GNSS-IR multisatellite combination for soil moisture retrieval based on wavelet analysis considering detection and repair of abnormal phases

Abstract

Signal-to-noise ratio (SNR) data received with standard geodetic instrumentation can be used to retrieve near-surface soil moisture (SM). However, low-quality SNR data usually cause abnormal phases in the fitting of nonlinear least squares (LLS) algorithms. This is not conducive to the effective use of multisatellite phases. In this paper, an SM retrieval method based on multisatellite combinations considering the detection and repair of abnormal phases is proposed. This method is aimed at using wavelet transform to separate the trend and modulation terms in SNR data, followed by detecting and repairing the abnormal phase for all satellites, finally constructing the multisatellite linear regression (MSLR) model for SM retrieval, and analysing the variation of accuracy with the increase of model testing days. The results indicate that with the coif5 wavelet, the trend and modulation terms of the SNR (compared to the traditional low-order polynomial) can be better separated. The abnormal phases can effectively be detected and repaired by combining the interquartile range and moving average filter, and further, the quality of the phases for each satellite can be improved. Furthermore, MSLR can fully combine the multisatellite phases to improve the accuracy of SM retrieval, and it is suitable for SM retrieval over different time periods.