How GPS works
How GPS works
HOW GPS WORKS
The GPS consists of three main segments: space, control, and user. Satellites, which are launched into specific orbits and monitored by ground station, transmit information in form of radio signals. Ground stations update the information from satellites and send update signals back to satellites. Users with GPS receivers can decode satellite signals to find helpful information such as a location in a desert.
The space segment includes 24 satellites that are launched into six specific orbits to ensure coverage of the whole globe. The six orbits are 20,200 km above the earth. They are 60 degrees apart with 55 degrees inclination with respect to the equatorial plane.
The satellites are equipped with atomic clocks that provide accurate time to within three billionths of a second. The satellites transmit signals on two L-band frequencies (L1 = 1575.42 MHz and L2 = 1227.6 MHz) to equipment on the ground. The satellite signals consist of a Pseudo Random Code (PRC), ephemeris, and almanac data. The Pseudo Random Code is a complicated sequence of pulse signals. This code ensures not only that the same frequency can be used for all satellites, but also that GPS receivers can receive the GPS signals without the need of big satellite dishes. Moreover, Pseudo Random Code also identifies which satellite is sending. When a receiver receives a Pseudo Random Code, the number of the satellite shows up on a screen. The ephemeris data reflects status of satellites, current date and time. The position of a satellite is encoded in almanac data. A receiver decodes almanac data of a satellite and finds out where that satellite is at any time.
As shown in Figure 6, the GPS control segment includes monitor stations, ground antenna stations, and a master control station. Monitor stations are located at five different places around the world. They are Hawaii and Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic Ocean, and Colorado Springs in Colorado. The monitor stations receive signals from all satellites and collect data. The master control station, which is located at Schriever Air Force Base in Colorado, processes the information from monitor stations to decode satellite message signals. Then, the master control station updates each satellite's message. The satellites receive updated information through the ground antenna stations and send it to receivers.
The user segment consists of numerous kinds of GPS receivers. The GPS receivers can be hand carried or installed on airplane, ships, submarines, and vehicles. The satellite transmits its update data to GPS receivers via radio signals. The GPS receiver converts these signals into position, velocity, and time information. The receiver's microprocessor decode signals from four satellites to determine three positions (X, Y and Z) and time.
Determination of Position
Based on precise measurement of distance between satellites and receivers, the GPS can locate a position with an accuracy to 20 meter. Locating a position on the Earth is illustrated in the triangulation method. A position on earth can be located on an imaginary sphere centered at a satellite. The radius of the sphere is distance from that position to the satellite. If distances from two satellites to a position on the Earth are determined, that position is located somewhere on the plane that is an intersect of the two spheres. With a third measurement, the position can be narrowed to two points where three spheres intersect. However, one of these two points is usually impossible and is rejected by mathematical methods in the receiver.
In fact, three satellites are needed to locate a 2D position. That means the information from three satellites can only reveal a precise latitude/longitude position. In case of an airplane position, a receiver needs information from at least four satellites to determine its latitude, longitude, and altitude (X, Y and Z locations).
Since the satellites transmit signals at the speed of light, the distance from a satellite to receiver is calculated by multiplying the speed of light by the travel time from satellite to receiver. For a difference of a thousandth of a second, an error will be 340 km. That means high precision atomic clocks need to be set up on satellites. With an imperfect clock in a receiver, a crosscheck of a fourth measurement is included. When the fourth measurement is not matched with the other three measurements of satellites, the receiver computer uses a correction factor to calculate the position.
In addition, other factors need to be considered in getting a precise position. For example, the atmosphere can cause a change in the speed of light and creates an error. Another inaccuracy source is Selective Availability (SA), which is an intentional error caused by the U.S. Department of Defense. The purpose of SA is to prevent hostile forces or terrorist groups from taking advantage of GPS for producing precise lethal weapons.