A differential coherent accumulation message demodulation algorithm with variable window length

To address the challenge of navigation message demodulation in extremely weak signal environments for satellite navigation receivers during deep space exploration missions, particularly when using a pure Frequency Lock Loop (FLL) tracking mode, this paper proposes a novel differential coherent accumulation message demodulation algorithm. In pure FLL tracking mode, it is difficult to lock the carrier phase, which causes the signal energy to be distributed evenly across the in-phase (I) and quadrature (Q) branches. This significantly degrades the performance of traditional message demodulation methods, which depend exclusively on energy detection in the I branch. This paper uses theoretical analysis to explain how phase ambiguity affects the coherent integration values of the I/Q branches. It also proposes a differential coherent accumulation algorithm to reduce the effect of phase unlocking on message demodulation. Two implementation architectures are innovatively introduced: fixed window length and variable window length. The paper details their specific implementations and compares their performance via simulations. The results indicate that for a bit error rate (BER) of 10-6, the carrier-to-noise ratio (C/N 0 ) thresholds for the variable and fixed window length methods are 24 dB and 27 dB, respectively. This demonstrates a 3 dB sensitivity improvement for the variable window approach. This research provides an effective solution for reliable message demodulation in ultra-weak signal environments for deep space navigation receivers, significantly expanding the applicable boundaries of satellite navigation systems.