reason the GPS
each satellite, since the signal travels				spacecraft transmit on at least two
at a known speed - the speed of light.				frequencies, L1 and L2. Ionospheric
These signals also carry information				delay is a well-defined function of
about the satellites' location and				frequency and the total electron content
general system health (known as almanac				(TEC) along the path, so measuring the
and ephemeris data). By determining the				arrival time difference between the
position of, and distance to, at least				frequencies determines TEC and thus the
three satellites, the receiver can				precise ionospheric delay at each
compute its position using				frequency.
trilateration.[3] Receivers typically do				Receivers with decryption keys can
not have perfectly accurate clocks and				decode the P(Y)-code transmitted on both
therefore track one or more additional				L1 and L2. However, these keys are
satellites, using their atomic clocks to				reserved for the military and
correct the receiver's own clock error.				"authorized" agencies and are not
[edit] Technical description				available to the public. Without keys,
Unlaunched GPS satellite on display at				it is still possible to use a codeless
the San Diego Aerospace museum				technique to compare the P(Y) codes on
Unlaunched GPS satellite on display at				L1 and L2 to gain much of the same error
the San Diego Aerospace museum				information. However, this technique is
[edit] System segmentation				slow, so it is currently limited to
The current GPS consists of three major				specialized surveying equipment. In the
segments. These are the space segment				future, additional civilian codes are
(SS), a control segment (CS), and a user				expected to be transmitted on the L2 and
segment (US).[4]				L5 frequencies (see GPS modernization,
[edit] Space segment				below). Then all users will be able to
The space segment (SS) is composed of				perform dual-frequency measurements and
the orbiting GPS satellites, or Space				directly compute ionospheric delay
Vehicles (SV) in GPS parlance. The GPS				errors.
design calls for 24 SVs to be				A second form of precise monitoring is
distributed equally among six circular				called Carrier-Phase Enhancement
orbital planes.[5] The orbital planes				(CPGPS). The error, which this corrects,
are centered on the Earth, not rotating				arises because the pulse transition of
with respect to the distant stars.[6]				the PRN is not instantaneous, and thus
The six planes have approximately 55Â°				the correlation (satellite-receiver
inclination (tilt relative to Earth's				sequence matching) operation is
equator) and are separated by 60Â°				imperfect. The CPGPS approach utilizes
right ascension of the ascending node				the L1 carrier wave, which has a period
(angle along the equator from a				1000 times smaller than that of the C/A
reference point to the orbit's				bit period, to act as an additional
intersection).[2]				clock signal and resolve the
Orbiting at an altitude of approximately				uncertainty. The phase difference error
20,200 kilometers (12,600 miles or				in the normal GPS amounts to between 2
10,900 nautical miles; orbital radius of				and 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 each				transition reduces this error to 3
sidereal day, so it passes over the same				centimeters (1 inch) of ambiguity. By
location on Earth once each day. The				eliminating this source of error, CPGPS
orbits are arranged so that at least six				coupled with DGPS normally realizes
satellites are always within line of				between 20 and 30 centimeters (8 to 12
sight from almost everywhere on Earth's				inches) of absolute accuracy.
surface.[7]				Relative Kinematic Positioning (RKP) is
As of September 2007, there are 31				another approach for a precise GPS-based
actively broadcasting satellites in the				positioning system. In this approach,
GPS constellation. The additional				determination of range signal can be
satellites improve the precision of GPS				resolved to an accuracy of less than 10
receiver calculations by providing				centimeters (4 in). This is done by
redundant measurements. With the				resolving the number of cycles in which
increased number of satellites, the				the signal is transmitted and received
constellation was changed to a				by the receiver. This can be
nonuniform arrangement. Such an				accomplished by using a combination of
arrangement was shown to improve				differential GPS (DGPS) correction data,
reliability and availability of the				transmitting GPS signal phase
system, relative to a uniform system,				information and ambiguity resolution
when multiple satellites fail.[8]				techniques via statistical
[edit] Control segment				tests-possibly with processing in
The flight paths of the satellites are				real-time (real-time kinematic
tracked by US Air Force monitoring				positioning, RTK).
stations in Hawaii, Kwajalein, Ascension				[edit] GPS time and date
Island, Diego Garcia, and Colorado				While most clocks are synchronized to
Springs, Colorado, along with monitor				Coordinated Universal Time (UTC), the
stations operated by the National				Atomic 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 the				time is not corrected to match the
Air Force Space Command's master control				rotation of the Earth, so it does not
station at Schriever Air Force Base in				contain leap seconds or other
Colorado Springs, which is operated by				corrections which are periodically added
the 2d Space Operations Squadron (2				to UTC. GPS time was set to match
SOPS) of the United States Air Force				Coordinated Universal Time (UTC) in
(USAF). 2 SOPS contacts each GPS				1980, but has since diverged. The lack
satellite regularly with a navigational				of corrections means that GPS time
update (using the ground antennas at				remains 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 on				performed on the on-board clocks to
board the satellites to within one				correct relativistic effects and keep
microsecond and adjust the ephemeris of				them synchronized with ground clocks.
