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Poseidon-4 is a dual frequency redundant radar altimeter, embarked on board of European Commission Copernicus Programme Sentinel-6 MF satellite, which represents a significant breakthrough with respect to its predecessors Jason-class altimeters thanks to its digital architecture (based on an on-board digital matched-filtering) and to novel internal calibrations modes. In this work, we assess Poseidon-4 main instrumental improvements and performances, with the presentation of the more important outcomes from the In-Flight internal calibration modes and from an external calibration analysis over a transponder. The instrumental performances of the radar altimeter, as verified from its internal calibrations, are excellent: Poseidon-4 delivers a range/azimuth instrument impulse response with the highest quality and fidelity in the age of space-borne radar altimetry and its thermal noise response is almost at level of random noise, and this both for its nominal and redundant side. A significant power decay of the level of the transmitted power in Ku Band has been detected both for the nominal and redundant side, which will not lead anyhow to a violation of the mission requirement of the minimum signal-to-noise ratio over ocean at the end of the expected satellite life (5.5 years). The novel CAL1 ECHO-CAL calibration mode allows to characterize very precisely the sensitivity of the instrument impulse response to the in-orbit temperature variations: this has been estimated to be of +0.3 mm for the range and of +0.01 dB for the power in Ku Band. Furthermore, the novel CAL1 INSTR mode successfully allows the monitoring of the transmitted chirp and, in case, to update the chirp replica used on-board for the pulse compression in order to recover for any significant degradation in the impulse response quality during the course of the mission life. The PTR sidelobes in Ku Band do not evolve in a perfect symmetrical manner between the left-hand side and right-hand side but some deviations have been registered for side-A (0.5 mm and 0.025 dB after 10 months). The impact of these dis-symmetries on the range measurement stability will be analyzed in a separate work (S6PP science results, abstract submitted in the same session). The external calibration by transponder analysis highlights a very small end-to-end range bias and a time-tag bias.

The Mission Performance Working Group is a body composed of experts from EUMETSAT, NASA, ESA, NOAA and CNES. The group meets on a regular basis, and it is aiming at monitoring the Sentinel-6MF overall altimetry mission performance. The group assesses on mission and payload performance budgets, data products quality, algorithm development, new processor verification and validation performance, calibration and validation planning, as well as provides scientific support to the reprocessing calibration and validation, key mission instrument meetings and mission performance meetings. The MPWG activities are similar to those done by the MSEs as per previous Jason mission. In this presentation, the MPWG will share with the OSTST the latest calibration and validation findings including any mission scientific challenges. Moreover, we will present the scientific plans for Sentinel-6MF evolutions, and provide an estimated timeline for these evolutions to be available to the Sentinel-6MF user community.

Launched on 21 November 2020, Sentinel-6 Michael Freilich is a Copernicus satellite designed to ensure the continuity to the mean sea level climate time series measured by the TOPEX/Poseidon and Jason satellites since 1992. The main payload carried on-board Sentinel-6 is the Poseidon-4 (POS4) dual frequency radar altimeter. POS4 uses an approximate9 kHz Pulse Repetition Frequency, and an innovative interleaved chronogram allowing for the optimization of the number of measurements acquired. Thanks to the interleaved design, POS4 offers for the first time the possibility to downlink simultaneous: Low Resolution (LR) data aiming at extending the legacy of the mean sea level record, and High Resolution (HR) or Synthetic Aperture Radar (SAR) data that significantly improves the along-track spatial resolution and reduces measurement noise. The on-board Range Migration Correction (RMC) algorithm , another innovation of POS4, allows reducing the HR data volume by half, in turn it facilitates the downlink of both LR and HR data all over the Globe. In this presentation we focus on the altimetry products from baseline F06, both the NRT/STC/NTC data disseminated by the EUMETSAT PDAP ground segment, as well as the full mission reprocessed data of the same baseline produced by EUMETSAT. All the NRT/STC/NTC timeliness products are assessed, and particular attention is given to the quality assessment of the NTC and reprocessed products. Both LR and HR performances are addressed. Moreover, POS4 instrument performances from the PDAP Long Term Monitoring are briefly summarized. For all latencies, LR requirements are fulfilled and data products show good continuity with Jason-3. GMSL inter-mission bias is well characterised after about 12 months, and GMSL continuity with Jason-3 is illustrated, although a longer time series is needed to fully demonstrate this. It is also shown that HR requirements are met for all latencies, except for SWH that are overestimated at high wave heights due vertical wave velocity impacts (a known issue in SAR altimetry) even after the number of looks was reduced in baseline F06. HR RMC is also assessed and presents performances very similar to the HR RAW data performances over open ocean, and in coastal regions.

