Extended Kalman Filter Tracking and Navigation Message Decoding of Weak GPS L2C and L5 Signals

摘要

Code and carrier tracking and navigation message decoding algorithms are developed in this paper to work with weak L2C and L5 GPS signals. The code and carrier tracking modules are based on the Extended Kalman filter (EKF). Long coherent integrations are used with both modules. The high bit error rate (BER) associated with weak signals is dealt with by estimating and weighting each data bit with a value derived from an estimated likelihood of the data combinations, and then the weighted data are used within the EKF equations. The L2C tracking combines the CM and CL signals to form the integration and the EKF equations. While the L5 tracking can work in three different modes: tracking the I5 channel only; tracking the Q5 channel only; (3) tracking the I5 and Q5 channels together. The performance of the tracking modules are monitored to detect large carrier and code delay errors. If such errors are detected, the EKFs are deactivated and a carrier re-initialization or acquisition-like modules are activated to reduce the errors. Thus avoiding losing lock and having to re-acquire the signal. Following that, the EKFs are reactivated. In addition, a method is developed to decode the convolutionally encoded navigation message. Once the message is decoded, it is re-encoded and passed to the tracking modules to be used as an internal aiding to remove the effect of the data signs before calculating the coherent integration. The results have shown the ability of the algorithms to work efficiently with signal as low as 10 dB-Hz. The tracking performances of the L2C and L5 are compared with the C/A L1 signal tracking.