ABSTRACT
Radar altimetry is an essential component for observing the complex ocean circulation. 95% of the ocean socioeconomic activity is developed in coastal areas. For this reason (and many others), understanding the physical processes driving the coastal ocean dynamics should be a priority (Vignudelli et al., 2011). The coastal altimetry community has the responsibility of providing the most accurate product to study the coastal processes.

One of the most novel concepts in coastal altimetry is the Fully Focused SAR (FF SAR) processing. FF SAR introduces improvements in terms of along-track resolution in high Pulse Repetition Frequency (PRF) radar altimeters. The processing is similar to the SAR altimetry, but with an unprecedented high along-track resolution up to the theoretical limit equal to half the antenna length (~0.5 m). This is in contrast to the ~300 m unfocused SAR along-track resolution given in coastal altimeter standard products (Egido and Smith 2017). The FF SAR footprint is SAR focused along-track and pulse-limited across-track. The main drawback of FF SAR is its high computational effort, but some works have been done to reduce the computational time without losing accuracy (Guccione et al., 2018).

In this work, the performance of FF SAR products in the Gulf of Cadiz (Spain) was analysed in terms of accuracy and precision. The analysis was done in the first 5 km from the coast. Two FF SAR algorithms still in development were used: (i) FF SAR Back Projection (BP) (S3 prototype version of Kleinherenbrink et al., 2020); and (ii) FF SAR Omega-Kappa (WK) (Guccione et al., 2018). Four retracking algorithms were used to estimate the retracked range: Threshold retracker (Davis 1993), SAMOSA (Ray et al., 2015), SAMOSA+ (Dinardo et al., 2018) and ALES+ SAR (Passaro et al., 2020). Also, the results were compared to unfocused SAR using data provided by the ESA Grid Processing On Demand (GPOD). Table 1 shows a description of the datasets used in this work.

Four tracks were processed in the study area according to the availability of in-situ measurements (Figure 1): Two tracks from Sentinel-3A and two from Sentinel-3B. The posting rate used was 80 Hz. A total of 45 cycles for S3A tracks and 15 cycles for S3B were analysed in the period 2016-2019. The products accuracy was obtained by comparing time series of Sea Level Anomaly (SLA) with those obtained from a radar tide gauge. The statistics used were the standard deviation of the difference (sdd). Also, an analysis of the Percentage of Cycles for High Correlation (PCHC) was done (Passaro et al., 2015). To evaluate the precision, the SLA differences between consecutive measurements along-track were calculated for each cycle. These differences were considered a good estimation of noise, since SLA is not expected to change significantly in 85 m along track, which is the distance between consecutive measurements at 80 Hz. Subsequently, using the average of these differences, the noise over a single cycle was obtained. Finally, the precision over each track segment was determined by averaging the noise of all the cycles.

Our preliminary results show percentages of PCHC higher in FF SAR than unfocused SAR. In terms of sdd, the result obtained comparing the in-situ data and the different S3 datasets were similar, ranging between 7 and 10 cm. Although it should be noted that lower values of sdd were obtained at 1-2 km of the coast in FF SAR. In addition, preliminary results indicate that the along-track noise is lower in FF SAR than in unfocused SAR. Moreover, some improvements in the results were observed when the ALES+ SAR retracker was applied.

Besides, in order to assess the potential of the different algorithms and retrackers for their use in coastal oceanographic applications, across-track surface current velocities, derived from the different SLA retrievals, will be compared with high-frequency radar (HFR) data.

The innovation of this study lies in the use of new products from FF SAR to improve our knowledge of coastal processes such as the coastal countercurrent observed in the Gulf of Cadiz (its origin is still on debate), taking advantage of the good quality of data closer to the coast (1-2 km) compared to unfocused SAR (up to 3 km) (Mulero-Martinez et al., 2021; Aldarias et al., 2020).

REFERENCES

Aldarias, A., Gómez-Enri, J., Laiz, I., Tejedor, B., Vignudelli, S., & Cipollini, P. (2020). Validation of Sentinel-3A SRAL coastal sea level data at high posting rate: 80 Hz. IEEE Transactions on Geoscience and Remote Sensing, 58(6), 3809-3821, doi: 10.1109/TGRS.2019.2957649

Davis, C. H. (1993). A surface and volume scattering retracking algorithm for ice sheet satellite altimetry. IEEE transactions on geoscience and remote sensing, 31(4), 811-818, doi: 10.1109/36.239903

Dinardo, S., Fenoglio-Marc, L., Buchhaupt, C., Becker, M., Scharroo, R., Fernandes, M. J., & Benveniste, J. (2018). Coastal SAR and PLRM altimetry in German Bight and West Baltic Sea. Advances in Space Research, 62(6), 1371-1404, doi: 10.1016/j.asr.2017.12.018

