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Global Positioning System

Global Positioning System

One hazard of human existence is being geographically lost, which can sometimes mean the difference between life and death. The ability to know one's position was considerably enhanced on February 22, 1978, when members of the U.S. Air Force (USAF) Space Division based in Los Angeles, California, launched the first NAVSTAR (Navigation Satellite with Timing and Ranging) satellite in the Global Positioning System (GPS). This satellite-based navigation system enables users anywhere on Earth to determine their location to a high degree of accuracy.

Components of the System

GPS is a satellite-based navigation system consisting of three segments: space, ground, and user. The space and ground segments are run by a military organization called the United States Space Command, which is located in Colorado Springs, Colorado. This command, composed of components of the USAF, the U.S. Army, and the U.S. Navy, launches the NAVSTAR satellites and is responsible for space and ground operations. The user segment includes any organization, ship, person, or airplane that uses GPS.

The space segment consists of a constellation of twenty-four satellites based in six different orbital planes at an altitude of 20,000 kilometers (12,400 miles). In this orbit, each satellite circles the planet twice in twenty-four hours and travels at the speed of 3.89 kilometers per second (8,640 miles per hour). Each satellite has an inclination of 55 degrees with respect to the equator, which means that it flies to a maximum of 55 degrees north latitude and 55 degrees south latitude during its orbits. The ground segment consists of the radar stations that monitor the satellites to determine the position and clock accuracy of each satellite. The locations of these ground stations are: Hawaii; Ascension Island, located in the southern Atlantic; Diego Garcia, an island in the Indian Ocean; Kwajalein, part of the Marshall Islands of the western Pacific; and Schriever Air Force Base, Colorado. The stations are staffed continuously to ensure that GPS broadcasts the most accurate data possible.

Each NAVSTAR satellite weighs about 1,000 kilograms (2,200 pounds) and is 5.25 meters (17 feet) long with its solar arrays extended. The spacecraft transmits its timing information to Earth with the power of 50 watts, obtained from the solar panels and augmented battery power. Using its 50 watts, the satellite transmits two signals called "Links," L1 and L2, shorthand for Link1 and Link2. L1 and L2 are "downlinks" because their signals go to Earth. Two cesium and two rubidium atomic clocks provide signal timing. Atomic clocks are not powered atomically; they measure the precise oscillations of cesium and rubidium atoms. These oscillation measurements are so accurate that an atomic clock, if left unadjusted, would gain or lose one second every 160,000 years. But how does accurate timing from a satellite at an altitude of 20,000 kilometers translate into a position within meters on Earth?

How Positions Are Determined

Distance to the satellitethe rangeis the key for determining positions on Earth. Time is related to range by a very simple formula: Range Velocity Time. For GPS, the range is the distance from the receiver to the satellite; the velocity equals the speed of light (300,000 kilometers per second [186,300 miles per second]); and the time is the time it takes to synchronize the satellite signal with the receiver. Because the speed of light is so fast, the key to measuring range is the accurate timing provided by the atomic clocks.

What is meant by synchronizing the satellite signal with the receiver? First, imagine that a GPS satellite begins to play the song "Twinkle, Twinkle, Little Star." Simultaneously, a GPS receiver starts playing the same song. The satellite's signal has to travel 20,000 kilometers to the receiver, and by the time it does, the words are so late that when the receiver says "Star" the satellite's signal starts its first "Twinkle." If two versions of the song were played simultaneously, they would interfere with one another. Consequently, the receiver determines the delay time when it receives the satellite's first "Twinkle" and then starts to play the receiver's tune with a delay time calculated, thereby synchronizing with the satellite's signal. The amount of delay time is the signal travel time. This signal travel time is multiplied by the speed of light to determine the range.

Obviously, the GPS does not use "Twinkle, Twinkle, Little Star," but rather it generates an electronic signal. This signal is similar to the interference heard on the radio when one cannot tune in the correct station or the "snow" one sees on one's television when the set is not on an operational channel. This electronic signal from the GPS satellite is called the Pseudo Random Code (PRC).

