INTENDED to provide highly accurate three‐dimensional position and velocity and precise time on a continual global basis, the NAVSTAR Global Positioning System (GPS) has been under development by the US Department of Defence since the early 1970's. It is a satellite based radio navigation system which, when it comes into operation in 1988, will consist of 18 satellites in six orbital planes inclined at 55°. Each plane will contain three satellites spaced 120° apart in 12 hour orbits, the relative phasing of the satellites from one plane to the next being 40°.