Global Positioning System

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 satellite—the range—is 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

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