About GPS

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About GPS

Global Positioning System				equipment. In the future, additional civilian
				codes are expected to be transmitted on the
The Global Positioning System (GPS) is the				L2 and L5 frequencies (see GPS modernization,
only fully functional Global Navigation				below). Then all users will be able to
Satellite System (GNSS). Utilizing a				perform dual-frequency measurements and
constellation of at least 24 medium Earth				directly compute ionospheric delay errors.
orbit satellites that transmit precise
microwave signals, the system enables a GPS				A second form of precise monitoring is called
receiver to determine its location, speed				Carrier-Phase Enhancement (CPGPS). The error,
direction, and time.				which this corrects, arises because the pulse
				transition of the PRN is not instantaneous,
Developed by the United States Department of				and thus the correlation (satellite-receiver
Defense, it is officially named NAVSTAR GPS				sequence matching) operation is imperfect.
(Contrary to popular belief, NAVSTAR is not				The CPGPS approach utilizes the L1 carrier
an acronym, but simply a name given by Mr.				wave, which has a period 1000 times smaller
John Walsh, a key decision maker when it came				than that of the C/A bit period, to act as an
to the budget for the GPS program[1]). The				additional clock signal and resolve the
satellite constellation is managed by the				uncertainty. The phase difference error in
United States Air Force 50th Space Wing. The				the normal GPS amounts to between 2 and 3
cost of maintaining the system is				meters (6 to 10 ft) of ambiguity. CPGPS
approximately US$750 million per year,[2]				working to within 1% of perfect transition
including the replacement of aging				reduces this error to 3 centimeters (1 inch)
satellites, and research and development.				of ambiguity. By eliminating this source of
Despite these costs, GPS is free for civilian				error, CPGPS coupled with DGPS normally
use as a public good.				realizes between 20 and 30 centimeters (8 to
				12 inches) of absolute accuracy.
GPS has become a widely used aid to
navigation worldwide, and a useful tool for				Relative Kinematic Positioning (RKP) is
map-making, land surveying, commerce, and				another approach for a precise GPS-based
scientific uses. GPS also provides a precise				positioning system. In this approach,
time reference used in many applications				determination of range signal can be resolved
including scientific study of earthquakes,				to an accuracy of less than 10 centimeters (4
and synchronization of telecommunications				in). This is done by resolving the number of
networks.				cycles in which the signal is transmitted and
				received by the receiver. This can be
Simplified method of operation				accomplished by using a combination of
				differential GPS (DGPS) correction data,
A GPS receiver calculates its position by				transmitting GPS signal phase information and
measuring the distance between itself and				ambiguity resolution techniques via
three or more GPS satellites. Measuring the				statistical tests-possibly with processing in
time delay between transmission and reception				real-time (real-time kinematic positioning,
of each GPS microwave signal gives the				RTK).
distance to each satellite, since the signal
travels at a known speed - the speed of				[edit] GPS time and date
light. These signals also carry information
about the satellites' location and general				While most clocks are synchronized to
system health (known as almanac and ephemeris				Coordinated Universal Time (UTC), the Atomic
data). By determining the position of, and				clocks on the satellites are set to GPS time.
distance to, at least three satellites, the				The difference is that GPS time is not
receiver can compute its position using				corrected to match the rotation of the Earth,
trilateration.[3] Receivers typically do not				so it does not contain leap seconds or other
have perfectly accurate clocks and therefore				corrections which are periodically added to
track one or more additional satellites,				UTC. GPS time was set to match Coordinated
using their atomic clocks to correct the				Universal Time (UTC) in 1980, but has since
receiver's own clock error.				diverged. The lack of corrections means that
				GPS time remains at a constant offset (19
[edit] Technical description				seconds) with International Atomic Time
				(TAI). Periodic corrections are performed on
Unlaunched GPS satellite on display at the				the on-board clocks to correct relativistic
San Diego Aerospace museum				effects and keep them synchronized with
				ground clocks.
Unlaunched GPS satellite on display at the
San Diego Aerospace museum				The GPS navigation message includes the
				difference between GPS time and UTC, which as
[edit] System segmentation				of 2006 is 14 seconds. Receivers subtract
				this offset from GPS time to calculate UTC
The current GPS consists of three major				and specific timezone values. New GPS units
segments. These are the space segment (SS), a				may not show the correct UTC time until after
control segment (CS), and a user segment				receiving the UTC offset message. The GPS-UTC
(US).[4]				offset field can accommodate 255 leap seconds
				(eight bits) which, at the current rate of
[edit] Space segment				change of the Earth's rotation, is sufficient
				to last until the year 2330.
The space segment (SS) is composed of the
orbiting GPS satellites, or Space Vehicles				As opposed to the year, month, and day format
(SV) in GPS parlance. The GPS design calls				of the Julian calendar, the GPS date is
for 24 SVs to be distributed equally among				expressed as a week number and a day-of-week
six circular orbital planes.[5] The orbital				number. The week number is transmitted as a
planes are centered on the Earth, not				ten-bit field in the C/A and P(Y) navigation
rotating with respect to the distant				messages, and so it becomes zero again every
stars.[6] The six planes have approximately				1,024 weeks (19.6 years). GPS week zero
55Ã‚Â° inclination (tilt relative to				started at 00:00:00 UTC (00:00:19 TAI) on
Earth's equator) and are separated by				January 6, 1980 and the week number became
60Ã‚Â° right ascension of the ascending				zero again for the first time at 23:59:47 UTC
node (angle along the equator from a				on August 21, 1999 (00:00:19 TAI on August
reference point to the orbit's				22, 1999). To determine the current Gregorian
intersection).[2]				date, a GPS receiver must be provided with
				the approximate date (to within 3,584 days)
Orbiting at an altitude of approximately				to correctly translate the GPS date signal.
20,200 kilometers (12,600 miles or 10,900				To address this concern the modernized GPS
nautical miles; orbital radius of 26,600 km				navigation messages use a 13-bit field, which
(16,500 mi or 14,400 NM)), each SV makes two				only repeats every 8,192 weeks (157 years),
complete orbits each sidereal day, so it				and will not return to zero until near the
passes over the same location on Earth once				year 2137.
each day. The orbits are arranged so that at
least six satellites are always within line				[edit] GPS modernization
of sight from almost everywhere on Earth's
surface.[7]				Main article: GPS modernization

