About GPS

Global Positioning System				Navigation Systems and Assisted GPS.
The Global Positioning System (GPS) is the only fully				[edit] Precise monitoring
functional Global Navigation Satellite System (GNSS).				The accuracy of a calculation can also be improved
Utilizing a constellation of at least 24 medium Earth				through precise monitoring and measuring of the
orbit satellites that transmit precise microwave				existing GPS signals in additional or alternate ways.
signals, the system enables a GPS receiver to				After SA, which has been turned off, the largest
determine its location, speed/direction, and time.				error in GPS is usually the unpredictable delay through
Developed by the United States Department of				the ionosphere. The spacecraft broadcast ionospheric
Defense, it is officially named NAVSTAR GPS				model parameters, but errors remain. This is one
(Contrary to popular belief, NAVSTAR is not an				reason the GPS spacecraft transmit on at least two
acronym, but simply a name given by Mr. John Walsh,				frequencies, L1 and L2. Ionospheric delay is a
a key decision maker when it came to the budget				well-defined function of frequency and the total
for the GPS program[1]). The satellite constellation is				electron content (TEC) along the path, so measuring
managed by the United States Air Force 50th Space				the arrival time difference between the frequencies
Wing. The cost of maintaining the system is				determines TEC and thus the precise ionospheric
approximately US$750 million per year,[2] including the				delay at each frequency.
replacement of aging satellites, and research and				Receivers with decryption keys can decode the
development. Despite these costs, GPS is free for				P(Y)-code transmitted on both L1 and L2. However,
civilian use as a public good.				these keys are reserved for the military and
GPS has become a widely used aid to navigation				"authorized" agencies and are not available to the
worldwide, and a useful tool for map-making, land				public. Without keys, it is still possible to use a
surveying, commerce, and scientific uses. GPS also				codeless technique to compare the P(Y) codes on L1
provides a precise time reference used in many				and L2 to gain much of the same error information.
applications including scientific study of earthquakes,				However, this technique is slow, so it is currently
and synchronization of telecommunications networks.				limited to specialized surveying equipment. In the
Simplified method of operation				future, additional civilian codes are expected to be
A GPS receiver calculates its position by measuring				transmitted on the L2 and L5 frequencies (see GPS
the distance between itself and three or more GPS				modernization, below). Then all users will be able to
satellites. Measuring the time delay between				perform dual-frequency measurements and directly
transmission and reception of each GPS microwave				compute ionospheric delay errors.
signal gives the distance to each satellite, since the				A second form of precise monitoring is called
signal travels at a known speed - the speed of light.				Carrier-Phase Enhancement (CPGPS). The error, which
These signals also carry information about the				this corrects, arises because the pulse transition of
satellites' location and general system health (known				the PRN is not instantaneous, and thus the correlation
as almanac and ephemeris data). By determining the				(satellite-receiver sequence matching) operation is
position of, and distance to, at least three satellites,				imperfect. The CPGPS approach utilizes the L1 carrier
the receiver can compute its position using				wave, which has a period 1000 times smaller than
trilateration.[3] Receivers typically do not have				that of the C/A bit period, to act as an additional
perfectly accurate clocks and therefore track one or				clock signal and resolve the uncertainty. The phase
more additional satellites, using their atomic clocks to				difference error in the normal GPS amounts to
correct the receiver's own clock error.				between 2 and 3 meters (6 to 10 ft) of ambiguity.
[edit] Technical description				CPGPS working to within 1% of perfect transition
Unlaunched GPS satellite on display at the San Diego				reduces this error to 3 centimeters (1 inch) of
Aerospace museum				ambiguity. By eliminating this source of error, CPGPS
Unlaunched GPS satellite on display at the San Diego				coupled with DGPS normally realizes between 20 and
Aerospace museum				30 centimeters (8 to 12 inches) of absolute accuracy.
[edit] System segmentation				Relative Kinematic Positioning (RKP) is another
The current GPS consists of three major segments.				approach for a precise GPS-based positioning system.
These are the space segment (SS), a control				In this approach, determination of range signal can be
segment (CS), and a user segment (US).[4]				resolved to an accuracy of less than 10 centimeters
[edit] Space segment				(4 in). This is done by resolving the number of cycles
The space segment (SS) is composed of the orbiting				in which the signal is transmitted and received by the
GPS satellites, or Space Vehicles (SV) in GPS parlance.				receiver. This can be accomplished by using a
The GPS design calls for 24 SVs to be distributed				combination of differential GPS (DGPS) correction
equally among six circular orbital planes.[5] The orbital				data, transmitting GPS signal phase information and
planes are centered on the Earth, not rotating with				ambiguity resolution techniques via statistical
respect to the distant stars.[6] The six planes have				tests-possibly with processing in real-time (real-time
approximately 55Ã‚Â° inclination (tilt relative				kinematic positioning, RTK).
to Earth's equator) and are separated by				[edit] GPS time and date
60Ã‚Â° right ascension of the ascending				While most clocks are synchronized to Coordinated
node (angle along the equator from a reference point				Universal Time (UTC), the Atomic clocks on the
to the orbit's intersection).[2]				satellites are set to GPS time. The difference is that
Orbiting at an altitude of approximately 20,200				GPS time is not corrected to match the rotation of
kilometers (12,600 miles or 10,900 nautical miles;				the Earth, so it does not contain leap seconds or
orbital radius of 26,600 km (16,500 mi or 14,400				other corrections which are periodically added to UTC.
NM)), each SV makes two complete orbits each				GPS time was set to match Coordinated Universal
sidereal day, so it passes over the same location on				Time (UTC) in 1980, but has since diverged. The lack
Earth once each day. The orbits are arranged so that				of corrections means that GPS time remains at a
at least six satellites are always within line of sight				constant offset (19 seconds) with International
from almost everywhere on Earth's surface.[7]				Atomic Time (TAI). Periodic corrections are
As of September 2007, there are 31 actively				performed on the on-board clocks to correct
broadcasting satellites in the GPS constellation. The				relativistic effects and keep them synchronized with
additional satellites improve the precision of GPS				ground clocks.
receiver calculations by providing redundant				The GPS navigation message includes the difference
measurements. With the increased number of				between GPS time and UTC, which as of 2006 is 14
satellites, the constellation was changed to a				seconds. Receivers subtract this offset from GPS
nonuniform arrangement. Such an arrangement was				time to calculate UTC and specific timezone values.
shown to improve reliability and availability of the				New GPS units may not show the correct UTC time
system, relative to a uniform system, when multiple				until after receiving the UTC offset message. The
satellites fail.[8]				GPS-UTC offset field can accommodate 255 leap
[edit] Control segment				seconds (eight bits) which, at the current rate of
The flight paths of the satellites are tracked by US				change of the Earth's rotation, is sufficient to last
Air Force monitoring stations in Hawaii, Kwajalein,				until the year 2330.
