Until recently, classical radar altimetry could not provide reliable sea level data within 10 km to the coast. However dedicated reprocessing of radar waveforms together with geophysical corrections adapted for the coastal regions now allows to fill this gap at a large number of coastal sites. In the context of the Climate Change Initiative Sea Level project of the European Space Agency, we have recently performed a complete reprocessing of high resolution (20 Hz, i.e., 350m) along-track altimetry data of the Jason-1, Jason-2 and Jason-3 missions over January 2002 to January 2020 along the coastal zones of Northeast Atlantic, Mediterranean Sea, whole African continent, North Indian Ocean, Southeast Asia, Australia and North and South America. This reprocessing has provided valid sea level data in the 0-15 km band from the coast. A total of nearly 1000 altimetry-based virtual coastal stations have been selected and sea level anomalies time series together with associated coastal sea level trends have been computed over the 2002-2020 time span. In the coastal regions devoid from tide gauges (e.g., African coastlines), these virtual stations offer a unique tool for estimating sea level change close to the coast (typically up to 3 km to the coast but in many instances up to 1 km or even closer). Results show that at about 80% of the virtual stations, the rate of sea level rise at the coast is similar to the rate offshore (15 km away from the coast). In the remaining 20%, the sea level rate in the last 3-4 km to the coast is either faster or slower than offshore. Understanding such a behavior will need further investigation. This new data set is freely available.

The Mean Dynamic Topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. The MDT is the missing component for the optimal assimilation of altimeter data into operational ocean system. However, in coastal areas, where in-situ measurements are sparse, mainly on the shelf, the global MDT solutions are often less accurate than in the open ocean. Considering the availability of long time-series of high-quality HF-Radar velocity measurements in the Mid Atlantic Bight, the main objective is to include these data to calculate an improved MDT is this area.
The prerequisites for the computation of this new Mean Dynamic Topography are to remove the non-geostrophic signal from the HF radar measured velocities. The first step is thus to pre-process these data. We used average currents from December 2006 through November 2016, estimated from HF radars and processed by Rutgers University (https://tds.marine.rutgers.edu/thredds/cool/codar/cat_totals.html ; Roarty et al, 2020). Then the mean wind-driven currents modelized as in Mulet et al (2021), were removed. Besides, the first guess MDT computed from altimetry and gravimetry was improved by using a dedicated filter along the coast. Finally, the improved MDT was estimated from the first guess MDT and the processed in-situ data (HF-radars, drifters and T/S profiles) through a multivariate objective analysis.
This new regional MDT defines better currents near the coast. In particular, the MDT shows more organized across-shelf gradient following the shelf-break, and a more continuous mean flow on the shelf.

Mulet, Sandrine, Marie-Hélène Rio, Hélène Etienne, Camilia Artana, Mathilde Cancet, Gérald Dibarboure, Hui Feng, et al. « The New CNES-CLS18 Global Mean Dynamic Topography ». Ocean Science 17, nᵒ 3 (17 juin 2021): 789‑808. https://doi.org/10.5194/os-17-789-2021.
Roarty, Hugh, Scott Glenn, Joseph Brodie, Laura Nazzaro, Michael Smith, Ethan Handel, Josh Kohut, et al. « Annual and Seasonal Surface Circulation Over the Mid-Atlantic Bight Continental Shelf Derived From a Decade of High Frequency Radar Observations ». Journal of Geophysical Research: Oceans 125, nᵒ 11 (2020): e2020JC016368. https://doi.org/10.1029/2020JC016368.

Since 2010, Synthetic Aperture Radar (SAR) Altimetry is an important tool to observe sea-surface, sea-ice, glaciers or inland waters. Due to the characteristics of SAR altimetry footprints the along-track resolution reduces from several kilometers for Conventional Altimetry (CA) to about a few hundred meters. This is beneficial in coastal areas where reflections from land surfaces contaminate the altimetry signal leading to a worse quality of retrieved geophysical parameters. However, in across track direction SAR altimetry signals behave like CA with a footprint width of several kilometers.
Due to the SAR processing scheme the backscattering of the sea surface is sampled in two dimensions, which are the range (or across-track) and the relative velocity (or along-track) with respect to the target. As the sea surface is not frozen but highly dynamic the position of a scattering surface location is blurred in range and relative velocity. The first is already well known and considered in CA signal models by a convolution with a Gaussian in range direction to describe the random elevation behavior of the sea surface. However, the random vertical velocities of scatterers are not considered in the state-of-the-art processing scheme yet, which leads to errors in the significant wave height estimation.
This study aims to improve the performance of SAR altimetry in coastal areas with a strategy to eliminate land contamination across the satellite track and to introduce vertical velocity blurring of sea surface scatterers. The latter will be considered with a 2D retracker (SINCS-OV) developed in Buchhaupt (2019) and refined in Buchhaupt et al (2020), which retracks the stack or Delay/Doppler-map instead of the multi-looked waveform. The across-track land contamination shall be addressed using an adaptive leading-edge retracking approach similar to the ALES retracker approach (Passaro et al., 2014) and adapted to 2D SAR retracking. Finally, we process CryoSat-2 L1A data in the North East Atlantic region and show the performance of our results with tide-gauges, buoys and wave model data.

Buchhaupt, C. (2019). Model Improvement for SAR Altimetry. PhD thesis, Technische Universität, Darmstadt. Also available as http://tuprints.ulb.tu-darmstadt.de/9015/.

Buchhaupt, C., Fenoglio, L., Becker, M., and Kusche, J. (2020). Impact of vertical water particle motions on focused SAR altimetry. Advances in Space Research.