As of September 2007, there are 31 actively				Having reached the program's requirements for
broadcasting satellites in the GPS				Full Operational Capability (FOC) on July 17,
constellation. The additional satellites				1995,[27] the GPS completed its original
improve the precision of GPS receiver				design goals. However, additional advances in
calculations by providing redundant				technology and new demands on the existing
measurements. With the increased number of				system led to the effort to modernize the GPS
satellites, the constellation was changed to				system. Announcements from the Vice President
a nonuniform arrangement. Such an arrangement				and the White House in 1998 initiated these
was shown to improve reliability and				changes, and in 2000 the U.S. Congress
availability of the system, relative to a				authorized the effort, referring to it as GPS
uniform system, when multiple satellites				III.
fail.[8]
				The project aims to improve the accuracy and
[edit] Control segment				availability for all users and involves new
				ground stations, new satellites, and four
The flight paths of the satellites are				additional navigation signals. New civilian
tracked by US Air Force monitoring stations				signals are called L2C, L5 and L1C; the new
in Hawaii, Kwajalein, Ascension Island, Diego				military code is called M-Code. Initial
Garcia, and Colorado Springs, Colorado, along				Operational Capability (IOC) of the L2C code
with monitor stations operated by the				is expected in 2008.[28] A goal of 2013 has
National Geospatial-Intelligence Agency				been established for the entire program, with
(NGA).[9] The tracking information is sent to				incentives offered to the contractors if they
the Air Force Space Command's master control				can complete it by 2011.
station at Schriever Air Force Base in
Colorado Springs, which is operated by the 2d				[edit] Applications
Space Operations Squadron (2 SOPS) of the
United States Air Force (USAF). 2 SOPS				The Global Positioning System, while
contacts each GPS satellite regularly with a				originally a military project, is considered
navigational update (using the ground				a dual-use technology, meaning it has
antennas at Ascension Island, Diego Garcia,				significant applications for both the
Kwajalein, and Colorado Springs). These				military and the civilian industry.
updates synchronize the atomic clocks on
board the satellites to within one				[edit] Military
microsecond and adjust the ephemeris of each
satellite's internal orbital model. The				Please help improve this article by expanding
updates are created by a Kalman filter which				this section.
uses inputs from the ground monitoring
stations, space weather information, and				See talk page for details. Please remove this
various other inputs.[10]				message once the section has been expanded.

GPS receivers come in a variety of formats,				The military use GPS for the following
from devices integrated into cars, phones,				purposes:
and watches, to dedicated devices such as
those shown here from manufacturers Trimble,				[edit] Navigation
Garmin and Leica (left to right).
				GPS allows soldiers to find objectives in the
GPS receivers come in a variety of formats,				dark or in unfamiliar territory, and to
from devices integrated into cars, phones,				coordinate the movement of troops and
and watches, to dedicated devices such as				supplies.
those shown here from manufacturers Trimble,
Garmin and Leica (left to right).				[edit] Target tracking

[edit] User segment				Various military weapons systems use GPS to
				track potential ground and air targets before
The user's GPS receiver is the user segment				they are flagged as hostile. These weapons
(US) of the GPS system. In general, GPS				systems pass GPS co-ordinates of targets to
receivers are composed of an antenna, tuned				precision-guided munitions to allow them to
to the frequencies transmitted by the				engage the targets accurately.
satellites, receiver-processors, and a
highly-stable clock (often a crystal				Military aircraft, particularly those used in
oscillator). They may also include a display				air-to-ground roles use GPS to find targets
for providing location and speed information				(for example, gun camera video from AH-1
to the user. A receiver is often described by				Cobras in Iraq show GPS co-ordinates that can
its number of channels: this signifies how				be looked up in Google Earth).
many satellites it can monitor
simultaneously. Originally limited to four or				[edit] Missile and projectile guidance
five, this has progressively increased over
the years so that, as of 2006, receivers				GPS allows accurate targeting of various
typically have between twelve and twenty				military weapons including ICBMs, cruise
channels.				missiles and precision-guided munitions.

