About GPS

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

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