The Global Positioning System (GPS) is the only fully
functional Global Navigation Satellite System (GNSS). Utilizing a constellation
of at least 24 medium Earth orbit satellites that transmit precise microwave
signals, the system enables a GPS receiver to determine its location, speed,
direction, and time. Other similar systems are the Russian GLONASS (incomplete
as of 2007) and the upcoming European Galileo positioning system.
Developed by the United States Department of Defense, GPS is officially named
NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but
simply a name given by Mr. John Walsh, a key decision maker when it came to the
budget for the GPS program).
The satellite constellation is managed by the United States Air Force 50th Space
Wing. The cost of maintaining the system is approximately US$750 million per
including the replacement of aging satellites, and research and development.
Following the shootdown of Korean Air Lines Flight 007 in 1983, President
Ronald Reagan issued a directive making the system available for free for
civilian use as a common good.
Since then, GPS has become a widely used aid to navigation worldwide, and a
useful tool for map-making, land surveying, commerce, and scientific uses. GPS
also provides a precise time reference used in many applications including
scientific study of earthquakes, and synchronization of telecommunications
Simplified method of operation
A typical GPS receiver calculates its position using the signals from four or
more GPS satellites. Four satellites are needed since the process needs a very
accurate local time, more accurate than a clock can provide, so the receiver
internally solves for time as well as position.
Each GPS satellite has an atomic clock, and continually transmits messages,
each containing the current time at the start of the message, parameters to
calculate the location of the satellite (the ephemeris), and the general system
health (the almanac). The signals travel at a known speed - the speed of light
through outer space, and slightly slower through the atmosphere. The receiver
uses the arrival time to compute the distance to each satellite, from which it
determines the position of the receiver using geometry and trigonometry. If the
local time is known very precisely, this process (known as trilateration)
can determine the receiver's position using three satellites. However, most
receivers do not contain clocks of this accuracy (an atomic clock would be
required), and so require tracking four or more satellites so that the receiver
can compute both the accurate time and its location.
The current GPS consists of three major segments. These are the space segment
(SS), a control segment (CS), and a user segment (US).
The space segment (SS) comprises the orbiting GPS satellites, or Space
Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs to be
distributed equally among six circular orbital planes.
The orbital planes are centered on the Earth, not rotating with respect to the
distant stars. The six planes have
approximately 55° inclination (tilt relative to Earth's equator) and are
separated by 60° right ascension of the ascending node (angle along the equator
from a reference point to the orbit's intersection).
Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or
10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)),
each SV makes two complete orbits each sidereal day.
The orbits are arranged so that at least six satellites are always within line
of sight from almost everywhere on Earth's surface.
As of September 2007, there are 31 actively broadcasting satellites in the
GPS constellation. The additional satellites improve the precision of GPS
receiver calculations by providing redundant measurements. With the increased
number of satellites, the constellation was changed to a nonuniform arrangement.
Such an arrangement was shown to improve reliability and availability of the
system, relative to a uniform system, when multiple satellites fail.
The flight paths of the satellites are tracked by US Air Force monitoring
stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado
Springs, Colorado, along with monitor stations operated by the National
Geospatial-Intelligence Agency (NGA).
The tracking information is sent to the Air Force Space Command's master control
station at Schriever Air Force Base in Colorado Springs, which is operated by
the 2d Space Operations Squadron (2 SOPS) of the United States Air Force (USAF).
2 SOPS contacts each GPS satellite regularly with a navigational update (using
the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado
Springs). These updates synchronize the atomic clocks on board the satellites to
within a few nanoseconds of each other, and adjust the ephemeris of each
satellite's internal orbital model. The updates are created by a Kalman filter
which uses inputs from the ground monitoring stations, space weather
information, and various other inputs.
The user's GPS receiver is the user segment (US) of the GPS system. In
general, GPS receivers are composed of an antenna, tuned to the frequencies
transmitted by the satellites, receiver-processors, and a highly-stable clock
(often a crystal oscillator). They may also include a display for providing
location and speed information to the user. A receiver is often described by its
number of channels: this signifies how many satellites it can monitor
simultaneously. Originally limited to four or five, this has progressively
increased over the years so that, as of 2006, receivers typically have between
twelve and twenty channels.
GPS receivers may include an input for differential corrections, using the
RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800
bit/s speed. Data is actually sent at a much lower rate, which limits the
accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers
can outperform those using external RTCM data. As of 2006, even low-cost units
commonly include Wide Area Augmentation System (WAAS) receivers.
