System and method for demodulating global positioning system signals

Abstract

A GPS receiver uses the energy of the L1 and L2 GPS satellite signals to track the phase of the L1 and L2 P-code and the carrier phase of the L2 signal. The in-phase channel of the L1 signal, after L1 carrier demodulation with the aid of the publicly known C/A code, is first correlated with the P-code from a P-code generator in the L1 channel of the receiver. The correlation output is filtered over two W-bit periods, using an integrate-and-dump (I&D) filter, to determine whether a polarity change has occurred. Similarly the in-phase and quadrature channels of the received L2 Y-code signal are correlated with the P-code from the P-code generator in the receiver L2 channel and I&D filtered over the corresponding period of two W-bits. Polarity transitions between the two W-bits over the I&D integration period are detected by a threshold comparison with the I&D output of the L1 punctual P-code correlation signal. If a polarity change has occurred, the L1 and L2 I&D outputs are not used for L2 carrier demodulation. When there is no polarity change, the I&D output of the L1 in-phase Punctual P-code correlation is combined with the L2 in-phase Punctual P-code correlation in accordance with the L1 and L2 signal power ratio. This summed signal is multiplied with the I&D output of the L2 quadrature channel early-late P-code correlation to obtain a product signal that provides an estimate of the L2 carrier phase.