each satellite's internal orbital model.				The GPS navigation message includes the
The updates are created by a Kalman				difference between GPS time and UTC,
filter which uses inputs from the ground				which as of 2006 is 14 seconds.
monitoring stations, space weather				Receivers subtract this offset from GPS
information, and various other				time to calculate UTC and specific
inputs.[10]				timezone values. New GPS units may not
GPS receivers come in a variety of				show the correct UTC time until after
formats, from devices integrated into				receiving the UTC offset message. The
cars, phones, and watches, to dedicated				GPS-UTC offset field can accommodate 255
devices such as those shown here from				leap seconds (eight bits) which, at the
manufacturers Trimble, Garmin and Leica				current rate of change of the Earth's
(left to right).				rotation, is sufficient to last until
GPS receivers come in a variety of				the year 2330.
formats, from devices integrated into				As opposed to the year, month, and day
cars, phones, and watches, to dedicated				format of the Julian calendar, the GPS
devices such as those shown here from				date is expressed as a week number and a
manufacturers Trimble, Garmin and Leica				day-of-week number. The week number is
(left to right).				transmitted as a ten-bit field in the C
[edit] User segment				A and P(Y) navigation messages, and so
The user's GPS receiver is the user				it 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 of				00:00:00 UTC (00:00:19 TAI) on January
an antenna, tuned to the frequencies				6, 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-stable				on August 21, 1999 (00:00:19 TAI on
clock (often a crystal oscillator). They				August 22, 1999). To determine the
may also include a display for providing				current Gregorian date, a GPS receiver
location and speed information to the				must be provided with the approximate
user. A receiver is often described by				date (to within 3,584 days) to correctly
its number of channels: this signifies				translate the GPS date signal. To
how many satellites it can monitor				address this concern the modernized GPS
simultaneously. Originally limited to				navigation messages use a 13-bit field,
four or five, this has progressively				which 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 between				until near the year 2137.
twelve and twenty channels.				[edit] GPS modernization
A typical OEM GPS receiver module, based				Main article: GPS modernization
on the SiRF Star III chipset, measuring				Having reached the program's
15Ã-17 mm, and used in many products.				requirements for Full Operational
A typical OEM GPS receiver module, based				Capability (FOC) on July 17, 1995,[27]
on the SiRF Star III chipset, measuring				the GPS completed its original design
15Ã-17 mm, and used in many products.				goals. However, additional advances in
GPS receivers may include an input for				technology and new demands on the
differential corrections, using the RTCM				existing system led to the effort to
SC-104 format. This is typically in the				modernize the GPS system. Announcements
form of a RS-232 port at 4,800 bit/s				from the Vice President and the White
speed. Data are actually sent at a much				House in 1998 initiated these changes,
lower rate, which limits the accuracy of				and in 2000 the U.S. Congress authorized
the signal sent using RTCM. Receivers				the effort, referring to it as GPS III.
with internal DGPS receivers can				The project aims to improve the accuracy
outperform those using external RTCM				and availability for all users and
data. As of 2006, even low-cost units				involves new ground stations, new
commonly include Wide Area Augmentation				satellites, and four additional
System (WAAS) receivers.				navigation signals. New civilian signals
Many GPS receivers can relay position				are called L2C, L5 and L1C; the new
data to a PC or other device using the				military code is called M-Code. Initial
NMEA 0183 protocol. NMEA 2000[11] is a				Operational 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 by				2013 has been established for the entire
the US-based National Marine Electronics				program, with incentives offered to the
Association. References to the NMEA				contractors if they can complete it by
protocols have been compiled from public				2011.
records, allowing open source tools like				[edit] Applications
gpsd to read the protocol without				The Global Positioning System, while
violating intellectual property laws.				originally a military project, is
Other proprietary protocols exist as				considered a dual-use technology,
well, such as the SiRF and MTK				meaning it has significant applications
protocols. Receivers can interface with				for both the military and the civilian
other devices using methods including a				industry.
serial connection, USB or Bluetooth.				[edit] Military
[edit] Navigation signals				Please help improve this article by
Main article: GPS signals				expanding this section.
GPS broadcast signal				See talk page for details. Please remove
GPS broadcast signal				this message once the section has been
Each GPS satellite continuously				expanded.
broadcasts a Navigation Message at 50				The military use GPS for the following
bit/s giving the time-of-day, GPS week				purposes:
number and satellite health information				[edit] Navigation
(all transmitted in the first part of				GPS allows soldiers to find objectives
the message), an ephemeris (transmitted				in the dark or in unfamiliar territory,
in the second part of the message) and				and 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 18				Various military weapons systems use GPS
seconds, repeating every 30 seconds. The				to track potential ground and air
ephemeris is updated every 2 hours and				targets before they are flagged as
is generally valid for 4 hours, with				hostile. These weapons systems pass GPS
provisions for 6 hour time-outs. The				co-ordinates of targets to
time needed to acquire the ephemeris is				precision-guided munitions to allow them
becoming a significant element of the				to engage the targets accurately.