Many GPS receivers can relay position data to a PC or other device using the
NMEA 0183 protocol. NMEA 2000 is a
newer and less widely adopted protocol. Both are proprietary and controlled by
the US-based National Marine Electronics Association. References to the NMEA
protocols have been compiled from public records, allowing open source tools
like gpsd to read the protocol without violating intellectual property laws.
Other proprietary protocols exist as well, such as the SiRF and MTK protocols.
Receivers can interface with other devices using methods including a serial
connection, USB or Bluetooth.
Each GPS satellite continuously broadcasts a Navigation Message at 50
bit/s giving the time-of-day, GPS week number and satellite health information
(all transmitted in the first part of the message), an ephemeris
(transmitted in the second part of the message) and an almanac (later
part of the message). The messages are sent in frames, each taking 30 seconds to
transmit 1500 bits.
The first 6 seconds of every frame contains data describing the satellite
clock and its relationship to GPS system time. The next 12 seconds contain the
ephemeris data, giving the satellite's own precise orbit. The ephemeris
is updated every 2 hours and is generally valid for 4 hours, with provisions for
updates every 6 hours or longer in non-nominal conditions. The time needed to
acquire the ephemeris is becoming a significant element of the delay to first
position fix, because, as the hardware becomes more capable, the time to lock
onto the satellite signals shrinks, but the ephemeris data requires 30 seconds
(worst case) before it is received, due to the low data transmission rate.
The almanac consists of coarse orbit and status information for each
satellite in the constellation, an ionospheric model, and information to relate
GPS derived time to Coordinated Universal Time (UTC). A new part of the almanac
is received for the last 12 seconds in each 30 second period. Time to collect
the complete almanac is 30 seconds for each satellite that is present in the
constellation (for example, 24 satellites would take 12 minutes). The almanac
serves several purposes. The first is to assist in the acquisition of satellites
at power-up by allowing the receiver to generate a list of visible satellites
based on stored position and time, while an ephemeris from each satellite is
needed to compute position fixes using that satellite. In older hardware, lack
of an almanac in a new receiver would cause long delays before providing a valid
position, because the search for each satellite was a slow process. Advances in
hardware have made the acquisition process much faster, so not having an almanac
is no longer an issue. The second purpose is for relating time derived from the
GPS system (called GPS time) to the international time standard of UTC. Finally,
the almanac allows a single frequency receiver to correct for ionospheric error
by using a global ionospheric model. The corrections are not as accurate as
augmentation systems like WAAS or dual frequency receivers. However it is often
better than no correction since ionospheric error is the largest error source
for a single frequency GPS receiver. An important thing to note about navigation
data is that each satellite transmits only its own ephemeris, but
transmits an almanac for all satellites.
GNU Free Documentation License -
Each satellite transmits its navigation message with at least two distinct
spread spectrum codes: the Coarse / Acquisition (C/A) code, which is
freely available to the public, and the Precise (P) code, which is
usually encrypted and reserved for military applications. The C/A code is a
1,023 chip pseudo-random (PRN) code at 1.023 million chips/sec so that it
repeats every millisecond. Each satellite has its own C/A code so that it can be
uniquely identified and received separately from the other satellites
transmitting on the same frequency. The P-code is a 10.23 megachip/sec PRN code
that repeats only every week. When the "anti-spoofing" mode is on, as it is in
normal operation, the P code is encrypted by the Y-code to produce the
P(Y) code, which can only be decrypted by units with a valid decryption key.
Both the C/A and P(Y) codes impart the precise time-of-day to the user.
Frequencies used by GPS include
- L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A)
code and encrypted precision P(Y) code, plus the new L1C on future Block III
- L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M
and newer satellites.
- L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection
System Payload (NDS) to signal detection of nuclear detonations and other
high-energy infrared events. Used to enforce nuclear test ban treaties.
- L4 (1379.913 MHz): Being studied for additional ionospheric correction.
- L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL)
signal (see GPS modernization). This frequency falls into an internationally
protected range for aeronautical navigation, promising little or no
interference under all circumstances. The first Block IIF satellite that
would provide this signal is set to be launched in 2008.