During the last years, different teams have devoted huge efforts to improve retracking algorithms for both conventional and Delay Doppler echoes. For LRM measurements, CLS/CNES has developed and successfully validated a solution called "Adaptive Retracker", implementing a new waveform model and a Nelder Mead optimization method with exact likelihood criterion. Compared to the reference MLE4, the Adaptive brings dramatic improvements over all surfaces. It has been presented several times to the OSTST community [Thibaut et al., OSTST 2021, Ollivier et al. OSTST 2019, Thibaut et al., OSTST 2017] This solution has been implemented as the reference algorithm for nadir echoes in the SWIM ground segment (CFOSAT mission) and in the new reprocessing CNES GDR-F of the Jason-3 mission, with excellent performances as described in [Tourain, 2021] and [Thibaut, 2021], respectively. In the context of different frameworks, historical missions are now being reprocessed with the Adaptive retracker allowing to have homogeneous long-term time-series processed with this up-to-date algorithm. Firstly, in the frame of the CNES GDR-F reprocessing, it is planned to reprocess the whole Jason-2 and Jason-1 time series with the Adaptive retracker, allowing to have more than 20 years of Jason data processed with this algorithm. Poseidon-3B (Jason-3 altimeter) is a replica of Poseidon-3 (Jason-2 altimeter), so the Adaptive can be directly applied to Jason-2. It is not the case for Jason-1’s altimeter Poseidon-2 that has two important characteristics that needs to be considered in a retracker: First, a compression and decompression algorithm has been applied to Jason-1 echoes, changing their shape and therefore the noise statistics, especially on the trailing edge. Second, Jason-1’s platform had pointing issues resulting in numerous periods of strong mispointing. As the Adaptive model is not valid for high mispointing values, and assumes a null mispointing in its current version, the impact of this mispointing must be carefully assessed. Finally, the impact of these two aspects on the neural-network waveform classification must also be analysed as it is used as an input of the Adaptive retracker. Secondly, in the frame of the ESA project FDR4ALT, aiming at reprocessing ERS-1, ERS-2 and ENVISAT Altimeter and Radiometer datasets based on the best state-of-the-art algorithms and corrections, several Thematic Data Products (TDP) will be provided at the end of the project, including Ocean & Coastal Topography dedicated products, and Ocean Waves dedicated products. In the frame of this project, the PTR (Point Target Response) arrays were handled in a new way to allow the Adaptive retracker to be applied on ENVISAT with excellent performances with respect to the current reference algorithm MLE3. After a round robin analysis, the Adaptive has therefore been selected to be the only algorithm providing the Sea Level Anomaly (SLA) for the Ocean & Coastal TDP, and the Significant Wave Heigh (SWH) for the Ocean Waves TDP. The full ENVISAT dataset has now been reprocessed with excellent results obtained for the SLA and SWH. The aim of this talk is to present the methods used to apply the Adaptive retracker on these historical missions in the frame of different projects, and then focus on the excellent performances and results obtained on ENVISAT and Jason-1 compared to the reference algorithms MLE3/MLE4. More specifically, this presentation will address the benefits of this algorithm on historical missions for the SLA and the SWH user communities.

