Patent ID: 8004459

Claim:
A method of determining a geolocation of an emitter from two time-difference of arrival measurements from different times, the method comprising: determining a first time-difference of arrival at an observer between a first signal from the emitter relayed by a first satellite and the first signal relayed by a second satellite, wherein the first time-difference of arrival is observed at the observer at a first time; determining a second time-difference of arrival at the observer between a second signal from the emitter relayed by the first satellite and the second signal relayed by a third satellite, wherein the second time-difference of arrival is observed at the observer at a second time, the second time being different from the first time; determining a first calibration factor from a third time-difference of arrival at the observer between a first calibration signal relayed by the first satellite and the first calibration signal relayed by the second satellite, wherein the first calibration factor is computed as a difference between the third time-difference of arrival and a value predicted for the third time-difference of arrival, the value predicted for the third time-difference of arrival being based on a known location of an emitter of the first calibration signal; determining a second calibration factor from a fourth time-difference of arrival at the observer between a second calibration signal relayed by the first satellite and the second calibration signal relayed by the third satellite, wherein the second calibration factor is computed as a difference between the fourth time-difference of arrival and a value predicted for the fourth time-difference of arrival, the value predicted for the fourth time-difference of arrival being based on a known location of an emitter of the second calibration signal; subtracting the first calibration factor from the first time-difference of arrival and the second calibration factor from the second time-difference of arrival to reduce effects of satellite position and velocity errors on the determined geolocation of the emitter; and determining the geolocation of the emitter based on the first and second time-difference of arrivals.