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Methods are developed to acquire and track Orthogonal Frequency Division Multiplexing (OFDM) digital FM radio broadcast signals using standard OFDM acquisition techniques and using a new Delay-Lock Loop (DLL) to determine digital symbol “code” start times and a Phase-Lock Loop (PLL) to determine beat carrier phase. These tracking algorithms are being developed with the goal of using their derived observables to navigate by fusing them with corresponding observables from a reference station and with pressure altimeter data. The DLL and PLL feedback discriminator outputs are computed by solving an optimal fitting problem in the frequency domain for each OFDM symbol. A second-order DLL is used to cope with significant transmitter-induced code/carrier divergence. The outputs of the DLL and the PLL and used to determine, respectively, standard pseudorange and accumulated delta range observables. Single-differencing of these observables between a roving user receiver and a reference station receiver at a known location can remove the transmitter clock drift effects to yield navigation observables. Wideband data collected in Roanoke, VA, and in Charlotte, NC, have been processed off-line and used to study the suitability of such signals for navigation purposes. Pseudorange measurement and bias errors can be on the order of 100 m, but accumulated delta range precision can be better than 0.1 m. Multipath, however, can cause significant degradation of a receiver’s tracking capability and of the quality of its navigation observables. Examples of these effects are presented as are potential mitigation techniques. A system that uses accumulated delta range may be able to yield 5-m level positioning accuracy if multipath effects can be compensated. Current pseudorange-only solutions, however, exhibit errors on the order of 100s of m. |