Since the launch of the Sentinel-3A satellite in February 2016, a new long-term series of Delay-Doppler/SAR altimetry SSH measurements over global ocean has begun with better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking. Data from an experimental mode, the so-called LR-RMC mode, are also available from the CNES S3PP prototype with a processing of the S3A data that differs from the classical SAR-mode processing in numerous ways. In-parallel, Jason-3 and now Sentinel-6/MF missions continue the long-term record of SSH measurements started in 1992 by the Topex-Poseidon satellite in the conventional low-resolution mode (LRM). In support of these various high-precision SSH measurements, SSB corrections need to be computed/adapted to each of these specific measurement modes to accurately correct all these range measurements for the sea-state effects because of its empirical nature. In this poster, we will compare the different SSB solutions to document the range/sea-state relationships from these different measurement modes.

Multidecadal efforts have made possible the current record of ocean sea surface height variations, 30-year long, provided by high-precision altimeter satellites. These observations have greatly improved our knowledge of the open ocean and are now an essential component of many operational marine systems and climate studies. It is not the case in the coastal ocean, where satellite altimetry encounters different issues that make it more difficult to derive accurate geophysical data. However, monitoring coastal sea level changes at global scale is now a critical need that the too rare in-situ data cannot fulfill. This has motivated many efforts to bring the sea level retrieval by satellite altimetry as close as possible to the shore. The result of these efforts is now the availability of many new algorithms for retracking radar altimeter data, correcting sea surface heights and finally deriving sea level variations. The main objective of this study, initiated and funded by the French space agency (CNES), is to objectively define the best set of altimetry algorithms for computing sea level anomalies (SLA) in coastal zones. We focus on Low Resolution Mode altimetry and aim for the creation of long-term time series spanning different missions. For the different processing components (retrackers, geophysical corrections, and auxiliary parameters, for a total of 21 algorithms), the relative quality of the different solutions is assessed with clearly defined metrics for variability, data availability, and impact on SLA as a function of distance to the nearest coast. The analysis is made at both global and regional scales for 3 specific zones: the Mediterranean Sea, the North East Atlantic and the East Australian coasts. In the regions considered, a comparison against available tide gauge data is also performed. This methodology is applied to Jason-2 and Jason-3 missions, and we determine which algorithms are the most limiting, and how results vary from one mission to the other. The set of best performing algorithms near the coast will finally be presented, with the criteria used for the selection. This set will constitute the baseline algorithms for the generation of a new Level-3 SLA product.

Wave breaking is a common phenomenon when there is high to medium wind speeds over the ocean. But the wave breaking can have various sources, being the wind only one of them. For instance, wave breaking occurs near inhomogeneous unsteady currents, when waves propagate into an opposing current steepen, shorten and may break; or more simply, as waves shoal over shallow bathymetry. Wave breaking limits the height of surface waves, mixes the ocean surface, generates currents, and enhances air-sea fluxes of heat, mass and momentum through the generation of turbulence, the entrainment of air and the creation of spray and aerosols (Melville, 2018). In this work, we study the wave breaking caused by large amplitude and nonlinear Internal Solitary Waves (ISWs). Internal Waves play an important role in determining the near-surface sea temperature structure and the air–sea exchange processes, being therefore important for understanding the evolution of the climate system. In the presence of strong ISW-Surface Wave interaction, breaking surface waves are known to occur and hence, it is expected that wave energy dissipates and the wave energy spectrum is altered. Furthermore, it has been recently shown that ISWs are successfully detected by using satellite altimetry. Here, we select two different regions of the ocean, namely the tropical Atlantic Ocean off the Amazon shelf and the Banda Sea in the Indian Ocean, where there are scenes of Sentinel-3 OLCI (Ocean Land Colour Instrument) acquired simultaneously with along-track SAR mode altimeter, which included signatures of large amplitude ISWs. New data of unfocused SAR (UF-SAR) and fully-focused SAR (FF-SAR) modes is analysed. It has been observed a strong decrease in normalized radar cross section (NRCS) over the rough part of the ISWs, and a small increase in the smooth part relatively to the unperturbed ocean background (Santos-Ferreira et al., 2018). Moreover, we demonstrate that the Significant Wave Height (SWH) parameter is significantly attenuated, after the passage of an ISW, considering length scales of about 10 km before and after the ISW crest (i.e. in 20 km length scales). It is suggested that the cause of this SWH attenuation is related to the wave breaking associated with the ISWs, characterized by surface wave energy dissipation, turbulence effects and air emulsion. Furthermore, Sentinel-2 images are analysed and provide insights admittedly into this same phenomenon: white-capping of two different kinds are reported, the first being a traditional radiance increase at all (visible) wavelengths extended in time scales of tens of seconds, and a second kind associated to quick transient “flashes” of enhanced radiance depicted in different coloured pixels in RGB composite images, with typical time scales of one second or less. Fraction of modulation of breaking waves in the presence of internal waves are presented.

