Pseudorange characteristics of soil-transmitted GNSS signals and their application in soil moisture content retrieval

The characteristic parameters of global navigation satellite system (GNSS) signals, such as the amplitude, phase, and pseudorange, will vary as the signals are transmitted through media. The power attenuation characteristics of soil-transmitted GNSS signals have been widely investigated and used in retrieving soil moisture content (SMC). However, few researchers have focused on the pseudorange characteristics of single-transmitted GNSS signals and GNSSs other than GPS. In this study, the pseudorange characteristics are investigated with theoretical simulations and 3-month experiments, and the pseudorange characteristics are implemented in SMC retrieval. Moreover, soil-transmitted signals from the BeiDou navigation satellite system (BDS) are analyzed for the first time. The results show that pseudorange variations caused by the soil layer, namely, pseudorange variances, have a negative correlation with the elevation angle of GNSS satellites but a positive correlation with the SMC and soil depth. In addition, due to a higher correlation with the SMC, the pseudorange variance can be used to achieve a better retrieval performance of the SMC than the power attenuation used in existing works, with correlation coefficients of 0.95 and 0.93 and RMSEs of 2.41 × 10^–2 and 2.74 × 10^–2 cm³/cm³ for BDS and GPS, respectively. Unfortunately, two GNSS antennas need to be installed on each side of the soil to estimate the pseudorange variance. A new observable, the multipath observation error (MP) from the GNSS receiver under the soil layer is explored to overcome this problem. The experimental results show that the MP can provide the same level of SMC retrieval performance and is less impacted by the soil depth than the pseudorange variance.