delay to first position fix, because, as				Military aircraft, particularly those
the hardware becomes more capable, the				used in air-to-ground roles use GPS to
time to lock onto the satellite signals				find targets (for example, gun camera
shrinks, but the ephemeris data requires				video from AH-1 Cobras in Iraq show GPS
30 seconds (worst case) before it is				co-ordinates that can be looked up in
received, due to the low data				Google Earth).
transmission rate. The almanac consists				[edit] Missile and projectile guidance
of coarse orbit and status information				GPS allows accurate targeting of various
for each satellite in the constellation				military weapons including ICBMs, cruise
and takes 12 seconds for each satellite				missiles and precision-guided munitions.
present, with information for a new				Artillery projectiles with embedded GPS
satellite being transmitted every 30				receivers able to withstand forces of
seconds (15.5 minutes for 31				12,000G have been developed for use in
satellites). The purpose of the data is				155 mm howitzers.[29]
to assist in the acquisition of				[edit] Search and Rescue
satellites at power-up by allowing the				Downed pilots can be located faster if
receiver to generate a list of visible				they have a GPS receiver.
satellites based on stored position and				[edit] Reconnaissance and Map Creation
time, while an ephemeris from each				The military use GPS extensively to aid
satellite is needed to compute position				mapping and reconnaissance.
fixes using that satellite. In older				[edit] Other
hardware, lack of an almanac in a new				The GPS satellites also carry nuclear
receiver would cause long delays before				detonation detectors, which form a major
providing a valid position, because the				portion of the United States Nuclear
search for each satellite was a slow				Detonation Detection System.[30]
process. Advances in hardware have made				[edit] Civilian
the acquisition process much faster, so				See also: GPS applications
not having an almanac is no longer an				This antenna is mounted on the roof of a
issue. An important thing to note about				hut containing a scientific experiment
navigation data is that each satellite				needing precise timing.
transmits only its own ephemeris, but				This antenna is mounted on the roof of a
transmits an almanac for all satellites.				hut containing a scientific experiment
Each satellite transmits its navigation				needing precise timing.
message with at least two distinct				Many civilian applications benefit from
spread spectrum codes: the Coarse /				GPS signals, using one or more of three
Acquisition (C/A) code, which is freely				basic components of the GPS; absolute
available to the public, and the Precise				location, relative movement, time
(P) code, which is usually encrypted and				transfer.
reserved for military applications. The				The ability to determine the receiver's
C/A code is a 1,023 chip pseudo-random				absolute location allows GPS receivers
(PRN) code at 1.023 million chips/sec so				to perform as a surveying tool or as an
that it repeats every millisecond. Each				aid to navigation. The capacity to
satellite has its own C/A code so that				determine relative movement enables a
it can be uniquely identified and				receiver to calculate local velocity and
received separately from the other				orientation, useful in vessels or
satellites transmitting on the same				observations of the Earth. Being able to
frequency. The P-code is a 10.23				synchronize clocks to exacting standards
megachip/sec PRN code that repeats only				enables time transfer, which is critical
every week. When the "anti-spoofing"				in large communication and observation
mode is on, as it is in normal				systems. An example is CDMA digital
operation, the P code is encrypted by				cellular. 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 units				spreading codes with other base stations
with a valid decryption key. Both the C				to facilitate inter-cell hand off and
A and P(Y) codes impart the precise				support hybrid GPS/CDMA positioning of
time-of-day to the user. Frequencies				mobiles for emergency calls and other
used by GPS include				applications.
* L1 (1575.42 MHz): Mix of Navigation				Finally, GPS enables researchers to
Message, coarse-acquisition (C/A) code				explore the Earth environment including
and encrypted precision P(Y) code, plus				the atmosphere, ionosphere and gravity
the new L1C on future Block III				field. GPS survey equipment has
satellites.				revolutionized tectonics by directly
* L2 (1227.60 MHz): P(Y) code, plus the				measuring the motion of faults in
new L2C code on the Block IIR-M and				earthquakes.
newer satellites.				To help prevent civilian GPS guidance
* L3 (1381.05 MHz): Used by the Nuclear				from being used in an enemy's military
Detonation (NUDET) Detection System				or improvised weaponry, the US
Payload (NDS) to signal detection of				Government controls the export of
nuclear detonations and other				civilian receivers. A US-based
high-energy infrared events. Used to				manufacturer cannot generally export a
enforce nuclear test ban treaties.				GPS receiver unless the receiver
* L4 (1379.913 MHz): Being studied for				contains 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 frequency				515 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 interference				expanding this section.
under all circumstances. The first Block				See talk page for details. Please remove
IIF satellite that would provide this				this message once the section has been
signal is set to be launched in 2008.				expanded.
