This study proposes a soil moisture retrieval method from ground reflection signals received by Global Positioning System (GPS) antenna modules consisting of an up-looking (UP) right-hand circular polarization (RHCP) and two down-looking (DW) RHCP and left-hand circular polarization (LHCP) signals. Field experiments at four different surface types (asphalt, grassland, dry soil, and moist soil) revealed that the DW RHCP and LHCP signals are affected by antenna height and multipath interference signals. The strength differences between the DW LHCP and UP RHCP signals were in good agreement with the DW LHCP signals. Methodologically, this study applied a spectrum analysis to the detrended surface-reflected signals for RHCP and LHCP. The study indicated that the down-looking antenna exhibited greater sensitivity to reflected GPS signals than the up-looking antenna. We demonstrated the feasibility of estimating soil moisture using GPS signals, by comparing LHCP signals received by the down-looking antenna with theoretical values. This study presents a novel method for estimating soil moisture in vegetated areas, leveraging the advantage of crosspolarization comparisons to achieve stronger signal strength than single-polarization reflection signals. With further research, including long-term observations and detailed analysis, the proposed method has the potential to enhance performance significantly.

1. Introduction
2. Data and Methods
    2.1. GPS receiver module
    2.2. Method
3. Results
4. Discussion
5. Conclusions
Acknowledgments
References

• Young-Joo Kwon(Moon Soul Graduate School of Future Strategy, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea)
• Hyun-Ju Ban(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Sumin Ryu(Satellite Application Division, Korea Aerospace Research Institute, Daejeon 34133, Republic of Korea)
• Suna Jo(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Han-Sol Ryu(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Yerin Kim(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Jeong-Eun Park(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Yun-Jeong Choi(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Kyung-Hoon Han(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Yeonjun Kim(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea)
• Sungwook Hong(Department of Environment, Energy, and Geoinfomatics, Sejong University, Seoul 05006, Republic of Korea) Corresponding author