Ascension Island, Diego Garcia, and Colorado Springs,				As opposed to the year, month, and day format of
Colorado, along with monitor stations operated by				the Julian calendar, the GPS date is expressed as a
the National Geospatial-Intelligence Agency (NGA).[9]				week number and a day-of-week number. The week
The tracking information is sent to the Air Force				number is transmitted as a ten-bit field in the C/A
Space Command's master control station at Schriever				and P(Y) navigation messages, and so it becomes
Air Force Base in Colorado Springs, which is operated				zero again every 1,024 weeks (19.6 years). GPS
by the 2d Space Operations Squadron (2 SOPS) of				week zero started at 00:00:00 UTC (00:00:19 TAI) on
the United States Air Force (USAF). 2 SOPS contacts				January 6, 1980 and the week number became zero
each GPS satellite regularly with a navigational update				again for the first time at 23:59:47 UTC on August
(using the ground antennas at Ascension Island, Diego				21, 1999 (00:00:19 TAI on August 22, 1999). To
Garcia, Kwajalein, and Colorado Springs). These				determine the current Gregorian date, a GPS receiver
updates synchronize the atomic clocks on board the				must be provided with the approximate date (to
satellites to within one microsecond and adjust the				within 3,584 days) to correctly translate the GPS
ephemeris of each satellite's internal orbital model.				date signal. To address this concern the modernized
The updates are created by a Kalman filter which				GPS navigation messages use a 13-bit field, which
uses inputs from the ground monitoring stations,				only repeats every 8,192 weeks (157 years), and will
space weather information, and various other				not return to zero until near the year 2137.
inputs.[10]				[edit] GPS modernization
GPS receivers come in a variety of formats, from				Main article: GPS modernization
devices integrated into cars, phones, and watches, to				Having reached the program's requirements for Full
dedicated devices such as those shown here from				Operational Capability (FOC) on July 17, 1995,[27] the
manufacturers Trimble, Garmin and Leica (left to				GPS completed its original design goals. However,
right).				additional advances in technology and new demands
GPS receivers come in a variety of formats, from				on the existing system led to the effort to
devices integrated into cars, phones, and watches, to				modernize the GPS system. Announcements from
dedicated devices such as those shown here from				the Vice President and the White House in 1998
manufacturers Trimble, Garmin and Leica (left to				initiated these changes, and in 2000 the U.S. Congress
right).				authorized the effort, referring to it as GPS III.
[edit] User segment				The project aims to improve the accuracy and
The user's GPS receiver is the user segment (US) of				availability for all users and involves new ground
the GPS system. In general, GPS receivers are				stations, new satellites, and four additional navigation
composed of an antenna, tuned to the frequencies				signals. New civilian signals are called L2C, L5 and L1C;
transmitted by the satellites, receiver-processors, and				the new military code is called M-Code. Initial
a highly-stable clock (often a crystal oscillator). They				Operational Capability (IOC) of the L2C code is
may also include a display for providing location and				expected in 2008.[28] A goal of 2013 has been
speed information to the user. A receiver is often				established for the entire program, with incentives
described by its number of channels: this signifies how				offered to the contractors if they can complete it by
many satellites it can monitor simultaneously. Originally				2011.
limited to four or five, this has progressively				[edit] Applications
increased over the years so that, as of 2006,				The Global Positioning System, while originally a
receivers typically have between twelve and twenty				military project, is considered a dual-use technology,
channels.				meaning it has significant applications for both the
A typical OEM GPS receiver module, based on the				military and the civilian industry.
SiRF Star III chipset, measuring 15Ãƒ-17 mm,				[edit] Military
and used in many products.				Please help improve this article by expanding this
A typical OEM GPS receiver module, based on the				section.
SiRF Star III chipset, measuring 15Ãƒ-17 mm,				See talk page for details. Please remove this
and used in many products.				message once the section has been expanded.
GPS receivers may include an input for differential				The military use GPS for the following purposes:
corrections, using the RTCM SC-104 format. This is				[edit] Navigation
typically in the form of a RS-232 port at 4,800 bit/s				GPS allows soldiers to find objectives in the dark or in
speed. Data are actually sent at a much lower rate,				unfamiliar territory, and to coordinate the movement
which limits the accuracy of the signal sent using				of troops and supplies.
RTCM. Receivers with internal DGPS receivers can				[edit] Target tracking
outperform those using external RTCM data. As of				Various military weapons systems use GPS to track
2006, even low-cost units commonly include Wide				potential ground and air targets before they are
Area Augmentation System (WAAS) receivers.				flagged as hostile. These weapons systems pass GPS
Many GPS receivers can relay position data to a PC				co-ordinates of targets to precision-guided munitions
or other device using the NMEA 0183 protocol. NMEA				to allow them to engage the targets accurately.
2000[11] is a newer and less widely adopted protocol.				Military aircraft, particularly those used in air-to-ground
Both are proprietary and controlled by the US-based				roles use GPS to find targets (for example, gun
National Marine Electronics Association. References to				camera video from AH-1 Cobras in Iraq show GPS
the NMEA protocols have been compiled from public				co-ordinates that can be looked up in Google Earth).
records, allowing open source tools like gpsd to read				[edit] Missile and projectile guidance
the protocol without violating intellectual property				GPS allows accurate targeting of various military
laws. Other proprietary protocols exist as well, such				weapons including ICBMs, cruise missiles and
as the SiRF and MTK protocols. Receivers can				precision-guided munitions.
interface with other devices using methods including a				Artillery projectiles with embedded GPS receivers
serial connection, USB or Bluetooth.				able to withstand forces of 12,000G have been
[edit] Navigation signals				developed for use in 155 mm howitzers.[29]
Main article: GPS signals				[edit] Search and Rescue
GPS broadcast signal				Downed pilots can be located faster if they have a
GPS broadcast signal				GPS receiver.
