A Code Phase Pull-In Method Based on the Zero-Crossing Point of the S-Curve Under the Strong Multipath Environment

The pull-in or traction module in the Global Navigation Satellite System (GNSS) receiver is an important bridge between the acquisition and tracking process, which can compress the code phase and carrier frequency estimation error introduced by the coarse acquisition stage. The method commonly used in the pull-in process of code phase fine-estimation is the interpolation based on multi-correlators, which has a better ability when the power of the multipath signal is lower than that of the direct signal. However, when there are strong multipath signals with higher power than direct signals or spoofing with small delay about one chip, both of them will cause large distortion of correlation peak. As a result, the code phase interpolation method directly leads to a large code phase estimation deviation. In this paper, a traction method based on the first zero crossing of the phase discrimination function is proposed. The S-curve is fitted by the code phase discrimination function based on the code correlation reference waveform (CCRW). Then the code phase value of the first zero crossing of the fitted S-curve is taken as the estimation result. The simulation results show that the code phase traction method proposed in this paper can work well in different multipath scenarios, and can mitigate the code phase estimation deviation introduced by the traditional interpolation method under strong multipath environments.