Encyclopedia
The
Global Positioning System, usually called
GPS, is the only fully-functional
satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio, allowing any
GPS receiver to accurately determine its location in any weather, day or night, anywhere on
Earth.
GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for
map-making and
land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes. GPS receivers can also gauge altitude and speed with a very high degree of accuracy.
The
United States Department of Defense developed the system, officially named
NAVSTAR GPS , and launched the first experimental satellite in 1978. The satellite constellation is managed by the
50th Space Wing. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.
In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called
L2C for enhanced accuracy and reliability. In the coming years, additional next-generation satellites will increase coverage of L2C and add a third and fourth civilian signal to the system, as well as advanced military capabilities.
The
Wide Area Augmentation System , available since August 2000, increases the accuracy of GPS signals to within 2
meters for compatible receivers. GPS accuracy can be improved further, to about 1 cm over short distances, using techniques such as Differential GPS .
Applications
The GPS is a "constellation" of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 1 to 100 meters depending on the type of equipment used. The GPS is owned and operated by the U.S. Department of Defense, but is available for general use around the world.
GPS works like this:
- 24 GPS satellites are in orbit at 20,200 kilometers above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon.
- Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, the satellite continually broadcasts its changing position and time.
- On the ground, any GPS receiver contains a computer that "triangulates" its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters.
- If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map.
- If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
- If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations.
Military
GPS allows accurate targeting of various military weapons including
cruise missiles and
precision-guided munitions, as well as improved command and control of forces through improved locational awareness. The satellites also carry nuclear detonation detectors, which form a major portion of the
United States Nuclear Detonation Detection System. Civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates; this is to prevent them from being used to create improvised missiles.
Navigation
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. Personal Navigation Devices such as hand-held GPS are used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turn points in competitions. Low cost GPS receivers are often combined with
PDAs,
cell phones, car computers, or vehicle tracking systems. Examples of GPS-based services are
MapQuest Mobile and TomTom digital maps. The system can be used to automate harvesters, mine trucks, and other vehicles. GPS equipment for the visually impaired is available.
Mobile Satellite Communications
Satellite communications systems permit "remotes" to communicate with "hubs" via satellites. A typical system uses satellites in geosynchronous orbit: this requires a directional antenna that is pointed at the satellite. When the "remote" is portable, as on a ship or a train, the antenna must be pointed based on its current location. Essentially all modern antenna controllers incorporate a GPS receiver to provide this location information.
The remote uses its location for two distinct purposes: first, to point the antenna at the satellite, and second, to compute the distance to the satellite. The distance to the satellite is crucial when deciding when to transmit a
TDMA burst.
In this application, there are two distinct types of satellites and two distinct antennas: the GPS satellies are MEO and the GPS antenna is typically a 2cm sq. "patch antenna." The communications satellites are GEO and the communications antenna is typically 1m or larger. To a first approximation, the GPS system is less than 1% of the total cost of the remote system.
Location-based services
GPS functionality can be used by emergency services and location-based services to locate mobile phones. Assisted GPS is a GPS technology often used by the mobile phone because it reduces the power requirements of the mobile phone and increases the accuracy of the location obtained. GPS provides a location solution which is less dependent on the telecommunications network
topology, but more dependent on the mobile phone than methods using radiolocation. The ability to locate a mobile phone to reasonable accuracy is mandated 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. The mobile phone location may also be used to provide location specific information to the mobile phone, such as location specific advertising, or providing service information specific to the phone user's geographic location.
Another location based service/application incorporating gps signals is a Gps tour guide
Location-based games
The availability of hand-held GPS receivers for a cost of about $90 and up has led to recreational applications including location-based games like the popular game
Geocaching. Geocaching 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. Other location-based games are played controversially by two or more teams on the streets of a city, but most of these are rather still in the stage of research prototypes than a commercial success.
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 inexpensive consumer GPS units have a minimal risk of interference, there is still a potential for interference. Because of this possibility, a few airlines disallow use of hand-held receivers for safety reasons. However, other airlines integrate aircraft tracking into the seat-back television entertainment system, available to all passengers even during takeoff and landing.