Each GPS satellite continuously broadcasts a				[edit] Reconnaissance and Map Creation
Navigation Message at 50 bit/s giving the				The military use GPS extensively to aid mapping and
time-of-day, GPS week number and satellite health				reconnaissance.
information (all transmitted in the first part of the				[edit] Other
message), an ephemeris (transmitted in the second				The GPS satellites also carry nuclear detonation
part of the message) and an almanac (later part of				detectors, which form a major portion of the United
the message). The ephemeris data gives the				States Nuclear Detonation Detection System.[30]
satellite's own precise orbit and is output over 18				[edit] Civilian
seconds, repeating every 30 seconds. The ephemeris				See also: GPS applications
is updated every 2 hours and is generally valid for 4				This antenna is mounted on the roof of a hut
hours, with provisions for 6 hour time-outs. The time				containing a scientific experiment needing precise
needed to acquire the ephemeris is becoming a				timing.
significant element of the delay to first position fix,				This antenna is mounted on the roof of a hut
because, as the hardware becomes more capable,				containing a scientific experiment needing precise
the time to lock onto the satellite signals shrinks, but				timing.
the ephemeris data requires 30 seconds (worst case)				Many civilian applications benefit from GPS signals,
before it is received, due to the low data				using one or more of three basic components of the
transmission rate. The almanac consists of coarse				GPS; absolute location, relative movement, time
orbit and status information for each satellite in the				transfer.
constellation and takes 12 seconds for each satellite				The ability to determine the receiver's absolute
present, with information for a new satellite being				location allows GPS receivers to perform as a
transmitted every 30 seconds (15.5 minutes for 31				surveying tool or as an aid to navigation. The
satellites). The purpose of the data is to assist in the				capacity to determine relative movement enables a
acquisition of satellites at power-up by allowing the				receiver to calculate local velocity and orientation,
receiver to generate a list of visible satellites based				useful in vessels or observations of the Earth. Being
on stored position and time, while an ephemeris from				able to synchronize clocks to exacting standards
each satellite is needed to compute position fixes				enables time transfer, which is critical in large
using that satellite. In older hardware, lack of an				communication and observation systems. An example
almanac in a new receiver would cause long delays				is CDMA digital cellular. Each base station has a GPS
before providing a valid position, because the search				timing receiver to synchronize its spreading codes
for each satellite was a slow process. Advances in				with other base stations to facilitate inter-cell hand
hardware have made the acquisition process much				off and support hybrid GPS/CDMA positioning of
faster, so not having an almanac is no longer an issue.				mobiles for emergency calls and other applications.
An important thing to note about navigation data is				Finally, GPS enables researchers to explore the Earth
that each satellite transmits only its own ephemeris,				environment including the atmosphere, ionosphere
but transmits an almanac for all satellites.				and gravity field. GPS survey equipment has
Each satellite transmits its navigation message with at				revolutionized tectonics by directly measuring the
least two distinct spread spectrum codes: the Coarse				motion of faults in earthquakes.
/ Acquisition (C/A) code, which is freely available to				To help prevent civilian GPS guidance from being
the public, and the Precise (P) code, which is usually				used in an enemy's military or improvised weaponry,
encrypted and reserved for military applications. The				the US Government controls the export of civilian
C/A code is a 1,023 chip pseudo-random (PRN) code				receivers. A US-based manufacturer cannot generally
at 1.023 million chips/sec so that it repeats every				export a GPS receiver unless the receiver contains
millisecond. Each satellite has its own C/A code so				limits restricting it from functioning when it is
that it can be uniquely identified and received				simultaneously (1) at an altitude above 18 kilometers
separately from the other satellites transmitting on				(60,000 ft) and (2) traveling at over 515 m/s (1,000
the same frequency. The P-code is a 10.23 megachip				knots).[31]
sec PRN code that repeats only every week. When				[edit] History
the "anti-spoofing" mode is on, as it is in normal				Please help improve this article by expanding this
operation, the P code is encrypted by the Y-code to				section.
produce the P(Y) code, which can only be decrypted				See talk page for details. Please remove this
by units with a valid decryption key. Both the C/A				message once the section has been expanded.
and P(Y) codes impart the precise time-of-day to the				The design of GPS is based partly on the similar
user. Frequencies used by GPS include				ground-based radio navigation systems, such as
* L1 (1575.42 MHz): Mix of Navigation Message,				LORAN and the Decca Navigator developed in the
coarse-acquisition (C/A) code and encrypted precision				early 1940s, and used during World War II. Additional
P(Y) code, plus the new L1C on future Block III				inspiration for the GPS system came when the
satellites.				Soviet Union launched the first Sputnik in 1957. A
* L2 (1227.60 MHz): P(Y) code, plus the new L2C				team of U.S. scientists led by Dr. Richard B. Kershner
code on the Block IIR-M and newer satellites.				were monitoring Sputnik's radio transmissions. They
* L3 (1381.05 MHz): Used by the Nuclear Detonation				discovered that, because of the Doppler effect, the
(NUDET) Detection System Payload (NDS) to signal				frequency of the signal being transmitted by Sputnik
detection of nuclear detonations and other				was higher as the satellite approached, and lower as
high-energy infrared events. Used to enforce nuclear				it continued away from them. They realized that
test ban treaties.				since they knew their exact location on the globe,
* L4 (1379.913 MHz): Being studied for additional				they could pinpoint where the satellite was along its
ionospheric correction.				orbit by measuring the Doppler distortion.
