The Radiodetermination Satellite Service (RDSS) provides radiolocation and limited communication for users of aeronautical, marine and terrestrial vehicles. A system of the RDSS is being developed in the United States by the Geostar Corporation (Rothblatt, 1988). In this system signals from a control center are transmitted to transceivers on users’ vehicles through one of a series of two or three satellites spaced along the geostationary orbit, and responses are received through each of these satellites. If three satellites are used, locations of vehicles can be determined from the measured times of transmission from the control center to the user and back via different satellite paths. If two satellites are used the timing data must be supplemented by information such as altitude data from a terrain map, or an altimeter reading from the user’s vehicle, which can be encoded in the response transmitted back. Computation of the vehicle location is performed at the control center. The equipment at the vehicle can be relatively simple since its basic function is that of a transponder, and intervention by the vehicle operator is not necessarily required. However, provision can also be made for the inclusion of short coded messages from the vehicle, so the unit on the vehicle is usually referred to as a transceiver. The system is designed to meet the needs of various users; for example, trucking companies which must periodically monitor the locations of vehicles in a widely distributed fleet. Position information can be relayed to the company headquarters as well as to the vehicle operator. The role played by the central control computer in determining the locations requires that all vehicles using the service identify themselves by a transmitted code. Thus developers and operators of RDSS systems can lease their services to vehicle users.