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About GPS

Global Positioning System problem is detected. The U.S. military
The Global Positioning System (GPS) is has also deployed their Selective
the only fully functional Global Availability / Anti-Spoofing Module
Navigation Satellite System (GNSS). (SAASM) in the Defense Advanced GPS
Utilizing a constellation of at least 24 Receiver (DAGR). In demonstration videos,
medium Earth orbit satellites that the DAGR is able to detect jamming and
transmit precise microwave signals, the maintain its lock on the encrypted GPS
system enables a GPS receiver to signals during interference which causes
determine its location, speed/direction, civilian receivers to lose lock.[26]
and time. [edit] Techniques to improve accuracy
Developed by the United States Department [edit] Augmentation
of Defense, it is officially named Main article: GNSS Augmentation
NAVSTAR GPS (Contrary to popular belief, Augmentation methods of improving
NAVSTAR is not an acronym, but simply a accuracy rely on external information
name given by Mr. John Walsh, a key being integrated into the calculation
decision maker when it came to the budget process. There are many such systems in
for the GPS program[1]). The satellite place and they are generally named or
constellation is managed by the United described based on how the GPS sensor
States Air Force 50th Space Wing. The receives the information. Some systems
cost of maintaining the system is transmit additional information about
approximately US$750 million per year,[2] sources of error (such as clock drift,
including the replacement of aging ephemeris, or ionospheric delay), others
satellites, and research and development. provide direct measurements of how much
Despite these costs, GPS is free for the signal was off in the past, while a
civilian use as a public good. third group provide additional
GPS has become a widely used aid to navigational or vehicle information to be
navigation worldwide, and a useful tool integrated in the calculation process.
for map-making, land surveying, commerce, Examples of augmentation systems include
and scientific uses. GPS also provides a the Wide Area Augmentation System,
precise time reference used in many Differential GPS, Inertial Navigation
applications including scientific study Systems and Assisted GPS.
of earthquakes, and synchronization of [edit] Precise monitoring
telecommunications networks. The accuracy of a calculation can also be
Simplified method of operation improved through precise monitoring and
A GPS receiver calculates its position by measuring of the existing GPS signals in
measuring the distance between itself and additional or alternate ways.
three or more GPS satellites. Measuring After SA, which has been turned off, the
the time delay between transmission and largest error in GPS is usually the
reception of each GPS microwave signal unpredictable delay through the
gives the distance to each satellite, ionosphere. The spacecraft broadcast
since the signal travels at a known speed ionospheric model parameters, but errors
- the speed of light. These signals also remain. This is one reason the GPS
carry information about the satellites' spacecraft transmit on at least two
location and general system health (known frequencies, L1 and L2. Ionospheric delay
as almanac and ephemeris data). By is a well-defined function of frequency
determining the position of, and distance and the total electron content (TEC)
to, at least three satellites, the along the path, so measuring the arrival
receiver can compute its position using time difference between the frequencies
trilateration.[3] Receivers typically do determines TEC and thus the precise
not have perfectly accurate clocks and ionospheric delay at each frequency.
therefore track one or more additional Receivers with decryption keys can decode
satellites, using their atomic clocks to the P(Y)-code transmitted on both L1 and
correct the receiver's own clock error. L2. However, these keys are reserved for
[edit] Technical description the military and "authorized" agencies
Unlaunched GPS satellite on display at and are not available to the public.
the San Diego Aerospace museum Without keys, it is still possible to use
Unlaunched GPS satellite on display at a codeless technique to compare the P(Y)
the San Diego Aerospace museum codes on L1 and L2 to gain much of the
[edit] System segmentation same error information. However, this
The current GPS consists of three major technique is slow, so it is currently
segments. These are the space segment limited to specialized surveying
(SS), a control segment (CS), and a user equipment. In the future, additional
segment (US).[4] civilian codes are expected to be
[edit] Space segment transmitted on the L2 and L5 frequencies
The space segment (SS) is composed of the (see GPS modernization, below). Then all
orbiting GPS satellites, or Space users will be able to perform
Vehicles (SV) in GPS parlance. The GPS dual-frequency measurements and directly
design calls for 24 SVs to be distributed compute ionospheric delay errors.