* L5 (1176.45 MHz): Proposed for use as a civilian				The first satellite navigation system, Transit, used by
safety-of-life (SoL) signal (see GPS modernization).				the United States Navy, was first successfully tested
This frequency falls into an internationally protected				in 1960. Using a constellation of five satellites, it could
range for aeronautical navigation, promising little or no				provide a navigational fix approximately once per
interference under all circumstances. The first Block				hour. In 1967, the U.S. Navy developed the Timation
IIF satellite that would provide this signal is set to be				satellite which proved the ability to place accurate
launched in 2008.				clocks in space, a technology the GPS system relies
[edit] Calculating positions				upon. In the 1970s, the ground-based Omega
[edit] Using the C/A code				Navigation System, based on signal phase
To start off, the receiver picks which C/A codes to				comparison, became the first world-wide radio
listen for by PRN number, based on the almanac				navigation system.
information it has previously acquired. As it detects				The first experimental Block-I GPS satellite was
each satellite's signal, it identifies it by its distinct C/A				launched in February 1978.[28] The GPS satellites
code pattern, then measures the time delay for each				were initially manufactured by Rockwell International
satellite. To do this, the receiver produces an identical				and are now manufactured by Lockheed Martin.
C/A sequence using the same seed number as the				[edit] Timeline
satellite. By lining up the two sequences, the receiver				* In 1972, the US Air Force Central Inertial Guidance
can measure the delay and calculate the distance to				Test Facility (Holloman AFB) conducted
the satellite, called the pseudorange[12].				developmental fight tests of two prototype GPS
Overlapping pseudoranges, represented as curves,				receivers over White Sands Missile Range, using
are modified to yield the probable position				ground-based pseudo-satellites.
Overlapping pseudoranges, represented as curves,				* In 1978 the first experimental Block-I GPS satellite
are modified to yield the probable position				was launched.
Next, the orbital position data, or ephemeris, from				* In 1983, after Soviet interceptor aircraft shot down
the Navigation Message is then downloaded to				the civilian airliner KAL 007 in restricted Soviet
calculate the satellite's precise position. A				airspace, killing all 269 people on board, U.S. President
more-sensitive receiver will potentially acquire the				Ronald Reagan announced that the GPS system
ephemeris data quicker than a less-sensitive receiver,				would be made available for civilian uses once it was
especially in a noisy environment.[13] Knowing the				completed.
position and the distance of a satellite indicates that				* By 1985, ten more experimental Block-I satellites
the receiver is located somewhere on the surface of				had been launched to validate the concept.
an imaginary sphere centered on that satellite and				* On February 14, 1989, the first modern Block-II
whose radius is the distance to it. Receivers can				satellite was launched.
substitute altitude for one satellite, which the GPS				* In 1992, the 2nd Space Wing, which originally
receiver translates to a pseudorange measured from				managed the system, was de-activated and replaced
the center of the earth.				by the 50th Space Wing.
Locations are calculated not in three-dimensional				* By December 1993 the GPS system achieved initial
space, but in four-dimensional spacetime, meaning a				operational capability[32]
measure of the precise time-of-day is very important.				* By January 17, 1994 a complete constellation of 24
The measured pseudoranges from four satellites				satellites was in orbit.
have already been determined with the receiver's				* Full Operational Capability was declared by
internal clock, and thus have an unknown amount of				NAVSTAR in April 1995.
clock error. (The clock error or actual time does not				* In 1996, recognizing the importance of GPS to
matter in the initial pseudorange calculation, because				civilian users as well as military users, U.S. President Bill
that is based on how much time has passed				Clinton issued a policy directive[33] declaring GPS to
between reception of each of the				be a dual-use system and establishing an Interagency
signals.[clarify][citation needed]) The four-dimensional				GPS Executive Board to manage it as a national
point that is equidistant from the pseudoranges is				asset.
calculated as a guess as to the receiver's location,				* In 1998, U.S. Vice President Al Gore announced
and the factor used to adjust those pseudoranges to				plans to upgrade GPS with two new civilian signals for
intersect at that four-dimensional point gives a guess				enhanced user accuracy and reliability, particularly with
as to the receiver's clock offset. With each guess, a				respect to aviation safety.
geometric dilution of precision (GDOP) vector is				* On May 2, 2000 "Selective Availability" was
calculated, based on the relative sky positions of the				discontinued as a result of the 1996 executive order,
satellites used. As more satellites are picked up,				allowing users to receive a non-degraded signal
pseudoranges from more combinations of four				globally.
satellites can be processed to add more guesses to				* In 2004, the United States Government signed a
the location and clock offset. The receiver then				historic agreement with the European Community
determines which combinations to use and how to				establishing cooperation related to GPS and Europe's
calculate the estimated position by determining the				planned Galileo system.
weighted average of these positions and clock				* In 2004, U.S. President George W. Bush updated
offsets. After the final location and time are				the national policy, replacing the executive board with
calculated, the location is expressed in a specific				the National Space-Based Positioning, Navigation, and
coordinate system, e.g. latitude/longitude, using the				Timing Executive Committee.
WGS 84 geodetic datum or a local system specific to				* November 2004, QUALCOMM announced
a country.				successful tests of Assisted-GPS system for mobile
[edit] Using the P(Y) code				phones.[3]
Calculating a position with the P(Y) signal is generally				* In 2005, the first modernized GPS satellite was
similar in concept, assuming one can decrypt it. The				launched and began transmitting a second civilian
encryption is essentially a safety mechanism: if a				signal (L2C) for enhanced user performance.
signal can be successfully decrypted, it is reasonable				* The most recent launch was on 17 November
to assume it is a real signal being sent by a GPS				2006. The oldest GPS satellite still in operation was
satellite.[citation needed] In comparison, civil receivers				launched in August 1991.
are highly vulnerable to spoofing since correctly				* On September 14, 2007, the aging
formatted C/A signals can be generated using readily				mainframe-based Ground Segment Control System
available signal generators. RAIM features do not				was transitioned to the new Architecture Evolution
protect against spoofing, since RAIM only checks the				Plan. [4]
signals from a navigational perspective.				[edit] Satellite numbers
[edit] Accuracy and error sources				Name Launch Period No of satellites launched, inc.
