The NAVSTAR-Clobal Positioning System (CPS) is a state-of-the-art, satellite-based, world-wide, navigation system which was declared operational this year. With GPS, the position of a user can be determined to within 100 m; by employing differential techniques this uncertainty can be reduced to 10 m or less. In real time, this requires a Differential GPS (DGPS) system to measure and communicate correction information to the user. This research considers nGPS systems employing modified marine radiobeacons and receivers, examining the factors affecting their coverage and performance. Existing techniques of coverage prediction have been examined and found to be inadequate. Improved methods of coverage and performance prediction are developed which take into account several new factors. The groundwave and skywave field strengths of a beacon are calculated, taking into account detailed information about the ground conductivity. A new method of determining own-skywave fading has been derived and the effect at night determined to be significant. Levels of interference from other beacons on the co- and adjacent channels, via groundwave and skywave, are shown to be dominant coverage limiting factors in the European environment. Atmospheric noise and its effect on receiver performance is evaluated and forms the basis of performance predictions. These new techniques have been built into a computer model which automatically evaluates coverage and performance contours for DGPS radiobeacons and which is being used world-wide in the planning and evaluation of these systems.