A PRC is a very complex electronic signal that repeats its pattern. The pattern of zeros and ones in the digital readouts ensures that the user segment receivers synchronize only on a NAVSTAR satellite downlink and not on some other electronic signal. Because each satellite has its own unique PRC, the twenty-four satellites do not jam each other's signals. This allows all the satellites to use the same GPS frequencies . Each satellite transmits two PRCs, over L1 and L2. The L1 PRC is known as Coarse Acquisition (CA), and it allows civilian receivers to determine position within 100 meters (330 feet). The second PRC is called the precise code, or "P," and is transmitted on L2. The P combines with the CA for orientation and then encrypts the signal to permit only personnel with the correct decoding mechanism, called a key, to use it. When L2 is encrypted, it is called the Y code and has an accuracy of 10 meters (33 feet).

Besides clock accuracy and PRC reception, the receiver needs to know the satellite's location. A typical receiver anywhere on Earth will see about five satellites in its field of view at any given instant. The USAF uses the GPS Master Plan for satellites to ensure that a minimum number are always in view anywhere on Earth. Additionally, all GPS receivers produce an almanac that is used to locate each GPS satellite in its orbital slot. The USAF, under the control of U.S. Space Command, monitors each satellite to check its altitude, position, and velocity at least twice a day. A position message, a clock correction, and an ephemeris (the satellite's predicted position) are also updated and uplinked to the GPS satellite daily.

A receiver needs ranges and satellite location information from three satellites to make a position determination. To obtain this, the receiver determines the range while synchronizing its internal clock on the first satellite's correct Universal time , which is based on the time in Greenwich, England. Once the clocks have been synchronized and the range to the first satellite has been determined, the receiver also determines the ranges to two other satellites. Each satellite's range can be assumed to be a sphere with the receiver at the center. The intersection of the three spheres yields two possible positions for the receiver. One of these positions must be invalid because it will place the user either in outer space or deep inside Earth, so the receiver has to be at the second position. Then the receiver compares the satellite's ephemeris and current almanac location to obtain the receiver's latitude and longitude. A fourth GPS satellite's range synchronizes the receiver's clock with all the atomic clocks aboard the spacecraft, narrows the accuracy of the receiver's position to only one intersecting point, and determines the receiver's altitude.

Selective Availability and Differential GPS

There are several errors in timing, ephemeris, and the speed of light for which the system must correct. However, the crews of U.S. Space Command occasionally must induce errors to keep the accuracy of the GPS system from falling into the hands of a hostile force. This error inducement is called "selective availability." To accomplish this, the crew inserts either intentional clock or ephemeris errors. On May 1, 2000, President Bill Clinton ordered the removal of selective availability, greatly enhancing the public use of GPS. However, the probability that access to data would be blocked in times of hostilities has led to a proposal for an independent European GPS-style system called Galileo.

When selective availability was introduced, a number of people wanted more accurate GPS readings, leading to the invention of Differential GPS. This system uses a known surveyed position, such as an airport tower, upon which is placed a GPS receiver. The GPS receiver determines its position constantly and compares the GPS position to the surveyed position and develops a "correction" factor that can be applied to make the accuracy of the GPS in the range of inches. Applications of Differential GPS include precision landings with aircraft and precision farming, which allows a farmer to know exactly where to apply fertilizer or pesticide, or both, within a field. Differential GPS is so accurate that it also permits scientists to accurately measure the movement of Earth's tectonic plates, which move at the speed of fingernail growth.

GPS receivers are currently on ships, trains, planes, cars, elephant collars, and even whales. This system promises to change the way we live, and satellite-based navigation is predicted to become a multibillion-dollar industry in the early twenty-first century.

Commercial Enterprises Involved in GPS

There are a number of commercial companies involved in the GPS industry. The largest are the companies that make the satellite itself, Lockheed Martin, Hughes (recently taken over by Boeing), Rockwell (also recently taken over by Boeing), and Boeing Space. The survivor of the takeover business will probably build the next block of GPS satellites, the 2-F block that will be without selective availability.