Passaro, Marcello & Cipollini, Paolo & Vignudelli, S. & Quartly, Graham & Snaith, Helen. (2014). ALES: A multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry. Remote Sensing of Environment. 145. 173–189. 10.1016/j.rse.2014.02.008.

Introduction
HYDROCOASTAL is a two year project funded by ESA, with the objective to maximise exploitation of SAR and SARin altimeter measurements in the coastal zone and inland waters, by evaluating and implementing new approaches to process SAR and SARin data from CryoSat-2, and SAR altimeter data from Sentinel-3A and Sentinel-3B. Optical data from Sentinel-2 MSI and Sentinel-3 OLCI instruments will also be used in generating River Discharge products.
New SAR and SARin processing algorithms for the coastal zone and inland waters will be developed and implemented and evaluated through an initial Test Data Set for selected regions. From the results of this evaluation a processing scheme will be implemented to generate global coastal zone and river discharge data sets.
A series of case studies will assess these products in terms of their scientific impacts.
All the produced data sets will be available on request to external researchers, and full descriptions of the processing algorithms will be provided

Objectives
The scientific objectives of HYDROCOASTAL are to enhance our understanding of interactions between the inland water and coastal zone, between the coastal zone and the open ocean, and the small scale processes that govern these interactions. Also the project aims to improve our capability to characterize the variation at different time scales of inland water storage, exchanges with the ocean and the impact on regional sea-level changes

The technical objectives are to develop and evaluate new SAR and SARin altimetry processing techniques in support of the scientific objectives, including stack processing, and filtering, and retracking. Also an improved Wet Troposphere Correction will be developed and evaluated.

Presentation
The presentation will describe the different SAR altimeter processing algorithms that are being evaluated in the first phase of the project, and present results from the evaluation of the initial test data set focusing on performance at the coast. It will also present the results of a study assessing regional tidal models.

Although the primary focus of CryoSat-2 is the cryosphere, the mission has provided over a decade of key altimetry observations across a diverse range of scientific domains (e.g. Bathymetry, Polar Oceanography, Coastal Ocean and Inland Waters). Due to its Interferometric Synthetic Aperture Radar (SARIn) altimeter, CryoSat-2 was a precursor of the development of Delay Doppler processing (the so called “SAR mode”) for ocean applications. Compared to traditional nadir altimeters, SARIn SAR mode observations are characterized by a much smaller footprint, and hence higher along-track resolution (~300-400 m instead of ~5-7 km), as well as reduced noise level. Over the oceans, these characteristics enable major advancements especially in coastal regions: the higher resolution allows a more detailed representation of the smaller structures ( < 100 km), while the smaller footprint improves data coverage and precision closer to the coast. Despite these advantages, CryoSat-2 ocean observations remain somehow underutilized in the oceanography community.

The overarching aim of the Cryo-TEMPO project is to develop agile, robust and state-of-the-art CryoSat-2 Products, which are dedicated to specific Thematic Areas, and which are accessible to a broad range of scientific and service users. The coastal ocean is one of the five thematic areas included in the project. The first goal of the Cryo-TEMPO Coastal Ocean Thematic Data Product (TDP) is to maximize the potential of CryoSat-2 observations over the ocean by implementing an ocean-specific processing chain based on the best performing methods and algorithms for coastal applications (and currently not implemented for the production of the official CryoSat-2 ocean products). These methods include, among others: the SAMOSA+ retracker, high-frequency adjustment (HFA) correction, iterative data editing, dedicated sea state bias (SSB) correction and regional mean dynamic topography (MDT). A second goal of Cryo-TEMPO is to expand the user base of its products beyond the traditional altimetry experts by providing an easy-to-use dataset. Thus, the Cryo-TEMPO Coastal Ocean TDP contains only few parameters (i.e. SLA and ADT, filtered SLA and ADT, flags) compared to traditional level 2 products. Furthermore, each variable is distributed with an associated transparent and traceable uncertainty.

The Cryo-TEMPO Coastal Ocean TDP is currently based on SAR and low resolution mode (LRM) CryoSat-2 observations over the Mediterranean region (-6.4 E to 36.5 E longitude; 30 N to 46 N latitude). Input data to generate the dataset are the official CryoSat-2 GOP baseline-C level1-b products. The Coastal Ocean TDP spans from the beginning of the mission (April 2010) to present, with new CryoSat-2 observations processed and added to the database on a monthly basis. The first version of the product (as well as monthly visualisations of the main variables) is currently available at the Cryo-TEMPO web portal (http://www.cpom.ucl.ac.uk/cryotempo/index.php?theme=coastaloceans). The data will be distributed and freely accessible via the ESA Cryosat science server (https://earth.esa.int/eogateway/missions/cryosat/data). Each year, the methods and algorithms used in the processing chain will be updated and optimized following altimetry expert and user feedbacks and the full Cryo-TEMPO Coastal Ocean TDP reprocessed.

The current version of the product has been compared to the official GOP baseline-C products. Our results show that the Cryo-TEMPO Coastal Ocean TDP is characterized by lower variance (especially in the LRM regions of the domain), indicating improved accuracy of the product. The largest impacts in the product are from the SSB and HFA corrections as well as from the iterative editing. Our analysis included also comparisons with in-situ observations. These include tide gauges sea-level records from the western part of the Mediterranean basin and surface velocities from the SOCIB HF-radar located in the Balearic Sea. Comparison between the Coastal Ocean TDS and tide gauges SLA indicates a good consistency between the two. Correlations are in line with those previously reported for Sentinel-3A and Jason-3 (although with larger root mean square error). Comparison between satellite-based across-track geostrophic currents and those from HF radar also shows good consistency. Furthermore, the errors between the two are smaller than previously reported in the same region from an analogous comparison using Saral-Altika altimeter observations, instead.