The position calculated by a GPS receiver requires				launch failures Currently in service
the current time, the position of the satellite and the				Block I 1978-1985 11 0
measured delay of the received signal. The position				Block II 1985-1990 9 0
accuracy is primarily dependent on the satellite				Block IIA 1990-1997 19 15+11
position and signal delay.				Block IIR 1997-2004 12 12
To measure the delay, the receiver compares the bit				Block IIR-M 2005- 3 3
sequence received from the satellite with an internally				Total 54 (plus one not launched) 30+1
generated version. By comparing the rising and trailing				1One test satellite
edges of the bit transitions, modern electronics can				[edit] Awards
measure signal offset to within about 1% of a bit				Two GPS developers have received the National
time, or approximately 10 nanoseconds for the C/A				Academy of Engineering Charles Stark Draper prize
code. Since GPS signals propagate nearly at the				year 2003:
speed of light, this represents an error of about 3				* Ivan Getting, emeritus president of The Aerospace
meters. This is the minimum error possible using only				Corporation and engineer at the Massachusetts
the GPS C/A signal.				Institute of Technology, established the basis for
Position accuracy can be improved by using the				GPS, improving on the World War II land-based radio
higher-chiprate P(Y) signal. Assuming the same 1% bit				system called LORAN (Long-range Radio Aid to
time accuracy, the high frequency P(Y) signal results				Navigation).
in an accuracy of about 30 centimeters.				* Bradford Parkinson, professor of aeronautics and
Electronics errors are one of several				astronautics at Stanford University, conceived the
accuracy-degrading effects outlined in the table				present satellite-based system in the early 1960s and
below. When taken together, autonomous civilian GPS				developed it in conjunction with the U.S. Air Force.
horizontal position fixes are typically accurate to				One GPS developer, Roger L. Easton, received the
about 15 meters (50 ft). These effects also reduce				National Medal of Technology on February 13, 2006
the more precise P(Y) code's accuracy.				at the White House.[34]
Sources of User Equivalent Range Errors (UERE)				On February 10, 1993, the National Aeronautic
Source Effect				Association selected the Global Positioning System
Ionospheric effects Ã‚Â± 5 meter				Team as winners of the 1992 Robert J. Collier
Ephemeris errors Ã‚Â± 2.5 meter				Trophy, the most prestigious aviation award in the
Satellite clock errors Ã‚Â± 2 meter				United States. This team consists of researchers
Multipath distortion Ã‚Â± 1 meter				from the Naval Research Laboratory, the U.S. Air
Tropospheric effects Ã‚Â± 0.5 meter				Force, the Aerospace Corporation, Rockwell
Numerical errors Ã‚Â± 1 meter				International Corporation, and IBM Federal Systems
[edit] Atmospheric effects				Company. The citation accompanying the
Inconsistencies of atmospheric conditions affect the				presentation of the trophy honors the GPS Team
speed of the GPS signals as they pass through the				"for the most significant development for safe and
Earth's atmosphere and ionosphere. Correcting these				efficient navigation and surveillance of air and
errors is a significant challenge to improving GPS				spacecraft since the introduction of radio navigation
position accuracy. These effects are smallest when				50 years ago."
the satellite is directly overhead and become greater				[edit] Other systems
for satellites nearer the horizon since the signal is				Main article: Global Navigation Satellite System
affected for a longer time. Once the receiver's				Other satellite navigation systems in use or various
approximate location is known, a mathematical model				states of development include:
can be used to estimate and compensate for these				* Beidou - China's regional system that China has
errors.				proposed to expand into a global system named
Because ionospheric delay affects the speed of				COMPASS.
microwave signals differently based on frequency-a				* Galileo - a proposed global system being developed
characteristic known as dispersion-both frequency				by the European Union, joined by China, Israel, India,
bands can be used to help reduce this error. Some				Morocco, Saudi Arabia and South Korea, Ukraine
military and expensive survey-grade civilian receivers				planned to be operational by 2011-12.
compare the different delays in the L1 and L2				* GLONASS - Russia's global system which is being
frequencies to measure atmospheric dispersion, and				restored to full availability in partnership with India.
apply a more precise correction. This can be done in				* Indian Regional Navigational Satellite System
civilian receivers without decrypting the P(Y) signal				(IRNSS) - India's proposed regional system.
carried on L2, by tracking the carrier wave instead of				* QZSS - Japanese proposed regional system, adding
the modulated code. To facilitate this on lower cost				better coverage to the Japanese islands.
receivers, a new civilian code signal on L2, called L2C,				[edit] See also
was added to the Block IIR-M satellites, which was				Satellite navigation systems Portal
first launched in 2005. It allows a direct comparison of				Nautical Portal
the L1 and L2 signals using the coded signal instead of				* RAIM
the carrier wave.				* SIGI
The effects of the ionosphere generally change				* radio navigation
slowly, and can be averaged over time. The effects				* High Sensitivity GPS
for any particular geographical area can be easily				* Degree Confluence Project Use GPS to visit integral
calculated by comparing the GPS-measured position				degrees of latitude and longitude.
to a known surveyed location. This correction is also				* Exif, GPS data transfer.
valid for other receivers in the same general location.				* Geotagging
Several systems send this information over radio or				* Geocaching
other links to allow L1 only receivers to make				* NaviTraveler.com, - a GPS point sharing community.
ionospheric corrections. The ionospheric data are				* GPS Drawing Digital mapping and drawing with GPS
transmitted via satellite in Satellite Based				tracks.
Augmentation Systems such as WAAS, which				* GPS tracking
transmits it on the GPS frequency using a special				* GPS/INS
pseudo-random number (PRN), so only one antenna				* Assisted GPS
and receiver are required.				* GPX (XML schema for interchange of waypoints)
Humidity also causes a variable delay, resulting in				* ID Sniper rifle
errors similar to ionospheric delay, but occurring in the				* OpenStreetMap, free content maps and street
troposphere. This effect is both more localized and				pictures (GFDL)
changes more quickly than ionospheric effects and is				* Telematics: Many telematics devices use GPS to
not frequency dependent. These traits making				determine the location of mobile equipment.
precise measurement and compensation of humidity				* The American Practical Navigator-Chapter 11
errors more difficult than ionospheric effects.				"Satellite Navigation"
Changes in altitude also change the amount of delay				* Point of Interest
due to the signal passing through less of the				* Automotive navigation system
atmosphere at higher elevations. Since the GPS				* NextGen
receiver computes its approximate altitude, this error				[edit] Notes
is relatively simple to correct.				1. ^ Parkinson, B.W. (1996), Global Positioning System:
[edit] Multipath effects				Theory and Applications, chap. 1: Introduction and
GPS signals can also be affected by multipath issues,				Heritage of NAVSTAR, the Global Positioning System.