equally among six circular orbital A second form of precise monitoring is
planes.[5] The orbital planes are called Carrier-Phase Enhancement (CPGPS).
centered on the Earth, not rotating with The error, which this corrects, arises
respect to the distant stars.[6] The six because the pulse transition of the PRN
planes have approximately 55° is not instantaneous, and thus the
inclination (tilt relative to Earth's correlation (satellite-receiver sequence
equator) and are separated by 60° matching) operation is imperfect. The
right ascension of the ascending node CPGPS approach utilizes the L1 carrier
(angle along the equator from a reference wave, which has a period 1000 times
point to the orbit's intersection).[2] smaller than that of the C/A bit period,
Orbiting at an altitude of approximately to act as an additional clock signal and
20,200 kilometers (12,600 miles or 10,900 resolve the uncertainty. The phase
nautical miles; orbital radius of 26,600 difference error in the normal GPS
km (16,500 mi or 14,400 NM)), each SV amounts to between 2 and 3 meters (6 to
makes two complete orbits each sidereal 10 ft) of ambiguity. CPGPS working to
day, so it passes over the same location within 1% of perfect transition reduces
on Earth once each day. The orbits are this error to 3 centimeters (1 inch) of
arranged so that at least six satellites ambiguity. By eliminating this source of
are always within line of sight from error, CPGPS coupled with DGPS normally
almost everywhere on Earth's surface.[7] realizes between 20 and 30 centimeters (8
As of September 2007, there are 31 to 12 inches) of absolute accuracy.
actively broadcasting satellites in the Relative Kinematic Positioning (RKP) is
GPS constellation. The additional another approach for a precise GPS-based
satellites improve the precision of GPS positioning system. In this approach,
receiver calculations by providing determination of range signal can be
redundant measurements. With the resolved to an accuracy of less than 10
increased number of satellites, the centimeters (4 in). This is done by
constellation was changed to a nonuniform resolving the number of cycles in which
arrangement. Such an arrangement was the signal is transmitted and received by
shown to improve reliability and the receiver. This can be accomplished by
availability of the system, relative to a using a combination of differential GPS
uniform system, when multiple satellites (DGPS) correction data, transmitting GPS
fail.[8] signal phase information and ambiguity
[edit] Control segment resolution techniques via statistical
The flight paths of the satellites are tests-possibly with processing in
tracked by US Air Force monitoring real-time (real-time kinematic
stations in Hawaii, Kwajalein, Ascension positioning, RTK).
Island, Diego Garcia, and Colorado [edit] GPS time and date
Springs, Colorado, along with monitor While most clocks are synchronized to
stations operated by the National Coordinated Universal Time (UTC), the
Geospatial-Intelligence Agency (NGA).[9] Atomic clocks on the satellites are set
The tracking information is sent to the to GPS time. The difference is that GPS
Air Force Space Command's master control time is not corrected to match the
station at Schriever Air Force Base in rotation of the Earth, so it does not
Colorado Springs, which is operated by contain leap seconds or other corrections
the 2d Space Operations Squadron (2 SOPS) which are periodically added to UTC. GPS
of the United States Air Force (USAF). 2 time was set to match Coordinated
SOPS contacts each GPS satellite Universal Time (UTC) in 1980, but has
regularly with a navigational update since diverged. The lack of corrections
(using the ground antennas at Ascension means that GPS time remains at a constant
Island, Diego Garcia, Kwajalein, and offset (19 seconds) with International
Colorado Springs). These updates Atomic Time (TAI). Periodic corrections
synchronize the atomic clocks on board are performed on the on-board clocks to
the satellites to within one microsecond correct relativistic effects and keep
and adjust the ephemeris of each them synchronized with ground clocks.
satellite's internal orbital model. The The GPS navigation message includes the
updates are created by a Kalman filter difference between GPS time and UTC,
which uses inputs from the ground which as of 2006 is 14 seconds. Receivers
monitoring stations, space weather subtract this offset from GPS time to
information, and various other calculate UTC and specific timezone
inputs.[10] values. New GPS units may not show the
GPS receivers come in a variety of correct UTC time until after receiving
formats, from devices integrated into the UTC offset message. The GPS-UTC
cars, phones, and watches, to dedicated offset field can accommodate 255 leap
devices such as those shown here from seconds (eight bits) which, at the
manufacturers Trimble, Garmin and Leica current rate of change of the Earth's
(left to right). rotation, is sufficient to last until the
GPS receivers come in a variety of year 2330.