A typical OEM GPS receiver module, based on				Artillery projectiles with embedded GPS
the SiRF Star III chipset, measuring				receivers able to withstand forces of 12,000G
15Ãƒ-17 mm, and used in many products.				have been developed for use in 155 mm
				howitzers.[29]
A typical OEM GPS receiver module, based on
the SiRF Star III chipset, measuring				[edit] Search and Rescue
15Ãƒ-17 mm, and used in many products.
				Downed pilots can be located faster if they
GPS receivers may include an input for				have a GPS receiver.
differential corrections, using the RTCM
SC-104 format. This is typically in the form				[edit] Reconnaissance and Map Creation
of a RS-232 port at 4,800 bit/s speed. Data
are actually sent at a much lower rate, which				The military use GPS extensively to aid
limits the accuracy of the signal sent using				mapping and reconnaissance.
RTCM. Receivers with internal DGPS receivers
can outperform those using external RTCM				[edit] Other
data. As of 2006, even low-cost units
commonly include Wide Area Augmentation				The GPS satellites also carry nuclear
System (WAAS) receivers.				detonation detectors, which form a major
				portion of the United States Nuclear
Many GPS receivers can relay position data to				Detonation Detection System.[30]
a PC or other device using the NMEA 0183
protocol. NMEA 2000[11] is a newer and less				[edit] Civilian
widely adopted protocol. Both are proprietary
and controlled by the US-based National				See also: GPS applications
Marine Electronics Association. References to
the NMEA protocols have been compiled from				This antenna is mounted on the roof of a hut
public records, allowing open source tools				containing a scientific experiment needing
like gpsd to read the protocol without				precise timing.
violating intellectual property laws. Other
proprietary protocols exist as well, such as				This antenna is mounted on the roof of a hut
the SiRF and MTK protocols. Receivers can				containing a scientific experiment needing
interface with other devices using methods				precise timing.
including a serial connection, USB or
Bluetooth.				Many civilian applications benefit from GPS
				signals, using one or more of three basic
[edit] Navigation signals				components of the GPS; absolute location,
				relative movement, time transfer.
Main article: GPS signals
				The ability to determine the receiver's
GPS broadcast signal				absolute location allows GPS receivers to
				perform as a surveying tool or as an aid to
GPS broadcast signal				navigation. The capacity to determine
				relative movement enables a receiver to
Each GPS satellite continuously broadcasts a				calculate local velocity and orientation,
Navigation Message at 50 bit/s giving the				useful in vessels or observations of the
time-of-day, GPS week number and satellite				Earth. Being able to synchronize clocks to
health information (all transmitted in the				exacting standards enables time transfer,
first part of the message), an ephemeris				which is critical in large communication and
(transmitted in the second part of the				observation systems. An example is CDMA
message) and an almanac (later part of the				digital cellular. Each base station has a GPS
message). The ephemeris data gives the				timing receiver to synchronize its spreading
satellite's own precise orbit and is output				codes with other base stations to facilitate
over 18 seconds, repeating every 30 seconds.				inter-cell hand off and support hybrid GPS
The ephemeris is updated every 2 hours and is				CDMA positioning of mobiles for emergency
generally valid for 4 hours, with provisions				calls and other applications.
for 6 hour time-outs. The time needed to
acquire the ephemeris is becoming a				Finally, GPS enables researchers to explore
significant element of the delay to first				the Earth environment including the
position fix, because, as the hardware				atmosphere, ionosphere and gravity field. GPS
becomes more capable, the time to lock onto				survey equipment has revolutionized tectonics
the satellite signals shrinks, but the				by directly measuring the motion of faults in
ephemeris data requires 30 seconds (worst				earthquakes.
case) before it is received, due to the low
data transmission rate. The almanac consists				To help prevent civilian GPS guidance from
of coarse orbit and status information for				being used in an enemy's military or
each satellite in the constellation and takes				improvised weaponry, the US Government
12 seconds for each satellite present, with				controls the export of civilian receivers. A
information for a new satellite being				US-based manufacturer cannot generally export
transmitted every 30 seconds (15.5 minutes				a GPS receiver unless the receiver contains
for 31 satellites). The purpose of the data				limits restricting it from functioning when
is to assist in the acquisition of satellites				it is simultaneously (1) at an altitude above
at power-up by allowing the receiver to				18 kilometers (60,000 ft) and (2) traveling
generate a list of visible satellites based				at over 515 m/s (1,000 knots).[31]
on stored position and time, while an
ephemeris from each satellite is needed to				[edit] History
compute position fixes using that satellite.
In older hardware, lack of an almanac in a				Please help improve this article by expanding
new receiver would cause long delays before				this section.
providing a valid position, because the
search for each satellite was a slow process.				See talk page for details. Please remove this
Advances in hardware have made the				message once the section has been expanded.
acquisition process much faster, so not
having an almanac is no longer an issue. An				The design of GPS is based partly on the
important thing to note about navigation data				similar ground-based radio navigation
is that each satellite transmits only its own				systems, such as LORAN and the Decca
ephemeris, but transmits an almanac for all				Navigator developed in the early 1940s, and
satellites.				used during World War II. Additional
				inspiration for the GPS system came when the
Each satellite transmits its navigation				Soviet Union launched the first Sputnik in
message with at least two distinct spread				1957. A team of U.S. scientists led by Dr.
spectrum codes: the Coarse / Acquisition (C				Richard B. Kershner were monitoring Sputnik's
A) code, which is freely available to the				radio transmissions. They discovered that,
public, and the Precise (P) code, which is				because of the Doppler effect, the frequency
usually encrypted and reserved for military				of the signal being transmitted by Sputnik
applications. The C/A code is a 1,023 chip				was higher as the satellite approached, and
pseudo-random (PRN) code at 1.023 million				lower as it continued away from them. They
chips/sec so that it repeats every				realized that since they knew their exact
millisecond. Each satellite has its own C/A				location on the globe, they could pinpoint
code so that it can be uniquely identified				where the satellite was along its orbit by
and received separately from the other				measuring the Doppler distortion.
satellites transmitting on the same
frequency. The P-code is a 10.23 megachip/sec				The first satellite navigation system,
PRN code that repeats only every week. When				Transit, used by the United States Navy, was
the "anti-spoofing" mode is on, as it is in				first successfully tested in 1960. Using a
normal operation, the P code is encrypted by				constellation of five satellites, it could
the Y-code to produce the P(Y) code, which				provide a navigational fix approximately once
can only be decrypted by units with a valid				per hour. In 1967, the U.S. Navy developed
decryption key. Both the C/A and P(Y) codes				the Timation satellite which proved the
impart the precise time-of-day to the user.				ability to place accurate clocks in space, a
Frequencies used by GPS include				technology the GPS system relies upon. In the
				1970s, the ground-based Omega Navigation
* L1 (1575.42 MHz): Mix of Navigation				System, based on signal phase comparison,
Message, coarse-acquisition (C/A) code and				became the first world-wide radio navigation
encrypted precision P(Y) code, plus the new				system.
L1C on future Block III satellites.
				The first experimental Block-I GPS satellite
* L2 (1227.60 MHz): P(Y) code, plus the new				was launched in February 1978.[28] The GPS
L2C code on the Block IIR-M and newer				satellites were initially manufactured by
satellites.				Rockwell International and are now
				manufactured by Lockheed Martin.
* L3 (1381.05 MHz): Used by the Nuclear
Detonation (NUDET) Detection System Payload				[edit] Timeline
(NDS) to signal detection of nuclear
detonations and other high-energy infrared				* In 1972, the US Air Force Central Inertial
events. Used to enforce nuclear test ban				Guidance Test Facility (Holloman AFB)
treaties.				conducted developmental fight tests of two
				prototype GPS receivers over White Sands
* L4 (1379.913 MHz): Being studied for				Missile Range, using ground-based
additional ionospheric correction.				pseudo-satellites.

* L5 (1176.45 MHz): Proposed for use as a				* In 1978 the first experimental Block-I GPS
civilian safety-of-life (SoL) signal (see GPS				satellite was launched.
modernization). This frequency falls into an
internationally protected range for				* In 1983, after Soviet interceptor aircraft
aeronautical navigation, promising little or				shot down the civilian airliner KAL 007 in
no interference under all circumstances. The				restricted Soviet airspace, killing all 269
first Block IIF satellite that would provide				people on board, U.S. President Ronald Reagan
this signal is set to be launched in 2008.				announced that the GPS system would be made
				available for civilian uses once it was
[edit] Calculating positions				completed.

