Asynchronous differential TDOA for non-GPS navigation using signals of opportunity

Non-GPS navigation techniques that use signals of opportunity (e.g. TV/AM broadcast signals with known source locations) without aid of satellite-based location systems (e.g. GPS) are promising since they use readily available strong signals that are not susceptible to blockage or jamming. However, problems such as large asynchronous clock offsets among receivers and large broadcast signal frequency offsets arise because broadcast signals are not dedicated for ranging purposes. In this paper, we propose an asynchronous differential time-difference-of-arrival (TDOA) positioning method to overcome the problems of biased signal frequencies and asynchronous receiver clocks. The proposed method eliminates the need for synchronous receiver clocks in the TDOA method and relaxes the tight timing constraint between receivers as required by the double-difference method in the differential GPS (dGPS).