[edit] Calculating positions				The design of GPS is based partly on the
[edit] Using the C/A code				similar ground-based radio navigation
To start off, the receiver picks which C				systems, 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 has				and used during World War II. Additional
previously acquired. As it detects each				inspiration for the GPS system came when
satellite's signal, it identifies it by				the Soviet Union launched the first
its distinct C/A code pattern, then				Sputnik in 1957. A team of U.S.
measures the time delay for each				scientists led by Dr. Richard B.
satellite. To do this, the receiver				Kershner were monitoring Sputnik's radio
produces an identical C/A sequence using				transmissions. They discovered that,
the same seed number as the satellite.				because of the Doppler effect, the
By lining up the two sequences, the				frequency of the signal being
receiver can measure the delay and				transmitted 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 as				that since they knew their exact
curves, are modified to yield the				location on the globe, they could
probable position				pinpoint where the satellite was along
Overlapping pseudoranges, represented as				its orbit by measuring the Doppler
curves, are modified to yield the				distortion.
probable position				The first satellite navigation system,
Next, the orbital position data, or				Transit, used by the United States Navy,
ephemeris, from the Navigation Message				was first successfully tested in 1960.
is then downloaded to calculate the				Using a constellation of five
satellite's precise position. A				satellites, it could provide a
more-sensitive receiver will potentially				navigational fix approximately once per
acquire the ephemeris data quicker than				hour. In 1967, the U.S. Navy developed
a less-sensitive receiver, especially in				the Timation satellite which proved the
a noisy environment.[13] Knowing the				ability to place accurate clocks in
position and the distance of a satellite				space, a technology the GPS system
indicates that the receiver is located				relies upon. In the 1970s, the
somewhere on the surface of an imaginary				ground-based Omega Navigation System,
sphere centered on that satellite and				based on signal phase comparison, became
whose radius is the distance to it.				the first world-wide radio navigation
Receivers can substitute altitude for				system.
one satellite, which the GPS receiver				The first experimental Block-I GPS
translates to a pseudorange measured				satellite was launched in February
from the center of the earth.				1978.[28] The GPS satellites were
Locations are calculated not in				initially manufactured by Rockwell
three-dimensional space, but in				International and are now manufactured
four-dimensional spacetime, meaning a				by 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 have				Inertial Guidance Test Facility
already been determined with the				(Holloman AFB) conducted developmental
receiver's internal clock, and thus have				fight tests of two prototype GPS
an unknown amount of clock error. (The				receivers over White Sands Missile
clock error or actual time does not				Range, using ground-based
matter in the initial pseudorange				pseudo-satellites.
calculation, because that is based on				* In 1978 the first experimental Block-I
how much time has passed between				GPS satellite was launched.
reception of each of the				* In 1983, after Soviet interceptor
signals.[clarify][citation needed]) The				aircraft shot down the civilian airliner
four-dimensional point that is				KAL 007 in restricted Soviet airspace,
equidistant from the pseudoranges is				killing all 269 people on board, U.S.
calculated as a guess as to the				President Ronald Reagan announced that
receiver's location, and the factor used				the GPS system would be made available
to adjust those pseudoranges to				for 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 clock				satellites had been launched to validate
offset. With each guess, a geometric				the concept.
dilution of precision (GDOP) vector is				* On February 14, 1989, the first modern
calculated, based on the relative sky				Block-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 of				de-activated and replaced by the 50th
four satellites can be processed to add				Space Wing.
more guesses to the location and clock				* By December 1993 the GPS system
offset. The receiver then determines				achieved initial operational
which combinations to use and how to				capability[32]
calculate the estimated position by				* By January 17, 1994 a complete
determining the weighted average of				constellation of 24 satellites was in
these positions and clock offsets. After				orbit.
the final location and time are				* Full Operational Capability was
calculated, the location is expressed in				declared by NAVSTAR in April 1995.
a specific coordinate system, e.g.				* In 1996, recognizing the importance of
latitude/longitude, using the WGS 84				GPS to civilian users as well as
geodetic datum or a local system				military users, U.S. President Bill
specific to a country.				Clinton issued a policy directive[33]
[edit] Using the P(Y) code				declaring 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. The				national asset.
encryption is essentially a safety				* In 1998, U.S. Vice President Al Gore
mechanism: if a signal can be				announced plans to upgrade GPS with two
successfully decrypted, it is reasonable				new civilian signals for enhanced user
to assume it is a real signal being sent				accuracy and reliability, particularly
by a GPS satellite.[citation needed] In				with respect to aviation safety.
comparison, civil receivers are highly				* On May 2, 2000 "Selective
vulnerable to spoofing since correctly				Availability" was discontinued as a
formatted C/A signals can be generated				result of the 1996 executive order,
using readily available signal				allowing users to receive a non-degraded
generators. RAIM features do not protect				signal globally.
against spoofing, since RAIM only checks				* In 2004, the United States Government
the signals from a navigational				signed a historic agreement with the
perspective.				European Community establishing
[edit] Accuracy and error sources				cooperation related to GPS and Europe's
The position calculated by a GPS				planned Galileo system.
receiver requires the current time, the				* In 2004, U.S. President George W. Bush
position of the satellite and the				updated the national policy, replacing
measured delay of the received signal.				the executive board with the National
The position accuracy is primarily				Space-Based Positioning, Navigation, and
dependent on the satellite position and				Timing Executive Committee.
