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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 o

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.

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1. A satellite communications system wherein a user terminal is operable to transmit to more than one satellite and each of the satellites is operable to transmit to the user terminal and to send and receive signals from an earth station, wherein the earth station is operable to exchange signals wit

1. A satellite communications system wherein a user terminal is operable to transmit to more than one satellite and each of the satellites is operable to transmit to the user terminal and to send and receive signals from an earth station, wherein the earth station is operable to exchange signals with the user terminal through the satellites and thereafter be operable to analyze the signals to determine the position of the user terminal on the surface of the earth, wherein each of the satellites is operable to provide a broadcast message at predetermined times, the user terminal being operable to measure and record the time of arrival of each broadcast message, the user terminal being operable to report back to the earth station the time of arrival of each received broadcast message, the earth station being operable to compare the reported time of arrival with the predetermined times to calculate the propagation delay between the user terminal and each of the satellites, and the earth station is operable to calculate the position of the user terminal relative to each satellite, based on a knowledge of the actual position of each satellite at the predetermined times. 2. A system according to claim 1 wherein said earth station is operable to send out a message, via each of said satellites, and where said user terminal is operable to return a message within a predetermined time of receipt of the message via said satellites said earth station being operable thereby to calculate the propagation delay between the satellites and said user terminal. 3. A system according to claim 2 wherein said earth station is operative to send said message via each of said satellites an optimum number of times, dependent upon the estimated position of said user terminal with respect to each of said satellites, and to take the average of the propagation delays derived therefrom. 4. A system according to claim 1 wherein said user terminal is operable to detect and record the time of arrival of broadcast messages from the satellites which are no longer in sight and to report the previous broadcast messages to said earth station, said earth station using knowledge of the position of said satellites, no longer in sight, at the time of receipt of the broadcast message by said user terminal to assist in the calculation of the position of said user terminal. 5. A system according to claim 1 wherein said earth station is operable to note the recorded time, reported by said user terminal, between two known intervals and is operable thereby to correct for drift and offset error in a timer in said user terminal. 6. A method for use in a satellite communications system wherein a user terminal is operable to transmit to more than one satellite and each of the satellites is operable to transmit to the user terminal, to send and receive signals from an earth station and to provide broadcast messages at predetermined times, the method including the earth station exchanging signals with the user terminal through the satellites and thereafter analyzing the signals to determine the position of the user terminal on the surface of the earth, the user terminal measuring and recording the time of arrival of each broadcast message, the user terminal reporting back to the earth station the time of arrival of each received broadcast message, the earth station comparing the reported time of arrival with the predetermined times to calculate the propagation delay between the user terminal and each of the satellites, and the earth station calculating the position of the user terminal relative to each satellite, based on a knowledge of the actual position of each satellite at the predetermined times. 7. A method according to claim 6 further including said earth station sending out a message via each of said satellites, and said user terminal returning another message within a predetermined time of receipt of the message via said satellites, the earth station calculating the propag ation delay between the satellites and said user terminal. 8. A method according to claim 7 further including sending the message via said each of said more than one satellites an optimum number of times, dependent upon the estimated position of said user terminal with respect to each of said satellites, and taking the average of the propagation delays derived therefrom. 9. A method according to claim 6 including the user terminal detecting and recording the time of arrival of broadcast messages from satellites which are no longer in sight of the earth station and reporting said previous broadcast messages to the earth station, said earth station using knowledge of the position of said satellites no longer in sight, at the time of receipt of the broadcast message by said user terminal to assist in the calculation of the position of said user terminal. 10. A method according to claim 6 including the earth station noting the apparent recorded time, reported by said user terminal, between two known intervals and thereby correcting for drift and offset error in the timer in said user terminal. 11. In a satellite communications system including an earth station configured to exchange signals with a user terminal via a plurality of satellites, the earth station being configured to receive reports of times of arrival at the user terminal of broadcast messages transmitted by the satellites at predetermined times, to compare the reported time of arrival with the predetermined times so as to calculate the propagation delay between the user terminal and the satellites and so determine the position of the user terminal on the surface of the earth.