
The Global Positioning System (GPS)
is currently the only fully functional Global Navigation Satellite
System (GNSS). More than two dozen GPS satellites are in medium Earth
orbit, transmitting signals allowing GPS receivers to determine the
receiver's location, speed and direction.
Since the first
experimental satellite was launched in 1978, GPS has become an
indispensable aid to navigation around the world, and an important tool
for map-making and land surveying. GPS also provides a precise time
reference used in many applications including scientific study of
earthquakes, and synchronization of telecommunications networks.
Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System).
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 year, including the replacement of aging satellites,
and research and development. Despite this fact, GPS is free for
civilian use as a public good.
Simplified method of operation
A
GPS receiver calculates its position by measuring the distance between
itself and three or more GPS satellites. Measuring the time delay
between transmission and reception of each GPS radio signal gives the
distance to each satellite, since the signal travels at a known speed.
The signals also carry information about the satellites' location. By
determining the position of, and distance to, at least three satellites,
the receiver can compute its position using trilateration, or the
determination of a distance from three points. Receivers typically do
not have perfectly accurate clocks and therefore track one or more
additional satellites to correct the receiver's clock error.
Technical description
System segmentation
The current GPS consists of three major segments: the space segment (SS), a control segment (CS), and a user segment (US).
Space segment
The
SS is composed of the orbiting GPS satellites, or Space Vehicles (SV)
in GPS parlance. The GPS design calls 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), each SV makes
two complete orbits each sidereal day (the length of time for Earth to
make a full rotation with respect to a fixed star, namely, 23 hours, 56
minutes, and 4.1 seconds), so it passes over the same location on Earth
once each day. The orbits are arranged so that at least six satellites
are always within line of sight from almost anywhere on Earth.
As
of February 2007, there are 30 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.
Control segment
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, Colorado Springs, Colorado, 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 one
microsecond and adjust the ephemeris (table of positions of celestial
bodies) 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 other various inputs.[
User segment
The
user's GPS receiver is the user segment (U.S.) 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.
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 by SiRF Technology Inc.
Receivers can interface with other devices using methods including a
serial connection, USB or Bluetooth.
Navigation signals
GPS satellites broadcast three different types of data in the primary navigation signal. The first is the almanac
which sends coarse time information along with status information about
the satellites. The second is the ephemeris, which contains orbital
information that allows the receiver to calculate the position of the
satellite. This data is included in the 37,500 bit Navigation Message, which takes 12.5 minutes to send at 50 bps.
The satellites also broadcast two forms of clock information, the Coarse / Acquisition code, or C/A which is freely available to the public, and the restricted Precise code, or P-code,
usually reserved for military applications. The C/A code is a 1,023 bit
long pseudo-random code broadcast at 1.023 MHz, repeating every
millisecond. Each satellite sends a distinct C/A code, which allows it
to be uniquely identified. The P-code is a similar code broadcast at
10.23 MHz, but it repeats only once a week. In normal operation, the
so-called "anti-spoofing mode" (spoofing, in GPS means a fake signal),
the P code is first encrypted into the Y-code, or P(Y), and then decrypted by units with a valid decryption key. Frequencies used by GPS include:
- L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.
- 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 Defense Support Program to signal detection of missile launches, nuclear detonations, and other high-energy infrared events.
- 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.
Calculating positions
The
coordinates are calculated according to the World Geodetic System WGS84
coordinate system. To calculate its position, a receiver needs to know
the precise time. The satellites are equipped with extremely accurate
atomic clocks, and the receiver uses an internal crystal
oscillator-based clock that is continually updated using the signals
from the satellites.
The receiver identifies each satellite's
signal by its distinct C/A code pattern, then measures the time delay
for each satellite. To do this, the receiver produces an identical C/A
sequence using the same seed number as the satellite. By lining up the
two sequences, the receiver can measure the delay and calculate the
distance to the satellite, called the pseudorange.
The orbital
position data from the Navigation Message is then used to calculate the
satellite's precise position. 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. When four satellites are measured
simultaneously, the intersection of the four imaginary spheres reveals
the location of the receiver. Earth-based users can substitute the
sphere of the planet for one satellite by using their altitude. Often,
these spheres will overlap slightly instead of meeting at one point, so
the receiver will yield a mathematically most-probable position (and
often indicate the uncertainty).
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 will
not help, 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 delay.
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 percent of a bit time, or approximately 10 nanoseconds for the
C/A code. Since GPS signals propagate nearly at the speed of light, this
represents an error of about 3 meters. This is the minimum error
possible using the GPS C/A signal.
Position accuracy can be
improved by using the higher-speed P(Y) signal. Assuming the same 1
percent accuracy, the faster 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.
