THE GLOBAL POSITIONING System (GPS) is a constellation of earth-orbiting satellites developed by the U.S. Department of Defense to provide global, all weather and 24-hour positioning capabilities to ground-based GPS receivers. Although the original system was intended for military use, it has also found important and widespread applications in civilian positioning, navigation, and mapping. Positioning is about determining an exact location based on a coordinate referencing system. Navigation is the process of moving from 1 location to a destination and knowing at each stage the current position with respect to the destination. Hence a route is a series of waypoints tracing out the path from the initial location to the destination. GPS is also increasingly used as data input in geographic information systems (GIS) for accurate positioning of geospatial map data and for the collection of field data. Effective use of a GPS receiver does not require an understanding of the technical details, but does require some training and an appreciation of the limitations of the system as a whole. GPS technology is collectively composed of a space segment, a control segment, and a user segment. The space segment of the system consists of 27 GPS satellites orbiting the Earth at a distance of about 12,427 mile (20,000 kilometer) above the surface. Twenty-four of these satellites are active, while 3 serve as replacement in case of failure. The satellites are positioned in 6 evenly spaced orbital planes and make 2 complete rotations each day. With this arrangement, at least 4 and as many as 8 GPS satellites can be detected above the horizon from any location on the surface. Each satellite has an expensive atomic clock that generates precise time, which together with its orbital location information, is broadcast to ground receivers along 2 coded carrier signals. Using a three-dimensional mathematical process called trilateration, the location of the ground GPS receiver can be determined if its distance from 4 satellites is known. The GPS receiver automatically calculates the distances to each satellite using the travel time information encoded in the detected signals and the speed at which the signal travels (3 x 108 ms-1). The GPS receiver does not contain an atomic clock to measure time differences precisely but uses instead an internal database called an almanac. The almanac gives the projected position of each satellite in its orbital plane from which time differences and the distance to the satellites can be determined. The almanac of GPS receivers is constantly updated by the satellites to adjust for any changes in predicted orbital positions. The control segment consists of a network of tracking ground stations that measure the satellite signals, evaluate orbital information, and upload maintenance data to the GPS satellites. The user segment consists of the GPS receivers and the user community. The GPS receiver is designed as a rugged and portable device with a small viewing screen and user controls and an antenna to receive the signals from the orbiting satellites.