Global Positioning System
In the early 1980s the US Department of Defense began deploying a satellite-based navigation system known as the Global Positioning System (GPS). This system enables an observer with the proper receiver to obtain instantaneous three-dimensional position information in virtually all parts of the globe with a precision of several meters. Under certain conditions this can be improved by up to three orders of magnitude. The scientific applications include precision ship and aircraft navigation for gravity measurements, precision aircraft and satellite positioning for altimetry, and high-precision geodesy for measurement of sea level, crustal strain and fault motion rates. This article briefly reviews these and other applications of high-precision GPS measurements. More comprehensive reviews can be found in Leick (1990), Dixon (1991) and Hager, King and Murray (1991).
The space segment of GPS is a constellation of satellites in high Earth orbit equipped with powerful