On April 7th, 2022, after 16 months of tandem flight with its predecessor, Jason-3, the Copernicus Sentinel-6 Michael Freilich (MF) satellite became the regional reference mission in the sea level climate record. Its on-board altimeter, POSeidon-4, is the first radar altimeter allowing a simultaneous and continuous acquisition in Low 2023, a new version of Sentinel-6MF ground segment (processing baseline F08) brought major improvements for the LR mode, with the implementation of a numerical retracker in addition to the hist sent work covers the full assessment of this new LR numerical retracker over open ocean, spanning from the retracker's outputs to their contribution to the Global Mean Sea Level. Improvements with respect to MLE4 appeared mainly in terms of sea-state related effects, leading to a 60 % reduction of the Sentinel-6MF/Jason-3 Sea Surface Height Anomaly bias correlated to Significant Wave Height. Such result improves an already very good continuity between the two tandem phase is also precisely assessed. The small remaining discrepancies are attributed to different components of the system, such as the orbit, the radiometer wet troposphere correction, C-band pro feature of the Sentinel-6MF numerical retracker is the use of t Resolution (LR) as well as in High Resolution (HR) mode. In March orical Maximum Likelihood Estimator-4 (MLE4) retracker. The pre- missions. The agreement between Jason-3 and Sentinel-6MF over the cessing or an MLE4-based empirical adjustment. Another important he in-flight Point Target Response to mitigate instrumental changes and thus improve long term stability. The Global Mean Sea Level with respect to Jason-3, once the radiometer wet troposphere correction impact is removed. (c) 2024 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/). analysis presented in this paper shows no significant trend difference

The Surface Water and Ocean Topography (SWOT) mission delivers unprecedented swath-altimetry products. Despite SWOT's 2D coverage and precision, its Level-2 ocean products suffer from the same limitations as their counterparts from nadir altimetry missions. To achieve the mission's primary science objectives, the space agencies generate Level-2 ocean products with SWOT alone. In contrast, some research domains and applications require consistent multi-mission observations, such as the Level-3 ocean products provided by the Data Unification and Altimeter Combination System (DUACS) for almost 3 decades and with 20 different satellites. In this paper, we describe how we extended the Level-3 algorithms to handle SWOT's unique swath-altimeter data. We also illustrate and discuss the benefits, relevance, and limitations of Level-3 swath-altimeter products for various research domains.

The SWOT mission was launched in December 2022. Its first of a kind KaRIn instruments provides two dimensionnal images of ocean surface topography over a 120 km wide swath. Ocean surface topography data quality and mission performance have been assessed and monitored over the first year of SWOT mission, as part of mission performance activities performed by the mission project. Here we present a synthesis of SWOT mission performances over ocean: from data availability and validity to end-to-end performance metrics (eg SSH differences at crossovers, wavenumber spectrum and comparison with the current nadir altimetry constellation). We also discuss the performance of cross-calibration algorithms at level 2 (based on crossovers) and level 3 (based on other altimeters). Results presented here are based on the analysis of the 2 km product during both the calval (1 day repeat orbit) and science (21 day repeat orbit) phases and focused on ocean surface topography retrievals. All mission CalVal metrics highlight the excellent performance of KaRIn measurements. We also present some known limitations of current SWOT products that are of interest to science users.