[edit] Using the C/A code				* By 1985, ten more experimental Block-I
				satellites had been launched to validate the
To start off, the receiver picks which C/A				concept.
codes to listen for by PRN number, based on
the almanac information it has previously				* On February 14, 1989, the first modern
acquired. As it detects each satellite's				Block-II satellite was launched.
signal, it identifies it by its distinct C/A
code pattern, then measures the time delay				* In 1992, the 2nd Space Wing, which
for each satellite. To do this, the receiver				originally managed the system, was
produces an identical C/A sequence using the				de-activated and replaced by the 50th Space
same seed number as the satellite. By lining				Wing.
up the two sequences, the receiver can
measure the delay and calculate the distance				* By December 1993 the GPS system achieved
to the satellite, called the pseudorange[12].				initial operational capability[32]

Overlapping pseudoranges, represented as				* By January 17, 1994 a complete
curves, are modified to yield the probable				constellation of 24 satellites was in orbit.
position
				* Full Operational Capability was declared by
Overlapping pseudoranges, represented as				NAVSTAR in April 1995.
curves, are modified to yield the probable
position				* In 1996, recognizing the importance of GPS
				to civilian users as well as military users,
Next, the orbital position data, or				U.S. President Bill Clinton issued a policy
ephemeris, from the Navigation Message is				directive[33] declaring GPS to be a dual-use
then downloaded to calculate the satellite's				system and establishing an Interagency GPS
precise position. A more-sensitive receiver				Executive Board to manage it as a national
will potentially acquire the ephemeris data				asset.
quicker than a less-sensitive receiver,
especially in a noisy environment.[13]				* In 1998, U.S. Vice President Al Gore
Knowing the position and the distance of a				announced plans to upgrade GPS with two new
satellite indicates that the receiver is				civilian signals for enhanced user accuracy
located somewhere on the surface of an				and reliability, particularly with respect to
imaginary sphere centered on that satellite				aviation safety.
and whose radius is the distance to it.
Receivers can substitute altitude for one				* On May 2, 2000 "Selective Availability" was
satellite, which the GPS receiver translates				discontinued as a result of the 1996
to a pseudorange measured from the center of				executive order, allowing users to receive a
the earth.				non-degraded signal globally.

Locations are calculated not in				* In 2004, the United States Government
three-dimensional space, but in				signed a historic agreement with the European
four-dimensional spacetime, meaning a measure				Community establishing cooperation related to
of the precise time-of-day is very important.				GPS and Europe's planned Galileo system.
The measured pseudoranges from four
satellites have already been determined with				* In 2004, U.S. President George W. Bush
the receiver's internal clock, and thus have				updated the national policy, replacing the
an unknown amount of clock error. (The clock				executive board with the National Space-Based
error or actual time does not matter in the				Positioning, Navigation, and Timing Executive
initial pseudorange calculation, because that				Committee.
is based on how much time has passed between
reception of each of the				* November 2004, QUALCOMM announced
signals.[clarify][citation needed]) The				successful tests of Assisted-GPS system for
four-dimensional point that is equidistant				mobile phones.[3]
from the pseudoranges is calculated as a
guess as to the receiver's location, and the				* In 2005, the first modernized GPS satellite
factor used to adjust those pseudoranges to				was launched and began transmitting a second
intersect at that four-dimensional point				civilian signal (L2C) for enhanced user
gives a guess as to the receiver's clock				performance.
offset. With each guess, a geometric dilution
of precision (GDOP) vector is calculated,				* The most recent launch was on 17 November
based on the relative sky positions of the				2006. The oldest GPS satellite still in
satellites used. As more satellites are				operation was launched in August 1991.
picked up, pseudoranges from more
combinations of four satellites can be				* On September 14, 2007, the aging
processed to add more guesses to the location				mainframe-based Ground Segment Control System
and clock offset. The receiver then				was transitioned to the new Architecture
determines which combinations to use and how				Evolution Plan. [4]
to calculate the estimated position by
determining the weighted average of these				[edit] Satellite numbers
positions and clock offsets. After the final
location and time are calculated, the				Name Launch Period No of satellites
location is expressed in a specific				launched, inc. launch failures Currently in
coordinate system, e.g. latitude/longitude,				service
using the WGS 84 geodetic datum or a local
system specific to a country.				Block I 1978-1985 11 0

[edit] Using the P(Y) code				Block II 1985-1990 9 0

Calculating a position with the P(Y) signal				Block IIA 1990-1997 19 15+11
is generally similar in concept, assuming one
can decrypt it. The encryption is essentially				Block IIR 1997-2004 12 12
a safety mechanism: if a signal can be
successfully decrypted, it is reasonable to				Block IIR-M 2005- 3 3
assume it is a real signal being sent by a
GPS satellite.[citation needed] In				Total 54 (plus one not launched) 30+1
comparison, civil receivers are highly
vulnerable to spoofing since correctly				1One test satellite
formatted C/A signals can be generated using
readily available signal generators. RAIM				[edit] Awards
features do not protect against spoofing,
since RAIM only checks the signals from a				Two GPS developers have received the National
navigational perspective.				Academy of Engineering Charles Stark Draper
				prize year 2003:
[edit] Accuracy and error sources
				* Ivan Getting, emeritus president of The
The position calculated by a GPS receiver				Aerospace Corporation and engineer at the
requires the current time, the position of				Massachusetts Institute of Technology,
the satellite and the measured delay of the				established the basis for GPS, improving on
received signal. The position accuracy is				the World War II land-based radio system
primarily dependent on the satellite position				called LORAN (Long-range Radio Aid to
and signal delay.				Navigation).

