Satellite radar altimetry provides a unique sea level data set that extends over more than 25 years back in time and that has an almost global coverage. However, when approaching the coasts, the ...extraction of correct sea level estimates is challenging due to corrupted waveforms and to errors in most of the corrections and in some auxiliary information used in the data processing. The development of methods dedicated to the improvement of altimeter data in the coastal zone dates back to the 1990s, but the major progress happened during the last decade thanks to progress in radar technology e.g., synthetic aperture radar (SAR) mode and Ka-band frequency, improved waveform retracking algorithms, the availability of new/improved corrections (e.g., wet troposphere and tidal models) and processing workflows oriented to the coastal zone. Today, a set of techniques exists for the processing of coastal altimetry data, generally called “coastal altimetry.” They have been used to generate coastal altimetry products. Altimetry is now recognized as part of the integrated observing system devoted to coastal sea level monitoring. In this article, we review the recent technical advances in processing and the new technological capabilities of satellite radar altimetry in the coastal zone. We also illustrate the fast-growing use of coastal altimetry data sets in coastal sea level research and applications, as high-frequency (tides and storm surge) and long-term sea level change studies.
The Sentinel-3A and Sentinel-3B satellites were launched, respectively, on 16 February 2016 and 25 April 2018 as part of the European Copernicus program. The Sentinel-3 Surface Topography Mission ...makes use of the altimeter instruments onboard Sentinel-3A and Sentinel-3B to provide elevation measurements not only of the ocean water level but also of the inland waters and ice caps. For the first time, the altimeters onboard Sentinel-3A and Sentinel-3B are operated in Synthetic Aperture Radar mode over all Earth surfaces. They also benefit from elevation priors (the Open-Loop Tracking Command) allowing them to precisely position their receiving window to track the backscattered signal from the inland water targets to be monitored rather than relying on the traditional Closed-Loop tracking mode. This paper makes use of the Sentinel-3A/Sentinel-3B tandem phase to assess the benefits of the Open-Loop tracking mode compared to Closed-Loop. Longer time series are also used to highlight the improvements in terms of the percentage of points over which the altimeter hooks on water surfaces and water surface height estimation brought by the switch of Sentinel-3A from the Closed-Loop to Open-Loop tracking mode as well as the successive Open-Loop Tracking Command updates. In particular, it is shown that from a Level-3 water level product service perspective, the increase in the number of water bodies with valid water surface height estimates is of the order of 25% in Open-Loop with respect to Closed-Loop with similar precision. It is also emphasized that the Open-Loop Tracking Command update onboard Sentinel-3A from v. 4.2 to v. 5.0 yielded a 30% increase in the number of water bodies over which valid water surface height could be estimated. Eventually, the importance of knowing whether a water target was associated with a fine-tuned Open-Loop Tracking Command or an interpolated one is stressed and the recommendation to provide such a flag in the Sentinel-3 Level2 Payload Data Ground Segment products is emitted.
The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the ...users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges.
During its commissioning phase, the Copernicus Sentinel-3B satellite has been placed in a tandem formation with Sentinel-3A for a period of 6 months. This configuration allowed a direct comparison of ...measurements obtained by the two satellites. The purpose of this paper was to present the range of analyses that can be performed from this dataset, highlighting methodology aspects and the main outcomes for each instrument. We examined, in turn, the benefit of the tandem in understanding instrument operational modes differences, in assessing inter-satellite differences, and in validating measurement uncertainties. The results highlighted the very good consistency of the Sentinel-3A and B instruments, ensuring the complete inter-operability of the constellation. Tandem comparisons also pave the way for further improvements through harmonization of the sensors (OLCI), correction of internal stray-light sources (SLSTR), or high-frequency processing of SRAL SARM data. This paper provided a comprehensive overview of the main results obtained, as well as insights into some of the results. Finally, we drew the main lessons learned from the Sentinel-3 tandem phase and provided recommendations for future missions.