signal delay.				* November 2004, QUALCOMM announced
To measure the delay, the receiver				successful tests of Assisted-GPS system
compares the bit sequence received from				for mobile phones.[3]
the satellite with an internally				* In 2005, the first modernized GPS
generated version. By comparing the				satellite was launched and began
rising and trailing edges of the bit				transmitting 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 10				November 2006. The oldest GPS satellite
nanoseconds for the C/A code. Since GPS				still in operation was launched in
signals propagate nearly at the speed of				August 1991.
light, this represents an error of about				* On September 14, 2007, the aging
3 meters. This is the minimum error				mainframe-based Ground Segment Control
possible using only the GPS C/A signal.				System was transitioned to the new
Position accuracy can be improved by				Architecture 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 results				launched, inc. launch failures
in an accuracy of about 30 centimeters.				Currently in service
Electronics errors are one of several				Block I 1978-1985 11 0
accuracy-degrading effects outlined in				Block II 1985-1990 9 0
the table below. When taken together,				Block IIA 1990-1997 19 15+11
autonomous civilian GPS horizontal				Block IIR 1997-2004 12 12
position fixes are typically accurate to				Block IIR-M 2005- 3 3
about 15 meters (50 ft). These effects				Total 54 (plus one not launched) 30+1
also reduce the more precise P(Y) code's				1One test satellite
accuracy.				[edit] Awards
Sources of User Equivalent Range Errors				Two GPS developers have received the
(UERE) Source Effect				National Academy of Engineering Charles
Ionospheric effects Â± 5 meter				Stark Draper prize year 2003:
Ephemeris errors Â± 2.5 meter				* Ivan Getting, emeritus president of
Satellite clock errors Â± 2 meter				The Aerospace Corporation and engineer
Multipath distortion Â± 1 meter				at the Massachusetts Institute of
Tropospheric effects Â± 0.5 meter				Technology, established the basis for
Numerical errors Â± 1 meter				GPS, improving on the World War II
[edit] Atmospheric effects				land-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's				aeronautics and astronautics at Stanford
atmosphere and ionosphere. Correcting				University, conceived the present
these errors is a significant challenge				satellite-based system in the early
to improving GPS position accuracy.				1960s and developed it in conjunction
These effects are smallest when the				with the U.S. Air Force.
satellite is directly overhead and				One GPS developer, Roger L. Easton,
become greater for satellites nearer the				received the National Medal of
horizon since the signal is affected for				Technology on February 13, 2006 at the
a longer time. Once the receiver's				White House.[34]
approximate location is known, a				On February 10, 1993, the National
mathematical model can be used to				Aeronautic Association selected the
estimate and compensate for these				Global Positioning System Team as
errors.				winners of the 1992 Robert J. Collier
Because ionospheric delay affects the				Trophy, the most prestigious aviation
speed of microwave signals differently				award in the United States. This team
based on frequency-a characteristic				consists of researchers from the Naval
known as dispersion-both frequency bands				Research Laboratory, the U.S. Air Force,
can be used to help reduce this error.				the Aerospace Corporation, Rockwell
Some military and expensive survey-grade				International Corporation, and IBM
civilian receivers compare the different				Federal Systems Company. The citation
delays in the L1 and L2 frequencies to				accompanying the presentation of the
measure atmospheric dispersion, and				trophy honors the GPS Team "for the most
apply a more precise correction. This				significant development for safe and
can be done in civilian receivers				efficient navigation and surveillance of
without decrypting the P(Y) signal				air and spacecraft since the
carried on L2, by tracking the carrier				introduction of radio navigation 50
wave instead of the modulated code. To				years ago."
facilitate this on lower cost receivers,				[edit] Other systems
a new civilian code signal on L2, called				Main article: Global Navigation
L2C, was added to the Block IIR-M				Satellite System
satellites, which was first launched in				Other satellite navigation systems in
2005. It allows a direct comparison of				use or various states of development
the L1 and L2 signals using the coded				include:
signal instead of the carrier wave.				* Beidou - China's regional system that
The effects of the ionosphere generally				China has proposed to expand into a
change slowly, and can be averaged over				global system named COMPASS.
time. The effects for any particular				* Galileo - a proposed global system
geographical area can be easily				being developed by the European Union,
calculated by comparing the GPS-measured				joined by China, Israel, India, Morocco,
position to a known surveyed location.				Saudi Arabia and South Korea, Ukraine
This correction is also valid for other				planned to be operational by 2011-12.
receivers in the same general location.				* GLONASS - Russia's global system which
Several systems send this information				is being restored to full availability
over radio or other links to allow L1				in partnership with India.
only receivers to make ionospheric				* Indian Regional Navigational Satellite
corrections. The ionospheric data are				System (IRNSS) - India's proposed
transmitted via satellite in Satellite				regional system.
Based Augmentation Systems such as WAAS,				* QZSS - Japanese proposed regional
which transmits it on the GPS frequency				system, adding better coverage to the
using a special pseudo-random number				Japanese 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 precise				visit integral degrees of latitude and
measurement and compensation of humidity				longitude.