where the radio signals reflect off surrounding terrain;				pp. 3-28, American Institute of Aeronautics and
buildings, canyon walls, hard ground, etc. These				Astronautics, Washington, D.C.
delayed signals can cause inaccuracy. A variety of				2. ^ a b GPS Overview from the NAVSTAR Joint
techniques, most notably narrow correlator spacing,				Program Office. Accessed December 15, 2006.
have been developed to mitigate multipath errors. For				3. ^ HowStuffWorks. How GPS Receivers Work.
long delay multipath, the receiver itself can recognize				Accessed May 14, 2006.
the wayward signal and discard it. To address shorter				4. ^ globalsecurity.org [1].
delay multipath from the signal reflecting off the				5. ^ Dana, Peter H. GPS Orbital Planes. August 8,
ground, specialized antennas may be used to reduce				1996.
the signal power as received by the antenna. Short				6. ^ What the Global Positioning System Tells Us
delay reflections are harder to filter out because they				about Relativity. Accessed January 2, 2007.
interfere with the true signal, causing effects almost				7. ^ USCG Navcen: GPS Frequently Asked Questions.
indistinguishable from routine fluctuations in				Accessed January 3, 2007.
atmospheric delay.				8. ^ Massatt, Paul and Brady, Wayne. "Optimizing
Multipath effects are much less severe in moving				performance through constellation management",
vehicles. When the GPS antenna is moving, the false				Crosslink, Summer 2002, pages 17-21.
solutions using reflected signals quickly fail to				9. ^ US Coast Guard General GPS News 9-9-05
converge and only the direct signals result in stable				10. ^ USNO. NAVSTAR Global Positioning System.
solutions.				Accessed May 14, 2006.
[edit] Ephemeris and clock errors				11. ^ NMEA NMEA 2000
The navigation message from a satellite is sent out				12. ^
only every 30 seconds. In reality, the data contained				13. ^ AN02 Network Assistance (HTML). Retrieved
in these messages tend to be "out of date" by an				on 2007-09-10.
even larger amount. Consider the case when a GPS				14. ^ a b Office of Science and Technology Policy.
satellite is boosted back into a proper orbit; for some				Presidential statement to stop degrading GPS. May 1,
time following the maneuver, the receiver's calculation				2000.
of the satellite's position will be incorrect until it				15. ^ FAA, Selective Availability. Retrieved Jan. 6,
receives another ephemeris update. The onboard				2007.
clocks are extremely accurate, but they do suffer				16. ^
from some clock drift. This problem tends to be very				17. ^ Rizos, Chris. University of New South Wales.
small, but may add up to 2 meters (6 ft) of				GPS Satellite Signals. 1999.
inaccuracy.				18. ^ The Global Positioning System by Robert A.
This class of error is more "stable" than ionospheric				Nelson Via Satellite, November 1999
problems and tends to change over days or weeks				19. ^ Ashby, Neil Relativity and GPS. Physics Today,
rather than minutes. This makes correction fairly				May 2002.
simple by sending out a more accurate almanac on a				20. ^ Space Environment Center. SEC Navigation
separate channel.				Systems GPS Page. August 26, 1996.
[edit] Selective availability				21. ^ The hunt for an unintentional GPS jammer. GPS
The GPS includes a feature called Selective Availability				World. January 1, 2003.
(SA) that introduces intentional, slowly changing				22. ^ Low Cost and Portable GPS Jammer. Phrack
random errors of up to a hundred meters (328 ft)				issue 0x3c (60), article 13]. Published December 28,
into the publicly available navigation signals to				2002.
confound, for example, guiding long range missiles to				23. ^ American Forces Press Service. CENTCOM
precise targets. Additional accuracy was available in				charts progress. March 25, 2003.
the signal, but in an encrypted form that was only				24. ^ [2]
available to the United States military, its allies and a				25. ^ Ruley, John. AVweb. GPS jamming. February 12,
few others, mostly government users.				2003.
SA typically added signal errors of up to about 10				26. ^ Commercial GPS Receivers: Facts for the
meters (32 ft) horizontally and 30 meters (98 ft)				Warfighter. Hosted at the Joint Chiefs website, linked
vertically. The inaccuracy of the civilian signal was				by the USAF's GPS Wing DAGR program website.
deliberately encoded so as not to change very				Accessed on 10 April, 2007
quickly, for instance the entire eastern U.S. area				27. ^ US Coast Guard news release. Global Positioning
might read 30 m off, but 30 m off everywhere and				System Fully Operational
in the same direction. To improve the usefulness of				28. ^ a b Hydrographic Society Journal. Developments
GPS for civilian navigation, Differential GPS was used				in Global Navigation Satellite Systems. Issue #104,
by many civilian GPS receivers to greatly improve				April 2002. Accessed April 5, 2007.
accuracy.				29. ^ XM982 Excalibur Precision Guided Extended
During the Gulf War, the shortage of military GPS				Range Artillery Projectile. GlobalSecurity.org
units and the wide availability of civilian ones among				(2007-05-29). Retrieved on 2007-09-26.
personnel resulted in a decision to disable Selective				30. ^ Sandia National Laboratory's Nonproliferation
Availability. This was ironic, as SA had been				programs and arms control technology.
introduced specifically for these situations, allowing				31. ^ Arms Control Association. Missile Technology
friendly troops to use the signal for accurate				Control Regime. Accessed May 17, 2006.
navigation, while at the same time denying it to the				32. ^ United States Department of Defense.
enemy. But since SA was also denying the same				Announcement of Initial Operational Capability.
accuracy to thousands of friendly troops, turning it				December 8, 1993.
off or setting it to an error of zero meters				33. ^ National Archives and Records Administration.
(effectively the same thing) presented a clear				U.S. GLOBAL POSITIONING SYSTEM POLICY. March
benefit.				29, 1996.