Commercial possibilities in GPS are in the following areas: aviation, geosciences, marine applications, mapping, survey, outdoor recreation, vehicle tracking, automobile navigation, and wireless communications. Since there are a number of companies involved in GPS, only four of these will be reviewed. Companies that are selling their GPS services for other than space support include Garmin, which is headquartered in Olathe, Kansas, and has subsidiary offices in the United Kingdom and Taiwan. Garmin sells navigation receivers that are portable and have brought navigation to the masses for hiking, motor boat operation, and other recreational vehicle arenas.

Another large company that employs over 500 workers in the manufacture of receivers is Magellan Systems Corporation, located in San Dimas, California. Magellan brought into market the world's first handheld commercial receiver for ordinary uses. Since 1989, Magellan has shipped more than one million of these units and has produced annual sales that now top $100 million. In 1995, Magellan introduced the first hand-held GPS receiver under $200 which led to even greater market expansion. Trimble Navigation Limited, located in Sunnyvale, California, offers services very similar to those of Garmin and Magellan. Trimble also has a subsidiary in the United Kingdom. Trimble has a particularly accurate receiver called the Scoutmaster, which has been used since 1993 with great success. The receiver allows an individual to not only determine latitude and longitude, but also speed on Earth's surface and distances to input navigation points.

Motorola Corporation has been very cooperative in their affiliation with universities putting payloads on satellites and on balloons. Using Motorola GPS units such as the Viceroy and the Monarch, university students have tracked balloon payloads over 240 miles and have used the navigation information to determine the jet stream speed and balloon altitudes over the United States. As the GPS system continues, so too, will ideas from small companies about how to use this information commercially, thus developing industries that people can only dream about at this time in our history.

see also Military Customers (volume 1); Navigation from Space (volume 1); Reconnaissance (volume 1); Remote Sensing Systems (volume 1).

John F. Graham

Bibliography

Larson, Wiley J., and James R. Wertz, eds. Space Mission Analysis and Design, 3rd ed.Torrance, CA: Microcosm Press, 1999.

Logsdon, Tom. The Navstar Global Positioning System. New York: Van Nostrand Reinhold, 1992.

Parkinson, Bradford W., and James J. Spilker Jr., eds. Global Positioning System: Theory and Applications, Vol. I. Washington, DC: American Institute of Aeronautics and Astronautics, Inc., 1996.

. Global Positioning System: Theory and Applications, Vol. II. Washington, DC:American Institute of Aeronautics and Astronautics, Inc., 1996.

Sellers, Jerry Jon. Understanding Space. New York: McGraw-Hill, Inc., 1994.

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Global Positioning System

Global Positioning System


Most people have been lost at one time or another, but what if it were possible to know where you are, anywhere on Earth, 24 hours a day? The Global Positioning System (GPS) can give that information, and it is free to anyone with the proper equipment and a basic knowledge of mathematics.

In the 1980s, the U. S. Department of Defense designed GPS to provide the military with accurate, round-the-clock positional information. Twenty-seven satellites orbiting over 10,000 miles above Earth regularly send information back to Earth. A small piece of equipment, called a GPS receiver, uses this information to compute its position to within a few yards. GPS receivers used for surveying can find positions to within less than one centimeter.

The "constellation" of satellites above the Earth is constantly changing; each orbits Earth twice a day. At any given time there are enough satellite

signals to accurately locate oneself in three dimensions: latitude, longitude, and elevation.

GPS is rapidly becoming a common technology, but it is still a mathematical wonder. Ancient sailors looked to the heavens to estimate their position in the vast oceans. Modern sailors also look to the sky for information, but the modern positioning information they receive is so accurate that any errors are less than the width of the pencil they use to mark their map.

Triangulation

The basic concept of GPS is triangulation. Suppose a person is standing in a valley surrounded by several towering mountain peaks. By using a compass to measure the direction to each peak, this person could locate his or her exact location on a map by using triangulation. After writing down the three measurements (remembering that there are 360 degrees in a circle), a line should be drawn from each peak in the opposite direction just measured.

Then 180 degrees is added or subtracted so that the direction the lines are drawn from each peak will fall between 0 and 360 degrees. For example, if one of the measurements is 270 degrees to peak A, the line from peak A back to the person's position would be 90 degrees. The point at which the three lines intersect is the point at which the person is standing.

