Research on carrier frequency tracking and message demodulation techniques for GNSS signals

Abstract Carrier frequency tracking is suitable for application scenarios where global navigation satellite system (GNSS) signals are weak and exhibit large dynamic changes. However, when using a frequency-locked loop (FLL) to track the carrier of a GNSS signal, it is difficult to apply the conventional coherent demodulation method of binary phase-shift keying-modulated messages because the carrier phase is not locked, which necessitates the use of differential demodulation. This study aims to explore the impact of different FLL combination designs on the performance of differential demodulation and decoding. Taking the B1I signal of the Beidou satellite navigation system as an example, a Monte Carlo simulation test was conducted. The test results indicate that when the carrier-to-noise ratio is below 24.5 dB-Hz, the combination design of the normalized CROSS discriminator and the ATAN discriminator achieves the best message demodulation performance. By utilizing the repeated broadcast characteristics of the B1I message, when six frames of messages are continuously accumulated, the demodulation threshold for the messages can be reduced to a carrier-to-noise ratio of 22.5 dB-Hz.