An Improved Dual-Antenna GNSS-IR Method for BDS-Based High-Elevation Soil Moisture Retrieval: Initial Results and Evaluation

GNSS Interferometric Reflectometry (GNSS-IR) has been proven effective for soil moisture monitoring by leveraging the interference effects between direct and reflected signals at low elevation angles. However, at high elevation angles, the polarization of reflected signals shifts to left-handed circular polarization (LHCP), which cannot be captured by right-handed circular polarization (RHCP) antennas—thus rendering conventional GNSS-IR technology ineffective. To develop more robust solutions for harnessing high-elevation GNSS data, this study proposes an improved GNSS-IR configuration that integrates two geodetic GNSS receivers, one upward-looking RHCP antenna, one downward-looking LHCP antenna, and two power couplers for soil moisture retrieval. This optimized setup enables the acquisition and utilization of high-elevation GNSS signals for GNSS-IR-based soil moisture estimation. To validate the proposed method, an observational campaign was conducted at a farmland site. Analyses of the amplitude and phase measurements derived from BDS reflected signals indicate that soil moisture solutions retrieved from high-elevation signals achieve accuracy comparable to those obtained from low-elevation signals. Specifically, the maximum correlation coefficient between retrievals and in-situ data reaches 0.95, and the mean RMSE across all frequencies is no greater than 0.02 m³ /m³. Overall, this method maximizes the utilization efficiency of received GNSS signals and provides a more comprehensive detection range compared with conventional GNSS-IR method.