GPS Receiver VFLL-Assisted PLL High Dynamic Weak Signal Tracking Based on a Maximum Likelihood Estimation

This paper proposes a GPS receiver vector frequency-locked loop-assisted phase-locked loop (VFAPLL) structure based on the maximum likelihood estimation (MLE) method for highly dynamic weak-signal scenarios. In this structure, the loop structure does not include a frequency discriminator, and the signal is directly input to the navigation filter after down-conversion, coherent integration, and other processing to avoid nonlinear noise error. Due to the high dimension and nonlinearity of the cost function of the MLE algorithm, the Levenberg Marquardt (LM) algorithm is used to optimize it. The proposed VFAPLL is compared with the VFAPLL implemented based on the extended Kalman filter (EKF) algorithm and the frequency locked loop assisted phase locked loop (FAPLL) implemented based on MLE. Through simulation verification, it was shown that the VFAPLL (MLE) has higher tracking accuracy, lower loss-of-lock threshold, and better robustness to the input signal than the other two loops.