Poseidon -4 is a dual -frequency redundant radar altimeter on board the European Commission Copernicus Programme Sentinel -6 Michael Freilich satellite, that represents a significant breakthrough with respect to its predecessors Jason -class altimeters due to its digital architecture and to its innovative measurements and calibration modes. In the framework of the Sentinel -6 Michael Freilich commissioning preparatory activities, CNES has contracted CLS for the development of a Sentinel -6 Processing Prototype (S6PP) application. S6PP is a multi -chain processing suite able to process Sentinel -6 Level1A and Level -1B data products up to Level -2. The novel algorithms developed in the CNES/CLS research and development activities are implemented within S6PP and validated to support the different thematic applications (in particular inland water and ocean) and in view of promoting them for possible implementation in the operational ground segment. The present work covers in particular the main results over open ocean for the main altimetric geophysical variables over the sea surface (sea surface height anomaly, significant wave -height, sigma -nought and wind speed) derived by the Low -Resolution Mode (LRM) and High -Resolution Mode (HRM) chains of S6PP in terms of precision, accuracy, spectral content and measurement stability. Given the reported variation of the payload in -orbit temperatures along with the reported instrumental ageing, and given the tight requirement to measure the GMSL (Global Mean Sea Level) in seamless continuity with Jason -3, the clear goal for S6PP was to process the S6-MF data with the minimum possible level of approximations along the processing pipeline but still maintaining a very efficient prototype from the computational point of view. For this scope, a novel and computationally efficient numerical retracking scheme with interface to the in-flight PTR (Point Target Response) provided by the instrument calibration chain has been put in place within S6PP for both the Low -Resolution and HighResolution modes whereas the Delay -Doppler beam -forming is carried out by applying the range walk correction based on a computationally efficient algorithm (Chirp Zeta -Transform). The impact of the range walk correction and of the in-flight PTR interface is assessed for HRM and LRM, respectively. The paper shows that the proposed processing baseline ensures a dataset robust from the currently known instrumental degradation or ageing issues, both in LRM and HRM mode and, once this is done, that Sentinel -6 Michael Freilich global mean sea level measurement is in line with the one measured by Jason -3. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

Abstract. The Surface Water and Ocean Topography (SWOT) mission delivers unprecedented swath altimetry products. Despite SWOT’s 2D coverage and precision, its Level-2 products suffer from the same limitations as their counterparts from nadir altimetry missions. Level-2 products are designed in a standalone ground-segment to meet the mission’s primary science objectives. In contrast, some research domains and applications require consistent multi-mission observations such as the Level-3 products provided by the Data Unification and Altimeter Combination System (DUACS) for almost 3 decades, and with 20 different satellites. In this paper, we describe how we extended the Level-3 algorithms to handle SWOT’s unique swath-altimeter data. We also illustrate and discuss the benefits, relevance, and limitations of Level-3 swath-altimeter products for various research domains.

The SWOT satellite, carrying the KaRIN first wide-swath onboard altimeter, was launched in December 2022, and has now delivered more than a year of surface water elevation data over the ocean and inland lakes/rivers. These data are affected by systematic errors which constitute the dominant part of the error budget at scales larger than a few thousands of kilometers. Some strategies for their estimation and calibration were explored during the pre-launch studies with performance estimations. Now, based on the real data, we propose in this study to assess the systematic error budget with statistical methods relying on spectral and co-spectral analysis. From this assessment, suggesting very low error levels (below requirements), we propose the implementation of the calibration algorithms at Level-2 and Level-3 with a few minor adjustments justified by the error spectra. The calibrated products are then validated with usual CalVal metrics.

This paper describes the demonstration of a regional high-resolution level-3 (L3) altimeter data unification and altimeter combination system (DUACS) developed with support from the French space agency (CNES). Deduced from full-rate (20 Hz to 40 Hz) level-2 (L2) altimeter measurements, this product provides sea level anomalies (SLA) and other essential physical variables at a spatial resolution of one sample every ~1 km over the North Atlantic Ocean. This allows us to resolve wavelengths from ~35 km to ~55 km depending on the altimeter considered. This was made possible by recent advances in radar altimeter processing for both synthetic aperture radar (SAR) and low-resolution-mode (LRM) measurements, as well as improvements made to different stages of the DUACS processing chain. Firstly, the new adaptive and low-resolution with range migration correction (LR-RMC) processing techniques were considered for Jason and Sentinel-3 (S3A), respectively. They significantly reduce errors at short wavelengths, and the adaptive processing also reduces possible land contamination near the coast. Next, up-to-date geophysical and environmental corrections were selected for this production. This includes specific corrections intended to reduce the measurement noise on LRM measurements and thus enhance the observability at short wavelengths. Compared with the 1 Hz product, the observable wavelengths reached with the demonstration high-resolution product are reduced by up to one third, or up to half in the northeast Atlantic region. The residual noises were optimally filtered from full-rate measurements, taking into consideration the different observing capabilities of the altimeters processed. A specific data recovery strategy was applied, significantly optimizing the data availability, both in the coastal and open ocean areas. This demonstration L3 product is thus better resolved than the conventional 1 Hz product, especially near the coast, where it is defined up to ~5 km against ~10 km for the 1 Hz version. Multi-mission cross-calibration processing was also optimized with an improved long-wavelength error (LWE) correction, leading to a better consistency between tracks, with a 9–15% reduction in SLA variance at cross-overs. The new L3 product improves the overall consistency with tide gauge measurements, with a reduction in SLA differences variance by 5 and 17% compared with the 1 Hz product from the S3A and Jason-3 (J3) measurements, respectively. Primarily intended for regional applications, this product can significantly contribute to improving high-resolution numerical model output via data assimilation. It also opens new perspectives for a better understanding of regional sea-surface dynamics, with an improved representation of the coastal currents and a refined spectral content revealing the unbalanced signal.

