Global Positioning System (GPS) - Applications, Technical description, Accuracy, Techniques to improve accuracy, Selective availability, Satellites, Receivers, Relativity, Awards, GPS tracking
A means of determining an exact position on the Earth using a GPS receiver and a system of satellites. Twenty-four satellites make up the American NAVSTAR system, orbiting about 20 000 km above the Earth. Each satellite makes a complete orbit of the Earth every 12 hours. Their positions are carefully calculated so that, from any point on the Earth, four or more of the satellites will be in direct line of sight to the GPS receiver. The GPS receiver picks up signals which are being transmitted continuously from the satellites. Each satellite carries four atomic clocks, so that the transmission time of the signals is known precisely. Using the time difference between the signals being transmitted and received, the GPS calculates the distance to that satellite. It has to lock on to at least three different satellite signals in order to locate itself in three-dimensional space. The Standard Positioning Service (SPS) is accurate to about 100 m (325 ft) and the Precise Positioning Service (PPS) is accurate to 20 m (65 ft). The Differential GPS (DGPS) also uses additional fixed stations on Earth and gives horizontal position accuracy to about 3 m (10 ft). NAVSTAR was begun in the 1970s and all 24 satellites were in operation by 1994. GPS was used to ensure that the two sets of teams digging the Channel Tunnel would meet in the centre.
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 (abbreviated to GPSr) to accurately determine its location (longitude, latitude, and altitude) 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 (Navigation Signal Timing and Ranging Global Positioning System), and launched the first experimental satellite in 1978. 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 (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft) for compatible receivers. GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).
Applications
The GPS (Global Positioning System) is a "constellation" of at least 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. 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:
A minimum of 24 GPS satellites are in orbit at 20,200 kilometers (12,600 miles) above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon. With an understanding of its own orbit and the clock, the satellite continually broadcasts its changing position and time. A number of times per day, depending upon various requirements, 2 SOPS of the USAF contacts each of the GPS space vehicles (or SV or satellite) and provides it with a new navigational upload. On the ground, any GPS receiver contains a computer that calculates its own position by getting time signals from three of the four satellites, using a process called trilateration (similar to triangulation). Some specialized GPS receivers can also store data for use in Geographic Information Systems (GIS) and map making.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. Civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates;
Navigation
GPS is used by people around the world for navigation in vehicles of all types and when moving about on foot.
Automobiles
An increasing number of automobiles are equipped with GPS receivers, either at the time of manufacture or as aftermarket equipment, to assist drivers in finding their way in areas with which they are unfamiliar.
Ships
Boats and ships are important users of GPS, which allows them to find their way anywhere on the world's seas and oceans, even in the absence of any other reference points. Maritime GPS units include functions specifically useful for use on water, such as "man overboard" functions that allow instant recording and retrieval of the location at which a person has fallen overboard, which simplifies rescue efforts. GPS capabilities may be integrated into larger systems for navigational or other use aboard the boat or ship.
Aircraft
Many aircraft use GPS for en-route navigation, although take-off, approach, and landing phases are still carried out using other types of navigational aids in most cases. Flight management systems in commercial airliners usually include GPS capability. Most aviation GPS receivers are built into aircraft, but handheld aviation receivers are available.
Aviation GPS receivers may use augmentation technologies such as WAAS or LAAS to increase accuracy and permit the use of GPS for approach and landing operations.
The relatively poor accuracy of altitude data in standard GPS (as compared to lateral navigation measurements or traditional barometric altimeters) has made GPS inappropriate as the sole means of altitude measurement in aircraft, but continuing development in GPS and augmentation technologies may lift this restriction in the future, particularly for certain critical phases of flight such as instrument landings.
Glider pilots use logged GPS data to verify their arrival at turn points in competitions.
On foot
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). Many models of inexpensive handheld GPS receivers are available for these uses, and they are one type of Personal Navigation Device (PND) used for these applications.
Other uses
Low-cost GPS receivers are often combined with PDAs, cell phones, car computers, or vehicle tracking systems. 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 (usually a "dish") that is pointed at the satellite. 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.
In this application, there are two distinct types of satellites and two distinct antennas: the GPS satellites are MEO and the GPS antenna is typically a 2cm sq. 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.
Location-based games
The availability of hand-held GPS receivers for a cost of about US$90 and up (as of March 2005) 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.
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.
Surveying
More costly and precise receivers are used by land surveyors to position boundaries, structures, and survey markers, and for road construction. Survey grade GPS receivers use the carrier wave signal from both the L1 and L2 GPS frequencies. 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 GIS
Mapping of resources and other less precise applications typiclally used with Geographic Information Systems often require greater precision than is possible with autonomous GPS receivers, but do not justify the expense of a survey grade receiver. 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.
