Performance Analysis and Evaluation of Frequency-Locked Loop for Weak GNSS Signals Based on Spectral Line Interpolation

In order to track weak signals, GNSS receivers often use a frequency-locked loop (FLL) to track the carrier frequency. The performance of the frequency discriminator directly affects the receiver's signal tracking capability in the FLL. In weak signals, FFT is currently used to estimate the carrier frequency. However, the FFT has a fence effect. The signal length affects the accuracy of the frequency estimation. According to the problem of FFT frequency estimation with low accuracy, we introduce three frequency estimators, which have good frequency estimation performances under low signal-to-noise ratio for GNSS weak signal tracking and propose a carrier frequency tracking method. The article compares and analyzes the estimation error performance of different frequency discriminators in detail. The simulation experiments are used to verify and analyze the tracking sensitivity of the tracking loop under different carrier-to-noise ratios. Theoretical analysis and simulation results show that these estimators effectively improve the accuracy of frequency estimation compared to traditional FFT estimator. Using the open-loop tracking structure, the tracking sensitivity can reach 24 dB-Hz when the coherent integration time is 20 ms and the number of signal samples is 16. Adding a loop filter, the tracking threshold can be improved to 20 dB-Hz.