To measure the delay, the receiver compares				* Bradford Parkinson, professor of
the bit sequence received from the satellite				aeronautics and astronautics at Stanford
with an internally generated version. By				University, conceived the present
comparing the rising and trailing edges of				satellite-based system in the early 1960s and
the bit transitions, modern electronics can				developed it in conjunction with the U.S. Air
measure signal offset to within about 1% of a				Force.
bit time, or approximately 10 nanoseconds for
the C/A code. Since GPS signals propagate				One GPS developer, Roger L. Easton, received
nearly at the speed of light, this represents				the National Medal of Technology on February
an error of about 3 meters. This is the				13, 2006 at the White House.[34]
minimum error possible using only the GPS C/A
signal.				On February 10, 1993, the National Aeronautic
				Association selected the Global Positioning
Position accuracy can be improved by using				System Team as winners of the 1992 Robert J.
the higher-chiprate P(Y) signal. Assuming the				Collier Trophy, the most prestigious aviation
same 1% bit time accuracy, the high frequency				award in the United States. This team
P(Y) signal results in an accuracy of about				consists of researchers from the Naval
30 centimeters.				Research Laboratory, the U.S. Air Force, the
				Aerospace Corporation, Rockwell International
Electronics errors are one of several				Corporation, and IBM Federal Systems Company.
accuracy-degrading effects outlined in the				The citation accompanying the presentation of
table below. When taken together, autonomous				the trophy honors the GPS Team "for the most
civilian GPS horizontal position fixes are				significant development for safe and
typically accurate to about 15 meters (50				efficient navigation and surveillance of air
ft). These effects also reduce the more				and spacecraft since the introduction of
precise P(Y) code's accuracy.				radio navigation 50 years ago."

Sources of User Equivalent Range Errors				[edit] Other systems
(UERE) Source Effect
				Main article: Global Navigation Satellite
Ionospheric effects Ã‚Â± 5 meter				System

Ephemeris errors Ã‚Â± 2.5 meter				Other satellite navigation systems in use or
				various states of development include:
Satellite clock errors Ã‚Â± 2 meter
				* Beidou - China's regional system that China
Multipath distortion Ã‚Â± 1 meter				has proposed to expand into a global system
				named COMPASS.
Tropospheric effects Ã‚Â± 0.5 meter
				* Galileo - a proposed global system being
Numerical errors Ã‚Â± 1 meter				developed by the European Union, joined by
				China, Israel, India, Morocco, Saudi Arabia
[edit] Atmospheric effects				and South Korea, Ukraine planned to be
				operational by 2011-12.
Inconsistencies of atmospheric conditions
affect the speed of the GPS signals as they				* GLONASS - Russia's global system which is
pass through the Earth's atmosphere and				being restored to full availability in
ionosphere. Correcting these errors is a				partnership with India.
significant challenge to improving GPS
position accuracy. These effects are smallest				* Indian Regional Navigational Satellite
when the satellite is directly overhead and				System (IRNSS) - India's proposed regional
become greater for satellites nearer the				system.
horizon since the signal is affected for a
longer time. Once the receiver's approximate				* QZSS - Japanese proposed regional system,
location is known, a mathematical model can				adding better coverage to the Japanese
be used to estimate and compensate for these				islands.
errors.
				[edit] See also
Because ionospheric delay affects the speed
of microwave signals differently based on				Satellite navigation systems Portal
frequency-a characteristic known as
dispersion-both frequency bands can be used				Nautical Portal
to help reduce this error. Some military and
expensive survey-grade civilian receivers				* RAIM
compare the different delays in the L1 and L2
frequencies to measure atmospheric				* SIGI
dispersion, and apply a more precise
correction. This can be done in civilian				* radio navigation
receivers without decrypting the P(Y) signal
carried on L2, by tracking the carrier wave				* High Sensitivity GPS
instead of the modulated code. To facilitate
this on lower cost receivers, a new civilian				* Degree Confluence Project Use GPS to visit
code signal on L2, called L2C, was added to				integral degrees of latitude and longitude.
the Block IIR-M satellites, which was first
launched in 2005. It allows a direct				* Exif, GPS data transfer.
comparison of the L1 and L2 signals using the
coded signal instead of the carrier wave.				* Geotagging

The effects of the ionosphere generally				* Geocaching
change slowly, and can be averaged over time.
The effects for any particular geographical				* NaviTraveler.com, - a GPS point sharing
area can be easily calculated by comparing				community.
the GPS-measured position to a known surveyed
location. This correction is also valid for				* GPS Drawing Digital mapping and drawing
other receivers in the same general location.				with GPS tracks.
Several systems send this information over
radio or other links to allow L1 only				* GPS tracking
receivers to make ionospheric corrections.
The ionospheric data are transmitted via				* GPS/INS
satellite in Satellite Based Augmentation
Systems such as WAAS, which transmits it on				* Assisted GPS
the GPS frequency using a special
pseudo-random number (PRN), so only one				* GPX (XML schema for interchange of
antenna and receiver are required.				waypoints)

Humidity also causes a variable delay,				* ID Sniper rifle
resulting in errors similar to ionospheric
delay, but occurring in the troposphere. This				* OpenStreetMap, free content maps and street
effect is both more localized and changes				pictures (GFDL)
more quickly than ionospheric effects and is
not frequency dependent. These traits making				* Telematics: Many telematics devices use GPS
precise measurement and compensation of				to determine the location of mobile
humidity errors more difficult than				equipment.
ionospheric effects.
				* The American Practical Navigator-Chapter 11
Changes in altitude also change the amount of				"Satellite Navigation"
delay due to the signal passing through less
of the atmosphere at higher elevations. Since				* Point of Interest
the GPS receiver computes its approximate
altitude, this error is relatively simple to				* Automotive navigation system
correct.
				* NextGen
[edit] Multipath effects
				[edit] Notes
GPS signals can also be affected by multipath
issues, where the radio signals reflect off				1. ^ Parkinson, B.W. (1996), Global
surrounding terrain; buildings, canyon walls,				Positioning System: Theory and Applications,
hard ground, etc. These delayed signals can				chap. 1: Introduction and Heritage of
cause inaccuracy. A variety of techniques,				NAVSTAR, the Global Positioning System. pp.
most notably narrow correlator spacing, have				3-28, American Institute of Aeronautics and
been developed to mitigate multipath errors.				Astronautics, Washington, D.C.
For long delay multipath, the receiver itself
can recognize the wayward signal and discard				2. ^ a b GPS Overview from the NAVSTAR Joint
it. To address shorter delay multipath from				Program Office. Accessed December 15, 2006.
the signal reflecting off the ground,
specialized antennas may be used to reduce				3. ^ HowStuffWorks. How GPS Receivers Work.
the signal power as received by the antenna.				Accessed May 14, 2006.
Short delay reflections are harder to filter
out because they interfere with the true				4. ^ globalsecurity.org [1].
signal, causing effects almost
indistinguishable from routine fluctuations				5. ^ Dana, Peter H. GPS Orbital Planes.
in atmospheric delay.				August 8, 1996.