In this paper, we propose a correction for the sea state dependent biases observed in the significant wave height (SWH) measurements obtained from high-resolution (HR) data of synthetic aperture radar (SAR) altimeters. Those biases have been linked to the effect of vertical wave motion on the radar signal, which creates a non-negligible Doppler spread that results in a loss of along-track resolution of the SAR altimeter (azimuth smearing). When this effect is not accounted for in the retracking process it results in sea state dependent biases, which can be very significant for high SWH values. The correction has a form of a look-up table (LUT), that depends on both sea state and mean zero up-crossing period. We calculated the LUT through numerical simulations of the SAR altimeter waveforms at different sea state conditions including the azimuth smearing effect through the SAR altimeter flat sea surface response, and later retracking those waveforms with a model that does not account for the vertical wave motion. To evaluate the LUT correction we use the low-resolution mode (LRM) SWH estimated values, and the mean up-crossing wave period, that we obtain from the Meteo France Wave Advanced Model. We evaluated the LUT correction using Sentinel-6A/MF HR and LR data and determined that the correction is able to eliminate most of the sea state dependent biases observed between both operation modes. The same results were also observed when the correction is applied to Sentinel-3A/B data, indicating the usefulness of this correction in providing consistent SWH measurements across all altimeter missions.

The improvement of retracking algorithms aiming at the optimal and efficient estimation of the geophysical parameters is a core activity within the altimetry community. The importance of having an optimal, robust and efficient retracking solution is indeed more and more critical with the increase of the amount of data and the high demanding requirements in terms of data quality and resolution. The accuracy of the retracking solution in retrieving the geophysical parameters has a strong impact on the accuracy of the estimation of Essential Climate Variables as the mean sea level, which is a key issue for trend studies and for the robust assessment of climate change at local and global scales. Currently, the most used retracking solution implemented in the ground segments for conventional altimetry is the MLE4 retracking algorithm, which is computationally fast but known to be sub-optimal and biased, therefore needing corrections that must be applied to avoid systematic bias in the retrieved parameters. Recently, lots of efforts have been done to improve this solution in terms of estimator, modelling and inclusion of instrumental-related effects (numerical PTR), leading to the development of the Adaptive retracker (Tourain et al 2021, Thibaut et al. OSTST 2017 & 2021) which is now successfully integrated in the ground segment of missions like Jason3 and CFOSAT. The Adaptive retracker is based on a Maximum Likelihood Estimator with the exact formulation of the likelihood function for a Gamma distributed multiplicative noise and provides huge improvements in the parameter estimation with respect to the MLE4 solution. Yet, while being optimal and unbiased, the Adaptive retracker is not numerically efficient as the optimisation of the exact likelihood criterion is done with the Nelder-Mead algorithm that has a very high computational cost. This can be a big issue, preventing for instance this innovative algorithm to be included in “near-real-time” and “real-time” official products. In this talk we present Fast-Adaptive: a new, optimal, unbiased and computationally efficient retracking approach for the analysis of conventional altimetry (Low Resolution Mode, LRM) data. We will present the formalism, describing the estimator, the optimisation method and the model, which is based on the four parameters “Adaptive-like” model with numerical PTR convolution. We will demonstrate that the proposed retracking solution provides with unbiased and optimal parameter estimation, compatible with the Cramer-Rao bounds, while keeping a low computational cost, comparable to the MLE4 retracker. We will present the validation of the Fast-Adaptive retracker on simulations and we will detail the results of the analysis of representative Sentinel-6 and Jason-3 LRM data sets with the new retracking solution and show the comparison with the existing solutions (MLE4 and Adaptive) on the same data. The Fast-Adaptive retracker is a new promising and powerful tool for the analysis of LRM data of current and future radar altimetry missions.