Using the C/A code
To start off, the receiver picks which C/A codes to listen for by PRN number,
based on the almanac information it has previously acquired. As it detects each
satellite's signal, it identifies it by its distinct C/A code pattern, then
measures the received time for each satellite. To do this, the receiver produces
an identical C/A sequence using the same seed number, referenced to its local
clock, starting at the same time the satellite sent it. It then computes the
offset to the local clock that generates the maximum correlation. This offset is
the time delay from the satellite to the receiver, as told by the receiver's
clock. Since the PRN repeats every millisecond, this offset is precise but
ambiguous, and the ambiguity is resolved by looking at the data bits, which are
sent at 50 Hz (20 ms) and aligned with the PRN code.
This data is used to solve for x,y,z and t. Many mathematical techniques can
be used. The following description shows a straightforward iterative way, but
receivers use more sophisticated methods. (see below)
Conceptually, the receiver calculates the distance to the satellite, called
Next, the orbital position data, or ephemeris, from the Navigation Message is
then downloaded to calculate the satellite's precise position. A more-sensitive
receiver will potentially acquire the ephemeris data more quickly than a
less-sensitive receiver, especially in a noisy environment.
Knowing the position and the distance of a satellite indicates that the receiver
is located somewhere on the surface of an imaginary sphere centered on that
satellite and whose radius is the distance to it. Receivers can substitute
altitude for one satellite, which the GPS receiver translates to a pseudorange
measured from the center of the Earth.
When pseudoranges have been determined for four satellites, a guess of the
receiver's location is calculated. Dividing the speed of light by the distance
adjustment required to make the pseudoranges come as close as possible to
intersecting results in a guess of the difference between UTC and the time
indicated by the receiver's on-board clock. With each combination of four
satellites, a geometric dilution of precision (GDOP) vector is
calculated, based on the relative sky positions of the satellites used. As more
satellites are picked up, pseudoranges from more combinations of four satellites
can be processed to add more guesses to the location and clock offset. The
receiver then determines which combinations to use and how to calculate the
estimated position by determining the weighted average of these positions and
clock offsets. After the final location and time are calculated, the location is
expressed in a specific coordinate system, e.g. latitude/longitude, using the
WGS 84 geodetic datum or a local system specific to a country.
here are many other alternatives and improvements to this process. If at
least 4 satellites are visible, for example, the receiver can eliminate time
from the equations by computing only time differences, then solving for position
as the intersection of hyperboloids. Also, with a full constellation and modern
receivers, more than 4 satellites can be seen and received at once. Then all
satellite data can be weighted by GDOP, signal to noise, path length through the
ionosphere, and other accuracy concerns, and then used in a least squares fit to
find a solution. In this case the residuals also gives an estimate of the
errors. Finally, results from other positioning systems such as GLONASS or the
upcoming Galileo can be used in the fit, or used to double-check the result. (By
design, these systems use the same bands, so much of the receiver circuitry can
be shared, though the decoding is different).
Using the P(Y) code
Calculating a position with the P(Y) signal is generally similar in concept,
assuming one can decrypt it. The encryption is essentially a safety mechanism:
if a signal can be successfully decrypted, it is reasonable to assume it is a
real signal being sent by a GPS satellite.
In comparison, civil receivers are highly vulnerable to spoofing since correctly
formatted C/A signals can be generated using readily available signal
generators. RAIM features do not protect against spoofing, since RAIM only
checks the signals from a navigational perspective.
Accuracy and error sources
The position calculated by a GPS receiver requires the current time, the
position of the satellite and the measured delay of the received signal. The
position accuracy is primarily dependent on the satellite position and signal
To measure the delay, the receiver compares the bit sequence received from
the satellite with an internally generated version. By comparing the rising and
trailing edges of the bit transitions, modern electronics can measure signal
offset to within about 1% of a bit time, or approximately 10 nanoseconds for the
C/A code. Since GPS signals propagate at the speed of light, this represents an
error of about 3 meters. This is the minimum error possible using only the GPS
Position accuracy can be improved by using the higher-chiprate P(Y) signal.
Assuming the same 1% bit time accuracy, the high frequency P(Y) signal results
in an accuracy of about 30 centimeters.
Electronics errors are one of several accuracy-degrading effects outlined in
the table below. When taken together, autonomous civilian GPS horizontal
position fixes are typically accurate to about 15 meters (50 ft). These effects
also reduce the more precise P(Y) code's accuracy.
Inconsistencies of atmospheric conditions affect the speed of the GPS signals
as they pass through the Earth's atmosphere and ionosphere. Correcting these
errors is a significant challenge to improving GPS position accuracy. These
effects are smallest when the satellite is directly overhead and become greater
for satellites nearer the horizon since the signal is affected for a longer
time. Once the receiver's approximate location is known, a mathematical model
can be used to estimate and compensate for these errors.