Surveying
More costly and precise receivers are used by land
surveyors to locate boundaries, structures, and survey markers, and for road construction. There is also a growing demand for Machine Guidance such as Automatic Grade Control systems that use GPS positions and 3D site plans to automatically control the blades and buckets of construction equipment.
Agriculture
GPS Machine Guidance is used for tractors and other large agricultural machines via auto steer or a visual aid displayed on a screen, which is extremely useful for controlled traffic and row crop operations and when spraying. As well as guidance, GPS used in harvesters with yield monitors can provide a yield map of the paddock being harvested.
Geophysics and geology
High precision measurements of crustal strain can be made with GPS by finding the relative displacement between GPS sites, one of which is assumed to be stationary. Multiple stations situated around an actively deforming area can be used to find strain and site velocities relative to a stable reference site. These measurements can then be inverted using the relationships between stress and strain to interpret the source and cause of the deformation. For example, measurements of ground deformation around a volcano can be used to interpret the source and cause—a dike, sill, or other body beneath the surface.
Precise time reference
Many systems that must be accurately synchronized use GPS as a source of accurate time. For instance, the GPS can be used as a
reference clock for time code generators or
NTP clocks. Also, when deploying sensors , GPS may be used to provide each recording apparatus with a precise time source, so that the time of events may be recorded accurately. Communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and
multiplexers.
The
atomic clocks on the satellites are set to "GPS time". GPS time is counted in days, hours, minutes, and seconds, in the manner that is conventional for most time standards. However, GPS time is not corrected to the rotation of the Earth, ignoring
leap seconds and other corrections. GPS time was set to read the same as
Coordinated Universal Time in 1980, but has since diverged as leap seconds were added to UTC.
The GPS day is identified in the GPS signals using a week number along with a day-of-week number. GPS week zero started at 00:00:00 UTC on January 6 1980. The week number is transmitted in a ten-bit field, and so it wraps round every 1,024 weeks . The transmitted week number returned to zero at 00:00:19 TAI on August 22, 1999 . GPS receivers thus need to know the time to within 3,584 days in order to correctly interpret the GPS time signal. A new field is being added to the GPS navigation message that supplies the calendar year number in a sixteen-bit field, thus performing this disambiguation for any receivers that know about the new field.
The GPS navigation message also includes the difference between GPS time and UTC, which is 14 seconds
as of 2006. Receivers subtract this offset from GPS time in order to display UTC time. They may further adjust the difference against the UTC time for a local time zone. New GPS units will initially show the incorrect UTC time, or not attempt to show UTC time at all, after achieving a GPS lock for the first time. However, this is usually corrected within 15 minutes, once the UTC offset message is received for the first time. The GPS-UTC offset field is only eight bits, and so it wraps round every 256 leap seconds. There is also a leap second warning bit, to help GPS receivers tick UTC correctly through a leap second, but its use is troublesome because of misunderstandings about its semantics.
History & Timeline
The design of GPS is based partly on the similar ground-based radio navigation systems, such as
LORAN 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 converse is also true: if the satellite's position were known, they could identify their own position on Earth.
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.
In 1983, after Soviet
interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board,
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. The first modern Block-II satellite was launched on February 14, 1989.
In 1992, the
2d Space Operations Squadron, which originally managed the system, was de-activated and replaced by the
50th Space Wing.
The system achieved initial operational capability by December 1993 A complete constellation of 24 satellites was in orbit by January 17, 1994.
In 1996, recognizing the importance of GPS to civilian users as well as military users, 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, 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.
In 2004, President
George W. Bush updated the national policy, replacing the board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.
The most recent launch was September 25, 2006. The oldest GPS satellite still in operation was launched in February 1989.
Technical description
Navigation signals
GPS satellites broadcast three different types of data in the primary navigation signals. The first is the
almanac which sends coarse time information with second precision 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 at any point in time. These bits of data are folded into the 37,500 bit
Navigation Message, or
NM, which takes 12.5 minutes to send at 50 Hz.