The GPS satellites are like mountain peaks; they are known points in space from which lines can be drawn in order to specify a location. Each satellite transmits a radio signal that can be received on Earth and recognized by a GPS receiver. Rather than measure direction, however, a GPS receiver uses the time it takes for each satellite's beacon to reach it and calculates a distance.

Because radio waves travel at the speed of light, the receiver divides the time the signal takes to reach the receiver by the speed of light (186,000 miles per second) and determines the distance. These distances can be used to form spheres around the satellites that will intersect at a specific position just as the lines drawn from the mountain peaks will intersect at a specific position.

Understanding GPS Measurements

Assume a GPS receiver is sitting in Nebraska. Once activated it begins to collect signals from GPS satellites 1, 2, 3, 4, 5, and 6. The distance to each satellite can be determined using the distance formula d = rt (distance, d, equals rate, r, multiplied by time, t, or distance equals velocity multiplied by time). Although all the satellites are 10,900 miles from the surface of the Earth, the distances to each one will vary according to its position in orbit. For example, all the street lights in a city may be 15 feet in the air but they are not all 15 feet from a specific point in the city.

The formula for determining these distances may be simple, but the calculations themselves are anything but simple. The satellites must be precisely timed so that each is synchronized with the other satellites in the constellation and with base stations on Earth. Although each satellite will be at a different distance from a particular point, the time it takes to cover those distances at the speed of light does not seem significant. In order to calculate distance, however, this time is significant to the GPS receiver.

Consider that a signal 10,900 miles from a receiver reaches that receiver in 0.058602 seconds. A signal 10,926 miles away, however, reaches the receiver in 0.0587419 seconds. A 26-mile difference translates into less than fourteen one hundred-thousandths (0.00014) of a second. Clearly, the GPS receiver has some very precise mathematics to work with, further complicated by the fact that the satellites are always moving.

After making these complex measurements, the distance to each satellite will be the hypotenuse of a right triangle created by the receiver's position, the satellite's position, and the position on Earth directly under the satellite. Once these distances are known, spheres can be created surrounding each satellite. Each sphere has a radius equal to the computed distance between the satellite and the receiver. The first sphere, around satellite 1, will have an infinite number of points along its surface so the receiver's position could fall anywhere on that sphere, including points in outer space.

Next, the sphere around satellite 2 is introduced, and the two spheres create an intersection that forms a circle. Now the GPS receiver could be anywhere on that circle, even points in space that, of course, it is not occupying. The third sphere, around satellite 3, intersects the first two spheres and limits the receiver's possible position to two points. The receiver is located at one of the two points. The other point is either in the air above the receiver or in the ground directly below the receiver. If the altitude of the receiver is known, then it is possible to determine which one of the two points is correct. The sphere around satellite number 4 will also reduce the two points to only one. It is amazing, yet basically simple, how one receiver and four satellites can reduce an infinite number of possible locations to only one.

Simulating GPS. To simulate the process of the Global Positioning System all that is needed is some string, scissors, tape, several coins, and four stationary points (the corners of a room will work). At any three-dimensional point in the room (on a desk, for example) a coin should be placed. The end of the string should then be taped into the corner of the room, with the other end pulled to the coin, cut, and then placed back in the corner. This process is repeated for the remaining three corners, and the extra coins are placed elsewhere in the room.

During this preparation, a volunteer waits outside the room. The volunteer should then enter the room and be alerted to the availability of the strings. The volunteer can then start pulling the cut ends of the string outward from the corners beginning with any two. By adding the third and fourth strings and finding where they all intersect, the volunteer should be able to eliminate all the extra coins and find the original coin. This is how GPS works in its most basic form.

Advantages and Disadvantages of GPS

Atmospheric inconsistencies can create inaccuracies in the positions computed by a GPS. Additionally, GPS is a "line of sight" system. Although a user cannot actually see the satellites in space, he or she does need an unobstructed view of the sky in order to utilize GPS. This poses serious challenges to those who choose to use GPS in canyons, cities, or other situations where large, solid objects mask out portions of the sky. When working where obstructions exist, careful planning must be done to ensure enough satellites are in "in view" for proper positioning.