Because ionospheric delay affects the speed of microwave signals differently
based on frequency—a characteristic known as dispersion—both frequency bands can
be used to help reduce this error. Some military and expensive survey-grade
civilian receivers compare the different delays in the L1 and L2 frequencies to
measure atmospheric dispersion, and apply a more precise correction. This can be
done in civilian receivers without decrypting the P(Y) signal carried on L2, by
tracking the carrier wave instead of the modulated code. To facilitate this on
lower cost receivers, a new civilian code signal on L2, called L2C, was added to
the Block IIR-M satellites, which was first launched in 2005. It allows a direct
comparison of the L1 and L2 signals using the coded signal instead of the
The effects of the ionosphere generally change slowly, and can be averaged
over time. The effects for any particular geographical area can be easily
calculated by comparing the GPS-measured position to a known surveyed location.
This correction is also valid for other receivers in the same general location.
Several systems send this information over radio or other links to allow L1 only
receivers to make ionospheric corrections. The ionospheric data are transmitted
via satellite in Satellite Based Augmentation Systems such as WAAS, which
transmits it on the GPS frequency using a special pseudo-random number (PRN), so
only one antenna and receiver are required.
Humidity also causes a variable delay, resulting in errors similar to
ionospheric delay, but occurring in the troposphere. This effect both is more
localized and changes more quickly than ionospheric effects, and is not
frequency dependent. These traits make precise measurement and compensation of
humidity errors more difficult than ionospheric effects.
Changes in receiver altitude also change the amount of delay, due to the
signal passing through less of the atmosphere at higher elevations. Since the
GPS receiver computes its approximate altitude, this error is relatively simple
GPS signals can also be affected by multipath issues, where the radio signals
reflect off surrounding terrain; buildings, canyon walls, hard ground, etc.
These delayed signals can cause inaccuracy. A variety of techniques, most
notably narrow correlator spacing, have been developed to mitigate multipath
errors. For long delay multipath, the receiver itself can recognize the wayward
signal and discard it. To address shorter delay multipath from the signal
reflecting off the ground, specialized antennas may be used to reduce the signal
power as received by the antenna. Short delay reflections are harder to filter
out because they interfere with the true signal, causing effects almost
indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe in moving vehicles. When the GPS
antenna is moving, the false solutions using reflected signals quickly fail to
converge and only the direct signals result in stable solutions.
Ephemeris and clock errors
While the ephemeris data is transmitted every 30 seconds, the information
itself may be up to two hours old. Data up to four hours old is considered valid
for calculating positions, but may not indicate the satellites actual position.
The satellite's atomic clocks experience noise and clock drift errors. The
navigation message contains corrections for these errors and estimates of the
accuracy of the atomic clock, however they are based on observations and may not
indicate the clock's current state.
These problems tend to be very small, but may add up to 2 meters (6 ft) of
GPS includes a (currently disabled) feature called Selective Availability
(SA) that can introduce intentional, slowly changing random errors of up
to a hundred meters (328 ft) into the publicly available navigation signals to
confound, for example, guiding long range missiles to precise targets. When
enabled, the accuracy is still available in the signal, but in an encrypted form
that is only available to the United States military, its allies and a few
others, mostly government users. Even those who have managed to acquire military
GPS receivers would still need to obtain the daily key, whose dissemination is
Prior to being turned off, SA typically added signal errors of up to about 10
meters (32 ft) horizontally and 30 meters (98 ft) vertically. The inaccuracy of
the civilian signal was deliberately encoded so as not to change very quickly.
For instance, the entire eastern U.S. area might read 30 m off, but 30 m off
everywhere and in the same direction. To improve the usefulness of GPS for
civilian navigation, Differential GPS was used by many civilian GPS receivers to
greatly improve accuracy.
