Abstract: The satellite system includes at least two satellites having on-board processing capability so that data messages may be exchanged and the position of mobiles may be determined with direct access from multiple ground control facilities. The satellites transmit data and poll individual mobiles upon command by one or more of the control facilities and receive replies from the mobiles including the mobiles' altitude and other data of interest to the control facility. The at least two satellites communicate with one another by means of a cross-link communication path. Computer processing apparatus is included on the satellites to compute the ranges between the mobiles and each of the satellites from the replies received from the mobiles, and to store and process messages. The position information and messages are transmitted to at least one of the control facilities. It is preferred that the control facilities employ TDMA or FDMA protocols in the uplinks to the satellites.