A position-estimating receiver, that receives ranging signals that are transmitted by multiple sources that modulate the signals with pseudo-random unique to the respective sources, includes data correlation channels, each of which correlates the in coming signals with a replica of the code used by a selected source to acquire the signal from the source in an acquisition mode and tracking the acquired signal in a tracking mode. The receiver detects possible spurious correlations in one or more monitor channels, each of which correlates the incoming signals with the code replica in use by a data channel in the tracking mode, stepping the replica through a succession of code phases other than the phase in the data channel and, if a significant correlation output is detected, causes the data channel to return to the acquisition mode.
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