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GPS: The Basics Introduction
The Global Position System (GPS) is a system for identifying three-dimensional position (horizontal coordinates and elevation), velocity, and time anywhere on earth. With a structural framework consisting of a satellite network and ground stations to track and coordinate the movement of these satellites, it provides anyone with a GPS receiver the ability to locate their position with accuracies ranging up to the centimeter level, at any time and in any weather. The system has become a fundamental piece in positioning and navigation technology, used in everything from transportation logistics and in-car navigation systems to ground surveying, flight navigation, and even sport fishing. This section will introduce the basic components of the system and provide a jumping off point to more detailed information on GPS and its uses. Fundamental Positioning Methodology
The system is based on a fundamental positioning procedure known as resection (often mislabeled triangulation), where knowing the distance from an unknown location to a certain number of known locations allows the determination of the unknown position's coordinates. In the GPS system, the 24 satellites orbiting the earth provide the known locations, and the unknown location is the position of the user with a GPS receiver. In determining a three-dimensional location on the surface of the earth, four known locations are theoretically required. The process works as follows: from the unknown location, we determine the distance to the first known location (satellite one). That measured distance thus establishes that the unknown point must lie somewhere on the surface of a sphere (with radius equal to the distance we determined) centered on satellite one. We next determine the distance to a second satellite, which gives us a second sphere. The intersection of the two spheres is a circle, so we know the unknown point lies somewhere on this circle. Then we determine the distance to a third satellite. The intersection of the resulting third sphere with the previous circle results in only two possible locations for our unknown point. A measurement to a fourth satellite would tell which of the two positions is correct; often, however, one of the positions is obviously wrong (e.g., in the wrong country or in space) so can be rejected without further measurement. We will see later however that a fourth measurement is needed for accurate positioning. There is more info including graphics at the link
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