This paper describes an innovative method for processing nadir altimeter data acquired in Synthetic Aperture Radar (SAR) mode, enhancing the system performances over open ocean. Similarly to the current SAR data processing scheme, the so-called LR-RMC (Low Resolution with Range Migration Correction) method, originally designed by Phalippou and Demeester (2011), includes Doppler beam forming, Doppler shift correction and range correction. In LR-RMC, however, an alternative and less complex averaging (stacking) operation is used so that all the Doppler beams produced in a radar cycle (4 bursts of 64 beams for the open-burst Sentinel-3-mode altimeter) are incoherently combined to form a multi-beam echo. In that manner, contrarily to the narrow-band SAR technique, the LRRMC processing enlarges the effective footprint to average out the effects of surface waves and particularly those from small submesoscale structures (<1 km) that are known to impact SAR-mode performances. On the other hand, the number of averaged beams is as high as in current SAR-mode processing, thus providing a noise reduction at least equally good. The LR-RMC method has the added benefit of reducing the incoherent integration time with respect to the SAR-mode processing (50 ms compared to 2.5 s) limiting possible surface movement effects. By processing one year of Sentinel-3A SRAL SAR-mode data using the LR-RMC method, it is shown that the swell impact on the SAR altimeter performances is totally removed and that an improvement of 10-50% is obtained in the measurement noise of the sea surface height and significant wave height with respect to SAR mode. Additionally, observational capabilities over the middle scales are enhanced potentially allowing the ocean mesoscale features to be retrieved and observations assimilated more usefully in ocean models. (C) 2021 COSPAR. Published by Elsevier Ltd. All rights reserved.

Being the very first SAR mode altimeter tandem phase, the Sentinel-3 A/B tandem phase has provided an unprecedented opportunity to better characterize the sensitivity of SAR altimetry retrievals to high-frequency processes, such as long ocean waves. In this paper, we show that for some sea-state conditions, that are still to be precisely characterized, long ocean waves are responsible for high-frequency (spatial and temporal) coherent Sea Level Anomaly (SLA) signals. It is found that the peak wavelength corresponds to the dominant swell wavelength. Furthermore, the short time lag between S3-A and S3-B acquisitions allows performing cross-spectral analyses that reveal phase shifts consistent with waves travelling according to the wave dispersion relation. It is also demonstrated that the classical 20 Hz sampling frequency is insufficient to properly sample most swell-induced SLA signals and that aliasing can generate errors over the entire frequency spectrum, including at long wavelengths. These results advocate for the use of azimuth oversampling (40 Hz or 80 Hz). Low-pass filtering should be applied prior to any down-sampling to 20 Hz, in order to prevent long-wavelength errors induced by spectral leakage. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

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.

