GPS stands for Global Positioning System, and is a fairly new technology that is starting to see widespread use in various fields. Originally made with military purposes in mind, GPS units utilize satellites which orbit the earth to track special transmitters on the planet's surface and find them, allowing a GPS computer to track the transmitter's location anywhere on the globe and even place it on a digitized map.
GPS systems are composed of three major parts. The first part is the space system, which is composed of the actual physical satellites which orbit the planet and whose purpose is to send and recieve data from the other two components, which are the Control system and the Transmitter system. The number of satellites for a full working GPS requires that at least 6 satellites at any one time have full line of sight to any and every location on the earth's surface.
The actual Control system of a GPS network is composed of a series of ground based installations which monitor frequencies from the satellites, as well as uploading commands to the satellites. The main base which acts as the hub for all worldwide GPS control centers is located in a Master Control Station in Colorado.
The user segment is perhaps the easiest part of a GPS system. It is a small transmitter which sends and receives signals from the satellites and allows the user to be tracked globally. Whereas before this technology was fairly bulky, recent advances have made transmitters so small that they can be integrated into car radios and cellular telephones.
One of the civilian uses for a GPS is navigation. Where before people who got lost in the wilderness had to rely on a conventional map and compass for finding out where they were, a modern GPS unit plugged into a laptop can display a fully digitized map and show the user's location. This use of the GPS has also found it's way into aviation and shipping travel, making it much easier for ships to navigate across oceans and aircraft to determine their location accurately.
Another use of the GPS is in modern cars; GPS units in cars allows people to not only find out where they are; it also includes rough street maps and even information on the approximate speed the car is travelling and distance travelled within a specified length of time. While this is no real replacement for a speedometer, it does have it's uses for people who want to regulate their gas mileage.
Naturally, the broadcast signals of a GPS satellite differ between civilian and military useage. Regular civilian signals also include a clock which gives the accurate time for the time zone where the transmitter is located. Civilian GPS units can also monitor the position of the satellites and the number of satellites accessible at a given location. Military signals are heavily coded and, for obvious reasons, transmit much more detailed data than civilian signals.
The process of determining a unit's location on a GPS isn't completely accurate, but is at least close enough to offer approximates. The transmitters send periodic pulses to all detected satellites in range, and the Control installations monitor the time spent from when the pulse is first emitted to when it "bounces back" to the user's unit. This gives the control tower a good estimate of the distance of the unit from ONE satellite. By meshing the data from all satellites which recieved the signal, the control computers can then triangulate the position of the user's unit on the global map.
While this offers only an approximate location of a GPS user unit, the approximate is still far more accurate than the general estimates made by more classic navigational methods. For example, triangulation using a map, a compass, and landmarks on the map will often place a person's location at a deviation of 50 to 100 meters (or more) from the actual location, depending on things like the scale of the map, the nearness of the landmarks, and the magnetic accuracy of the compass. this doesn;t even take into account human error.
GPS units, on the other hand, have a deviation of only 5 meters at most for atmpspheric interference, 2 meters for satellite clock timing errors, 1 meter for distortion errors causes by other radio signals, and half a meter for local terrain interference from buildings and trees. So assuming worst case scenario, a user's location will be 8 and a half (maybe 10 on a bad day) meters away from his actual position. Compared to fumbling with a compass, map, or star and tidal charts, this makes the GPS an essential modern tool for navigation.