Method and apparatus for determining the position of a vehicle

A vehicle positioning system in a satellite-based system for communicating with and determining the position of a vehicle comprising:

• a vehicle transponder which is carried by each of a plurality of vehicles which travel on or above the earth's surface and which transmits a predetermined response signal in response to reception of an interrogation signal;
• at least two or more satellites each of which have repeaters for receiving the response signal transmitted from said vehicle transponder and for retransmitting the response signal, said satellites positioned at spaced orbital locations above the earth's surface;
• and a ground station for receiving the response signal from said vehicle transponder which has been retransmitted from each of the satellites and for measuring the position of each vehicle which has a vehicle transponder using the transmit time required for signals from the transmission of said interrogation signal to the reception of said response signal, wherein said vehicle transponder has receiving means for receiving said interrogation signal, decoding means for decoding the interrogation signal received by said receiving means;
• encoding means for forming the response signal responsive to the interrogation signal which is synchronized with a strobe signal which is supplied from said decoding means, transmitting means for transmitting an output signal of said encoding means and which uses a carrier having a predetermined carrier frequency, and a time lag storing means for previously storing information regarding the time lag from the reception time of said interrogation signal to the transmission time of said response signal of said vehicle transponder, and wherein the time lag information of said vehicle transponder which has been stored in said time lag storing means is included in the response signal and such response signal is then transmitted by said vehicle transponder.