*Article* **Retrieval of Soil Moisture Content Based on Multisatellite Dual-Frequency Combination Multipath Errors**

**Shihai Nie, Yanxia Wang \*, Jinsheng Tu, Peng Li, Jianhui Xu, Nan Li, Mengke Wang, Danni Huang and Jia Song**

College of Geographic Information and Tourism, Chuzhou University, Chuzhou 239000, China; nsh1017@chzu.edu.cn (S.N.); tujinsheng@chzu.edu.cn (J.T.); lipeng@chzu.edu.cn (P.L.); xujh@chzu.edu.cn (J.X.); linan@chzu.edu.cn (N.L.); mengkewang@sdust.edu.cn (M.W.); hdn325@chzu.edu.cn (D.H.); songjia@chzu.edu.cn (J.S.)

**\*** Correspondence: lidarcenter@chzu.edu.cn; Tel.: +86-159-5501-2916

**Abstract:** Global navigation satellite system interferometric reflectometry (GNSS-IR) is a new type of microwave remote sensing technology that can measure soil moisture content (SMC). GNSS-IR soil moisture retrieval methods based on the satellite signal-to-noise ratio (SNR) and triple-frequency signal combination have the following shortcomings: SNR does not always exist in the original GNSS file, and the number of triple-frequency signal observation satellites is small, resulting in GNSS-IR soil moisture observation time resolution being low. Based on the above problems, in this study, we constructed a soil moisture inversion method based on multisatellite dual-frequency combined multipath error is proposed: the multipath error calculation model of dual-frequency carrier phase (L4 Ionosphere Free, L4\_IF) and dual-frequency pseudorange (DFP) without ionospheric effect is constructed. We selected the data of the five epochs before and after the time point of the effective satellite period to construct the multipath error model and error equation, and we solved the delay phase for soil moisture retrieval. We verified the method using Plate Boundary Observatory (PBO) P041 site data. The results showed that the Pearson correlation coefficients (R) of L4\_IF and DFP methods at P041 station are 0.97 and 0.91, respectively. To better verify the results' reliability and the proposed method's effectiveness, the soil moisture data of the MFLE station about 210 m away from P041 station are used as the verification data in this paper. The results showed that the delay phase solved by multipath error and soil moisture strongly correlate. Pearson correlation coefficients (R) of L4\_IF and DFP methods at MFLE station are 0.93 and 0.86, respectively. In order to improve the inversion accuracy of GNSS-IR soil moisture, this paper constructs the prediction model of soil moisture by using the linear regression (ULR), back propagation neural network (BPNN) and radial basis function neural network (RBFNN), and evaluates the accuracy of each model. The results showed that the soil moisture retrieval method based on multisatellite dual-frequency combined multipath error can replace the traditional retrieval method and effectively improve the time resolution of GNSS-IR soil moisture estimation. To perform highly dynamic monitoring of soil moisture, higher retrieval accuracy can only be obtained with a small epoch multipath error.

**Keywords:** GNSS-IR; soil moisture content; multisatellite combination; dual-frequency pseudorange; dual-frequency carrier phase combination; multipath error; phase delay