The satellites also broadcast two forms of accurate clock information, the
Coarse Acquisition code, or
C/A, and the
Precise code, or
P-code. The former is normally used for most civilian navigation. It consists of 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 them to be identified. The P-code is a similar code broadcast at 10.23 MHz, but it repeats only once per week. In normal operation, the so-called "anti-spoofing mode", the P code is first encrypted into the
Y-code, or
P, which can only be decrypted by units with a valid decryption key. All three signals, NM, C/A and P, are mixed together and sent on the primary radio channel, L1, at 1575.42 MHz. The P signal is also broadcast alone on the L2 channel, 1227.60 MHz. Several additional frequencies are used for unrelated purposes.
Calculating positions
GPS allows receivers to accurately calculate their distance from the GPS satellites. The receivers do this by measuring the time delay between when the satellite sent the signal and the local time when the signal was received. This delay, multiplied by the speed of light, gives the distance to that satellite. The receiver also calculates the position of the satellite based on information periodically sent in the same signal. By comparing the two, position and range, the receiver can discover its own location.
To calculate its position, a receiver first needs to know the precise time. To do this, it uses an internal
crystal oscillator-based clock that is continually updated by the signals being sent in L1 from various satellites. At that point the receiver identifies the visible satellites by the distinct pattern in their C/A codes. It then looks up the ephemeris data for each satellite, which was captured from the NM and stored in memory. This data is used in a formula that calculates the precise location of the satellites at that point in time.
Finally the receiver must calculate the time delay to each satellite. To do this, it produces an identical C/A sequence from a known seed number. The time delay is calculated by increasingly delaying the local signal and comparing it to the one received from the satellite; at some point the two signals will match up, and that delay is the time needed for the signal to reach the receiver. The delay is generally between 65 and 85 milliseconds. The distance to that satellite can then be calculated directly, the so-called pseudorange.
The receiver now has two key pieces of information: an accurate estimate of the position of the satellite, and an accurate measurement of the distance to that satellite. This tells the receiver that it lies on the surface of an imaginary sphere whose radius is that distance. To calculate the precise position, at least four such measurements are taken simultaneously. This places the receiver at the intersection of the four imaginary spheres. Since the C/A pattern repeats every millisecond, it can only be used to place the user within 300 kilometers . Thus the multiple measurements are also needed to determine whether the receiver has lined up its internal C/A code properly, or is "one off".
The calculation of the position of the satellite, and thus the time delay and range to it, all depend on the accuracy of the local clock. The satellites themselves are equipped with extremely accurate
atomic clocks, but this is not economically feasible for a receiver. Instead, the system takes redundant measurements to re-capture the correct clock information.
To understand how this works, consider a local clock that is off by 0.1 microseconds, or about 30 meters when converted to distance. When the position is calculated using this clock, the range measurements to each of the satellites will read 30 meters too long. In this case the four spheres will not overlap at a point, instead each sphere will intersect at a different point, resulting in several potential positions about 30 meters apart. The receiver then uses a mathematical technique to calculate the clock error that would produce this offset, in this case 0.1 microseconds, adjusts the range measurements by this amount, and then updates the internal clock to make it more accurate.
This technique can be applied with any four satellites. Commercial receivers therefore attempt to "tune in" to as many satellites as possible, and repeatedly make this correction. In doing so, clock errors can be reduced almost to zero. In practice, anywhere from six to ten measurements are taken in order to round out errors, and civilian receivers generally have 10 to 12 channels in total.
Calculating a position with the P 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 a set of navigationally consistent C/A signals can be generated using readily available off the shelf signal generators. Even if the victim receiver incorporates RAIM features it will still "buy In' to the spoofing signals since RAIM only checks to make sure the signals make sense from a navigational perspective.
Accuracy
Best case
The position calculated by a GPS receiver relies on three accurate measurements: the current time, the position of the satellite, and the time delay for the signal. Errors in the clock signal can be reduced using the method above, meaning that the overall accuracy of the system is generally based on the accuracy of the position and delay.
The measurement of the delay requires the receiver to "lock onto" the same sequence of bits being sent from the satellite. This can be made relatively accurate by timing comparing the rising or trailing edges of the bits. Modern electronics can lock the two signals to about 1% of a bit time, or in this case about 1% of a microsecond. Since light travels at 299,792,458 m/s, this represents an error of about 3 meters , the minimum error possible given the timing of the C/A signal.