Fortunately, GPS has a built-in feature, the almanac, to aid in identifying the location of satellites. Each satellite "knows" the location and direction of every other satellite. Along with the signal used to provide positions, satellites also transmit the almanac to a GPS receiver. Common GPS planning software can use the almanac to plot the entire constellation of satellites so users can plan ahead for their needs.

For example, if one needed to work in a canyon, planning software may indicate the only feasible time would be from noon until 2:00 p.m. Only during that time will the receiver have an unobstructed path to a sufficient number of satellites, all very high above the horizon, from within the canyon.

see also Flight, Measurements of; Maps and Mapmaking; Navigation.

Elizabeth Sweeney

Internet Resources

GPS Primer. The Aerospace Corporation. <http://www.aero.org/publications/GPSPRIMER>.


APPLICATIONS OF GPS

Vehicle tracking is one of the fastest-growing GPS applications. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations. Many public service units are using GPS to determine the police car, fire truck, or ambulance nearest to an emergency.

Mapping and surveying companies use GPS extensively. GPS-equipped balloons are monitoring holes in the ozone layer. Buoys tracking major oil spills transmit data using GPS.

These are just a few examples. New applications will continue to be created as GPS technology continues to evolve.


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GPS

GPS

Global Positioning System (GPS) is a navigation system consisting of a constellation of 24 navigational satellites orbiting Earth, launched and maintained by the U.S. military. GPS satellites orbit at approximately 11,000 mi (17,700 km) above Earth, with orbit periods of approximately 10 hours. The final satellite was placed in orbit in 1993. Because each satellite houses cesium and rubidium atomic clocks that are periodically updated and synchronized with a ground station in Colorado, GPS receivers can decode signals from the satellites to calculate location and exact time.

To overcome shortcomings in earlier navigation systems, United States developed another system: Navstar (Navigation Satellite for Time and Ranging) Global Positioning System. This system consists of 24 operational satellites equally divided into six different orbital planes (each containing four satellites) spaced at 60° intervals. The new system can measure to within 33 ft, (10 m), whereas earlier systems (e.g. Transit) were accurate only to 0.1 mi (0.16 km). Military users have access to systems with still greater accuracy.

Ground users commonly rely on GPS receivers. The receivers are small, hand-held devices that receive and decode GOS satellite signals. Small differences in the time lapse between signal receptions from three orbiting satellite signals (allowing triangulation of signals) are mathematically converted to latitude, longitude, and altitude. Sophisticated hand-held units are capable of determining latitude and longitude to a thousandth of an arc minute; these units show changes in reading as vehicles move very short distances).

With GPS, two types of systems are available with different frequencies and levels of accuracy. The Standard Positioning System (SPS) is used primarily by civilians and commercial agencies. As of midnight, May 1, 2000, the SPS system became 30 times more accurate when President William Jefferson Clinton ordered that the Selective Availability (SA) component of SPS be discontinued. SA was the deliberate decrease of accurate positioning information available for commercial or civilian use. The SPS obtains information from a frequency labeled GPS L1. The United States military has access to GPS L1 and a second frequency, L2. The use of L1 and L2 permits the transfer of data with a higher level of security. In addition to heightened security, the United States military also has access to much more accurate positioning by using the Precise Positioning System (PPS). Use of the PPS is usually limited to the U.S. military and other domestic government agencies.

Long before the space age, people used the heavens for navigation. Besides relying on the sun, moon, and stars, the early travelers invented the magnetic compass, the sextant, and the seagoing chronometer. Eventually,

radio navigation in which a position could be determined by receiving radio signals broadcast from multiple transmitters came into existence. Improved high frequency signals gave greater accuracy of position, but were sometimes blocked by high terrain and could not bend over the horizon. This limitation was overcome by moving the transmitters into space on Earth-orbiting satellites, where high frequency signals could accurately cover wide areas.

The principle of early satellite navigation was relatively simple. When a transmitter moves toward an observer, the Doppler shifted radio waves have a higher frequency, just like a train's horn sounds higher as it approaches a listener. A transmitter's signal will have a lower frequency when it moves away from an observer. If measurements of the amount of shift in frequency of a satellite radiating a fixed frequency signal with an accurately known orbit are carefully made, the observer can determine a correct position on Earth.