Egido, A., & Smith, W. H. (2016). Fully focused SAR altimetry: theory and applications. IEEE Transactions on Geoscience and Remote Sensing, 55(1), 392-406, doi: 10.1109/TGRS.2016.2607122

Guccione, P., Scagliola, M., & Giudici, D. (2018). 2D Frequency domain fully focused SAR processing for high PRF radar altimeters. Remote Sensing, 10(12), 1943, doi: 10.3390/rs10121943

Kleinherenbrink, M., Naeije, M., Slobbe, C., Egido, A., & Smith, W. (2020). The performance of CryoSat-2 fully-focussed SAR for inland water-level estimation. Remote Sensing of Environment, 237, 111589, doi: 10.1016/j.rse.2019.111589

Mulero-Martínez, R., Gómez-Enri, J., Mañanes, R., & Bruno, M. (2021). Assessment of near-shore currents from CryoSat-2 satellite in the Gulf of Cádiz using HF radar-derived current observations. Remote Sensing of Environment, 256, 112310, doi: 10.1016/j.rse.2021.112310

Passaro, M., Fenoglio-Marc, L., & Cipollini, P. (2015). Validation of significant wave height from improved satellite altimetry in the German Bight. IEEE Transactions on Geoscience and Remote Sensing, 53(4), 2146-2156, doi: 10.1109/TGRS.2014.2356331

Passaro, M., Restano, M., Sabatino, G., Orrú, C., & Benveniste, J. (2020). The ALES+ SAR Service for Cryosat-2 and Sentinel-3 at ESA GPOD. In OSTST 2020.

Ray, C., Martin-Puig, C., Clarizia, M. P., Ruffini, G., Dinardo, S., Gommenginger, C., & Benveniste, J. (2015). SAR altimeter backscattered waveform model. IEEE Transactions on Geoscience and Remote Sensing, 53(2), 911-919, doi: 10.1109/TGRS.2014.2330423

Vignudelli, A. G. Kostianoy, P. Cipollini, and J. Benveniste, Eds., Coastal Altimetry. Berlin, Germany: Springer, 2011, doi: 10.1007/978-3-642-12796-0

The dynamics of estuaries and coastal zone is affected by a strong variability of the water level at various temporal and spatial scales due to the interaction of different water bodies (river, tributaries, groundwater and sea) and phenomena, like river discharge, tide, waves, surges and sea level rise.
The freshwater discharge to the coast is a fundamental component of global water cycle and can impact sea level on a broad range of spatial and temporal scales.

In this study, water level and discharge change and extremes in the Elbe tidal river, estuary and at the coast nearby are examined based on enhanced quality SAR altimeter data and an unstructured model simulation, the Geesthacht Coupled Ocean Model System 3D (GCOAST-SCHISM).
In-situ water level and wave height (from the federal agencies BfG, BKG and BSH), physical and water quality parameters are measured in campaigns (CMEMS databank) and simulated in models with regular grids, like the BSHmodel, at resolution 900 m, and NEMO-WAM with 3 km for North Sea-Baltic Sea area and 400 m for the southern North Sea resolution.
First aim is to quantify the advantages in modelling achieved with the unstructured model GCOAST-SCHISM where the mean resolution is 200m. The German Bight setup used to solve for the wave- and hydrodynamics resolves the Elbe with on average 30-50m/100-200m in cross/along channel direction and 21 vertical terrain layers. Horizontal resolution is 150 m in tidal inlets and estuaries, and 400 m in open sea. Dedicated runs of SCHISM are performed in the Elbe, Ems, Weser and German Bight region of interest. Hourly simulations start in 2010 and include strong variabilities in river discharge, shorter scenario runs of coastal sea level rise are conducted.

For the observational part, a multi-mission along-Track altimeter database in SAR mode for German Bight and Estuaries (MATSAR-GBE) is build starting from year 2010 for the missions CryoSat-2, Sentinel-3 and Sentinel-6. The data are from the unfocused SAR, SARin, Migration Correction (RMC-SAR) and Fully Focused SAR (FF-SAR) processing. All processors are running at the ESA Network of Resources, included the in-house TUDaBo SAR processor. The STARS processor instead, which is developed within the HYDROCOASTAL ESA funded project, is running at the University of Bonn.

Water level, wave height and wind are evaluated against in-situ and model data. The study aims also at the preparation for the exploitation of the future SWOT data and is related to the CONWEST-DYCO project of the SWOT Science Team. The potential of wide-swath altimetry is investigated considering along track observations for along track altimetry and SWOT in its two sampling phases.

