In a GPS-denied environment, the exploration of signals of opportunity (SOOP) for positioning is a viable alternative. The locations of SOOP and their characteristics are assumed to be known. Examples include digital television (DTV) transmission towers and wireless local area network (WLAN) access points (AP). However, the number of independent SOOP may not be sufficient for a solution and their geometric distribution may be poor. More importantly, the SOOP are not synchronized, each subject to a different clock bias and drift. In the past, monitoring stations at known locations were used to estimate the clock errors of SOOP and a network server provides time corrections to subscribed users. In this paper, a self-calibrating scheme for positioning with SOOP is set forth, in which a user makes a number of displacements and at each leg, the user measures the time of arrival (TOA) of SOOP. Based on the measured displacements and TOA, the user determines its initial location and clock errors of SOOP jointly via a fixed-point smoothing algorithm. The positioning mechanism is described in the paper together with an analysis of geometric dilution of precision (GDOP). Simulation results are presented to show the effects of timing (ranging) errors and GDOP on the position location accuracy.
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