GPS satellite in orbit, image courtesy NASA
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 to GPS receivers, allowing them to accurately determine their 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.
The United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging 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 (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).
Magellan GPS receiver in a marine application.
Over fifty GPS satellites such as this NAVSTAR have been launched since 1978.
Applications
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
This taxi in Kyoto, equipped with GPS navigation, is an example of how GPS technology can be applied in routine activities.
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. Hand-held GPS receivers can be 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.
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. 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.
Location-based games
GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such those shown here from manufacturers Trimble, Garmin and Leica (respectively, left to right).
The availability of hand-held GPS receivers for a cost of about $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. 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.
Even fixed systems may use GPS, in order to get precise time. This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.
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 Automatic Grade Control systems that use GPS positions and 3D site plans to automatically control the blades and buckets of construction equipment.
Agriculture
GPS is used for the guidance of 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 (such as a volcano or fault zone) 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 Network Time Protocol 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. 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 (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. The week number is transmitted in a ten-bit field, and so it wraps round every 1,024 weeks (7,168 days). The transmitted week number returned to zero at 00:00:19 TAI on August 22, 1999 (23:59:47 UTC on August 21, 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 UTC time adjust 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
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 inactivated 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 in September 2005. The oldest GPS satellite still in operation was launched in February 1989.
Technical description
Navigation signals
GPS broadcast signal
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(Y), which can only be decrypted by units with a valid decryption key. All three signals, NM, C/A and P(Y), are mixed together and sent on the primary radio channel, L1, at 1575.42 MHz. The P(Y) 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.
Pseudorange
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 (180 mi). 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 .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. 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 .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(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, the C/A signal can be generated fairly easily, allowing an unscrupulous user to send out their own fake signal, which would be difficult to distinguish from the original. Mathematical techniques can be used here as well, making spoofing of the C/A signal a very difficult prospect for any modern receiver equipped with some sort of RAIM system.
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