formats, from devices integrated into As opposed to the year, month, and day
cars, phones, and watches, to dedicated format of the Julian calendar, the GPS
devices such as those shown here from date is expressed as a week number and a
manufacturers Trimble, Garmin and Leica day-of-week number. The week number is
(left to right). transmitted as a ten-bit field in the C/A
[edit] User segment and P(Y) navigation messages, and so it
The user's GPS receiver is the user becomes zero again every 1,024 weeks
segment (US) of the GPS system. In (19.6 years). GPS week zero started at
general, GPS receivers are composed of an 00:00:00 UTC (00:00:19 TAI) on January 6,
antenna, tuned to the frequencies 1980 and the week number became zero
transmitted by the satellites, again for the first time at 23:59:47 UTC
receiver-processors, and a highly-stable on August 21, 1999 (00:00:19 TAI on
clock (often a crystal oscillator). They August 22, 1999). To determine the
may also include a display for providing current Gregorian date, a GPS receiver
location and speed information to the must be provided with the approximate
user. A receiver is often described by date (to within 3,584 days) to correctly
its number of channels: this signifies translate the GPS date signal. To address
how many satellites it can monitor this concern the modernized GPS
simultaneously. Originally limited to navigation messages use a 13-bit field,
four or five, this has progressively which only repeats every 8,192 weeks (157
increased over the years so that, as of years), and will not return to zero until
2006, receivers typically have between near the year 2137.
twelve and twenty channels. [edit] GPS modernization
A typical OEM GPS receiver module, based Main article: GPS modernization
on the SiRF Star III chipset, measuring Having reached the program's requirements
15Ã-17 mm, and used in many products. for Full Operational Capability (FOC) on
A typical OEM GPS receiver module, based July 17, 1995,[27] the GPS completed its
on the SiRF Star III chipset, measuring original design goals. However,
15Ã-17 mm, and used in many products. additional advances in technology and new
GPS receivers may include an input for demands on the existing system led to the
differential corrections, using the RTCM effort to modernize the GPS system.
SC-104 format. This is typically in the Announcements from the Vice President and
form of a RS-232 port at 4,800 bit/s the White House in 1998 initiated these
speed. Data are actually sent at a much changes, and in 2000 the U.S. Congress
lower rate, which limits the accuracy of authorized the effort, referring to it as
the signal sent using RTCM. Receivers GPS III.
with internal DGPS receivers can The project aims to improve the accuracy
outperform those using external RTCM and availability for all users and
data. As of 2006, even low-cost units involves new ground stations, new
commonly include Wide Area Augmentation satellites, and four additional
System (WAAS) receivers. navigation signals. New civilian signals
Many GPS receivers can relay position are called L2C, L5 and L1C; the new
data to a PC or other device using the military code is called M-Code. Initial
NMEA 0183 protocol. NMEA 2000[11] is a Operational Capability (IOC) of the L2C
newer and less widely adopted protocol. code is expected in 2008.[28] A goal of
Both are proprietary and controlled by 2013 has been established for the entire
the US-based National Marine Electronics program, with incentives offered to the
Association. References to the NMEA contractors if they can complete it by
protocols have been compiled from public 2011.
records, allowing open source tools like [edit] Applications
gpsd to read the protocol without The Global Positioning System, while
violating intellectual property laws. originally a military project, is
Other proprietary protocols exist as considered a dual-use technology, meaning
well, such as the SiRF and MTK protocols. it has significant applications for both
Receivers can interface with other the military and the civilian industry.
devices using methods including a serial [edit] Military
connection, USB or Bluetooth. Please help improve this article by
[edit] Navigation signals expanding this section.
Main article: GPS signals See talk page for details. Please remove
GPS broadcast signal this message once the section has been
GPS broadcast signal expanded.