The United States Navy developed such a system, named Transit, in the late 1960s and early 1970s. Transit helped submarines update their on-board inertial navigation systems. After nearly ten years of perfecting the system, the Navy released it for civilian use. However, a major drawback to Transit was that it was not accurate enough; a user had to wait until the satellite passed overhead, position fixes required some time to be determined, and an accurate fix was difficult to obtain on a moving platform.

Both Transit and Navstar use instantaneous satellite position data to help users traveling from one place to another. But another satellite system uses positioning data to report where users have been. This system, called Argos, is a little more complicated: an object on the ground sends a signal to a satellite, which then retransmits the signal to the ground. Argos can locate the object to within 0.5 mi (0.8 km). It is used primarily for environmental studies. Ships and buoys can collect and send data on weather, currents, winds, and waves. Land-based stations can send weather information, as well as information about hydrologic, volcanic, and seismic activity. Argos

can be used with balloons to study weather and the physical and chemical properties of the atmosphere. In addition, the system is being perfected to track animals, including marine life.

In addition to GPS use in weapons systems and for navigation, use of the GPS system in everyday life is becoming more frequent. Equipment providing and utilizing GPS is shrinking both in size and cost, while it increases in reliability. The number of people able to use the systems is also increasing. GPS devices are being installed in cars to provide directional, tracking, and emergency information. Emergency personnel can respond more quickly to 911 calls using to tracking signal devices in their vehicles and in the cell phones of the person making the call. As technology continues to advance the accuracy of navigational satellite and without the impedance of Selective Availability, the uses for GPS will continue to develop.

FURTHER READING:

BOOKS:

Balazs, G. H. "Homeward bound: satellite tracking of Hawaiian green turtles from nesting beaches to foraging pastures." Proceedings of the Thirteenth Annual Symposium on Sea Turtle Biology and Conservation. U.S. Dep. Commer., NOAA Tech. Memo. NOAA-TM-NMFS-SEFSC-341, (1994):205208.

El-Rabbany, Ahmed. Inroduction to GPS: The Global Positioning System Norwood, MA: Artech Publishing, 2002.

ELECTRONIC:

Dana, Peter H. "Global Positioning Overview" The Geographer's Craft Project. May 1, 2000. University of Colorado. <http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html> (March 29, 2003).

SEE ALSO

Mapping Technology

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GPS (Global Positioning System)

GPS (global positioning system)

Long before the space age, people used the heavens for navigation. Besides relying on the Sun , Moon , and stars, the early travelers invented the magnetic compass, the sextant, and the seagoing chronometer. Eventually, radio navigation in which a position could be determined by receiving radio signals broadcast from multiple transmitters came into existence. Improved high frequency signals gave greater accuracy of position, but they were blocked by mountains and could not bend over the horizon. This limitation was overcome by moving the transmitters into space on Earth-orbiting satellites, where high frequency signals could accurately cover wide areas.

The principle of satellite navigation is relatively simple. When a transmitter moves toward an observer, radio waves have a higher frequency, just like a train's horn sounds higher as it approaches a listener. A transmitter's signal will have a lower frequency when it moves away from an observer. If measurements of the amount of shift in frequency of a satellite radiating a fixed frequency signal with an accurately known orbit are carefully made, the observer can determine a correct position on Earth.

The United States Navy developed such a system, called Transit, in the late 1960s and early 1970s. Transit helped submarines update their on-board inertial navigation systems. After nearly ten years of perfecting the system, the Navy released it for civilian use. It is now used in surveying, fishing, private and commercial maritime activities, offshore oil exploration, and drifting buoys. However, a major drawback to Transit was that it was not accurate enough; a user had to wait until the satellite passed overhead, position fixes required some time to be determined, and an accurate fix was difficult to obtain on a moving platform.

As a result of these shortcomings, the United States military developed another system: Navstar (Navigation Satellite for Time and Ranging) Global Positioning System. This system consists of 24 operational satellites equally divided into six different orbital planes (each containing four satellites) spaced at 60° intervals. The new system can measure to within 33 ft (10 m), whereas Transit was accurate only to 528 ft (161 m).