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 atmosphere				sharing community.
at higher elevations. Since the GPS				* GPS Drawing Digital mapping and
receiver computes its approximate				drawing 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 radio				waypoints)
signals reflect off surrounding terrain;				* ID Sniper rifle
buildings, canyon walls, hard ground,				* OpenStreetMap, free content maps and
etc. These delayed signals can cause				street 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 mitigate				mobile equipment.
multipath errors. For long delay				* The American Practical
multipath, the receiver itself can				Navigator-Chapter 11 "Satellite
recognize the wayward signal and discard				Navigation"
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 reflections				1. ^ Parkinson, B.W. (1996), Global
are harder to filter out because they				Positioning System: Theory and
interfere with the true signal, causing				Applications, chap. 1: Introduction and
effects almost indistinguishable from				Heritage of NAVSTAR, the Global
routine fluctuations in atmospheric				Positioning System. pp. 3-28, American
delay.				Institute of Aeronautics and
Multipath effects are much less severe				Astronautics, Washington, D.C.
in moving vehicles. When the GPS antenna				2. ^ a b GPS Overview from the NAVSTAR
is moving, the false solutions using				Joint Program Office. Accessed December
reflected signals quickly fail to				15, 2006.
converge and only the direct signals				3. ^ HowStuffWorks. How GPS Receivers
result in stable solutions.				Work. Accessed May 14, 2006.
[edit] Ephemeris and clock errors				4. ^ globalsecurity.org [1].
The navigation message from a satellite				5. ^ Dana, Peter H. GPS Orbital Planes.
is sent out only every 30 seconds. In				August 8, 1996.
reality, the data contained in these				6. ^ What the Global Positioning System
messages tend to be "out of date" by an				Tells Us about Relativity. Accessed
even larger amount. Consider the case				January 2, 2007.
when a GPS satellite is boosted back				7. ^ USCG Navcen: GPS Frequently Asked
into a proper orbit; for some time				Questions. Accessed January 3, 2007.
following the maneuver, the receiver's				8. ^ Massatt, Paul and Brady, Wayne.
calculation of the satellite's position				"Optimizing performance through
will be incorrect until it receives				constellation management", Crosslink,
another ephemeris update. The onboard				Summer 2002, pages 17-21.
clocks are extremely accurate, but they				9. ^ US Coast Guard General GPS News
do suffer from some clock drift. This				9-9-05
problem tends to be very small, but may				10. ^ 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 to				12. ^
change over days or weeks rather than				13. ^ AN02 Network Assistance (HTML).
minutes. This makes correction fairly				Retrieved on 2007-09-10.
simple by sending out a more accurate				14. ^ a b Office of Science and
almanac on a separate channel.				Technology Policy. Presidential
[edit] Selective availability				statement to stop degrading GPS. May 1,
The GPS includes a feature called				2000.
Selective Availability (SA) that				15. ^ FAA, Selective Availability.
introduces intentional, slowly changing				Retrieved Jan. 6, 2007.
random errors of up to a hundred meters				16. ^
(328 ft) into the publicly available				17. ^ Rizos, Chris. University of New
navigation signals to confound, for				South Wales. GPS Satellite Signals.
example, guiding long range missiles to				1999.
precise targets. Additional accuracy was				18. ^ The Global Positioning System by
available in the signal, but in an				Robert A. Nelson Via Satellite, November
encrypted form that was only available				1999
to the United States military, its				19. ^ Ashby, Neil Relativity and GPS.
allies and a few others, mostly				Physics Today, May 2002.
government users.				20. ^ Space Environment Center. SEC
SA typically added signal errors of up				Navigation Systems GPS Page. August 26,
to about 10 meters (32 ft) horizontally				1996.
and 30 meters (98 ft) vertically. The				21. ^ The hunt for an unintentional GPS
inaccuracy of the civilian signal was				jammer. GPS World. January 1, 2003.
deliberately encoded so as not to change				22. ^ Low Cost and Portable GPS Jammer.
very quickly, for instance the entire				Phrack 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 same				23. ^ American Forces Press Service.
direction. To improve the usefulness of				CENTCOM charts progress. March 25, 2003.
GPS for civilian navigation,				24. ^ [2]
Differential GPS was used by many				25. ^ Ruley, John. AVweb. GPS jamming.
civilian GPS receivers to greatly				February 12, 2003.
improve accuracy.				26. ^ Commercial GPS Receivers: Facts
During the Gulf War, the shortage of				for the Warfighter. Hosted at the Joint
military GPS units and the wide				Chiefs website, linked by the USAF's GPS
availability of civilian ones among				Wing DAGR program website. Accessed on
personnel resulted in a decision to				10 April, 2007
disable Selective Availability. This was				27. ^ US Coast Guard news release.
ironic, as SA had been introduced				Global Positioning System Fully
specifically for these situations,				Operational
allowing friendly troops to use the				28. ^ a b Hydrographic Society Journal.
signal for accurate navigation, while at				Developments in Global Navigation
the same time denying it to the enemy.				Satellite Systems. Issue #104, April
But since SA was also denying the same				2002. Accessed April 5, 2007.
accuracy to thousands of friendly				29. ^ XM982 Excalibur Precision Guided
troops, turning it off or setting it to				Extended Range Artillery Projectile.
an error of zero meters (effectively the				GlobalSecurity.org (2007-05-29).
same thing) presented a clear benefit.				Retrieved on 2007-09-26.
