GLOBAL POSITIONING SYSTEM (GPS) RECEIVER FOR RECOVERY AND TRACKING OF SIGNALS MODULATED WITH P-CODE

摘要

Method and apparatus for improving the speed and accuracy of processing signals from global positioning system (GPS) receivers (50, 52) by ensuring access to GPS carrier signals that have been modulated with an encrypted P-code sequence. In one disclosed embodiment of the invention, GPS L1 and L2 signals are correlated with a locally generated P-code signal (64, 62), bandpass filtered to enhance signal-to-noise ratio performance, and then cross-correlated (66) to obtain a signal with an L1-L2 frequency component that facilitates the resolution of carrier cycle ambiguity. In another embodiment, received GPS signals are immediately converted to digital form (114), then digitally correlated with in-phase and quadrature components of a locally generated P-code signal. Signals resulting from the correlation are then integrated (124, 126) over timing intervals corresponding to a previously determined encryption period to provide in-phase (I) and quadrature (Q) samples (130, 132).