Sentinel-3 radar altimetry for river monitoring – a catchment-scale evaluation of satellite water surface elevation from Sentinel-3A and Sentinel-3B

Abstract

Abstract. Sentinel-3 is the first satellite altimeter to operate in Synthetic Aperture Radar (SAR) mode and in open-loop tracking mode nearly globally. Both features are expected to improve the ability of the altimeters to observe inland water bodies. In this study we evaluate the possibility to extract river water surface elevation (WSE) at catchment level from Sentinel-3A and Sentinel-3B radar altimetry, using Level-1b and Level-2 data from two public platforms. The objectives of the study are to evaluate the density of valuable observations and establish a WSE monitoring network. Additionally, we demonstrate the potential application of Sentinel-3 for monitoring river interactions with wetlands and floodplains. In the Zambezi basin, 175 virtual stations (VS) contain useful WSE information in both datasets, far exceeding the number of VS available in standard databases. The RMSD is between 2.7 cm and 31.2 cm at six in-situ stations and the VS reflect the observed WSE climatology throughout the basin. Additional VS are available in both the Copernicus Open Access Hub and GPOD (Grid Processing on Demand), highlighting the value of considering multiple processing options. In particular, we show that the processing options available on GPOD strongly affect the number of useful VS; in particular, extending the size of the receiving window, considerably improved data at 13 Sentinel-3 VS. The number of VS delivering usable data increased after the Open-Loop Tracking Command (OLTC) on-board Sentinel-3A was updated. However, the open-loop tracking mode poses two new challenges: correct on-board elevation information is crucial, and steep changes in the receiving window position can have detrimental effects on the WSE observations. Finally, we extract Sentinel-3 observations over key wetlands in the Zambezi basin. We show that clear seasonal patterns are captured in the Sentinel-3 WSE, reflecting flooding events in the floodplains. These results highlight the potential of using Sentinel-3 as a SWOT (Surface Water and Ocean Topography) surrogate while awaiting the mission launch. The results show the benefit of the high-resolution Synthetic Aperture Radar (SAR) altimeter, as well as the benefits and disadvantages of the open-loop tracking mode.

On the Performance of Sentinel‐3 Altimetry Over New Reservoirs: Approaches to Determine Onboard A Priori Elevation

“…Here the first outlier removal procedure is based on the backscatter coefficient ( σ 0 ). A backscatter coefficient ( σ 0 ) threshold of 30 dB is used to remove obvious outliers (Kittel et al, 2020). Note that σ 0 and the orthometric WSE are estimated using the OCOG retracker.…”

On the Contribution of Satellite Altimetry-Derived Water Surface Elevation to Hydrodynamic Model Calibration in the Han River

“…The ESA level 2 product contains 20Hz measurements including the waveform, tracker range, satellite altitude, and corrections. The waveforms are retracked with a traditional offset center of gravity retracker [7]. For CryoSat-2, we use the ESA level 1b product from baseline C (https://science-pds.cryosat.esa.int/).…”

“…The RMSE values are 0.14 m and 0.71 m for the VS3 and VS7, respectively. According to the previous studies [7,51], the RMSE values for satellite altimetry below 0.3 m are taken as good, and below 0.6 m are moderate. Sentinel-3A and CryoSat-2 data are not only found to be good and moderate in the literature [7,34,43] but also have been proved to be good and moderate in our case study river, a medium river (around 400 m) with braided and single-threaded reaches.…”

“…According to the previous studies [7,51], the RMSE values for satellite altimetry below 0.3 m are taken as good, and below 0.6 m are moderate. Sentinel-3A and CryoSat-2 data are not only found to be good and moderate in the literature [7,34,43] but also have been proved to be good and moderate in our case study river, a medium river (around 400 m) with braided and single-threaded reaches. By discarding outliers based on DEM approaches, a good or moderate accuracy of altimetry observations can be obtained in the case study, indicating that the land relief issue can be well addressed by discarding invalid measurements (e.g., a careful analysis of satellite waveforms identification).…”

“…One of the key hydraulic variables is the water surface elevation (WSE), which refers to the height of surface water above a given datum (i.e., 1985 National Height Datum of China for in-situ records, and EGM2008 datum for satellite WSE) [5]. Reliable and timely estimation of WSE is important for flood and water resources management tasks [6,7]. Traditionally, WSE measurements relied on in-situ gauging stations and often suffered from insufficient spatial coverage and data access constraints [8,9].…”