Abstract: A method of compensating for or correcting satellite ephemeris error involves measuring time difference of arrival (TDOA) and frequency difference of arrival (FDOA) for signal replicas received via two satellites (34, 46) from calibration transmitters (42a to 42d) at different geographical locations. An initial satellite ephemeris consisting of position and velocity vectors is used to calculate ephemeris changes yielding estimated TDOA and FDOA values providing a best fit to measured TDOA and FDOA values. This provides estimated changes required to compensate for or to correct errors in the initial satellite ephemeris. The method may be iterated to deal with large ephemeris changes: i.e. the changes obtained in one iteration may be used to correct ephemeris for use as a new initial ephemeris in a following iteration. The method may be used to correct ephemeris errors in one or both satellites, if so a greater number of calibration transmitters EphemCal 1 to EphemCal 10 may be used.

Abstract: A method of locating the source of an unknown signal is provided that includes calculating a differential offset for a signal for each of a plurality of positions within a region in which the transmitter must lie, for each of a series of times m with respect to first and second signal relays and respective first and second receivers, the positions of the signal relays and receivers being known, generating a cross-ambiguity function (CAF) using data corresponding to samples of the unknown signal received at the first and second receivers via the first and second signal relays respectively, estimating the level of noise in the CAF, and using this data to obtain a measure of the likelihood that the source is located within defined areas within the region, where the differential offsets are differential time offsets, or differential frequency offsets, or both. The method provides location which is unconditionally convergent.

Abstract: A method of locating the source of an unknown signal comprises the steps of: (i) calculating a differential offset for a signal for each of a plurality of positions within a region in which the transmitter must lie, for each of a series of times m with respect to first and second signal relays and respective first and second receivers, the positions of the signal relays and receivers being known; (ii) generating a cross-ambiguity function (CAF) using data corresponding to samples of the unknown signal received at the first and second receivers via the first and second signal relays respectively; (iii) estimating the level of noise in the CAF; and (iv) using data generated in steps (i), (ii) and (iii) to obtain a measure of the likelihood that the source is located within defined areas within said region; wherein the differential offsets are differential time offsets (DTOs), or differential frequency offsets (DFOs), or both DTOs and DFOs.

Abstract: A method of compensating for or correcting satellite ephemeris error involves measuring time difference of arrival (TDOA) and frequency difference of arrival (FDOA) for signal replicas received via two satellites (34, 46) from calibration transmitters (42a to 42d) at different geographical locations. An initial satellite ephemeris consisting of position and velocity vectors is used to calculate ephemeris changes yielding estimated TDOA and FDOA values providing a best fit to measured TDOA and FDOA values. This provides estimated changes required to compensate for or to correct errors in the initial satellite ephemeris. The method may be iterated to deal with large ephemeris changes: i.e. the changes obtained in one iteration may be used to correct ephemeris for use as a new initial ephemeris in a following iteration. The method may be used to correct ephemeris errors in one or both satellites, if so a greater number of calibration transmitters EphemCal 1 to EphemCal 10 may be used.

Abstract: A method of determining the location of an unknown source (10) transmitting an unknown signal to satellite relays (14 and 16) comprises receiving the signal from the relays at respective receivers (18). The receivers (18) receive reference signals via respective relays from a common source (22). The unknown signal and reference signal received by each receiver (18) are processed coherently to preserve their timing and phase information relative to one another independently of signals received elsewhere. The signals are frequency downconverted and digitised, and transferred to a common processing computer (150). The computer (150) performs cross ambiguity function processing of the reference signals to determine their relative Differential Time Offset (DTO) and Differential Frequency Offset (DFO).