Validating Sentinel 3 altimetry over the Neckar River using GNSS Interferometric Reflectometry

Abstract

GNSS-IR is a technique that enables the constant observation of water surface height using reflected GNSS signals from water surface. It offers a simple monitoring approach compared to other techniques, requiring only a GNSS receiver near the water. The principle of the technique involves analyzing signal-to-noise data by converting signals from the time domain to the frequency domain. Satellite altimetry is another powerful technique for long-term water monitoring, providing extensive spatial coverage. The retrieval of water level from altimetry waveforms, known as retracking, is susceptible to errors due to various factors, despite the development of multiple retracking algorithms for different waveform types. The Sentinel-3 satellite mission, operated by ESA and EUMETSAT, is designed to monitor Earth’s surface topography and climate while providing altimetry data. In this study, the altimetry results for the Neckar river from Sentinel-3 mission will be validated using the GNSS-IR technique. Due to the absence of a permanent GNSS receiver at the ideal measurement point, the measurement campaigns have a limited duration of a few hours each time. To better receive the reflected signals from water, GNSS antennas are rotated in the last 2 campaigns. To maximize the utilization of GNSS signal-to-noise ratio (SNR) data and capture the dynamic water level changes during observations, a novel technique is developed. This technique involves splitting the data according to time and multiplying the Lomb-Scargle Periodograms(LSP) from different satellites within specific time ranges. By extracting the peaks of the multiplied periodograms, a time series can be generated. The altimetry results from the Sentinel-3 mission will then be validated using this time series. To enhance the quality of GNSS-IR results, various methods have been implemented, including selecting different campaign locations, rotating GNSS receivers, and applying data filters such as elevation angle. GNSS-IR is proved to be a able to monitor the inland small water body and rotating the GNSS antenna can improve the result quality. All seven retrackers from the Sentinel-3 mission are validated using water level data obtained from GNSS-IR. The altimetry water level is higher as the result from GNSS-IR and this offset varies for different retrackers from about 0.1 to 0.4 meters. In the challenging Neckar area with a narrow river width and complex environment, OCOG has demonstrated the best performance in terms of both availability and accuracy, the difference is from 0.03 to 0.17 meters in all experiments.