Abstract: A satellite communications system and method, where a satellite 10 can pass messages between a user terminal 44 and an earth station 38, as the satellite moves in an orbit 12 12′ as indicated by arrow 46, comprises means for measuring the position of the user terminal 44, on the surface of the earth 14, relative to the nadir 50, by means of doppler shift measurements and propagation delay measurements between the satellite 10 and the user terminal 44. Where more than one satellite 10 is visible to the user terminal 44, combination means are employed using propagation delays to plural satellites 10, and/or recordal of the receipt times, by the user terminal 44, of timed broadcasts from one or more satellites 10 to determine the propagation delay between the one or more satellites 10 and the user terminal 44.

Abstract: In a communication system, a base station (1) transmits to a mobile station (5) in different channels. One channel is relayed to the mobile station (5) by a first satellite (3) and the other is relayed by a second satellite (4). The mobile unit (5) compares the qualities of the signals relayed by the satellites (3, 4) and transmits back to the base station (1) using the satellite (3,4) providing the best quality signals. The use of satellite links is not essential. The system has the advantages of diversity communication without increasing power demand at the mobile station.

Abstract: In a cellular satellite mobile telecommunication system, cell reception measurements for cell broadcast channels (BCCH) at a user terminal (UT 1) are made in an idle mode when no call is being made. At the commencement of a call, the measurement information (31, FIG. 8) relating to the measurement made in the idle mode is encoded onto a slow control channel (SACCH up) associated with a speech channel (TCH) for speech communication from the user terminal to a remote ground station (SAN 1) via a satellite (3a), and the power of the speech channel transmitted to the user terminal from the satellite, and from the UT to the satellite, for the call, is set in dependence upon the report. The reported data can also be used to determine whether to make a cell or satellite handover.

Abstract: In a satellite communications system, where a satellite 10 provides a plurality of radio beams 30, and where distance and/or angle measurement from a satellite 10 or satellites 10 enables the position of the user terminal 44, on the surface of the earth 14, to be measured to within an uncertainty of being in one out of a possible plurality of positions 142, when some out of the plurality of positions fall within a single beam 30A, which one of the plurality of possible positions 142 is actually occupied by the user terminal 44 is determined by assessing the quality of signals received from those other, adjacent, overlapping beams 30B-30G which surround the beam 30A.

Abstract: A terminal 44 44A 44B using standard navigational satellite 142 position determination to find its position on the surface of the earth 14, has its performance enhanced by being linked to a communications satellite system 10 38. In one variant, the communications satellite 10 broadcasts on each of a plurality of spot beams 30, each interactive with a different part of the earth, the broadcast indicating which navigational satellites 142 are accessible for a terminal within that beam 30. The terminal 44 44A 44B receives the broadcast and adjusts the navigational satellite receiver 174 to look for signals only from the nominated navigational satellites 142. In another variant, the communications satellite system 10 38 approximately measures the position of the terminal 44 44B and then informs the terminal 44 44B which navigational satellites are accessible from the terminal 44 44B, the navigational satellite receiver 174 again only looking for signals from the nominated navigational satellites 142.

Abstract: A communications satellite communications system and method, where a communications satellite 10 can pass messages between a user terminal 44 and an earth station 38, as the communications satellite moves in an orbit 12 12' as indicated by arrow 46, comprises means for estimating the position of the user terminal 44, on the surface of the earth 14, relative to the nadir 50, by means of doppler shift measurements and propagation delay measurements between the communications satellite 10 and the user terminal 44. Where more than one communications satellite 10 is visible to the user terminal 44, combination means are employed using propagation delays to plural communications satellites 10, and/or recordal of the receipt times, by the user terminal 44, of timed broadcasts from one or more communications satellites 10 to determine the propagation delay between the one or more communications satellites 10 and the user terminal 44.