This can be improved by using the higher-speed P signal, assuming the same 1% accuracy in locking the retrieved P-code to the internally generated version. In this case the same calculation results in an accuracy of about 30 centimeters . Since the P-code repeats at 10.23 MHz, it has a "repeat range" of about 30 kilometers . This explains the terminology; when using the P-code, it was first necessary to calculate a coarse position with the C/A code in order to determine how to line up the P-code with the internally generated copy.
However, several "real world" effects intrude and degrade the accuracy of the system. These are outlined in the table below, with descriptions following. When all of these effects are added up, GPS is typically accurate to about 15 meters . These effects also overwhelm the P code's added accuracy.
Sources of error| 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
One of the biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the
ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, since the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error.
Because ionospheric delay affects the speed of radio waves differently based on their frequencies, the second frequency band can be used to help eliminate this type of error. Some military and expensive survey-grade civilian receivers can compare the difference between the P signal carried in the L1 and L2 frequencies to measure atmospheric delay and apply precise corrections. This correction can be applied even without decrypting the P signal, as long as the encryption key is the same on both channels. In order to make this easier, the military is considering broadcasting the C/A signal on L2 starting with the Block III-R satellites. This would allow a direct comparison of the L1 and L2 signals using the same circuitry that already decodes the C/A on L1.
The effects of the ionosphere are generally slow-moving and can easily be tracked. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction, say, "10 meters to the east" is also valid for other receivers in the same general location. Several systems send this information over radio or other links to the receivers, allowing them to make better corrections than a comparison of L1 and L2 alone could.
The amount of humidity in the air also has a delaying effect on the signal, resulting in errors similar to those generated in the ionosphere but located much closer to the ground in the
troposphere. The areas affected by these problems tend to be smaller in area and faster moving than the billows in the ionosphere, making accurate correction for these effects more difficult.
Multipath effects
GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. This delay in reaching the receiver causes inaccuracy. A variety of receiver 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. This form of multipath is harder to filter out since it is only slightly delayed as compared to the direct signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe in dynamic applications such as cars and planes. 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 tends to be "out of date" by an even larger amount. 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. Additionally, the amount of accuracy sent in the ephemeris is limited by the bandwidth; using the data from the satellites alone limits its accuracy.
Further, while it is true that the onboard clocks are extremely accurate, they do suffer from clock drift. This problem tends to be very small, but may add up to 2 meters of inaccuracy.
These sorts of errors are even more "stable" than ionospheric problems and tend to change on the order of days or weeks, as opposed to minutes. This makes correcting for these errors fairly simple by sending out a more accurate almanac on a separate channel.
Techniques to improve accuracy
The accuracy of GPS can be improved several ways:
- Differential GPS can improve the normal GPS accuracy of 4-20 meters to 1-3 meters . DGPS uses a network of stationary GPS receivers to calculate the difference between their actual known position and the position as calculated by their received GPS signal. The "difference" is broadcast as a local FM signal, allowing many civilian GPS receivers to "fix" the signal for greatly improved accuracy. The US Coast Guard maintains a similar system on marine longwave radio near ports and major waterways, supplemented by additional sites in Canada.
- The Wide Area Augmentation System . This system uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays and individual satellite clock drift. Although only a few WAAS satellites are currently available as of 2004
...
, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation . The current WAAS only works for North America , and because of the satellite location, the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS are being developed in Europe and Japan , which are virtually identical to WAAS.
- A Local Area Augmentation System . This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometer radius around the transmitter.
- Exploitation of DGPS for Guidance Enhancement is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition
...
.