Sources of errors
Source | Effect |
---|---|
Ionospheric effects | ± 5 meter |
Ephemeris errors | ± 2.5 meter |
Satellite clock errors | ± 2 meter |
Multipath distortion | ± 1 meter |
Tropospheric effects | ± 0.5 meter |
Numerical errors | ± 1 meter or less |
Atmospheric effects
Changing atmospheric conditions
change 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
minimized 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 radio
waves differently based on frequency, a characteristic known as
dispersion (a phenomenon where a light signal is separated into its
constituent elements), 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, first launched
in 2005. It allows a direct comparison of the L1 and L2 signals using
the coded signal instead of the carrier wave.
The effects of the
ionosphere are generally slow-moving, 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 only L1 receivers to make ionospheric corrections. The
ionospheric data are transmitted via satellite in Satellite Based
Augmentation Systems such as Wide Area Augmentation System (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 is much more
localized, and changes more quickly than the ionospheric effects, making
precise compensation for humidity more difficult. Altitude also causes a
variable delay, as the signal passes through less atmosphere at higher
elevations. Since the GPS receiver measures altitude directly, this is a
much simpler correction to apply.
Multipath effects
GPS
signals can also be affected by multipath issues, where the radio
signals reflect off surrounding terrain such as buildings, canyon walls,
and hard ground. 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. Short delay reflections are
harder to filter out since they are only slightly delayed, 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
The
navigation message from a satellite is sent out only every 12.5
minutes. In reality, the data contained in these messages tend to be
"out of date" by a greater distance. Consider the case when a GPS
satellite is boosted back into a proper orbit; for some time following
the maneuver, the receiver's calculation of the satellite's position
will be incorrect until it receives another ephemeris update. The
onboard clocks are extremely accurate, but they do suffer from some
clock drift. This problem tends to be very small, but may add up to 2
meters (~6 ft) of inaccuracy.
This class of error is more
"stable" than ionospheric problems and tends to change over days or
weeks rather than minutes. This makes correction fairly simple by
sending out a more accurate almanac on a separate channel.
Selective availability
The GPS includes a feature called Selective Availability (SA)
that introduces intentional errors between 0 meters and up to a hundred
meters (300 ft) into the publicly available navigation signals, making
it difficult to use for guiding long range missiles to precise targets.
Additional accuracy was available in the signal, but in an encrypted
form that was only available to the United States military, its allies,
and a few others, mostly government users.
SA typically added
signal errors of up to about 10 meters (30 ft) horizontally and 30
meters (100 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. In order 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 wide
availability of civilian ones 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 since SA was also denying the same accuracy
to thousands of friendly troops, turning it off or setting it to an
error of 0 meters (effectively the same thing) presented a clear
benefit.
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, but SA was eventually
"discontinued;" 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 an undergraded L1 signal. Per the directive,
the induced error of SA was changed to add no error to the public
signals (C/A code). 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 U.S.
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 developed the
ability to locally 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.
Relativity
According
to the theory of relativity, due to their constant movement and height
relative to the Earth-centered 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 45,900 nanoseconds
(ns) 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, by
about 7,200 ns per day, than stationary ground clocks. When combined,
the discrepancy is 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.
Another relativistic
effect to be compensated for in GPS observation processing is 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 system.
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 overpower 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 at near the poles of the Earth's magnetic field.
Human-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 Afghanistan War and the U.S. military
claimed to destroy a GPS jammer with a GPS-guided bomb during the Iraq
War. While a jammer is relatively easy to detect and locate, making it
an attractive target for anti-radiation missiles, there is the
possibility that these jammers would then be located near non-combatant
infrastructure and used to attract the precision-guided munitions
towards; a tactic known as using a human shield.
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 military has also deployed
the Selective Availability / Anti-Spoofing Module (SAASM) in its Defense
Advanced GPS Receiver, which as shown in demonstration videos, is able
to detect the jamming and maintain a lock on the GPS signals.
Techniques to improve accuracy
Augmentation
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 WAAS, Differential GPS, and Inertial Navigation Systems.
Precise monitoring
The
accuracy of a calculation can also be improved through precise
monitoring and measuring of the existing GPS signals in additional or
alternate ways.
The first is called Dual Frequency
monitoring, and refers to systems that can compare two or more signals,
such as the L1 frequency to the L2 frequency. Since these are two
different frequencies, they are affected in different, yet predictable
ways by the atmosphere and objects around the receiver. After monitoring
these signals, it is possible to calculate how much error is being
introduced and then nullify that error.
Receivers that have the
correct decryption key can relatively easily decode the P(Y)-code
transmitted on both L1 and L2 to measure the error. Receivers that do
not possess the key can still use a process called codeless to
compare the encrypted information on L1 and L2 to gain much of the same
error information. However, this technique is currently limited to
specialized surveying equipment. In the future, additional civilian
codes are expected to be transmitted on the L2 and L5 frequencies. When
these become operational, non-encrypted users will be able to make the
same comparison and directly measure some errors.
