Adaptively Robust Phase Lock Loop for Low C/N Carrier Tracking in a GPS Software Receiver

An important issue in GPS applications is how to track GPS (global positioning system) signal precisely and continuously under low carrier-to-noise ratio (C/N). In this paper, an adaptively robust filter based low C/N carrier phase lock loop (PLL) is developed under a GPS software receiver platform. Considering the effect of low C/N carrier signal on the traditional tracking loop, a parallel correlation tracking loop is established. A linear optimal estimator is designed to deal with the dependent noises in kinematics model and measurements. Furthermore, an adaptively robust filter is designed based on a three segment function adjust factor. When received signals are under favorable conditions, the performance of the new filter is very similar to a standard Kalman filter. For a practical weak carrier tracking, this new enhanced PLL intelligently tunes the loop parameters according to the total phase jitter analysis. It successfully resists the outliers and dynamic model errors by adaptively balancing the influence of updated dynamic model state and the measurements. The robustness and efficiency of the new filter is demonstrated by some real data testing experiments. The results verify that the standard deviation of the phase errors with this adaptively robust phase tracking loop can reach 0.01 cycles with satellite C/N ratios around 24 dB-Hz, which improves the performance significantly.