In the 1990s, the FAA started pressuring				30. ^ Sandia National Laboratory's
the military to turn off SA permanently.				Nonproliferation programs and arms
This would save the FAA millions of				control technology.
dollars every year in maintenance of				31. ^ Arms Control Association. Missile
their own radio navigation systems. The				Technology Control Regime. Accessed May
military resisted for most of the 1990s,				17, 2006.
and it ultimately took an executive				32. ^ United States Department of
order to have SA removed from the GPS				Defense. Announcement of Initial
signal. The amount of error added was				Operational 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 users				Administration. U.S. GLOBAL POSITIONING
access to the error-free L1 signal. Per				SYSTEM POLICY. March 29, 1996.
the directive, the induced error of SA				34. ^ United States Naval Research
was changed to add no error to the				Laboratory. National Medal of Technology
public signals (C/A code). Selective				for GPS. November 21, 2005
Availability is still a system				[edit] External links
capability of GPS, and error could, in				Wikimedia Commons has media related to:
theory, be reintroduced at any time. In				Global Positioning System
practice, in view of the hazards and				Government links
costs this would induce for US and				* GPS.gov-General public education
foreign shipping, it is unlikely to be				website created by the U.S. Government
reintroduced, and various government				* National Space-Based PNT Executive
agencies, including the FAA,[15] have				Committee-Established in 2004 to oversee
stated that it is not intended to be				management 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 other				GPS constellation, government policy,
navigation services) to hostile forces				and links to other references. Also
in a specific area of crisis without				includes 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 the				acquiring the system on behalf of the US
Selective Availability hardware is the				Government.
capability to correct the frequency of				* U.S. Naval Observatory's GPS
the GPS caesium and rubidium atomic				constellation status
clocks to an accuracy of approximately 2				* U.S. Army Corps of Engineers manual:
Ã- 10-13 (one in five trillion). This				NAVSTAR HTML and PDF (22.6 MB, 328
represented a significant improvement				pages)
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 that				Edition-The official Standard
they would not procure any more				Positioning Signal specification.
satellites capable of implementing SA.				* Federal Aviation Administration's GPS
[16]				FAQ
[edit] Relativity				Introductory / tutorial links
According to the theory of relativity,				* How does GPS work? TomTom explains
due to their constant movement and				GPS, navigation, and digital maps
height relative to the Earth-centered				* GPS Academy Garmin interactive video
inertial reference frame, the clocks on				web site explaing what exactly GPS is
the satellites are affected by their				and what it can do for you
speed (special relativity) as well as				* HowStuffWorks' Simplified explanation
their gravitational potential (general				of GPS and video about how GPS works.
relativity). For the GPS satellites,				* Trimble's Online GPS Tutorial Tutorial
general relativity predicts that the				designed to introduce you to the
atomic clocks at GPS orbital altitudes				principles behind GPS
will tick more rapidly, by about 45,900				* GPS and GLONASS Simulation(Java
nanoseconds (ns) per day, because they				applet) Simulation and graphical
are in a weaker gravitational field than				depiction of space vehicle motion
atomic clocks on Earth's surface.				including computation of dilution of
Special relativity predicts that atomic				precision (DOP)
clocks moving at GPS orbital speeds will				Technical, historical, and ancillary
tick more slowly than stationary ground				topics links
clocks by about 7,200 ns per day. When				* Dana, Peter H. "Global Positioning
combined, the discrepancy is 38				System 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 onboard				and working of GPS, touching on the
each satellite is given a rate offset				upcoming Galileo
prior to launch, making it run slightly				* History of GPS, including information
slower than the desired frequency on				about each satellite's configuration and
Earth; specifically, at 10.22999999543				launch.
MHz instead of 10.23 MHz.[18]				* Chadha, Kanwar. "The Global
GPS observation processing must also				Positioning System: Challenges in
compensate for another relativistic				Bringing GPS to Mainstream Consumers"
effect, the Sagnac effect. The GPS time				Technical 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 the				navigation.
two systems modifies the signal run				* Improved weather predictions from
time, a correction having opposite				COSMIC GPS satellite signal occultation
algebraic signs for satellites in the				data.