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 feet) of
ambiguity. CPGPS working to within 1 percent of perfect transition
reduces this error to 3 cm (1 inch) of ambiguity. By eliminating this
source of error, CPGPS coupled with DGPS normally realizes between 20
and 30 cm (8 to 12 in) 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 cm (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 synchronized with the
International Atomic Time (TAI).
The GPS navigation message
includes the difference between GPS time and UTC, which as of 2006 is 14
seconds. Receivers subtract this offset from GPS time to calculate UTC
and 'local' time. 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
2330.
As opposed to the year, month, and day format of the Julian
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). In order to determine the current Gregorian date, a GPS receiver
must be provided with the approximate date (to within 3,584 days) in
order to correctly translate the GPS date signal. To address this
concern the modernized GPS navigation messages use a 13-bit field, which
repeats every 8,192 weeks (157 years), and will not return to zero
until near the year 2137.
GPS modernization
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 Presidential and the White House in 1998
heralded the beginning of these changes, and in 2000 the U.S. Congress
reaffirmed the effort; referred 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.
Applications
Military
GPS
allows accurate targeting of various military weapons including cruise
missiles and precision-guided munitions. To help prevent GPS guidance
from being used in enemy or improvised weaponry, the US Government
controls the export of civilian receivers. A US-based manufacturer
cannot generally export a receiver unless the receiver contains limits
restricting it from functioning when it is simultaneously (1) at an
altitude above 18 kilometers (60,000ft) and (2) traveling at over 515
m/s (1,000 knots).
The GPS satellites also carry nuclear
detonation detectors, which form a major portion of the United States
Nuclear Detonation Detection System.
Navigation
- Automobiles can be equipped with GPS receivers at the factory or as after-market equipment. Units often display moving maps and information about location, speed, direction, and nearby streets and landmarks.
- Aircraft navigation systems usually display a "moving map" and are often connected to the autopilot for en-route navigation. Cockpit-mounted GPS receivers and glass cockpits are appearing in general aviation aircraft of all sizes, using technologies such as WAAS or Local Area Augmentation System (LAAS) to increase accuracy. Many of these systems may be certified for instrument flight rules navigation, and some can also be used for final approach and landing operations. Glider pilots use GNSS Flight Recorders to log GPS data verifying their arrival at turn points in gliding competitions. Flight computers installed in many gliders also use GPS to compute wind speed aloft, and glide paths to waypoints such as alternate airports or mountain passes, to aid en route decision making for cross-country soaring.
- Boats and ships can use GPS to navigate all of the world's lakes, seas, and oceans. Maritime GPS units include functions useful on water, such as “man overboard” (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts. GPS may be connected to the ships self-steering gear and Chartplotters using the NMEA 0183 interface. GPS can also improve the security of shipping traffic by enabling AIS.
- Heavy Equipment can use GPS in construction, mining and precision agriculture. The blades and buckets of construction equipment are controlled automatically in GPS-based machine guidance systems. Agricultural equipment may use GPS to steer automatically, or as a visual aid displayed on a screen for the driver. This is very useful for controlled traffic and row crop operations and when spraying. Harvesters with yield monitors can also use GPS to create a yield map of the paddock being harvested.
- Bicycles often use GPS in racing and touring. GPS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps. Some GPS receivers are specifically adapted for cycling with special mounts and housings.
- Hikers, climbers, and even ordinary pedestrians in urban or rural environments can use GPS to determine their position, with or without reference to separate maps. In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers).
- GPS equipment for the visually impaired is available. For more detailed information see the article GPS for the visually impaired
- Spacecraft are now beginning to use GPS as a navigational tool. The addition of a GPS receiver to a spacecraft allows precise orbit determination without ground tracking. This, in turn, enables autonomous spacecraft navigation, formation flying, and autonomous rendezvous. The use of GPS in MEO, GEO, HEO, and highly elliptical orbits is feasible only if the receiver can acquire and track the much weaker (15 - 20 dB) GPS side-lobe signals. This design constraint, and the radiation environment found in space, prevents the use of COTS receivers.
Surveying and mapping
- Surveying—Survey-Grade GPS receivers can be used to position survey markers, buildings, and road construction. These units use the signal from both the L1 and L2 GPS frequencies. Even though the L2 code data are encrypted, the signal's carrier wave enables correction of some ionospheric errors. These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimeter or less when used in carrier phase differential GPS mode.
- Mapping and geographic information systems (GIS)—Most mapping grade GPS receivers use the carrier wave data from only the L1 frequency, but have a precise crystal oscillator which reduces errors related to receiver clock jitter. This allows positioning errors on the order of one meter or less in real-time, with a differential GPS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially post-processing the data, positioning errors on the order of 10 cm are possible with these receivers.
