High-precision GNSS-R Altimetry based on Carrier Phase Measurement Combination

Abstract: Global Navigation Satellite System-Reflectometry (GNSS-R) technique has been used to obtain sea surface height (SSH) since the 1990s. Due to the short wavelengths and low power of GNSS signals, the continuously tracked carrier phase measurements of reflected signals are usually unavailable for sea surfaces with big roughness, varying over space and time. Under high sea states, the phase difference cannot be well retrieved from carrier phase measurements, especially for the signals with high elevation angles. To overcome these shortcomings related to temporal incoherence, we propose an improved algorithm to extract the combined interferometric phase difference measurements between direct and reflected signals. We improve the configuration of GNSS-R altimetry software-defined receiver (SDR) by reconstructing ‘clean’ direct signals to compute phase differences between direct and reflected signals. The interferometric phase differences are combined in the complex domain and the resulting interferometric signal is refined through open-loop tracking with 60-s coherent integration before the phase difference measurements are extracted, without tracking their respective carrier phase measurements in advance.  In order to verify our method, a coastal experiment under different sea conditions was conducted. Raw intermediate frequency data of Quasi-Zenith Satellite System were collected and processed by SDR to compute the path delay measurements of L1 and L5. Under high sea states, the phase delay measurements of L1 and L5 were random over time, while phase delay can still be well recovered based on the proposed method even in the case of high elevation angles. The altimetry solutions were compared with the in situ observations from a radar altimeter instrument. The results show that centimeter-level altimetry accuracy can be achieved under high sea states using the proposed method The same SDR and method are applied in the shipborne altimetry experiment, the interferometric phase observations are successfully extracted on both ship motion and statics. Also, the integer ambiguity in the interferometric phase observations is well estimated. The differences in the SSH measurements between different satellites is at centimeter level. The coastal and shipborne experiments demonstrate that the dual-antenna GNSS-R phase altimetry technique can be used for low-cost tide gauges on different platforms to monitor sea levels.

Acknowledgments: This work was supported by Natural Science Foundation of China (42192534) and Key Research and Development Program of Shandong Province (Major Technological Innovation Project, 2021ZDSYS01).