Radiolocation Using AM Broadcast Signals

I have designed, built, and evaluated a passive radiolocation system that uses only signals of opportunity, that is, signals that exist for purposes other than radiolocation. The system estimates the relative position vector between a base station, which is a navigation receiver at a known location, and a rover, which is like the base station but free to move about. The relative position vector, called the baseline vector, is determined by multilateration from observations of the carrier phases of signals received from AM broadcast stations. This system determines the horizontal components of the baseline with about ten-meter uncertainties for baseline lengths up to about 35 kilometers. The navigation receivers are implemented as software radios on standard Intel-based personal computers. The signals received by a one-meter vertical whip antenna are bandpass-filtered, amplified, and digitized. The entire AM band is digitized so simultaneous observation of all available signals is achieved. All further processing of the signals, including carrier-phase determination, is implemented in software run on the personal computer. The base station and rover record observed phase, frequency, and amplitude data on their local hard drives; and navigation algorithms are implemented in post-realtime. The interpretation of a carrier-phase observation in terms of position is ambiguous because one cycle of a carrier wave is virtually indistinguishable from the next. Previous attempts at signal-of-opportunity navigation using carrier phase sidestepped the ambiguity problem by requiring that the initial position of the rover be known and that phase variations be tracked without interruption. I designed and implemented an ambiguity-function method that enables the phase ambiguity to be resolved instantaneously without position initialization or signal-tracking continuity. I encountered several impediments to AM-broadcast-based radiolocation that, if not dealt with appropriately, reduce positioning accuracy, reduce ambiguity-resolution robustness, or both. AM transmitter position uncertainty directly causes receiver positiondetermination uncertainty. Since the error in published antenna positions sometimes exceeds 100 meters, I conducted sub-meter-accuracy geodetic surveys of 29 Boston-area