Abstract: A Global Positioning System (GPS) commercial receiver including a digital processor disposed to utilize the energy of both the L1 and L2 GPS satellite signals in order to derive an estimate of an unknown security code used to modulate the signals. In processing the signal energy from the received L1 and L2 signals in accordance with statistical Maximum A Posteriori (MAP) estimation theory, the received L1 signal is correlated with a locally generated replica of the P-code, and passed through a bandpass filter having a bandwidth approximating the bandwidth of the unknown modulation code. The received L2 signal is similarly correlated and filtered, and the decorrelated signals are then latched in such a way as to account for the differential ionospheric refraction of the L1 and L2 signals. The bandlimited L2 signal is used to produce quadrature error signals related to phase difference between the L2 signal and a locally generated L2 replica.