Global Positioning System (GPS)

The Global Positioning System (GPS) is a worldwide, satellite-based, radio navigation system which gives the exact position of vehicles (in which the GPS device is installed), no matter where they are, what time it is, or what the weather is like. There are a total of 24 satellites orbiting the Earth. Each satellite transmits a unique code, its exact location and a very precise time code. On earth, the GPS receivers take this satellite data from multiple satellites and - using an algorithm & the time stamps - calculate the individual distances of each satellite from the GPS receiver. With some clever calculations, these can then point the precise location on Earth where the signal is received. The precise location is then transmitted by any of the available methods such as mobile phones (GSM/GPRS, WCDMA), Wireless (RF, WiFi, WiMax) and even a Satellite uplink for remote locations.

Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors.  GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board.

Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.

There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit.

 Determining Position - A GPS receiver "knows" the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect.

GPS Accuracy - The accuracy of a position determined with GPS depends on the type of receiver. Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.

When the system was created, timing errors were inserted into GPS transmissions to limit the accuracy of non-military GPS receivers to about 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000.