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

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.

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Mapping Water Surface Elevation and Slope in the Mississippi River Delta Using the AirSWOT Ka-Band Interferometric Synthetic Aperture Radar

Remote Sensing ◽

10.3390/rs11232739 ◽

2019 ◽

Vol 11 (23) ◽

pp. 2739 ◽

Cited By ~ 2

Author(s):

Michael Denbina ◽

Marc Simard ◽

Ernesto Rodriguez ◽

Xiaoqing Wu ◽

Albert Chen ◽

...

Keyword(s):

Synthetic Aperture Radar ◽

Mississippi River ◽

Water Surface ◽

Surface Elevation ◽

Synthetic Aperture ◽

Ka Band ◽

Water Surface Elevation ◽

Phase Calibration ◽

Aperture Radar

AirSWOT is an airborne Ka-band synthetic aperture radar, capable of mapping water surface elevation (WSE) and water surface slope (WSS) using single-pass interferometry. AirSWOT was designed as a calibration and validation instrument for the forthcoming Surface Water and Ocean Topography (SWOT) mission, an international spaceborne synthetic aperture radar mission planned for launch in 2022 which will enable global mapping of WSE and WSS. As an airborne instrument, capable of quickly repeating overflights, AirSWOT enables measurement of high frequency and fine scale hydrological processes encountered in coastal regions. In this paper, we use data collected by AirSWOT in the Mississippi River Delta and surrounding wetlands of coastal Louisiana, USA, to investigate the capabilities of Ka-band interferometry for mapping WSE and WSS in coastal marsh environments. We introduce a data-driven method to estimate the time-varying interferometric phase drift resulting from radar hardware response to environmental conditions. A system of linear equations based on AirSWOT measurements is solved for elevation bias and time-varying phase calibration parameters using weighted least squares. We observed AirSWOT WSE uncertainty of 12 cm RMS compared to in situ water level measurements when averaged over an area of 0.5 km 2 at incidence angles below 15 ∘ . At higher incidence angles, the observed AirSWOT elevation bias is possibly due to residual phase calibration errors or radar backscatter from vegetation. Elevation profiles along the Wax Lake Outlet river channel indicate AirSWOT can measure WSS over a 24 km distance with uncertainty below 0.3 cm/km, 8% of the true water surface slope as measured by in situ data. The data analysis and results presented in this paper demonstrate the potential of AirSWOT to measure water surface elevation and slope within highly dynamic and spatially complex coastal environments.

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

Hydrology and Earth System Sciences ◽

10.5194/hess-25-333-2021 ◽

2021 ◽

Vol 25 (1) ◽

pp. 333-357

Author(s):

Cecile M. M. Kittel ◽

Liguang Jiang ◽

Christian Tøttrup ◽

Peter Bauer-Gottwein

Keyword(s):

Water Surface ◽

Open Loop ◽

Surface Elevation ◽

Catchment Scale ◽

Spatiotemporal Resolution ◽

Radar Altimetry ◽

Water Surface Elevation ◽

Satellite Constellation ◽

In Situ Data ◽

Wide Range

Abstract. Sentinel-3 is the first satellite altimetry mission to operate both 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. Additionally, the two-satellite constellation offers a unique compromise between spatial and temporal resolution with over 65 000 potential water targets sensed globally. In this study, we evaluate the possibility of extracting 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 Copernicus Open Access Hub (SciHub) and Grid Processing on Demand (GPOD). The objectives of the study are to demonstrate that by using publicly available processing platforms, such databases can be created to suit specific study areas for any catchment and with a wide range of applications in hydrology. We select the Zambezi River as a study area. In the Zambezi basin, 156 virtual stations (VSs) contain useful WSE information in both datasets. The root-mean-square deviation (RMSD) is between 2.9 and 31.3 cm at six VSs, where in situ data are available, and all VSs reflect the observed WSE climatology throughout the basin. Some VSs are exclusive to either the SciHub or GPOD datasets, highlighting the value of considering multiple processing options beyond global altimetry-based WSE databases. In particular, we show that the processing options available on GPOD affect the number of useful VSs; specifically, extending the size of the receiving window considerably improved data at 13 Sentinel-3 VSs. This was largely related to the implementation of GPOD parameters. While correct on-board elevation information is crucial, the postprocessing options must be adapted to handle the steep changes in the receiving window position. 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 benefit of the high spatiotemporal resolution of the dual-satellite constellation.

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Evaluation of Sentinel-3A SAR Altimetry Observations over the Taiwan coastal region

10.5194/egusphere-egu2020-12039 ◽

2020 ◽

Author(s):

Huan Chin Kao ◽

Chung Yen Kuo ◽

Ck Shum ◽

Yuchan Yi

Keyword(s):

Synthetic Aperture Radar ◽

Tide Gauge ◽

European Space Agency ◽

Coastal Region ◽

Open Loop ◽

Ocean Dynamics ◽

Synthetic Aperture ◽

European Organization ◽

Tracking Mode ◽

Aperture Radar

Pulse-limited radar altimeters have been proven to be an excellent data source in oceanography for monitoring sea surface heights and inland water surface elevations since the 1990s. However, the measurements of conventional altimetry missions in coastal areas present the principal problems related to the inherent limitations of this technique such as wider footprint resulting in contaminated waveforms and relatively unreliable media and geophysical corrections. The European Space Agency (ESA) and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) joint mission Sentinel-3A, launched in February 2016, is the first altimetry mission to provide 100% global coverage of ocean observations in Synthetic Aperture Radar (SAR) mode. The Sentinel-3A carries a dual-frequency (Ku- and C-band) Synthetic Aperture Radar Altimeter (SRAL) with a new on-board tracking system (open-loop tracking mode) to employ SAR altimetry technologies providing finer along-track spatial resolution up to ~300 m. Compared with the similar mission Cryosat-2, Sentinel-3A has a better ability to observe the global monitoring of ocean dynamics with a shorter repeat cycle of 27 days and less affected by topography in contaminated waveforms from coastal regions due to open-loop tracking mode with a good prior surface elevation estimate on-board. In this study, the SAR altimetry observations of Sentinel-3A over the Taiwan coastal region were reprocessed by a proposed retracking strategy to obtain more accurately retrieved sea level observations. The main objective of this study is to evaluate the performance of Sentinel-3A in coastal observation by using a near-by tide gauge measurements or other altimetry mission like SARAL/Altika and Jason-3.

