*Article* **Enhancing Spatial Resolution of GNSS-R Soil Moisture Retrieval through XGBoost Algorithm-Based Downscaling Approach: A Case Study in the Southern United States**

**Qidi Luo, Yueji Liang \*, Yue Guo, Xingyong Liang, Chao Ren, Weiting Yue, Binglin Zhu and Xueyu Jiang**

College of Geomatics and Geoinformation, Guilin University of Technology, Guilin 541004, China; luoqidi@glut.edu.cn (Q.L.); guoyue@nnct.edu.cn (Y.G.); liangxy@glut.edu.cn (X.L.); renchao@glut.edu.cn (C.R.); yueweiting@glut.edu.cn (W.Y.); zhubinglin@glut.edu.cn (B.Z.); xiaojiang@glut.edu.cn (X.J.) **\*** Correspondence: lyjayq@glut.edu.cn; Tel.: +86-158-783-57721

**Abstract:** The retrieval of soil moisture (SM) using the Global Navigation Satellite System-Reflectometry (GNSS-R) technique has become a prominent topic in recent years. Although prior research has reached a spatial resolution of up to 9 km through the Cyclone Global Navigation Satellite System (CYGNSS), it is insufficient to meet the requirements of higher spatial resolutions for hydrological or agricultural applications. In this paper, we present an SM downscaling method that fuses CYGNSS and SMAP SM. This method aims to construct a dataset of CYGNSS observables, auxiliary variables, and SMAP SM (36 km) products. It then establishes their nonlinear relationship at the same scale and finally builds a downscale retrieval model of SM using the eXtreme Gradient Boosting (XGBoost) algorithm. Focusing on the southern United States, the results indicate that the SM downscaling method exhibits robust performance during both the training and testing processes, enabling the generation of a CYGNSS SM product with a 1 day/3 km resolution. Compared to existing methods, the spatial resolution is increased threefold. Furthermore, in situ sites are utilized to validate the downscaled SM, and spatial correlation analysis is conducted using MODIS EVI and MODIS ET products. The CYGNSS SM obtained by the downscaling model exhibits favorable correlations. The high temporal and spatial resolution characteristics of GNSS-R are fully leveraged through the downscaled method proposed. Furthermore, this work provides a new perspective for enhancing the spatial resolution of SM retrieval using the GNSS-R technique.

**Keywords:** GNSS-R; CYGNSS; SMAP; downscaled; soil moisture
