Extraction and Application of Subcarrier Phase Measurements in GNSS-R Altimetry

Subcarrier modulated signals, such as BDS-3 B2 and Galileo E5, are widely used to improve the spectral compatibility and ranging accuracy of Global Navigation Satellite Systems (GNSS). However, designing signal processing and observation extraction techniques for subcarrier modulated signals is still challenging for the navigation community. In GNSS-R application, GNSS-R phase altimetry is proposed to provide sea surface height information as an economical and accurate technology to solve the resolution problem. However, phase unwrapping and phase integer ambiguity pose significant challenges for the application of GNSS-R phase altimetry. To overcome these obstacles, the utilization of subcarrier modulated signals is noticed for the first time, presenting a novel opportunity for GNSS-R phase altimetry due to the substantial subcarrier wavelength of 19.5 meters. In the work, we developed a subcarrier modulated signal processing strategy that can track both the upper and lower band signals. By combining the two signals, we construct virtual signals whose phase matches the subcarrier phase. These virtual signals undergo processing using long-time coherent integration and sliding filtering to enhance the signal-to-noise ratio and minimize errors. Subsequently, subcarrier phase measurements are extracted from these virtual signals. To validate the effectiveness, subcarrier phase measurements are applied to a GNSS-R altimetry station and drone platform. When compared with accurate in-situ SSH results, it is evident that the phase is much easier to unwrap and the phase integer ambiguity is easier to fix. Furthermore, the accuracy can achieve centimeter-level precision.