Each GPS satellite continuously The military use GPS for the following
broadcasts a Navigation Message at 50 bit purposes:
s giving the time-of-day, GPS week number [edit] Navigation
and satellite health information (all GPS allows soldiers to find objectives in
transmitted in the first part of the the dark or in unfamiliar territory, and
message), an ephemeris (transmitted in to coordinate the movement of troops and
the second part of the message) and an supplies.
almanac (later part of the message). The [edit] Target tracking
ephemeris data gives the satellite's own Various military weapons systems use GPS
precise orbit and is output over 18 to track potential ground and air targets
seconds, repeating every 30 seconds. The before they are flagged as hostile. These
ephemeris is updated every 2 hours and is weapons systems pass GPS co-ordinates of
generally valid for 4 hours, with targets to precision-guided munitions to
provisions for 6 hour time-outs. The time allow them to engage the targets
needed to acquire the ephemeris is accurately.
becoming a significant element of the Military aircraft, particularly those
delay to first position fix, because, as used in air-to-ground roles use GPS to
the hardware becomes more capable, the find targets (for example, gun camera
time to lock onto the satellite signals video from AH-1 Cobras in Iraq show GPS
shrinks, but the ephemeris data requires co-ordinates that can be looked up in
30 seconds (worst case) before it is Google Earth).
received, due to the low data [edit] Missile and projectile guidance
transmission rate. The almanac consists GPS allows accurate targeting of various
of coarse orbit and status information military weapons including ICBMs, cruise
for each satellite in the constellation missiles and precision-guided munitions.
and takes 12 seconds for each satellite Artillery projectiles with embedded GPS
present, with information for a new receivers able to withstand forces of
satellite being transmitted every 30 12,000G have been developed for use in
seconds (15.5 minutes for 31 satellites). 155 mm howitzers.[29]
The purpose of the data is to assist in [edit] Search and Rescue
the acquisition of satellites at power-up Downed pilots can be located faster if
by allowing the receiver to generate a they have a GPS receiver.
list of visible satellites based on [edit] Reconnaissance and Map Creation
stored position and time, while an The military use GPS extensively to aid
ephemeris from each satellite is needed mapping and reconnaissance.
to compute position fixes using that [edit] Other
satellite. In older hardware, lack of an The GPS satellites also carry nuclear
almanac in a new receiver would cause detonation detectors, which form a major
long delays before providing a valid portion of the United States Nuclear
position, because the search for each Detonation Detection System.[30]
satellite was a slow process. Advances in [edit] Civilian
hardware have made the acquisition See also: GPS applications
process much faster, so not having an This antenna is mounted on the roof of a
almanac is no longer an issue. An hut containing a scientific experiment
important thing to note about navigation needing precise timing.
data is that each satellite transmits This antenna is mounted on the roof of a
only its own ephemeris, but transmits an hut containing a scientific experiment
almanac for all satellites. needing precise timing.
Each satellite transmits its navigation Many civilian applications benefit from
message with at least two distinct spread GPS signals, using one or more of three
spectrum codes: the Coarse / Acquisition basic components of the GPS; absolute
(C/A) code, which is freely available to location, relative movement, time
the public, and the Precise (P) code, transfer.
which is usually encrypted and reserved The ability to determine the receiver's
for military applications. The C/A code absolute location allows GPS receivers to
is a 1,023 chip pseudo-random (PRN) code perform as a surveying tool or as an aid
at 1.023 million chips/sec so that it to navigation. The capacity to determine
repeats every millisecond. Each satellite relative movement enables a receiver to
has its own C/A code so that it can be calculate local velocity and orientation,
uniquely identified and received useful in vessels or observations of the
separately from the other satellites Earth. Being able to synchronize clocks
transmitting on the same frequency. The to exacting standards enables time
P-code is a 10.23 megachip/sec PRN code transfer, which is critical in large
that repeats only every week. When the communication and observation systems. An
"anti-spoofing" mode is on, as it is in example is CDMA digital cellular. Each
normal operation, the P code is encrypted base station has a GPS timing receiver to
by the Y-code to produce the P(Y) code, synchronize its spreading codes with
which can only be decrypted by units with other base stations to facilitate
a valid decryption key. Both the C/A and inter-cell hand off and support hybrid
P(Y) codes impart the precise time-of-day GPS/CDMA positioning of mobiles for
to the user. Frequencies used by GPS emergency calls and other applications.
