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Abstract

A Low Earth Orbiting satellite system provides location and data communications services to mobile users equipped with a receiver/transmitter. The receiver/transmitter acts as a transponder that responds to a query transmitted over the satellite network. The response is sent after a precisely controlled time interval after the transponder receives the query so that the ground station can estimate the length of the propagation path from the satellite to the transponder. The transponder also transmits the response at a frequency that is proportional to the frequency of the received query so that the ground station can estimate the first and second derivatives of the length of the propagation path according to the measured Doppler shift. The ground station also estimates the satellite positioning using telemetry from the satellite obtained from the on-board GPS receiver. The position of the user terminal relative to the satellite position is then determined from the path length measurements. Given the satellite position and velocity, the measured path length and first and second derivatives determine the angle between the direction of satellite motion and the line of bearing to the user terminal. This angle defines a cone with the satellite at the origin. The user terminal position is somewhere on the circle defined by the cone and the estimated path length. The intersection of this circle with the surface of the Earth yields two possible user positions, which ambiguity can be resolved by three techniques: (1) use of knowledge of which beam the signal was received in; (2) use of earlier position data; or (3) using nearby satellites to receive the signal. The user terminal uses a single frequency reference to provide timing for the receive and transmit frequency synthesizers and for the analog-to digital and digital-to-analog converters. The frequency tracking algorithm shifts the baseband frequency of the response by a factor k so that the output frequency is related to the frequency of the received signal by the same factor k. This eliminates the absolute frequency of the reference as a source of error and allows the use of less expensive oscillators in the user terminal. In addition to the above, the present invention is able to rapidly acquire the signal at very low signal levels in the presence of large frequency uncertainties.

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First Claim