- Geophysics and geology—High precision measurements of crustal strain can be made with differential GPS by finding the relative displacement between GPS sensors. Multiple stations situated around an actively deforming area (such as a volcano or fault zone) can be used to find strain and ground movement. These measurements can then be used to interpret the cause of the deformation, such as a dike or sill beneath the surface of an active volcano.
Other uses
- Precise time reference—Many systems that must be accurately synchronized use GPS as a source of accurate time. GPS can be used as a reference clock for time code generators or NTP clocks. Sensors (for seismology or other monitoring application), can use GPS as a precise time source, so events may be timed accurately. TDMA communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and multiplexers.
- Mobile Satellite Communications—Satellite communications systems use a directional antenna (usually a "dish") pointed at a satellite. The antenna on a moving ship or train, for example, must be pointed based on its current location. Modern antenna controllers usually incorporate a GPS receiver to provide this information.
- Emergency and Location-based services—GPS functionality can be used by emergency services to locate cell phones. The ability to locate a mobile phone is required in the United States by E911 emergency services legislation. However, as of September 2006 such a system is not in place in all parts of the country. GPS is less dependent on the telecommunications network topology than radiolocation for compatible phones. Assisted GPS reduces the power requirements of the mobile phone and increases the accuracy of the location. A phone's geographic location may also be used to provide location-based services including advertising, or other location-specific information.
- Location-based games—The availability of hand-held GPS receivers has led to games such as Geocaching, which involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects hidden by other geocachers. This popular activity often includes walking or hiking to natural locations. Geodashing is an outdoor sport using waypoints.
- Aircraft passengers—Most airlines allow passenger use of GPS units on their flights, except during landing and take-off when other electronic devices are also restricted. Even though consumer GPS receivers have a minimal risk of interference, a few airlines disallow use of hand-held receivers during flight. Other airlines integrate aircraft tracking into the seat-back television entertainment system, available to all passengers even during takeoff and landing.
- Heading information—The GPS system can be used to determine heading information, even though it was not designed for this purpose. A "GPS compass" uses a pair of antennas separated by about 50 cm to detect the phase difference in the carrier signal from a particular GPS satellite. Given the positions of the satellite, the position of the antenna, and the phase difference, the orientation of the two antennas can be computed. More expensive GPS compass systems use three antennas in a triangle to get three separate readings with respect to each satellite. A GPS compass is not subject to magnetic declination as a magnetic compass is, and doesn't need to be reset periodically like a gyrocompass. It is, however, subject to multipath effects.
- GPS tracking systems use GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a log of movements. The data can be stored inside the unit, or sent to a remote computer by radio or cellular modem. Some systems allow the location to be viewed in real-time on the Internet with a web-browser.
- Weather Prediction Improvements—Measurement of atmospheric bending of GPS satellite signals by specialized GPS receivers in orbital satellites can be used to determine atmospheric conditions such as air density, temperature, moisture and electron density. Such information from a set of six micro-satellites, launched in April 2006, called the Constellation of Observing System for Meteorology, Ionosphere and Climate COSMIC has been proven to improve the accuracy of weather prediction models.
- Photograph annotation—Combining GPS position data with photographs taken with a (typically digital) camera, allows one to lookup the locations where the photographs were taken in a gazeteer, and automatically annotate the photographs with the name of the location they depict. The GPS device can be integrated into the camera, or the timestamp of a picture's metadata can be combined with a GPS track log.
- Skydiving—Most commercial drop zones use a GPS to aid the pilot to "spot" the plane to the correct position relative to the dropzone that will allow all skydivers on the load to be able to fly their canopies back to the landing area. The "spot" takes into account the number of groups exiting the plane and the upper winds. In areas where skydiving through cloud is permitted the GPS can be the sole visual indicator when spotting in overcast conditions, this is referred to as a "GPS Spot."
- Marketing—Some market research companies have combined GIS systems and survey based research to help companies to decide where to open new branches, and to target their advertising according to the usage patterns of roads and the socio-demographic attributes of residential zones.
History
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 and are now manufactured by
Lockheed Martin.
Timeline
- In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS system 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 system achieved initial operational capability
- By January 17, 1994 a complete constellation of 24 satellites was in orbit.
- 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, allowing users outside the US military to receive a full quality signal.
- In 2004, U.S. President George W. Bush updated the national policy, replacing the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.
- The most recent launch was on November 17, 2006. The oldest GPS satellite still in operation was launched in August 1991.
Awards
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 systems
- GLONASS (GLObal NAvigation Satellite System) is operated by Russia, although with only twelve active satellites as of 2004. In Russia, Northern Europe and Canada, at least four GLONASS satellites are visible 45 percent of time. There are plans to restore GLONASS to full operation by 2008 with assistance from India.
- Galileo is being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2010.
- Beidou may be developed independently by China.