In the 1990s, the FAA started pressuring the military				34. ^ United States Naval Research Laboratory.
to turn off SA permanently. This would save the				National Medal of Technology for GPS. November 21,
FAA millions of dollars every year in maintenance of				2005
their own radio navigation systems. The military				[edit] External links
resisted for most of the 1990s, and it ultimately took				Wikimedia Commons has media related to:
an executive order to have SA removed from the				Global Positioning System
GPS signal. The amount of error added was "set to				Government links
zero"[14] at midnight on May 1, 2000 following an				* GPS.gov-General public education website created
announcement by U.S. President Bill Clinton, allowing				by the U.S. Government
users access to the error-free L1 signal. Per the				* National Space-Based PNT Executive
directive, the induced error of SA was changed to				Committee-Established in 2004 to oversee
add no error to the public signals (C/A code).				management of GPS and GPS augmentations at a
Selective Availability is still a system capability of GPS,				national level.
and error could, in theory, be reintroduced at any				* USCG Navigation Center-Status of the GPS
time. In practice, in view of the hazards and costs				constellation, government policy, and links to other
this would induce for US and foreign shipping, it is				references. Also includes satellite almanac data.
unlikely to be reintroduced, and various government				* The GPS Joint Program Office (GPS
agencies, including the FAA,[15] have stated that it is				JPO)-Responsible for designing and acquiring the
not intended to be reintroduced.				system on behalf of the US Government.
The US military has developed the ability to locally				* U.S. Naval Observatory's GPS constellation status
deny GPS (and other navigation services) to hostile				* U.S. Army Corps of Engineers manual: NAVSTAR
forces in a specific area of crisis without affecting				HTML and PDF (22.6 MB, 328 pages)
the rest of the world or its own military systems.[14]				* PNT Selective Availability Announcements
One interesting side effect of the Selective				* GPS SPS Signal Specification, 2nd Edition-The official
Availability hardware is the capability to correct the				Standard Positioning Signal specification.
frequency of the GPS caesium and rubidium atomic				* Federal Aviation Administration's GPS FAQ
clocks to an accuracy of approximately 2 Ãƒ-				Introductory / tutorial links
10-13 (one in five trillion). This represented a				* How does GPS work? TomTom explains GPS,
significant improvement over the raw accuracy of				navigation, and digital maps
the clocks.[citation needed]				* GPS Academy Garmin interactive video web site
On 19 September 2007, the United States				explaing what exactly GPS is and what it can do for
Department of Defense announced that they would				you
not procure any more satellites capable of				* HowStuffWorks' Simplified explanation of GPS and
implementing SA. [16]				video about how GPS works.
[edit] Relativity				* Trimble's Online GPS Tutorial Tutorial designed to
According to the theory of relativity, due to their				introduce you to the principles behind GPS
constant movement and height relative to the				* GPS and GLONASS Simulation(Java applet)
Earth-centered inertial reference frame, the clocks on				Simulation and graphical depiction of space vehicle
the satellites are affected by their speed (special				motion including computation of dilution of precision
relativity) as well as their gravitational potential				(DOP)
(general relativity). For the GPS satellites, general				Technical, historical, and ancillary topics links
relativity predicts that the atomic clocks at GPS				* Dana, Peter H. "Global Positioning System
orbital altitudes will tick more rapidly, by about 45,900				Overview"
nanoseconds (ns) per day, because they are in a				* Satellite Navigation: GPS & Galileo (PDF)-16-page
weaker gravitational field than atomic clocks on				paper about the history and working of GPS,
Earth's surface. Special relativity predicts that atomic				touching on the upcoming Galileo
clocks moving at GPS orbital speeds will tick more				* History of GPS, including information about each
slowly than stationary ground clocks by about 7,200				satellite's configuration and launch.
ns per day. When combined, the discrepancy is 38				* Chadha, Kanwar. "The Global Positioning System:
microseconds per day; a difference of 4.465 parts in				Challenges in Bringing GPS to Mainstream Consumers"
1010.[17]. To account for this, the frequency standard				Technical Article (1998)
onboard each satellite is given a rate offset prior to				* GPS Weapon Guidance Techniques
launch, making it run slightly slower than the desired				* RAND history of the GPS system (PDF)
frequency on Earth; specifically, at 10.22999999543				* GPS Anti-Jam Protection Techniques
MHz instead of 10.23 MHz.[18]				* Crosslink Summer 2002 issue by The Aerospace
GPS observation processing must also compensate				Corporation on satellite navigation.
for another relativistic effect, the Sagnac effect. The				* Improved weather predictions from COSMIC GPS
GPS time scale is defined in an inertial system but				satellite signal occultation data.
observations are processed in an Earth-centered,				* David L. Wilson's GPS Accuracy Web Page A
Earth-fixed (co-rotating) system, a system in which				thorough analysis of the accuracy of GPS.
simultaneity is not uniquely defined. The Lorentz				* Innovation: Spacecraft Navigator, Autonomous GPS
transformation between the two systems modifies				Positioning at High Earth Orbits Example of GPS
the signal run time, a correction having opposite				receiver designed for high altitude spaceflight.
algebraic signs for satellites in the Eastern and				* The Navigator GPS Receiver GSFC's Navigator
Western celestial hemispheres. Ignoring this effect will				spaceflight receiver.