During the Gulf War, the shortage of military GPS units and the ready
availability of civilian ones caused many troops to buy their own civilian GPS
units: their wide use among personnel resulted in a decision to disable
Selective Availability. This was ironic, as SA had been introduced specifically
for these situations, allowing friendly troops to use the signal for accurate
navigation, while at the same time denying it to the enemy—but the assumption
underlying this policy was that all U.S. troops and enemy troops would have
military-specification GPS receivers and that civilian receivers would not exist
in war zones. But since many American soldiers were using civilian devices, SA
was also denying the same accuracy to thousands of friendly troops; turning it
off (by removing the added-in error) presented a clear benefit to friendly
In the 1990s, the FAA started pressuring the military to turn off SA
permanently. This would save the FAA millions of dollars every year in
maintenance of their own radio navigation systems. The military resisted for
most of the 1990s, and it ultimately took an executive order to have SA removed
from the GPS signal. The amount of error added was "set to zero"
at midnight on May 1, 2000 following an announcement by U.S. President Bill
Clinton, allowing users access to the error-free L1 signal. Per the directive,
the induced error of SA was changed to add no error to the public signals (C/A
code). Clinton's executive order required SA to be set to zero by 2006; it
happened in 2000.
Selective Availability is still a system capability of GPS, and error could,
in theory, be reintroduced at any time. In practice, in view of the hazards and
costs this would induce for US and foreign shipping, it is unlikely to be
reintroduced, and various government agencies, including the FAA,
have stated that it is not intended to be reintroduced.
The US military has since developed a new system that provides the ability to
deny GPS (and other navigation services) to hostile forces in a specific area of
crisis without affecting the rest of the world or its own military systems.
One interesting side effect of the Selective Availability hardware is the
capability to correct the frequency of the GPS cesium and rubidium atomic clocks
to an accuracy of approximately 2 × 10-13 (one in five trillion).
This represented a significant improvement over the raw accuracy of the clocks.
On 19 September 2007, the United States Department of Defense announced that
they would not procure any more satellites capable of implementing SA.
According to the theory of relativity, due to their constant movement and
height relative to the Earth-centred inertial reference frame, the clocks on the
satellites are affected by their speed (special relativity) as well as their
gravitational potential (general relativity). For the GPS satellites, general
relativity predicts that the atomic clocks at GPS orbital altitudes will tick
more rapidly, by about 45.9 microseconds (μs) per day, because they are in a
weaker gravitational field than atomic clocks on Earth's surface. Special
relativity predicts that atomic clocks moving at GPS orbital speeds will tick
more slowly than stationary ground clocks by about 7.2 μs per day. When
combined, the discrepancy is about 38 microseconds per day; a difference of
4.465 parts in 1010..
To account for this, the frequency standard onboard each satellite is given a
rate offset prior to launch, making it run slightly slower than the desired
frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.
GPS observation processing must also compensate for another relativistic
effect, the Sagnac effect. The GPS time scale is defined in an inertial system
but observations are processed in an Earth-centered, Earth-fixed (co-rotating)
system, a system in which simultaneity is not uniquely defined. The Lorentz
transformation between the two systems modifies the signal run time, a
correction having opposite algebraic signs for satellites in the Eastern and
Western celestial hemispheres. Ignoring this effect will produce an east-west
error on the order of hundreds of nanoseconds, or tens of meters in position.
The atomic clocks on board the GPS satellites are precisely tuned, making the
system a practical engineering application of the scientific theory of
relativity in a real-world environment.
GPS interference and jamming
Since GPS signals at terrestrial receivers tend to be relatively weak, it is
easy for other sources of electromagnetic radiation to desensitize the receiver,
making acquiring and tracking the satellite signals difficult or impossible.
Solar flares are one such naturally occurring emission with the
potential to degrade GPS reception, and their impact can affect reception over
the half of the Earth facing the sun. GPS signals can also be interfered with by
naturally occurring geomagnetic storms, predominantly found near the poles of
the Earth's magnetic field. GPS
signals are also subjected to interference from Van Allen Belt radiation when
satellites pass through the South Atlantic Anomaly. Another source of problems
is the metal embedded in some car windscreens to prevent icing, degrading
reception just inside the car.
Man-made interference can also disrupt, or jam, GPS signals. In one well
documented case, an entire harbor was unable to receive GPS signals due to
unintentional jamming caused by a malfunctioning TV antenna preamplifier.
Intentional jamming is also possible. Generally, stronger signals can interfere
with GPS receivers when they are within radio range, or line of sight. In 2002,
a detailed description of how to build a short range GPS L1 C/A jammer was
published in the online magazine Phrack.
The U.S. government believes that such jammers were used occasionally during
the 2001 war in Afghanistan and the U.S. military claimed to destroy a GPS
jammer with a GPS-guided bomb during the Iraq War.
