Introduction

The Global Positioning System (GPS) is a satellite-based positioning system operated by the United States Department of Defense. It has been operational since 1993, and although it is primarily intended for military use, it is, subject to one or two provisos, also available for world-wide civilian use. It is now widely used in remote sensing – in field work, to determine the position of training areas for image classification (see section 11.3.4.2), and also by some remote sensing satellites, for precise determination of the satellite's own position. This appendix therefore presents a brief introduction to GPS and its capabilities. A much fuller discussion is given by, for example, Leick (1995).

The fundamental idea behind GPS is not a new one. It is a radio-positioning system in which timing signals are transmitted at known times from a number of radio beacons at known locations. By measuring the times at which these signals are received, the distances to the various beacons can be calculated, and hence the position of the receiver can be deduced. What sets GPS apart from its predecessors is that the beacons are carried on satellites, providing genuinely global coverage.

Space segment

The ‘space segment’ of the GPS system consists of 24 satellites in circular orbits around the Earth. The semi-major axis (radius) of these orbits is about 26 600 km and the inclination is 55°, giving them a nodal period (see equation (10.13)) of 43 082 s, or exactly half a sidereal day.