The Copernicus Sentinel-6A mission was successfully launched on the 21st November 2020. This mission is taking the responsibility to extend the global sea level record on the reference low inclination orbit started in 1992 with TOPEX/Poseidon mission and continued by the Jason series. This new Copernicus constellation (Sentinel-6B launch expected in 2026) will ensure the continuity of this unique high-precision coverage of altimetry data in support to climate change monitoring, research, and forecasting, as well as operational oceanography applications. In the frame of the CNES/EUMETSAT commissioning activities, Sentinel-6A LR (LRM) and HR (SAR) data from PDAP have been fully validated thanks to the tandem flight with Jason-3. Both LR datasets are in line allowing a smooth transition between Jason-3 and Sentinel-6A as a reference mission. Thanks to Poseidon-4 altimeter on board Sentinel-6A and its interleaved mode, co-located LR and HR measurements are provided, allowing a precise estimation of the performances between the modes by direct comparison. In this presentation, we focus on the assessment of Sentinel-6A HR data over ocean. We aim at listing the known small limitations and draw a first error budget of Sentinel-6A HR data, with the objective to identify potential processing improvements. The assessment has been compared to Sentinel-3A SAR performances and its remaining small errors (Raynal et al 0STST 2019 and Cadier et al OSTST 2020). Analyses show some similarities, in term of swell dependencies or wind impact, but also some different behaviours: it has been shown that the impact of ocean vertical velocity is more important on Sentinel-6A HR than on Sentinel-3.

IASI-A has been in operation on board MetOp-A since June 2007 and is now considered as a reference sensor for the radiometric calibration at high spectral resolution. IASI-B on board MetOp-B was launched on September 17th, 2012, and finished its commissioning phase in April 2013. CNES is in charge of the performance monitoring of the two IASI instruments. One of the associated tasks is to compare the calibration of IASI-A with IASI-B, and to compare each one with other similar sensors (AIRS and CrIS). Our goal is to check the IASI data quality with an external reference and to establish the high radiometric accuracy required for climate data records. After more than one year of IASI-B data, we are now in long-term routine monitoring. Our inter-comparisons are now valid on every season and observation condition. We first present the radiometric inter-calibration between IASI-A and IASI-B. An original method has been developed based on the observations of common regions with a 50 minutes temporal gap and in different viewing conditions due to the orbital configuration. The input dataset filtering has been tuned to minimize the geophysical biases. The sounding pixels are spatially averaged on a 300x300km² area. Radiometry is compared at full spectral resolution. We also present the radiometric inter-calibration of IASI (A and B) with Aqua/AIRS and NPP/CrIS. Our method is based on simultaneous nadir overpasses, occurring at high altitudes. The difference in spectral sampling and resolution is handled through either the use of spectrally broad pseudo-channels or the reconvolution of IASI spectra. Results for with more than one year of data for IASI-A / AIRS, IASI-B / AIRS, IASI-A / CRIS, IASI-B / CRIS and IASI-A / IASI-B show small biases of the order of 0 to 0.2K. The five sounders show very stable inter-calibration. Several diagnosis tools are presented to understand the small residuals of inter-calibration. An indirect inter-comparison IASI-B / IASI-A is also presented using double differences with AIRS and CRIS. We also present a long-term monitoring of the spectral cross-calibration, based on the cross-correlation of spectra on several spectrally broad pseudo-channels.