Such a jammer is relatively easy to detect and locate, making it an attractive
target for anti-radiation missiles. The UK Ministry of Defence tested a jamming
system in the UK's West Country on 7 and 8 June 2007.
Some countries allow the use of GPS repeaters to allow for the reception of
GPS signals indoors and in obscured locations, however, under EU and UK laws,
the use of these is prohibited as the signals can cause interference to other
GPS receivers that may receive data from both GPS satellites and the repeater.
Due to the potential for both natural and man-made noise, numerous techniques
continue to be developed to deal with the interference. The first is to not rely
on GPS as a sole source. According to John Ruley, "IFR pilots should have a
fallback plan in case of a GPS malfunction".
Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in
some receivers, which is designed to provide a warning to the user if jamming or
another problem is detected. The U.S. military has also deployed their Selective
Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver
(DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain
its lock on the encrypted GPS signals during interference which causes civilian
receivers to lose lock.
Techniques to improve accuracy
Augmentation methods of improving accuracy rely on external information being
integrated into the calculation process. There are many such systems in place
and they are generally named or described based on how the GPS sensor receives
the information. Some systems transmit additional information about sources of
error (such as clock drift, ephemeris, or ionospheric delay), others provide
direct measurements of how much the signal was off in the past, while a third
group provide additional navigational or vehicle information to be integrated in
the calculation process.
Examples of augmentation systems include the Wide Area Augmentation System,
Differential GPS, Inertial Navigation Systems and Assisted GPS.
The accuracy of a calculation can also be improved through precise monitoring
and measuring of the existing GPS signals in additional or alternate ways.
After SA, which has been turned off, the largest error in GPS is usually the
unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric
model parameters, but errors remain. This is one reason the GPS spacecraft
transmit on at least two frequencies, L1 and L2. Ionospheric delay is a
well-defined function of frequency and the total electron content (TEC) along
the path, so measuring the arrival time difference between the frequencies
determines TEC and thus the precise ionospheric delay at each frequency.
Receivers with decryption keys can decode the P(Y)-code transmitted on both
L1 and L2. However, these keys are reserved for the military and "authorized"
agencies and are not available to the public. Without keys, it is still possible
to use a codeless technique to compare the P(Y) codes on L1 and L2 to
gain much of the same error information. However, this technique is slow, so it
is currently limited to specialized surveying equipment. In the future,
additional civilian codes are expected to be transmitted on the L2 and L5
frequencies (see GPS modernization, below). Then all users will be able to
perform dual-frequency measurements and directly compute ionospheric delay
A second form of precise monitoring is called Carrier-Phase Enhancement
(CPGPS). The error, which this corrects, arises because the pulse transition of
the PRN is not instantaneous, and thus the correlation (satellite-receiver
sequence matching) operation is imperfect. The CPGPS approach utilizes the L1
carrier wave, which has a period 1000 times smaller than that of the C/A bit
period, to act as an additional clock signal and resolve the uncertainty. The
phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to
10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces
this error to 3 centimetres (1 inch) of ambiguity. By eliminating this source of
error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters
(8 to 12 inches) of absolute accuracy.
Relative Kinematic Positioning (RKP) is another approach for a precise
GPS-based positioning system. In this approach, determination of range signal
can be resolved to an accuracy of less than 10 centimeters (4 in). This is done
by resolving the number of cycles in which the signal is transmitted and
received by the receiver. This can be accomplished by using a combination of
differential GPS (DGPS) correction data, transmitting GPS signal phase
information and ambiguity resolution techniques via statistical tests—possibly
with processing in real-time (real-time kinematic positioning, RTK).
GPS time and date
While most clocks are synchronized to Coordinated Universal Time (UTC), the
atomic clocks on the satellites are set to GPS time. The difference is
that GPS time is not corrected to match the rotation of the Earth, so it does
not contain leap seconds or other corrections which are periodically added to
UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has
since diverged. The lack of corrections means that GPS time remains at a
constant offset (19 seconds) with International Atomic Time (TAI). Periodic
corrections are performed on the on-board clocks to correct relativistic effects
and keep them synchronized with ground clocks.