With the new Global Positioning System (GPS), two types of systems are available with different frequencies and levels of accuracy. The Standard Positioning System (SPS) is used primarily by civilians and commercial agencies. As of midnight, May 1, 2000, the SPS system became 30 times more accurate when President Bill Clinton ordered that the Selective Availability (SA) component of SPS be discontinued. SA was the deliberate decrease of accurate positioning information available for commercial or civilian use. The SPS obtains information from a frequency labeled GPS L1. The United States military has access to GPS L1 and a second frequency, L2. The use of L1 and L2 permits the transfer of data with a higher level of security. In addition to heightened security, the United States military also has access to much more accurate positioning by using the Precise Positioning System (PPS). Use of the PPS is usually limited to the U.S. military and other domestic government agencies.

Both Transit and Navstar use instantaneous satellite position data to help users traveling from one place to another. But another satellite system uses positioning data to report where users have been. This system, called Argos, is a little more complicated: an object on the ground sends a signal to a satellite, which then retransmits the signal to the ground. Argos can locate the object to within 0.5 mi (0.8 km). It is used primarily for environmental studies. Ships and buoys can collect and send data on weather , currents, winds, and waves. Land-based stations can send weather information, as well as information about hydrologic, volcanic, and seismic activity. Argos can be used with balloons to study weather and the physical and chemical properties of the atmosphere. In addition, the system is being perfected to track animals.

Use of the GPS system in our everyday lives is becoming more frequent. Equipment providing and utilizing GPS is shrinking both in size and cost, while it increases in reliability. The number of people able to use the systems is also increasing. GPS devices are being installed in cars to provide directional, tracking, and emergency information. People who enjoy the outdoors can pack hand held navigational devices that show their position while exploring uncharted areas. Emergency personnel can respond more quickly to 911 calls thanks to tracking signal devices in their vehicles and in the cell phones of the person making the call. As technology continues to advance the accuracy of navigational satellite and without the impedance of Selective Availability, the uses for GPS will continue to develop.

See also Archeological mapping; Weather satellite

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global positioning system

global positioning system (GPS) Navigation system based on a network of Earth-orbiting satellites. There are six satellites, each with a 24-hour orbit and its own identification signal. The satellites transmit continuous time signals. A user needs to be within range of four satellites to fix a position. One satellite sends out a reference signal and the other three give bearings indicating direction. A computer uses the minute time differences in receiving the signals from each satellite to calculate the position. Ordinary users can find their position to within 100m, although the military can pinpoint position to within 10m.

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Global Positioning System

Global Positioning System (GPS) A total of 18 satellites, in different orbits, so that at least four are visible at any time from all points on the Earth's surface. They give a locational accuracy of less than 2 cm on baselines of 1000 km. It is planned to use them for measuring the motions of small blocks of the Earth's crust, e.g. opposite sides of a fault system, microplates, etc. (The satellites also have obvious military applications.)

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"Global Positioning System." A Dictionary of Earth Sciences. . Encyclopedia.com. 23 Aug. 2017 <http://www.encyclopedia.com>.

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"Global Positioning System." A Dictionary of Earth Sciences. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/global-positioning-system

Global Positioning System

Global Positioning System: see navigation satellite.

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"Global Positioning System." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 23 Aug. 2017 <http://www.encyclopedia.com>.

"Global Positioning System." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/global-positioning-system

"Global Positioning System." The Columbia Encyclopedia, 6th ed.. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/global-positioning-system

GPS

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"GPS." A Dictionary of Earth Sciences. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/gps

GPS

GPS (or gps) gallons per second
• Global Positioning System (US defence satellite)
• Graduated Pension Scheme
• (Australia) Great Public Schools (indicating a group of mainly nonstate schools, and of sporting competitions between them)

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"GPS." The Oxford Dictionary of Abbreviations. . Encyclopedia.com. 23 Aug. 2017 <http://www.encyclopedia.com>.

"GPS." The Oxford Dictionary of Abbreviations. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/gps

"GPS." The Oxford Dictionary of Abbreviations. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/gps