Tracking Digital FM OFDM Signals for the Determination of Navigation Observables

Methods are developed to acquire and track orthogonal frequency division multiplexing (OFDM) digital FM radio signals. These methods are being developed with the goal of using FM signals' pseudorange and accumulated delta-range observables to navigate. Delay lock loop and phase lock loop discriminator outputs are computed by solving an optimal fitting problem in the frequency domain for each OFDM symbol. Single-differencing of the signals' observables between a roving user receiver and a reference station receiver can remove transmitter clock drift effects. Wideband data collected in Roanoke, Virginia, and in Charlotte, North Carolina, have been processed offline and used to study these signals' suitability for navigation. Single-differenced pseudorange measurement and bias errors relative to a base station can be on the order of 100 m, but single-differenced accumulated delta-range precision can be better than 0.1 m. A system that uses accumulated delta range may be able to yield 5-m level positioning accuracy if multipath effects can be compensated for. The present study represents an initial effort toward the goal of achieving this level of accuracy. Only pseudorange-based navigation is tested here, however, and its observed errors are on the order of 500 m.