Machine Guidance
There is also a growing demand for GPS 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. GPS is also used in precision agriculture and can be coupled to an aircraft autopilot or self-steering gear for a ship.
GPS Machine Guidance is used for tractors and other large Agricultural equipment 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 differential GPS by finding the relative displacement between GPS sites, one of which is assumed to be stationary.
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 (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with a precise time source, so that the time of events may be recorded accurately.
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 (UTC) 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 (00:00:19 TAI) on January 6, 1980. 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. 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. 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. 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.
The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. 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.
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.
In 1992, the 2nd Space Wing, 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.
On 2 May 2000, "Selective Availability" was discontinued, allowing users outside the US military to receive a full quality signal.
The most recent launch was on 17 November 2006. The oldest GPS satellite still in operation was launched in October 1990.
Technical description
Navigation signals
GPS satellites broadcast three different types of data in the primary navigation signals. 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.
The satellites also broadcast two forms of accurate clock information, the Coarse Acquisition code, or C/A, and the Precise code, or P-code.
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. The receiver also calculates the position of the satellite based on information periodically sent in the same signal.
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. 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. The time delay is calculated by increasingly delaying the local signal and comparing it to the one received from the satellite;
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.
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.
To understand how this works, consider a local clock that is off by 0.1 microseconds, or about 30 meters (100 ft) 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.
This technique can be applied with any four satellites.
Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.
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 improved by using the higher-speed P(Y) signal, assuming the same 1% accuracy in locking the retrieved P-code to the internally generated version.
However, several "real world" effects intrude and degrade the accuracy of the system. When all of these effects are added up, GPS is typically accurate to about 15 meters (50 ft).
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.
Because ionospheric delay affects the speed of radio waves differently based on their frequencies, the second frequency band (L2) can be used to help eliminate this type of error. In order to make this easier, the U.S. Government has added a new civilian signal on L2, called L2C, starting with the Block IIR-M satellites.
The effects of the ionosphere are generally slow-moving and can easily be tracked.
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.
Multipath effects
GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain;
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. 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.
Further, while it is true that the onboard clocks are extremely accurate, they do suffer from clock drift.
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.
Techniques to improve accuracy
The accuracy of GPS can be improved several ways:
Differential GPS (DGPS) can improve the normal GPS accuracy of 4-20 meters (13-65 ft) to 1-3 meters (3-10 ft). 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. 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 (landing an airplane in conditions of little or no visibility). The transmitters in this application, costing as little as US$50,000, are sometimes called pseudolites (pseudo-satellites), far cheaper than actual orbiting GPS satellites. The phase difference error in the normal GPS amounts to a 2-3 meter (6-10 ft) ambiguity. Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers. 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). 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. Quasi-Zenith Satellite SystemSelective 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.
SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. 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.
In the 1990s, the FAA started pressuring the military to turn off SA permanently. Selective Availability is still a system capability of GPS, and error could be reintroduced at any time.
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. Such Navigation Warfare uses techniques such as local jamming to replace the blunt, worldwide degradation of civilian GPS service that SA represented.
Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy.
Satellites
As of August 2006 the GPS system used a satellite constellation of 29 active Block II/IIA/IIR/IIR-M satellites (for the global coverage 24 is enough) in intermediate circular orbits.
The flight paths of the satellites are measured by five monitor stations around the world (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs). 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 (1575.42 MHz):Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted precision P(Y) code. L3 (1381.05 MHz):
Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events.
Receivers
GPS receivers vary widely in accuracy because of the expense of adding more radio receivers needed to tune in more satellites.
Another major factor in the accuracy of a GPS fix is the amount of processing applied to the received signals.
GPS receivers may include an input for differential corrections, using the RTCM SC-104 format.
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol.
Relativity
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 (special relativity) as well as their gravitational potential (general relativity).
Neil Ashby presented in Physics Today (May 2002) an account how these relativistic corrections are applied, and their orders of magnitude. The GPS system also makes adjustments for the relativistic drift of the atomic clocks in the satellites.
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).One GPS developer, Roger L.
On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. 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.
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. 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". 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; 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 (GLObal NAvigation Satellite System), although with only twelve active satellites as of 2004, the system is of limited usefulness. At least four GLONASS satellites 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.
The European Union is developing Galileo as an alternative to the United States-owned-and-operated GPS system.
According to a rumour China might be abandoning the Galileo project and proceeding with its own Beidou navigation system.
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