Multipath effects are much less severe in				6. ^ What the Global Positioning System Tells
moving vehicles. When the GPS antenna is				Us about Relativity. Accessed January 2,
moving, the false solutions using reflected				2007.
signals quickly fail to converge and only the
direct signals result in stable solutions.				7. ^ USCG Navcen: GPS Frequently Asked
				Questions. Accessed January 3, 2007.
[edit] Ephemeris and clock errors
				8. ^ Massatt, Paul and Brady, Wayne.
The navigation message from a satellite is				"Optimizing performance through constellation
sent out only every 30 seconds. In reality,				management", Crosslink, Summer 2002, pages
the data contained in these messages tend to				17-21.
be "out of date" by an even larger amount.
Consider the case when a GPS satellite is				9. ^ US Coast Guard General GPS News 9-9-05
boosted back into a proper orbit; for some
time following the maneuver, the receiver's				10. ^ USNO. NAVSTAR Global Positioning
calculation of the satellite's position will				System. Accessed May 14, 2006.
be incorrect until it receives another
ephemeris update. The onboard clocks are				11. ^ NMEA NMEA 2000
extremely accurate, but they do suffer from
some clock drift. This problem tends to be				12. ^
very small, but may add up to 2 meters (6 ft)
of inaccuracy.				13. ^ AN02 Network Assistance (HTML).
				Retrieved on 2007-09-10.
This class of error is more "stable" than
ionospheric problems and tends to change over				14. ^ a b Office of Science and Technology
days or weeks rather than minutes. This makes				Policy. Presidential statement to stop
correction fairly simple by sending out a				degrading GPS. May 1, 2000.
more accurate almanac on a separate channel.
				15. ^ FAA, Selective Availability. Retrieved
[edit] Selective availability				Jan. 6, 2007.

The GPS includes a feature called Selective				16. ^
Availability (SA) that introduces
intentional, slowly changing random errors of				17. ^ Rizos, Chris. University of New South
up to a hundred meters (328 ft) into the				Wales. GPS Satellite Signals. 1999.
publicly available navigation signals to
confound, for example, guiding long range				18. ^ The Global Positioning System by Robert
missiles to precise targets. Additional				A. Nelson Via Satellite, November 1999
accuracy was available in the signal, but in
an encrypted form that was only available to				19. ^ Ashby, Neil Relativity and GPS. Physics
the United States military, its allies and a				Today, May 2002.
few others, mostly government users.
				20. ^ Space Environment Center. SEC
SA typically added signal errors of up to				Navigation Systems GPS Page. August 26, 1996.
about 10 meters (32 ft) horizontally and 30
meters (98 ft) vertically. The inaccuracy of				21. ^ The hunt for an unintentional GPS
the civilian signal was deliberately encoded				jammer. GPS World. January 1, 2003.
so as not to change very quickly, for
instance the entire eastern U.S. area might				22. ^ Low Cost and Portable GPS Jammer.
read 30 m off, but 30 m off everywhere and in				Phrack issue 0x3c (60), article 13].
the same direction. To improve the usefulness				Published December 28, 2002.
of GPS for civilian navigation, Differential
GPS was used by many civilian GPS receivers				23. ^ American Forces Press Service. CENTCOM
to greatly improve accuracy.				charts progress. March 25, 2003.