The objective of this study is to compare the characteristics of the oceanic teleconnection with the linear equatorial dynamics of two upwelling systems along the southwestern South American and African continents at subseasonal time scales (<120 days). Altimetric data analysis shows that the coastal variability remains coherent with the equatorial signal until 27 degrees S in the southeastern Pacific (SEP), while in the southeastern Atlantic (SEA) it fades out south of 12 degrees S. To explain this striking difference, our methodology is based on the experimentation with twin regional model configurations of the SEP and SEA Oceans. The estimation of free Coastal-Trapped Waves (CTWs) modal structures and associated contribution to coastal variability allows inferring and comparing the characteristics of each CTW mode in the two systems; namely, their forcings, amplitude, dissipation rate, and scattering. Results show that the Pacific subseasonal equatorial forcing is only 20% larger than in the Atlantic, but important differences in the relative contribution of each baroclinic mode are reported. The first baroclinic mode dominates the eastern equatorial Pacific variability, while in the eastern equatorial Atlantic, the second mode is the most energetic. This leads to a drastic increase in the dissipation and scattering of the remotely forced CTW in the SEA sector, compared to the coastal SEP. Concomitantly, south of 15 degrees S, the subseasonal coastal wind stress forcing is substantially more energetic in the SEA and participates in breaking the link between the equatorial forcing and the coastal variability. Our results are consistent with the solutions of a simple multimode CTW model. Plain Language Summary The Humboldt and the Benguela upwelling systems are connected to the equatorial variability. Part of the incoming eastward equatorial wave energy is transmitted southward along the South American and African coasts as Coastal-Trapped Waves, where they imprint on the ecosystem variability. At subseasonal time scales (<120 days), altimetry reveals that the coastal variability remains coherent with the equatorial signal until 27 degrees S in the southeastern Pacific, while in the Atlantic counterpart it fades out south of 12 degrees S. To explain this striking difference, we compare the characteristics of coastal waves between the two systems: their forcing at the equator, their dissipation and scattering along their propagation, and the energization by the coastal wind stress. We use a variety of ocean models of different complexity ranging from regional general circulation models to simple linear coastal models. Results show that the difference between the two systems regarding the connection with the equatorial variability can be attributed to the distinct characteristics of their equatorial forcing. The latter favors fast and weakly dissipative coastal wave in the Humboldt. Off southwestern Africa, the equatorially-forced coastal-trapped waves dissipate at approximate to 13 degrees S and the subseasonal coastal wind stress forcing which is energetic south of 15 degrees S, participates in breaking the link between the equatorial and coastal variabilities.

The Humboldt and the Benguela upwelling systems are connected to the equatorial variability through the coastal waveguide, so that a large variance of the coastal sea level and current variability can be described as an infinite sum of orthonormal free Coastal-Trapped Wave (CTW) modes. The objective of this study is to infer the CTW mode contributions to the coastal variability in both systems at subseasonal timescales (<120 days) from regional ocean circulation model simulations. We develop and validate twin regional model configurations of the southeastern Pacific and Atlantic Oceans. Cross-shore spatial structures of the first four free CTW modes are then derived from model mean stratification and topography along the southwestern African and South American continents. We introduce and validate a new methodology to estimate the gravest CTW mode contributions to model pressure and alongshore current. Our formulation draws on the orthonormality of the CTW modal structures, and uses a simple projection of the coastal and bottom model pressure onto each CTW structure. Results give confidence in the ability of this modal decomposition methodology to disentangle CTW mode contributions from complex nonlinear coastal processes that control the coastal subseasonal variability. In both systems, it allows to successfully extract the gravest poleward propagating CTW modes with velocities close to the theoretical values and amplitudes consistent with the solutions of a simple multimode linear CTW model. Furthermore, results show that both systems exhibit relatively different CTW dynamics and forcings which are discussed in the companion paper (Illig et al., 2018). Plain Language Summary Coastal-trapped waves propagate in the ocean along the continental shelves, with the coast on their left in the southern hemisphere. They exert an important influence on the coastal circulation and mixing, with notable implications for productive ecosystems. We introduce a new methodology to estimate the amplitude of these waves and their contribution to the coastal sea level and alongshore current variability. It benefits from a relatively simple implementation and is adapted to ocean model solutions. We validate this method using twin regional ocean model configurations of the southeastern Pacific and Atlantic Oceans. This novel approach allows to successfully extract the contribution of the southward propagating coastal waves in these two different systems, with velocities close to the theoretical values and amplitudes consistent with the dynamics of a simple linear coastal model. This gives confidence in the ability of this new technique to disentangle coastal wave contributions from complex nonlinear processes that control the ocean variability in coastal fringes. Furthermore, results show that both systems exhibit drastic differences in the coastal wave dynamics at subseasonal timescales (<120 days) which are discussed in the companion paper (Illig et al., 2018).