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Evaluation of Sentinel-3 SRAL SAR Altimetry over Recently Constructed Global Reservoirs

10.5194/egusphere-egu2020-8012 ◽

2020 ◽

Author(s):

Xingxing Zhang ◽

Liguang jiang ◽

Zhijun Yao ◽

Zhaofei Liu ◽

Rui Wang ◽

...

Keyword(s):

Water Surface ◽

Tracking System ◽

A Priori ◽

Open Loop ◽

Water Bodies ◽

Surface Elevation ◽

Radar Altimetry ◽

Inland Water ◽

Water Surface Elevation ◽

Inland Water Bodies

Satellite radar altimetry is increasingly being used for hydrological studies. However, it is still challenging to deliver high quality data over inland water bodies, i.e. lakes, rivers and reservoirs. One of the reasons is that the positioning of the range window is difficult due to highly variable topography and water surface elevation. To address this issue, Sentinel-3, the first SAR altimeter operating at global scale, is configured with a new on-board tracking system, i.e. open-loop mode. An open-loop tracking system can position the range window very efficiently based on a priori surface elevation stored on-board. In this context, a suitable a priori surface elevation of inland water bodies is very important.Sentinel-3 is operating based on a pseudo-DEM controlled through the Open-Loop Tracking Command (OLTC). &#160;The current OLTC V5 (operated after March 2019) is generated based on an inland water mask and Altimeter corrected Elevations (ACE-2), which is created using multi-mission Satellite Radar Altimetry from 1994-2005 in combination with the Shuttle Radar Topography Mission (SRTM). However, OLTC V5 still misses some inland water bodies and contains some incorrect surface elevations, especially over newly built reservoirs, where the difference between water surface elevation and ACE-2 can exceed 100m.In this study, a comprehensive evaluation of Sentinel-3A (S3A) is conducted at 26 globally-distributed recently constructed large reservoirs. The results show that S3A fails to deliver useful data over most new reservoirs in open loop due to the incorrect a priori elevations stored in OLTC V5. On the contrary, S3A closed-loop (operated before March 2019) can deliver useful data in many cases.To improve the OLTC table, we propose two approaches. The first one is to use dam height to correct the a priori surface elevation, which is relevant for very recently completed dams or dams under construction. The other is to use water surface elevation from Cryosat-2 to update the OLTC table. The approaches are demonstrated for reservoirs located on the Lancang and Nu rivers in the Southwest of China. The updated OLTC table will help exploit the Sentinel-3 radar altimetry mission to its full potential, enabling it to correctly track water surface elevation in a larger number of water bodies.

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Evaluation of the Performances of Radar and Lidar Altimetry Missions for Water Level Retrievals in Mountainous Environment: The Case of the Swiss Lakes

Remote Sensing ◽

10.3390/rs13112196 ◽

2021 ◽

Vol 13 (11) ◽

pp. 2196

Author(s):

Frédéric Frappart ◽

Fabien Blarel ◽

Ibrahim Fayad ◽

Muriel Bergé-Nguyen ◽

Jean-François Crétaux ◽

...

Keyword(s):

Synthetic Aperture Radar ◽

Water Level ◽

Open Loop ◽

Water Levels ◽

Synthetic Aperture ◽

Low Resolution ◽

Radar Altimetry ◽

Constant Bias ◽

Aperture Radar

Radar altimetry is now commonly used to provide long-term monitoring of inland water levels in complement to or for replacing disappearing in situ networks of gauge stations. Recent improvements in tracking and acquisition modes improved the quality the water retrievals. The newly implemented Open Loop mode is likely to increase the number of monitored water bodies owing to the use of an a priori elevation, especially in hilly and mountainous areas. The novelty of this study is to provide a comprehensive evaluation of the performances of the past and current radar altimetry missions according to their acquisition (Low Resolution Mode or Synthetic Aperture Radar) and tracking (close or open loop) modes, and acquisition frequency (Ku or Ka) in a mountainous area where tracking losses of the signal are likely to occur, as well as of the recently launched ICESat-2 and GEDI lidar missions. To do so, we evaluate the quality of water level retrievals from most radar altimetry missions launched after 1995 over eight lakes in Switzerland, using the recently developed ALtimetry Time Series software, to compare the performances of the new tracking and acquisition modes and also the impact of the frequency used. The combination of the Open Loop tracking mode with the Synthetic Aperture Radar acquisition mode on SENTINEL-3A and B missions outperforms the classical Low Resolution Mode of the other missions with a lake observability greater than 95%, an almost constant bias of (−0.17 ± 0.04) m, a RMSE generally lower than 0.07 m and a R most of the times higher than 0.85 when compared to in situ gauge records. To increase the number of lakes that can be monitored and the temporal sampling of the water level retrievals, data acquired by lidar altimetry missions were also considered. Very accurate results were also obtained with ICESat-2 data with RMSE lower than 0.06 and R higher than 0.95 when compared to in situ water levels. An almost constant bias (0.42 ± 0.03) m was also observed. More contrasted results were obtained using GEDI. As these data were available on a shorter time period, more analyses are necessary to determine their potential for retrieving water levels.

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