include Finally, GPS enables researchers to
* L1 (1575.42 MHz): Mix of Navigation explore the Earth environment including
Message, coarse-acquisition (C/A) code the atmosphere, ionosphere and gravity
and encrypted precision P(Y) code, plus field. GPS survey equipment has
the new L1C on future Block III revolutionized tectonics by directly
satellites. measuring the motion of faults in
* L2 (1227.60 MHz): P(Y) code, plus the earthquakes.
new L2C code on the Block IIR-M and newer To help prevent civilian GPS guidance
satellites. from being used in an enemy's military or
* L3 (1381.05 MHz): Used by the Nuclear improvised weaponry, the US Government
Detonation (NUDET) Detection System controls the export of civilian
Payload (NDS) to signal detection of receivers. A US-based manufacturer cannot
nuclear detonations and other high-energy generally export a GPS receiver unless
infrared events. Used to enforce nuclear the receiver contains limits restricting
test ban treaties. it from functioning when it is
* L4 (1379.913 MHz): Being studied for simultaneously (1) at an altitude above
additional ionospheric correction. 18 kilometers (60,000 ft) and (2)
* L5 (1176.45 MHz): Proposed for use as a traveling at over 515 m/s (1,000
civilian safety-of-life (SoL) signal (see knots).[31]
GPS modernization). This frequency falls [edit] History
into an internationally protected range Please help improve this article by
for aeronautical navigation, promising expanding this section.
little or no interference under all See talk page for details. Please remove
circumstances. The first Block IIF this message once the section has been
satellite that would provide this signal expanded.
is set to be launched in 2008. The design of GPS is based partly on the
[edit] Calculating positions similar ground-based radio navigation
[edit] Using the C/A code systems, such as LORAN and the Decca
To start off, the receiver picks which C Navigator developed in the early 1940s,
A codes to listen for by PRN number, and used during World War II. Additional
based on the almanac information it has inspiration for the GPS system came when
previously acquired. As it detects each the Soviet Union launched the first
satellite's signal, it identifies it by Sputnik in 1957. A team of U.S.
its distinct C/A code pattern, then scientists led by Dr. Richard B. Kershner
measures the time delay for each were monitoring Sputnik's radio
satellite. To do this, the receiver transmissions. They discovered that,
produces an identical C/A sequence using because of the Doppler effect, the
the same seed number as the satellite. By frequency of the signal being transmitted
lining up the two sequences, the receiver by Sputnik was higher as the satellite
can measure the delay and calculate the approached, and lower as it continued
distance to the satellite, called the away from them. They realized that since
pseudorange[12]. they knew their exact location on the
Overlapping pseudoranges, represented as globe, they could pinpoint where the
curves, are modified to yield the satellite was along its orbit by
probable position measuring the Doppler distortion.
Overlapping pseudoranges, represented as The first satellite navigation system,
curves, are modified to yield the Transit, used by the United States Navy,
probable position was first successfully tested in 1960.
Next, the orbital position data, or Using a constellation of five satellites,
ephemeris, from the Navigation Message is it could provide a navigational fix
then downloaded to calculate the approximately once per hour. In 1967, the
satellite's precise position. A U.S. Navy developed the Timation
more-sensitive receiver will potentially satellite which proved the ability to
acquire the ephemeris data quicker than a place accurate clocks in space, a
less-sensitive receiver, especially in a technology the GPS system relies upon. In
noisy environment.[13] Knowing the the 1970s, the ground-based Omega
position and the distance of a satellite Navigation System, based on signal phase
indicates that the receiver is located comparison, became the first world-wide
somewhere on the surface of an imaginary radio navigation system.
sphere centered on that satellite and The first experimental Block-I GPS
whose radius is the distance to it. satellite was launched in February
Receivers can substitute altitude for one 1978.[28] The GPS satellites were
satellite, which the GPS receiver initially manufactured by Rockwell
translates to a pseudorange measured from International and are now manufactured by
the center of the earth. Lockheed Martin.
Locations are calculated not in [edit] Timeline
three-dimensional space, but in * In 1972, the US Air Force Central
four-dimensional spacetime, meaning a Inertial Guidance Test Facility (Holloman
measure of the precise time-of-day is AFB) conducted developmental fight tests
very important. The measured pseudoranges of two prototype GPS receivers over White
from four satellites have already been Sands Missile Range, using ground-based
determined with the receiver's internal