GPS Navigation

GPS System

Accuracy

processes the information to determine which satellites are “visible” to the receiver’s antenna. With this determination made, the receiver chooses satellites to calculate a position fix. Using information transmitted from three or more satellites, the unit can calculate latitude and longitude (usually abbreviated Lat/Lon); with four or more satellites, GPS altitude can also be calculated.

The GPS allows a high degree of position fix accuracy. The system can produce a position fix accurate to within less than one meter under ideal conditions. Due to concern for national security, the U.S. Department of Defense introduces constantly changing errors to the transmitted satellite data to degrade the accuracy of the system. Called Selective Availability, this practice limits GPS position fix accuracy to about 100 meters, although the relative position of the satellites to one another, their elevation above the horizon, and other factors can also affect accuracy of the position fix. Under optimal conditions, accuracy can improve to within 10 meters. GPS position fix accuracy is not affected by atmospheric conditions.

GPS altitude is based on a mathematical model of the shape of the earth, the WGS-84 ellipsoid. Including intentional degradation, GPS altitude may differ from barometric altitude by several hundred feet.

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