During the Gulf War, the shortage of military				24. ^ [2]
GPS units and the wide availability of
civilian ones among personnel resulted in a				25. ^ Ruley, John. AVweb. GPS jamming.
decision to disable Selective Availability.				February 12, 2003.
This was ironic, as SA had been introduced
specifically for these situations, allowing				26. ^ Commercial GPS Receivers: Facts for the
friendly troops to use the signal for				Warfighter. Hosted at the Joint Chiefs
accurate navigation, while at the same time				website, linked by the USAF's GPS Wing DAGR
denying it to the enemy. But since SA was				program website. Accessed on 10 April, 2007
also denying the same accuracy to thousands
of friendly troops, turning it off or setting				27. ^ US Coast Guard news release. Global
it to an error of zero meters (effectively				Positioning System Fully Operational
the same thing) presented a clear benefit.
				28. ^ a b Hydrographic Society Journal.
In the 1990s, the FAA started pressuring the				Developments in Global Navigation Satellite
military to turn off SA permanently. This				Systems. Issue #104, April 2002. Accessed
would save the FAA millions of dollars every				April 5, 2007.
year in maintenance of their own radio
navigation systems. The military resisted for				29. ^ XM982 Excalibur Precision Guided
most of the 1990s, and it ultimately took an				Extended Range Artillery Projectile.
executive order to have SA removed from the				GlobalSecurity.org (2007-05-29). Retrieved on
GPS signal. The amount of error added was				2007-09-26.
"set to zero"[14] at midnight on May 1, 2000
following an announcement by U.S. President				30. ^ Sandia National Laboratory's
Bill Clinton, allowing users access to the				Nonproliferation programs and arms control
error-free L1 signal. Per the directive, the				technology.
induced error of SA was changed to add no
error to the public signals (C/A code).				31. ^ Arms Control Association. Missile
Selective Availability is still a system				Technology Control Regime. Accessed May 17,
capability of GPS, and error could, in				2006.
theory, be reintroduced at any time. In
practice, in view of the hazards and costs				32. ^ United States Department of Defense.
this would induce for US and foreign				Announcement of Initial Operational
shipping, it is unlikely to be reintroduced,				Capability. December 8, 1993.
and various government agencies, including
the FAA,[15] have stated that it is not				33. ^ National Archives and Records
intended to be reintroduced.				Administration. U.S. GLOBAL POSITIONING
				SYSTEM POLICY. March 29, 1996.
The US military has developed the ability to
locally deny GPS (and other navigation				34. ^ United States Naval Research
services) to hostile forces in a specific				Laboratory. National Medal of Technology for
area of crisis without affecting the rest of				GPS. November 21, 2005
the world or its own military systems.[14]
				[edit] External links
One interesting side effect of the Selective
Availability hardware is the capability to				Wikimedia Commons has media related to:
correct the frequency of the GPS caesium and
rubidium atomic clocks to an accuracy of				Global Positioning System
approximately 2 Ãƒ- 10-13 (one in five
trillion). This represented a significant				Government links
improvement over the raw accuracy of the
clocks.[citation needed]				* GPS.gov-General public education website
				created by the U.S. Government
On 19 September 2007, the United States
Department of Defense announced that they				* National Space-Based PNT Executive
would not procure any more satellites capable				Committee-Established in 2004 to oversee
of implementing SA. [16]				management of GPS and GPS augmentations at a
				national level.
[edit] Relativity
				* USCG Navigation Center-Status of the GPS
According to the theory of relativity, due to				constellation, government policy, and links
their constant movement and height relative				to other references. Also includes satellite
to the Earth-centered inertial reference				almanac data.
frame, the clocks on the satellites are
affected by their speed (special relativity)				* The GPS Joint Program Office (GPS
as well as their gravitational potential				JPO)-Responsible for designing and acquiring
(general relativity). For the GPS satellites,				the system on behalf of the US Government.
general relativity predicts that the atomic
clocks at GPS orbital altitudes will tick				* U.S. Naval Observatory's GPS constellation
more rapidly, by about 45,900 nanoseconds				status
(ns) per day, because they are in a weaker
gravitational field than atomic clocks on				* U.S. Army Corps of Engineers manual:
Earth's surface. Special relativity predicts				NAVSTAR HTML and PDF (22.6 MB, 328 pages)
that atomic clocks moving at GPS orbital
speeds will tick more slowly than stationary				* PNT Selective Availability Announcements
ground clocks by about 7,200 ns per day. When
combined, the discrepancy is 38 microseconds				* GPS SPS Signal Specification, 2nd
per day; a difference of 4.465 parts in				Edition-The official Standard Positioning
1010.[17]. To account for this, the frequency				Signal specification.
standard onboard each satellite is given a
rate offset prior to launch, making it run				* Federal Aviation Administration's GPS FAQ
slightly slower than the desired frequency on
Earth; specifically, at 10.22999999543 MHz				Introductory / tutorial links
instead of 10.23 MHz.[18]
				* How does GPS work? TomTom explains GPS,
GPS observation processing must also				navigation, and digital maps
compensate for another relativistic effect,
the Sagnac effect. The GPS time scale is				* GPS Academy Garmin interactive video web
defined in an inertial system but				site explaing what exactly GPS is and what it
observations are processed in an				can do for you
Earth-centered, Earth-fixed (co-rotating)
system, a system in which simultaneity is not				* HowStuffWorks' Simplified explanation of
uniquely defined. The Lorentz transformation				GPS and video about how GPS works.
between the two systems modifies the signal
run time, a correction having opposite				* Trimble's Online GPS Tutorial Tutorial
algebraic signs for satellites in the Eastern				designed to introduce you to the principles
and Western celestial hemispheres. Ignoring				behind GPS
this effect will produce an east-west error
on the order of hundreds of nanoseconds, or				* GPS and GLONASS Simulation(Java applet)
tens of meters in position.[19]				Simulation and graphical depiction of space
				vehicle motion including computation of
The atomic clocks on board the GPS satellites				dilution of precision (DOP)
are precisely tuned, making the system a
practical engineering application of the				Technical, historical, and ancillary topics
scientific theory of relativity in a				links
real-world environment.
				* Dana, Peter H. "Global Positioning System
[edit] GPS interference and jamming				Overview"

Since GPS signals at terrestrial receivers				* Satellite Navigation: GPS & Galileo
tend to be relatively weak, it is easy for				(PDF)-16-page paper about the history and
other sources of electromagnetic radiation to				working of GPS, touching on the upcoming
desensitize the receiver, making acquiring				Galileo
and tracking the satellite signals difficult
or impossible.				* History of GPS, including information about
				each satellite's configuration and launch.
Solar flares are one such naturally occurring
emission with the potential to degrade GPS				* Chadha, Kanwar. "The Global Positioning
reception, and their impact can affect				System: Challenges in Bringing GPS to
reception over the half of the Earth facing				Mainstream Consumers" Technical Article
the sun. GPS signals can also be interfered				(1998)
with by naturally occurring geomagnetic
storms, predominantly found near the poles of				* GPS Weapon Guidance Techniques
the Earth's magnetic field.[20] Another
source of problems is the metal embedded in				* RAND history of the GPS system (PDF)
some car windscreens to prevent icing,
degrading reception just inside the car.				* GPS Anti-Jam Protection Techniques