Coastal Processing from the Copernicus Altimeters: the CORS processor outcomes

The Copernicus constellation has today 3 altimeters in flight, the two Sentinel-3, A & B, and the most recent altimetric mission Sentinel-6, as a continuation of a long historical series. All of them are operating the new standard Synthetic Aperture Radar mode, enabling for the first time an enhanced resolution sea surface monitoring over the global Ocean (CryoSat-2 was operating in SAR / SARin only over dedicated areas).
Coastal zones are crucial for the human development, and the characterisation of the ocean processes near to the coast is a must. The sea level rise is one of the most pressing climate change impacts, and although it is driven by global ocean forcing, refining this variable, along with sub-mesoscale ocean dynamics in the last 10 km to the coast, is still a challenge in the altimetry field.
Dedicated processing evolutions are needed for these areas when using altimetry data to derive the geophysical retrievals Sea Surface Height (SSH) and Significant Wave Height (SWH). The contribution of undesired targets is to be addressed and the retracking process is to be adapted. This has been the goal of this study, developed over the recent years, refining the processing steps of an algorithm that has been designed, implemented, and validated over a variety of complex coastal topography areas and different sea states.
The validation outcomes give a consistent SSH noise reduction of around 50% over different validation areas, such as the Mediterranean Sea. In the other hand, Power Spectral Density (PSD) studies show a better (denoised) SWH PSD over the full range of wavelengths from the largest (real geophysical signal) to the smallest scales (measurements noise). The SSH median bias between the ocean and coastal isardSAT retracking outputs is about 5 mm, while the SWH median bias is about 10 cm.
SAR mode data from the 3 Copernicus altimeters is used for this investigation. The coastal processing is focussed at filtering out as much as possible the sea surface signal contamination, with the minimum degradation of the sea surface scientific information. This specific processing comes from the idea explained at (Garcia et al.,2018, https://doi.org/10.1016/j.asr.2018.03.015) using CryoSat-2 SARin data, adapted to Copernicus altimeters SAR data.
isardSAT has developed this study as ESA Expert Support Laboratory within the Sentinel-3A Mission Performance Centre team and within the CORS (Coastal Ocean Retracking for Sentinels) project as contribution to the Sentinel-6 Validation Team.

Satellite altimetry allows for the determination of sea level, wind speed, and significant wave height through remote sensing. We present a novel coastal retracking algorithm for SAR altimetry to estimate the significant wave height. A novel and innovative adaptive interference masking scheme senses and masks spurious interfering signals that arise from strongly reflective targets in the coastal zone. The objective of the retracker is to significantly increase the number of valid records without sacrificing quality. The performance of the novel retracker is validated with the methodology recently developed in the framework of the European Space Agency Sea State Climate Change Initiative project. As an external reference for validation, an ERA5-based hindcast wave model, a set of in-situ buoy data gathered from the European Centre for Medium-Range Weather Forecasts, and three external datasets from competing retracking algorithms were used. Different metrics were assessed as functions of sea state and distance to the nearest coast: outliers, number of valid records, intrinsic noise, wave spectral variability, and correlation statistics of the comparison against wave model and in-situ buoy data. The validation results show that the presented retracker significantly increases the number of valid 20-Hz records in the near coastal zone of less than 5 km off the coast by more than 25% as compared to the best competing retracker with no degradation of quality. We highlight the importance of the correct choice of the quality flag that is provided together with the significant wave height. Our findings suggest that the strategy for significant wave height quality flag of the official baseline L2 product of Sentinel-3 can be redefined to get more robust significant wave height estimates in the coastal zone.

More than 600 million people (about 10% of the world's population) live in coastal areas that are less than 10 m above sea level. Despite the urgent need to monitor coastal waters, in-situ measuring stations including wave buoys around the world do not provide sufficient insight into coastal water level variations, and in particular, they cannot provide sufficient information on one of the essential properties of water surfaces, namely the Significant Wave Height (SWH). Satellite altimetry plays an increasingly important role, especially after operating in Synthetic Aperture Radar (SAR) mode. However, due to the complexity of the coastal water surfaces, the performance of the satellite altimeters over the coastal area falls behind the open ocean surfaces. In addition, the well-known direct relationship between waveform rise time and SWH does not hold for SAR waveforms due to a different processing scheme. This study proposes a data-driven method to determine SWH using the Sentinel-3 data for both oceanic and coastal zones. For this purpose, we propose a method based on the rise time ($\delta r$) and the width of a waveform, called RiwiSAR-SWH (rise time width model for SAR-SWH), which is free from the complexity of the SAR physical model and estimates SWH over the coastal area and open ocean in a relatively straightforward manner. We have employed our method over different regions in the coastal zone of the North Sea. The results are validated against in-situ buoy data and compared with SWH estimates from SAMOSA+, SAMOSA++ and the Sentinel-3 Ocean retracker. The validation shows that the proposed method can determine SWH with accuracy ranging from 0.25 m to 0.91 m for different locations in the North Sea. Moreover, we obtain reliable SWH to within 1\ut{km} from distance from the coast, which is an improvement of more than 40\% compared to existing methodologies.