produce an east-west error on the order of hundreds				* Neil Ashby's Relativity in the Global Positioning
of nanoseconds, or tens of meters in position.[19]				System
The atomic clocks on board the GPS satellites are				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢
precisely tuned, making the system a practical				e
engineering application of the scientific theory of				Satellite navigation systems
relativity in a real-world environment.				Historical Flag of the United States Transit
[edit] GPS interference and jamming				Operational Flag of the Soviet Union / Flag of Russia
Since GPS signals at terrestrial receivers tend to be				GLONASS Ã‚Â· Flag of the United States
relatively weak, it is easy for other sources of				GPS
electromagnetic radiation to desensitize the receiver,				Developmental Flag of the People's Republic of China
making acquiring and tracking the satellite signals				Beidou/COMPASS Ã‚Â· Flag of Europe
difficult or impossible.				Galileo Ã‚Â· Flag of India IRNSS
Solar flares are one such naturally occurring emission				Ã‚Â· Flag of Japan QZSS
with the potential to degrade GPS reception, and				Related topics EGNOS Ã‚Â· GAGAN
their impact can affect reception over the half of the				Ã‚Â· GPSÃ‚Â·C Ã‚Â·
Earth facing the sun. GPS signals can also be				LAAS Ã‚Â· MSAS Ã‚Â· WAAS
interfered with by naturally occurring geomagnetic				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢
storms, predominantly found near the poles of the				e
Earth's magnetic field.[20] Another source of				Time signal stations
problems is the metal embedded in some car				Longwave DCF77 Ã‚Â· HBG Ã‚Â·
windscreens to prevent icing, degrading reception				JJY Ã‚Â· MSF Ã‚Â· TDF
just inside the car.				Ã‚Â· WWVB
Man-made interference can also disrupt, or jam, GPS				Shortwave BPM Ã‚Â· CHU Ã‚Â·
signals. In one well documented case, an entire harbor				RWM Ã‚Â· WWV Ã‚Â· WWVH
was unable to receive GPS signals due to				Ã‚Â· YVTO
unintentional jamming caused by a malfunctioning TV				GNSS time transfer Beidou Ã‚Â· Galileo
antenna preamplifier.[21] Intentional jamming is also				Ã‚Â· GLONASS Ã‚Â· GPS
possible. Generally, stronger signals can interfere with				Ã‚Â· IRNSS
GPS receivers when they are within radio range, or				Defunct time stations OMA Ã‚Â· VNG
line of sight. In 2002, a detailed description of how to				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢
build a short range GPS L1 C/A jammer was				e
published in the online magazine Phrack.[22]				Global structure in Systems, Systems sciences and
The U.S. government believes that such jammers				Systems scientists
were used occasionally during the 2001 war in				Categories Category:Conceptual systems
Afghanistan and the U.S. military claimed to destroy a				Ã‚Â· Category:Physical systems
GPS jammer with a GPS-guided bomb during the Iraq				Ã‚Â· Category:Social systems
War.[23] Such a jammer is relatively easy to detect				Ã‚Â· Category:Systems Ã‚Â·
and locate, making it an attractive target for				Category:Systems science Ã‚Â·
anti-radiation missiles. The UK Ministry of Defence				Category:Systems scientists Ã‚Â·
tested a jamming system in the UK's West Country				Category:Systems theory
on 7 and 8 June 2007. [24]				Systems Biological system Ã‚Â· Complex
Some countries allow the use of GPS repeaters to				system Ã‚Â· Complex adaptive system
allow for the reception of GPS signals indoors and in				Ã‚Â· Conceptual system Ã‚Â·
obscured locations, however, under EU and UK laws,				Cultural system Ã‚Â· Dynamical system
the use of these is prohibited as the signals can				Ã‚Â· Economic system Ã‚Â·
cause interference to other GPS receivers that may				Ecosystem Ã‚Â· Formal system
receive data from both GPS satellites and the				Ã‚Â· Global Positioning System
repeater.				Ã‚Â· Human organ systems Ã‚Â·
Due to the potential for both natural and man-made				Information systems Ã‚Â· Legal system
noise, numerous techniques continue to be developed				Ã‚Â· Metric system Ã‚Â· Nervous
to deal with the interference. The first is to not rely				system Ã‚Â· Non-linear system
on GPS as a sole source. According to John Ruley,				Ã‚Â· Operating system Ã‚Â·
"IFR pilots should have a fallback plan in case of a				Physical system Ã‚Â· Political system
GPS malfunction".[25] Receiver Autonomous Integrity				Ã‚Â· Sensory system Ã‚Â· Social
Monitoring (RAIM) is a feature now included in some				system Ã‚Â· Solar System Ã‚Â·
receivers, which is designed to provide a warning to				System Ã‚Â· Systems of measurement
the user if jamming or another problem is detected.				Fields of theory Chaos theory Ã‚Â·
The U.S. military has also deployed their Selective				Complex systems Ã‚Â· Control theory
Availability / Anti-Spoofing Module (SAASM) in the				Ã‚Â· Cybernetics Ã‚Â· Holism in
Defense Advanced GPS Receiver (DAGR). In				science Ã‚Â· Sociotechnical systems theory
demonstration videos, the DAGR is able to detect				Ã‚Â· Systems biology Ã‚Â·
jamming and maintain its lock on the encrypted GPS				System dynamics Ã‚Â· Systems ecology
signals during interference which causes civilian				Ã‚Â· Systems engineering Ã‚Â·
receivers to lose lock.[26]				Systems theory Ã‚Â· Systems science
[edit] Techniques to improve accuracy				Systems scientists Russell L. Ackoff Ã‚Â·
[edit] Augmentation				William Ross Ashby Ã‚Â· Gregory Bateson
Main article: GNSS Augmentation				Ã‚Â· Ludwig von Bertalanffy Ã‚Â·
Augmentation methods of improving accuracy rely on				Kenneth E. Boulding Ã‚Â· Peter Checkland
external information being integrated into the				Ã‚Â· C. West Churchman Ã‚Â·
calculation process. There are many such systems in				Heinz von Foerster Ã‚Â· Charles
place and they are generally named or described				FranÃƒÂ§ois Ã‚Â· Jay Wright
based on how the GPS sensor receives the				Forrester Ã‚Â· Ralph W. Gerard
information. Some systems transmit additional				Ã‚Â· Debora Hammond Ã‚Â·
information about sources of error (such as clock				George Klir Ã‚Â· Niklas Luhmann
drift, ephemeris, or ionospheric delay), others provide				Ã‚Â· Humberto Maturana Ã‚Â·
direct measurements of how much the signal was				Donella Meadows Ã‚Â· Mihajlo D. Mesarovic
off in the past, while a third group provide additional				Ã‚Â· Howard T. Odum Ã‚Â·
navigational or vehicle information to be integrated in				Talcott Parsons Ã‚Â· Ilya Prigogine
the calculation process.				Ã‚Â· Anatol Rapoport Ã‚Â·
Examples of augmentation systems include the Wide				Francisco Varela Ã‚Â· John N.
Area Augmentation System, Differential GPS, Inertial

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