Man-made interference can also disrupt, or				* Crosslink Summer 2002 issue by The
jam, GPS signals. In one well documented				Aerospace Corporation on satellite
case, an entire harbor was unable to receive				navigation.
GPS signals due to unintentional jamming
caused by a malfunctioning TV antenna				* Improved weather predictions from COSMIC
preamplifier.[21] Intentional jamming is also				GPS satellite signal occultation data.
possible. Generally, stronger signals can
interfere with GPS receivers when they are				* David L. Wilson's GPS Accuracy Web Page A
within radio range, or line of sight. In				thorough analysis of the accuracy of GPS.
2002, a detailed description of how to build
a short range GPS L1 C/A jammer was published				* Innovation: Spacecraft Navigator,
in the online magazine Phrack.[22]				Autonomous GPS Positioning at High Earth
				Orbits Example of GPS receiver designed for
The U.S. government believes that such				high altitude spaceflight.
jammers were used occasionally during the
2001 war in Afghanistan and the U.S. military				* The Navigator GPS Receiver GSFC's Navigator
claimed to destroy a GPS jammer with a				spaceflight receiver.
GPS-guided bomb during the Iraq War.[23] Such
a jammer is relatively easy to detect and				* Neil Ashby's Relativity in the Global
locate, making it an attractive target for				Positioning System
anti-radiation missiles. The UK Ministry of
Defence tested a jamming system in the UK's				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢ e
West Country on 7 and 8 June 2007. [24]
				Satellite navigation systems
Some countries allow the use of GPS repeaters
to allow for the reception of GPS signals				Historical Flag of the United States Transit
indoors and in obscured locations, however,
under EU and UK laws, the use of these is				Operational Flag of the Soviet Union / Flag
prohibited as the signals can cause				of Russia GLONASS Ã‚Â· Flag of the
interference to other GPS receivers that may				United States GPS
receive data from both GPS satellites and the
repeater.				Developmental Flag of the People's Republic
				of China Beidou/COMPASS Ã‚Â· Flag of
Due to the potential for both natural and				Europe Galileo Ã‚Â· Flag of India IRNSS
man-made noise, numerous techniques continue				Ã‚Â· Flag of Japan QZSS
to be developed to deal with the
interference. The first is to not rely on GPS				Related topics EGNOS Ã‚Â· GAGAN
as a sole source. According to John Ruley,				Ã‚Â· GPSÃ‚Â·C Ã‚Â· LAAS
"IFR pilots should have a fallback plan in				Ã‚Â· MSAS Ã‚Â· WAAS
case of a GPS malfunction".[25] Receiver
Autonomous Integrity Monitoring (RAIM) is a				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢ e
feature now included in some receivers, which
is designed to provide a warning to the user				Time signal stations
if jamming or another problem is detected.
The U.S. military has also deployed their				Longwave DCF77 Ã‚Â· HBG Ã‚Â· JJY
Selective Availability / Anti-Spoofing Module				Ã‚Â· MSF Ã‚Â· TDF Ã‚Â· WWVB
(SAASM) in the Defense Advanced GPS Receiver
(DAGR). In demonstration videos, the DAGR is				Shortwave BPM Ã‚Â· CHU Ã‚Â· RWM
able to detect jamming and maintain its lock				Ã‚Â· WWV Ã‚Â· WWVH Ã‚Â· YVTO
on the encrypted GPS signals during
interference which causes civilian receivers				GNSS time transfer Beidou Ã‚Â· Galileo
to lose lock.[26]				Ã‚Â· GLONASS Ã‚Â· GPS Ã‚Â·
				IRNSS
[edit] Techniques to improve accuracy
				Defunct time stations OMA Ã‚Â· VNG
[edit] Augmentation
				[show]v Ã¢Â€Â¢ d Ã¢Â€Â¢ e
Main article: GNSS Augmentation
				Global structure in Systems, Systems sciences
Augmentation methods of improving accuracy				and Systems scientists
rely on external information being integrated
into the calculation process. There are many				Categories Category:Conceptual systems
such systems in place and they are generally				Ã‚Â· Category:Physical systems Ã‚Â·
named or described based on how the GPS				Category:Social systems Ã‚Â·
sensor receives the information. Some systems				Category:Systems Ã‚Â· Category:Systems
transmit additional information about sources				science Ã‚Â· Category:Systems scientists
of error (such as clock drift, ephemeris, or				Ã‚Â· Category:Systems theory
ionospheric delay), others provide direct
measurements of how much the signal was off				Systems Biological system Ã‚Â· Complex
in the past, while a third group provide				system Ã‚Â· Complex adaptive system
additional navigational or vehicle				Ã‚Â· Conceptual system Ã‚Â·
information to be integrated in the				Cultural system Ã‚Â· Dynamical system
calculation process.				Ã‚Â· Economic system Ã‚Â· Ecosystem
				Ã‚Â· Formal system Ã‚Â· Global
Examples of augmentation systems include the				Positioning System Ã‚Â· Human organ
Wide Area Augmentation System, Differential				systems Ã‚Â· Information systems
GPS, Inertial Navigation Systems and Assisted				Ã‚Â· Legal system Ã‚Â· Metric
GPS.				system Ã‚Â· Nervous system Ã‚Â·
				Non-linear system Ã‚Â· Operating system
[edit] Precise monitoring				Ã‚Â· Physical system Ã‚Â· Political
				system Ã‚Â· Sensory system Ã‚Â·
The accuracy of a calculation can also be				Social system Ã‚Â· Solar System
improved through precise monitoring and				Ã‚Â· System Ã‚Â· Systems of
measuring of the existing GPS signals in				measurement
additional or alternate ways.
				Fields of theory Chaos theory Ã‚Â·
After SA, which has been turned off, the				Complex systems Ã‚Â· Control theory
largest error in GPS is usually the				Ã‚Â· Cybernetics Ã‚Â· Holism in
unpredictable delay through the ionosphere.				science Ã‚Â· Sociotechnical systems
The spacecraft broadcast ionospheric model				theory Ã‚Â· Systems biology Ã‚Â·
parameters, but errors remain. This is one				System dynamics Ã‚Â· Systems ecology
reason the GPS spacecraft transmit on at				Ã‚Â· Systems engineering Ã‚Â·
least two frequencies, L1 and L2. Ionospheric				Systems theory Ã‚Â· Systems science
delay is a well-defined function of frequency
and the total electron content (TEC) along				Systems scientists Russell L. Ackoff
the path, so measuring the arrival time				Ã‚Â· William Ross Ashby Ã‚Â·
difference between the frequencies determines				Gregory Bateson Ã‚Â· Ludwig von
TEC and thus the precise ionospheric delay at				Bertalanffy Ã‚Â· Kenneth E. Boulding
each frequency.				Ã‚Â· Peter Checkland Ã‚Â· C. West
				Churchman Ã‚Â· Heinz von Foerster
Receivers with decryption keys can decode the				Ã‚Â· Charles FranÃƒÂ§ois Ã‚Â·
P(Y)-code transmitted on both L1 and L2.				Jay Wright Forrester Ã‚Â· Ralph W.
However, these keys are reserved for the				Gerard Ã‚Â· Debora Hammond Ã‚Â·
military and "authorized" agencies and are				George Klir Ã‚Â· Niklas Luhmann
not available to the public. Without keys, it				Ã‚Â· Humberto Maturana Ã‚Â· Donella
is still possible to use a codeless technique				Meadows Ã‚Â· Mihajlo D. Mesarovic
to compare the P(Y) codes on L1 and L2 to				Ã‚Â· Howard T. Odum Ã‚Â· Talcott
gain much of the same error information.				Parsons Ã‚Â· Ilya Prigogine Ã‚Â·
However, this technique is slow, so it is				Anatol Rapoport Ã‚Â· Francisco Varela
currently limited to specialized surveying				Ã‚Â· John N.

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