The GPS navigation message includes the difference between GPS time and UTC,
which as of 2007 is 14 seconds. Receivers subtract this offset from GPS time to
calculate UTC and specific timezone values. New GPS units may not show the
correct UTC time until after receiving the UTC offset message. The GPS-UTC
offset field can accommodate 255 leap seconds (eight bits) which, at the current
rate of change of the Earth's rotation, is sufficient to last until the year
As opposed to the year, month, and day format of the Gregorian calendar, the
GPS date is expressed as a week number and a day-of-week number. The week number
is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and
so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started
at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980 and the week number became
zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI
on August 22, 1999). To determine the current Gregorian date, a GPS receiver
must be provided with the approximate date (to within 3,584 days) to correctly
translate the GPS date signal. To address this concern the modernized GPS
navigation messages use a 13-bit field, which only repeats every 8,192 weeks
(157 years), and will not return to zero until near the year 2137.
Having reached the program's requirements for Full Operational Capability
(FOC) on July 17, 1995, the GPS
completed its original design goals. However, additional advances in technology
and new demands on the existing system led to the effort to modernize the GPS
system. Announcements from the Vice President and the White House in 1998
initiated these changes, and in 2000 the U.S. Congress authorized the effort,
referring to it as GPS III.
The project aims to improve the accuracy and availability for all users and
involves new ground stations, new satellites, and four additional navigation
signals. New civilian signals are called L2C, L5 and L1C;
the new military code is called M-Code. Initial Operational Capability
(IOC) of the L2C code is expected in 2008.
A goal of 2013 has been established for the entire program, with incentives
offered to the contractors if they can complete it by 2011.
The Global Positioning System, while originally a military project, is
considered a dual-use technology, meaning it has significant applications
for both the military and the civilian industry.
The military applications of GPS span many purposes:
- Navigation: GPS allows soldiers to find objectives in the dark or in
unfamiliar territory, and to coordinate the movement of troops and supplies.
The GPS-receivers commanders and soldiers use are respectively called the
Commanders Digital Assistant and the Soldier Digital Assistant.
- Target tracking: Various military weapons systems use GPS to track
potential ground and air targets before they are flagged as hostile.
These weapons systems pass GPS co-ordinates of targets to precision-guided
munitions to allow them to engage the targets accurately. Military aircraft,
particularly those used in air-to-ground roles use GPS to find targets (for
example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates
that can be looked up in Google Earth).
- Missile and projectile guidance: GPS allows accurate targeting of
various military weapons including ICBMs, cruise missiles and
precision-guided munitions. Artillery projectiles with embedded GPS
receivers able to withstand accelerations of 12,000G have been developed for
use in 155 mm howitzers.
- Search and Rescue: Downed pilots can be located faster if they have a
- Reconnaissance and Map Creation: The military use GPS extensively to aid
mapping and reconnaissance.
- The GPS satellites also carry nuclear detonation detectors, which form a
major portion of the United States Nuclear Detonation Detection System.
Many civilian applications benefit from GPS signals, using one or more of
three basic components of the GPS: absolute location, relative movement, and
The ability to determine the receiver's absolute location allows GPS
receivers to perform as a surveying tool or as an aid to navigation. The
capacity to determine relative movement enables a receiver to calculate local
velocity and orientation, useful in vessels or observations of the Earth. Being
able to synchronize clocks to exacting standards enables time transfer, which is
critical in large communication and observation systems. An example is CDMA
digital cellular. Each base station has a GPS timing receiver to synchronize its
spreading codes with other base stations to facilitate inter-cell hand off and
support hybrid GPS/CDMA positioning of mobiles for emergency calls and other
Finally, GPS enables researchers to explore the Earth environment including
the atmosphere, ionosphere and gravity field. GPS survey equipment has
revolutionized tectonics by directly measuring the motion of faults in
To help prevent civilian GPS guidance from being used in an enemy's military
or improvised weaponry, the US Government controls the export of civilian
receivers. A US-based manufacturer cannot generally export a GPS receiver unless
the receiver contains limits restricting it from functioning when it is
simultaneously (1) at an altitude above 18 kilometers (60,000 ft) and (2)
traveling at over 515 m/s (1,000 knots).
These parameters are well above the operating characteristics of the typical
cruise missile, but would be characteristic of the reentry vehicle from a
The design of GPS is based partly on the similar ground-based radio
navigation systems, such as LORAN and the Decca Navigator developed in the early
1940s, and used during World War II. Additional inspiration for the GPS system
came when the Soviet Union launched the first Sputnik in 1957. A team of U.S.
scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio
transmissions. They discovered that, because of the Doppler effect, the
frequency of the signal being transmitted by Sputnik was higher as the satellite
approached, and lower as it continued away from them. They realized that since
they knew their exact location on the globe, they could pinpoint where the
satellite was along its orbit by measuring the Doppler distortion.