Eastern and Western celestial				* David L. Wilson's GPS Accuracy Web
hemispheres. Ignoring this effect will				Page A thorough analysis of the accuracy
produce an east-west error on the order				of 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 GPS				Orbits Example of GPS receiver designed
satellites are precisely tuned, making				for high altitude spaceflight.
the system a practical engineering				* The Navigator GPS Receiver GSFC's
application of the scientific theory of				Navigator spaceflight receiver.
relativity in a real-world environment.				* Neil Ashby's Relativity in the Global
[edit] GPS interference and jamming				Positioning System
Since GPS signals at terrestrial				[show]v â¢ d â¢ e
receivers tend to be relatively weak, it				Satellite navigation systems
is easy for other sources of				Historical Flag of the United States
electromagnetic radiation to desensitize				Transit
the receiver, making acquiring and				Operational Flag of the Soviet Union /
tracking the satellite signals difficult				Flag of Russia GLONASS Â· Flag of the
or impossible.				United States GPS
Solar flares are one such naturally				Developmental Flag of the People's
occurring emission with the potential to				Republic of China Beidou/COMPASS Â·
degrade GPS reception, and their impact				Flag of Europe Galileo Â· Flag of
can affect reception over the half of				India IRNSS Â· Flag of Japan QZSS
the Earth facing the sun. GPS signals				Related topics EGNOS Â· GAGAN Â·
can also be interfered with by naturally				GPSÂ·C Â· LAAS Â· MSAS Â· WAAS
occurring geomagnetic storms,				[show]v â¢ d â¢ e
predominantly found near the poles of				Time signal stations
the Earth's magnetic field.[20] Another				Longwave DCF77 Â· HBG Â· JJY Â·
source of problems is the metal embedded				MSF Â· TDF Â· WWVB
in some car windscreens to prevent				Shortwave BPM Â· CHU Â· RWM Â·
icing, degrading reception just inside				WWV Â· WWVH Â· YVTO
the car.				GNSS time transfer Beidou Â· Galileo
Man-made interference can also disrupt,				Â· GLONASS Â· GPS Â· IRNSS
or jam, GPS signals. In one well				Defunct time stations OMA Â· VNG
documented case, an entire harbor was				[show]v â¢ d â¢ e
unable to receive GPS signals due to				Global structure in Systems, Systems
unintentional jamming caused by a				sciences and Systems scientists
malfunctioning TV antenna				Categories Category:Conceptual systems
preamplifier.[21] Intentional jamming is				Â· Category:Physical systems Â·
also possible. Generally, stronger				Category:Social systems Â·
signals can interfere with GPS receivers				Category:Systems Â· Category:Systems
when they are within radio range, or				science Â· Category:Systems scientists
line of sight. In 2002, a detailed				Â· Category:Systems theory
description of how to build a short				Systems Biological system Â· Complex
range GPS L1 C/A jammer was published in				system Â· Complex adaptive system Â·
the online magazine Phrack.[22]				Conceptual system Â· Cultural system
The U.S. government believes that such				Â· Dynamical system Â· Economic
jammers were used occasionally during				system Â· Ecosystem Â· Formal
the 2001 war in Afghanistan and the U.S.				system Â· Global Positioning System
military claimed to destroy a GPS jammer				Â· Human organ systems Â·
with a GPS-guided bomb during the Iraq				Information systems Â· Legal system
War.[23] Such a jammer is relatively				Â· Metric system Â· Nervous system
easy to detect and locate, making it an				Â· Non-linear system Â· Operating
attractive target for anti-radiation				system Â· Physical system Â·
missiles. The UK Ministry of Defence				Political system Â· Sensory system
tested a jamming system in the UK's West				Â· Social system Â· Solar System
Country on 7 and 8 June 2007. [24]				Â· System Â· Systems of measurement
Some countries allow the use of GPS				Fields of theory Chaos theory Â·
repeaters to allow for the reception of				Complex systems Â· Control theory Â·
GPS signals indoors and in obscured				Cybernetics Â· Holism in science Â·
locations, however, under EU and UK				Sociotechnical systems theory Â·
laws, the use of these is prohibited as				Systems biology Â· System dynamics
the signals can cause interference to				Â· Systems ecology Â· Systems
other GPS receivers that may receive				engineering Â· Systems theory Â·
data from both GPS satellites and the				Systems science
repeater.				Systems scientists Russell L. Ackoff
Due to the potential for both natural				Â· William Ross Ashby Â· Gregory
and man-made noise, numerous techniques				Bateson Â· Ludwig von Bertalanffy
continue to be developed to deal with				Â· Kenneth E. Boulding Â· Peter
the interference. The first is to not				Checkland Â· C. West Churchman Â·
rely on GPS as a sole source. According				Heinz von Foerster Â· Charles
to John Ruley, "IFR pilots should have a				FranÃ§ois Â· Jay Wright Forrester
fallback plan in case of a GPS				Â· Ralph W. Gerard Â· Debora Hammond
malfunction".[25] Receiver Autonomous				Â· George Klir Â· Niklas Luhmann
Integrity Monitoring (RAIM) is a feature				Â· Humberto Maturana Â· Donella
now included in some receivers, which is				Meadows Â· Mihajlo D. Mesarovic Â·
designed to provide a warning to the				Howard T. Odum Â· Talcott Parsons Â·
user if jamming or another problem is				Ilya Prigogine Â· Anatol Rapoport
detected. The U.S. military has also				Â· Francisco Varela Â· John N.
deployed their Selective Availability /

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