- A Carrier-Phase Enhancement . This technique utilizes the 1.575 GHz L1 carrier wave to act as a sort of additional clock signal, resolving ambiguity caused by variations in the location of the pulse transition of the C/A PRN signal. The problem arises because the transition from 0-1 or 1-0 on the C/A signal is not instantaneous, it takes a non-zero amount of time, and thus the correlation operation is imperfect. A successful correlation could be defined in a number of various places along the rising/falling edge of the pulse, which imparts timing errors. CPGPS solves this problem by using the L1 carrier, which has a period 1/1000 that of the C/A bit width, to define the transition point instead. The phase difference error in the normal GPS amounts to a 2-3 meter ambiguity. CPGPS working to within 1% of perfect transition matching can achieve 3 mm ambiguity; in reality, CPGPS coupled with DGPS normally realizes 20-30 centimeter accuracy.
- Wide Area GPS Enhancement is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris data to specially-equipped receivers.
- Relative Kinematic Positioning is another approach for a precise GPS-based positioning system. In this approach, accurate determination of range signal can be resolved to an accuracy of less than 10 centimeters . 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 correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time .
- Many automobiles that use the GPS combine the GPS unit with a gyroscope and speedometer
...
pickup, allowing the computer to maintain a continuous navigation solution by dead reckoning when buildings, terrain, or tunnels block the satellite signals. This is similar in principle to the combination of GPS and
inertial navigation used in ships and aircraft, but less accurate and less expensive because it only fills in for short periods.
Selective availability
When it was first deployed, GPS included a feature called
Selective Availability that introduced intentional errors of up to a hundred meters 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 horizontally and 30 meters 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 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 turned off in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded L1 signal.
The US military has developed the ability to locally deny GPS to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems. Such
Navigation Warfare uses techniques such as local jamming to replace the blunt, world-wide degradation of civilian GPS service that SA represented.
Military users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere.
Satellites
As of August 2006 the GPS system used a satellite constellation of 29 active Block II/IIA/IIR/IIR-M satellites in intermediate circular orbits. The constellation includes three spare satellites in orbit, in case of any failure. Each satellite circles the Earth twice each day at an altitude of about 20,200 kilometers . The orbits are aligned so at least four satellites are always within line of sight from almost any place on Earth.
There are four active satellites in each of six orbital planes. Each orbit is
inclined 55 degrees from the
equatorial plane, and the
right ascension of the ascending nodes is separated by sixty degrees.
The flight paths of the satellites are measured by five monitor stations around the world . The master control station, at Schriever Air Force Base, processes their combined observations and sends updates to the satellites through the stations at Ascension Island, Diego Garcia, and Kwajalein. The updates synchronize the atomic clocks on board each satellite to within one microsecond, and also adjust the ephemeris of the satellites' internal orbital model to match the observations of the satellites from the ground.
Frequencies used
Several frequencies make up the GPS
electromagnetic spectrum:
- L1 :
Carries a publicly usable coarse-acquisition code as well as an encrypted precision P code. - L2 :
Usually carries only the P code, but will also carry a second C/A code on the Block III-R satellites. - L3 :
Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches , nuclear detonations, and other high-energy infrared events.
Two new signals are also being studied:
- L4 :
Being studied for additional ionospheric correction. - L5 :
Proposed for use as a civilian safety-of-life signal. 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.
Receivers
GPS receivers vary widely in accuracy because of the expense of adding more radio receivers needed to tune in more satellites. For instance, early consumer-grade receivers typically included six to eight receivers for the L1 C/A signal. As the computer industry has improved the state of the art in chipmaking, the cost of implementing these receivers has fallen dramatically, and even low-cost hand held receivers typically have twelve receivers today. More expensive units, known as "dual-frequency receivers", also tune in the L2 signals in order to correct for ionospheric delays.
Another major factor in the accuracy of a GPS fix is the amount of processing applied to the received signals. This is a function of the performance of the electronics and the required battery life. These factors have also been dramatically affected by improved chip making, allowing even low cost modern receivers to outperform much more expensive earlier models.
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 bps 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. The cost of implementing these receivers is also falling dramatically, and even low-cost units are commonly including
WAAS receivers today.
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 are controlled on a for-profit basis 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 protocol. Receivers can interface with external devices via a number of means, such as a serial connection, a USB connection or even a
Bluetooth wireless connection.
