Extended Kalman-Filter (EKF) based methods have been developed for semi-codeless tracking of the P(Y) code on weak dual-frequency GPS signals. Optimal Kalman filtering methods are essential to maintain lock with a semi-codeless approach during ionospheric scintillations, when deep power fades can occur. EKF-based semicodeless techniques use maximum a posteriori estimation techniques to estimate the unknown W code that gets modulo-2 added to the P code in order to provide antispoofing protection. These tracking techniques take advantage of the known W-bit chip timing, whose chipping rate is approximately 500 KHz. They also use a posteriori cost functions that involve -log[cosh()] terms, which allow seamless transition between various W-bit decision techniques. The algorithm estimates the P(Y) carrier amplitudes, accounts for signal dynamics in the optimization process, and optimally couples its code and carrier tracking loops. The algorithm has been successfully tested with real data under normal signal strength conditions.
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