Initially developed to monitor the performance of TOPEX/Poseidon and to follow the Jason legacy satellite altimeters at Senetosa Cape, Corsica, this calibration/validation site has been extended to ...include a new location at Ajaccio. This addition enables the site to monitor Envisat and ERS missions, CryoSat-2 and, more recently, the SARAL/AltiKa mission and Sentinel-3A satellites. Sentinel-3A and CryoSat-2 carry altimeters that use a synthetic aperture radar (SAR) mode that is different to the conventional pulse-bandwidth limited altimeters often termed “low resolution mode” (LRM). The aim of this study is to characterize the sea surface height (SSH) bias of the new SAR altimeter instruments and to demonstrate the improvement of data quality close to the coast. Moreover, some passes of Sentinel-3A and CryoSat-2 overfly both Senetosa and Ajaccio with only a few seconds time difference, allowing us to evaluate the reliability and homogeneity of both ground sites in term of geodetic datum. The Sentinel-3A and CryoSat-2 SSH biases for the SAR mode are respectively +22 ± 7 mm and −73 ± 5 mm (for CryoSat-2 baseline C products). The results show that the stability of the SAR SSH bias time series is better than standard LRM altimetry. Moreover, compared to standard LRM data, for which the measurements closer than ~10 km from the coast were generally unusable, SAR mode altimeters provide measurements that are reliable at less than few hundred meters from the coast. View Full-Text
Inspired by the synthetic aperture radar (SAR) technique, a nadir radar altimeter concept called the "Delay/Doppler altimeter" or "SAR mode altimeter" provides better precision and resolution ...capabilities than conventional pulse-limited altimeters (i.e., low-resolution mode). This concept was initially carried on board the CryoSat-2 satellite, then used on Sentinel-3, initially for user requirements on ice or inland water monitoring. This paper addresses geophysical parameter retrieval from Delay/Doppler altimetry over ocean surfaces. For the inversion of geophysical parameters (sea surface height, significant wave height, and backscatter coefficient), we developed an inversion method based on the numerical computation of the radar power-return equation, including instrument design features, such as the range and azimuth impulse responses. To compare this technique with respect to conventional altimetry, we also generated reduced SAR (RDSAR) measurements from the same input data. Geophysical parameter retrieval from low- and high-resolution techniques was then performed for cross-comparison, demonstrating consistency for both techniques, but with a constant 3-cm bias. The proposed processing strategy was then validated using two years of CryoSat-2 data over oceans. The SAR mode provides significant benefits for the observation of small-scale signals (below 50 km) and performs as accurately as conventional altimetry for basin or global scales. The results demonstrate what is expected from the upcoming Sentinel-3 and Sentinel-6/Jason-CS missions.
Monitoring the dynamics and surface conditions of lakes is essential for the understanding of climate change dynamics. Lake water level (LWL) is one of the variables that needs a particular ...vigilance, requiring a global and consistent monitoring that can be achieved by spaceborne remote sensing. Satellite radar altimetry has been widely used in the monitoring of inland waters in the past decades, with recent great improvements in the spatial resolution through the new generation of synthetic aperture radar (SAR) altimeters. However, current radar altimetry constellations limit the space-time sampling necessary for a systematic and regular mapping of lakes. GNSS-reflectometry (GNSS-R) could provide complementary observations to densify the spatiotemporal coverage, allowing hydrologists to access to finer details on the lake surface evolution. In particular, grazing GNSS-R broadens the use of opportunistic GNSS signals due to the smaller elevation angles of the reflecting signals. In this letter, we address this issue by comparing the altimetry profiles over Great Lakes of grazing GNSS-R data provided by Spire constellation satellites and SAR altimeter data provided by Sentinel-3 (S-3) constellation satellites, as a first approach in preparation of intertechniques hydrology LWL products. This letter confirms the advantages of fusing observations from both types of remote sensing techniques attaining a precision in LWL estimations better than 6 cm, provided that an accurate regional geoid model is available. Furthermore, GNSS-R data could also provide an independent LWL information to validate the radar altimetry measurements over sites, which are not equipped with ground means.