Time dilation
According to Einstein's
Theory of relativity, because of their constant movement and height relative to the Earth Centered Inertial
reference frame the clocks on the satellites are affected by their speed as well as their gravitational potential . Consequently it was expected that when observed from the Earth's reference frame, satellite clocks would be perceived as running at a slightly faster rate than clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, when observed from the Earth. To account for this, the frequency standard on-board the satellites is set to run slightly slower than its desired frequency on Earth, at 10.22999999543 MHz instead of 10.23 MHz—a difference of 0.00457 Hz. The satellite clocks are claimed to be well tuned when in orbit, making it a practical demonstration of the theory of relativity in a real-world system.
Neil Ashby presented in
Physics Today an account how these relativistic corrections are applied, and their orders of magnitude.
Note that in GPS the theory of relativity is a mere correction to classical wave theory; with new positioning algorithms that are meant to eliminate noise effects such corrections have become less important. Thus in his book GPS Satellite Surveying, Alfred Leick writes :
- Relativistic effects are important in GPS surveying, but fortunately can be accurately computed. The atomic frequency standards in the satellites are affected by both special relativity and general relativity
Therefore, geometrical differencing in precise positioning cancels out most of the relativistic effects.
Awards
Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:
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."
GPS tracking
A GPS tracking system uses GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a track file or log of activities. The recorded data can be stored within the tracking unit, or it may be transmitted to a central location, or Internet-connected computer, using a cellular modem, 2-way
radio, or satellite. This allows the data to be reported in real-time, using either web browser based tools or customized software.
GPS jamming
Jamming of any radio navigation system, including satellite based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in Phrack issue 60 by an anonymous author. There has also been at least one well-documented case of unintentional jamming, tracing back to a malfunctioning TV antenna preamplifier. If stronger signals were generated intentionally, they could potentially interfere with aviation GPS receivers within line of sight. According to John Ruley, of AVweb, "IFR pilots should have a fallback plan in case of a GPS malfunction". Receiver Autonomous Integrity Monitoring , a feature of some aviation and marine receivers, is designed to provide a warning to the user if jamming or another problem is detected. GPS signals can also be interfered with by natural
geomagnetic storms, predominantly at high latitudes.
GPS jammers the size of a cigarette box are allegedly available from
Russia; their effectiveness is in question following their use in the
Iraq War. The
U.S. government believes that such jammers were also used occasionally during the
2001 war in Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards non-combatant infrastructure; other officials believe that the jammers are completely ineffective. In either case, the jammers may be attractive targets for
anti-radiation missiles. Low power jammers would have limited military usefulness and high power jammers would be easy to locate and destroy. During the Iraq War, the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb.
Other systems
Russia operates an independent system called
GLONASS , although with only twelve active satellites
as of 2004, the system is of limited usefulness. Availability in Russia, Northern Europe and Canada is above 90%. At least 4 GLONASS satelites are visible 90% of time, which is notable considering
GLONASS operates only 12 of 24 required satellites. Dual-band GPS and GLONASS receivers can use satellites from either system, increasing availability in regions where satellite visibility is a problem.
There are plans to restore GLONASS to full operation by 2008 with assistance from India. The
European Union is developing
Galileo as an alternative to USA owned operated GPS system. The
People's Republic of China,
Israel,
India,
Morocco,
Saudi Arabia and
South Korea joined the
EU in this project. It is planned to be operational by 2010.
See also
- adding
geographic coordinates to WikiPedia articles.
.
,
Wikimedia proposed project.
- Satellite navigation, Chapter 11
- Hafele-Keating experiment#GPS
References
External links
- , updated daily
- - Large amount of technical information and discussion.
- at howstuffworks.com
- - The official signal specification.
- — 16-page paper about the history and working of GPS, touching on the upcoming Galileo
- , including information about each satellite's configuration and launch.
- U.S. Army Corps of Engineers manual: and
- Technical Article by Kanwar Chadha, BSEE
- , Status of the GPS constellation, government policy, and links to other references. Also includes satellite almanac data.
- - Established in 2004 to oversee management of GPS and GPS augmentations at a national level.
- - Responsible for designing and acquiring the system on behalf of the US Government.
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-
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- — Tutorial designed to introduce you to the principles behind GPS
- — excellent introduction for newbies
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