The first satellite navigation system, Transit, used by the United States
Navy, was first successfully tested in 1960. Using a constellation of five
satellites, it could provide a navigational fix approximately once per hour. In
1967, the U.S. Navy developed the Timation satellite which proved the ability to
place accurate clocks in space, a technology the GPS system relies upon. In the
1970s, the ground-based Omega Navigation System, based on signal phase
comparison, became the first world-wide radio navigation system.
The first experimental Block-I GPS satellite was launched in February 1978.
The GPS satellites were initially manufactured by Rockwell International (now
part of Boeing) and are now manufactured by Lockheed Martin (IIR/IIR-M) and
In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman
AFB) conducted developmental flight tests of two prototype GPS receivers over
White Sands Missile Range, using ground-based pseudo-satellites.
In 1978 the first experimental Block-I GPS satellite was launched.
In 1983, after Soviet interceptor aircraft shot down the civilian airliner
KAL 007 that strayed into prohibited airspace due to navigational errors,
killing all 269 people on board, U.S. President Ronald Reagan announced that the
GPS would be made available for civilian uses once it was completed.
By 1985, ten more experimental Block-I satellites had been launched to
validate the concept.
On February 14, 1989, the first modern Block-II satellite was launched.
In 1992, the 2nd Space Wing, which originally managed the system, was
de-activated and replaced by the 50th Space Wing.
By December 1993 the GPS achieved initial operational capability.
By January 17, 1994 a complete constellation of 24 satellites was in orbit.
Full Operational Capability was declared by NAVSTAR in April 1995.
In 1996, recognizing the importance of GPS to civilian users as well as
military users, U.S. President Bill Clinton issued a policy directive
declaring GPS to be a dual-use system and establishing an Interagency GPS
Executive Board to manage it as a national asset.
In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two
new civilian signals for enhanced user accuracy and reliability, particularly
with respect to aviation safety.
On May 2, 2000 "Selective Availability" was discontinued as a result of the
1996 executive order, allowing users to receive a non-degraded signal globally.
In 2004, the United States Government signed a historic agreement with the
European Community establishing cooperation related to GPS and Europe's planned
In 2004, U.S. President George W. Bush updated the national policy and
replaced the executive board with the National Space-Based Positioning,
Navigation, and Timing Executive Committee.
November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile
In 2005, the first modernized GPS satellite was launched and began
transmitting a second civilian signal (L2C) for enhanced user performance.
On September 14, 2007, the aging mainframe-based Ground Segment Control
System was transitioned to the new Architecture Evolution Plan.
The most recent launch was on March 15, 2008. The oldest GPS satellite
still in operation was launched on November 26, 1990, and became operational on
December 10, 1990.
Two GPS developers have received the National Academy of Engineering Charles
Stark Draper prize year 2003:
- Ivan Getting, emeritus president of The Aerospace Corporation and
engineer at the Massachusetts Institute of Technology, established the basis
for GPS, improving on the World War II land-based radio system called LORAN
(Long-range Radio Aid to Navigation).
- Bradford Parkinson, professor of aeronautics and astronautics at
Stanford University, conceived the present satellite-based system in the
early 1960s and developed it in conjunction with the U.S. Air Force.
One GPS developer, Roger L. Easton, received the National Medal of Technology
on February 13, 2006 at the White House.
On February 10, 1993, the National Aeronautic Association selected the Global
Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the
most prestigious aviation award in the United States. This team consists of
researchers from the Naval Research Laboratory, the U.S. Air Force, the
Aerospace Corporation, Rockwell International Corporation, and IBM Federal
Systems Company. The citation accompanying the presentation of the trophy honors
the GPS Team "for the most significant development for safe and efficient
navigation and surveillance of air and spacecraft since the introduction of
radio navigation 50 years ago."
Other satellite navigation systems in use or various states of development
- Beidou — China's regional system that China has proposed to
expand into a global system named COMPASS.
- Galileo — a proposed global system being developed by the
European Union, joined by China, Israel, India, Morocco, Saudi Arabia, South
Korea, and Ukraine, planned to be operational by 2011–12.
- GLONASS — Russia's global system which is being restored to full
availability in partnership with India.
- Indian Regional Navigational Satellite System (IRNSS) — India's
proposed regional system.
- QZSS - Japanese proposed regional system, adding better coverage
to the Japanese islands.