This paper presents an assessment and comparison of recent mean sea surface (MSS) models. Using a new approach and independent altimeter data sets, we quantify the major improvement of the CNES_CLS15 ...and the DTU15 models. We observe a reduction in the amplitude of omission errors thanks to the use of new geodetic altimeter data sets (i.e., Cryosat‐2 and Jason‐1 geodetic): they are reduced by a factor of 2 compared with previous generations (CNES_CLS11). We also quantify commission errors resulting from the leakage of residual ocean variability and altimeter noise into the MSS models. For wavelengths shorter than 250 km, the error is of the order of 1–2 cm2, i.e., ∼10% to 20% of the sea level anomaly (SLA) variance. The global error of both 2015 models has similar orders of magnitude and spectral power densities, although the commission errors of the CNES_CLS15 model are about half as large as those of the DTU15 model. Its absolute error is also slightly smaller than for the DTU15 model in coastal regions and at high latitudes. Conversely, the DTU15 model produces smaller omission errors, especially in the open ocean over strong bathymetric features. More importantly, the MSS errors still have a substantial impact on altimetry products for wavelengths ranging from 30 to 100 km: the error explains ∼30% of the global SLA variance, and the error can be 2.5 times higher on uncharted ground tracks (e.g., Sentinel‐3) over rugged bathymetry.
Key Points
The use of geodetic measurements in the new MSS (CNES_CLS15, DTU15) contributes to reduce the omission errors by a factor of 2
The commission errors represent between 10% (CNES_CLS15) and 20% (DTU15) of the SLA variance for wavelengths shorter than 250 km
MSS errors on uncharted tracks (e.g., Sentinel‐3A) explain 30% of the SLA variance at short wavelengths (i.e., 30–100 km)
The MicroWave Expertise Center has first been developed to provide a work environment supporting the calibration/validation (CalVal) activities of the Surface Water Ocean Topography (SWOT) mission, ...launched on December 16th, 2022 1. Onboard of SWOT, the new instrument 'KaRIn', is a revolution for both oceanography and hydrology communities and gives access to small scale measurements over ocean, worldwide river heights and flows, and lake heights. Given the high resolution, and thus the large volumetry, the data access is more complex and can no longer be downloaded on private computers.The challenge was to design and set up a work environment that facilitates the data access, providing free computing and storage resources as well as all the tools needed to transform, analyze and visualize such voluminous data. Key features have been addressed in the perspective of reaching this goal.The expertise center is now operational and ensure SWOT CalVal activities. It also addresses prospect studies and could be foreseen for wider activities such as hydrological, oceanographic or coastal research and multi-sensor comparison.
We show the error in water-limited yields simulated by crop models which is associated with spatially aggregated soil and climate input data. Crop simulations at large scales (regional, national, ...continental) frequently use input data of low resolution. Therefore, climate and soil data are often generated via averaging and sampling by area majority. This may bias simulated yields at large scales, varying largely across models. Thus, we evaluated the error associated with spatially aggregated soil and climate data for 14 crop models. Yields of winter wheat and silage maize were simulated under water-limited production conditions. We calculated this error from crop yields simulated at spatial resolutions from 1 to 100 km for the state of North Rhine-Westphalia, Germany. Most models showed yields biased by <15% when aggregating only soil data. The relative mean absolute error (rMAE) of most models using aggregated soil data was in the range or larger than the inter-annual or inter-model variability in yields. This error increased further when both climate and soil data were aggregated. Distinct error patterns indicate that the rMAE may be estimated from few soil variables. Illustrating the range of these aggregation effects across models, this study is a first step towards an ex-ante assessment of aggregation errors in large-scale simulations.
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