Investigation on GNSS-A precise point positioning based on adaptively robust filter considering the horizontal heterogeneity of sound speed structure

The Global Navigation Satellite System-Acoustic ranging combination technique (GNSS-A) is a seafloor geodetic technique to provide underwater positioning information, which is affected by several kinds of positioning error sources. The uncertainty related to the sound speed structure (SSS), resulting from the temporal and spatial variation, significantly affects the positioning results. Specifically, the horizontal heterogeneity of SSS directly limits the horizontal positioning accuracy. In this paper, we use the equivalent gradient parameters in east-west and north-south components to depict the horizontal features of SSS and reconstruct the acoustic timing observation equation model. And then the extended Kalman filter (EKF) is applied for parameters estimation considering the superiority in dynamic measuring data processing. However, the filter accuracy is deteriorated and even divergent due to the pollution of unexpected measurement outliers. Therefore, the adaptively robust filter is presented and adopted to suppress noises effect in state-observation equation systems. To evaluate the performance of the above positioning methods, a series of simulation test were carried out. Testing results indicate that the positioning model considering of the horizontal heterogeneity of SSS performs better in extracting the synthetic time delays, and the adaptively robust filter shows advantage in resisting the effect of outliers and guarantees the positioning accuracy.