A radio position determination and message transfer system is implemented using a number of satellites in geostationary orbit for relaying interrogation and reply signals between a ground station and a user-carried transceiver. Message information can be exchanged between a given user transceiver and the ground station, as well as between different user transceivers. The user transceiver is provided with means for monitoring the quality of the radio communication link between the transceiver and one or more of the satelites, based on errors detected in the received interrogation signals. The transmission of a reply signal by the transceiver is enabled only when the link quality is found to be acceptable. The reply signal may contain message information or may constitute a request for a position fix. In the latter case, the transceiver may be configured to await favorable link quality to more than one satellite before the reply signal is transmitted.