An FLL structure based on a DFT interpolation discriminator for tracking weak GNSS signal

In a GNSS receiver, carrier tracking is the most challenging part, especially in a harsh environment. This paper presents an open tracking loop structure to overcome the matter of losing lock under weak-signal conditions. In order to enhance the stability and sensitivity of weak GNSS signal tracking, a frequency-locked loop structure based on DFT interpolated frequency estimation is proposed. The IF signals are split into two branches in the structure, and each signal is correlated with an equally frequency-spaced local carrier. Then, a new DFT interpolation algorithm of two-point DFT interpolation is used as a frequency discriminator to calculate the carrier frequency deviation. The bias and RMSE performances of the frequency discriminator are comparatively analyzed under different SNRs using computer simulations. Simulation results show that this discriminator is characterized by small computational effort and high estimation accuracy. The signal-tracking performances of the structure are verified by an experiment using a GNSS simulator. Results demonstrate that this method is able to track 26 dB-Hz signals using only 8 samples, which improves the tracking threshold by 4 dB-Hz over the FFT discriminator, and its tracking error jitter is less than 0.5 Hz. The structure needs only two-point interpolation, resulting in a low computation load in a real-time receiver.