Abstract. Global observation of ocean and sea ice dynamics relies heavily on space altimeters. However, due to their high latitudes, which are less covered by space altimeters, and the ice cover, which hides the sea level, polar oceans are still poorly observed and modeled. The SWOT altimeter significantly increases the density and quality of measurements up to 77° latitude, covering the entire Southern Ocean and a large part of the Arctic Ocean. Still, these observations require a distinction to be made between measurements taken on water and measurements taken on ice. The CNES L3 Unsmoothed 250m v2.0.1 product includes a new flag that identifies the type of surface observed for each pixel, enabling processing tailored to the object under study. Here we present the methodology used to calculate this flag and evaluate the results obtained using space imagery, nadir altimetry and OSI SAF concentration products.

River water level observations with satellite-based remote sensing have historically relied on nadir altimetry measurements. However, the spacing between satellite ground-tracks constrains the spatial sampling achievable over rivers. In this paper, we develop an approach to automatically retrieve river water heights off-nadir, as long as the targets fall within the altimeter’s reception window. Using Unfocused SAR data from Sentinel-3 and Sentinel-6A altimeters at along-track resolutions of 420 m and 300 m respectively, we demonstrate the ability to retrieve water heights with a STD consistently below 20 cm for stations located up to 5 km away from the nadir measurement. These results are validated along the Creuse, Garonne, Sarthe, and Vilaine rivers. Additionally, we demonstrate that 35 of the 116 in situ stations analyzed across France —nearly one third— can be successfully monitored using off-nadir altimetry observations, achieving an STD below 30 cm. This validation paves the way for extending the approach to additional virtual stations in more favorable locations worldwide.

The upcoming Sentinel-3 Next Generation Topography (S3NG-T) mission, designed to succeed the current Sentinel-3 (S3) mission, will operate on the same ground tracks as the current S3 constellation to maximise continuity of measurements, but with a fixed 4 h temporal lag due to satellite design constraints. This configuration prevents the implementation of a classical near-simultaneous tandem phase, traditionally used for inter-mission cross-calibration, and raises concerns regarding the impact of short-term oceanic variability on continuity assessment. In this study, we evaluate the feasibility and expected performance of a 4 h delayed tandem phase for cross-calibrating S3 and S3NG-T. Using tandem datasets from Sentinel-3A/B and Jason-3/Sentinel-6 missions, combined with SWOT KaRIn observations, we develop a methodology to quantify the oceanic variability introduced by a 4 h delay and to evaluate its effect on the accuracy of inter-mission offset estimates. Results indicate that the classical tandem configuration achieves regional inter-mission Sea Level Anomaly (SLA) offset uncertainties of approximately 2 mm over a three-month period. In contrast, a 4 h delayed tandem phase increases this uncertainty to about 7 mm in the same period, but still performs significantly better than non-tandem scenarios. Extending the 4 h tandem phase to one year enables the detection of systematic instrumental errors of ±3.5 mm amplitude, sufficient to ensure continuity between S3 and S3NG-T. These findings demonstrate that, despite additional oceanic variability, a 4 h tandem configuration remains a viable and effective strategy for cross-calibration, especially when supported by improved environmental corrections and by extending the observation duration to a full year.

The launch of the altimetric satellite SWOT (Surface Water and Ocean Topography) was a revolution in oceanography and hydrology. With its120 km swath width, a spatial resolution of 500m² (in Low Resolution acquisition mode) and an instrumental random error significantly lower than the one from nadir altimetry, the Ka-Band Radar Interferometer (KaRIn) onboard SWOT mission also present a huge potential to develop applications in the polar regions. Indeed, the SWOT product enables the observation of leads, icebergs and polynyas (Dibarboure and al 2024) through the measures of surface topography and backscatter coefficient. The surface discrimination between leads and floes is the first step toward polar ocean monitoring, ice thickness and snow depth estimations. However because of the complexity of the surface (different surface roughness properties in the leads, presence of melt pounds) added to residual sensing errors (residual KaRIn random error, residual systematic errors, …) this first required achieved is not straightforward. Therefore several classification approaches were developed : one based on a statistical method (Markov Random Field), one based on an unsupervised machine learning method (Kmeans) and another one based on a supervised machine learning method (XGBoost). The objective of this paper is thus to present the results of these methods (their robustness, strengths and weaknesses) through local comparisons with respect to optical, SAR images and global comparison with existing state of the art products (OSISAF ice concentration products).

Over the past three decades, five satellites have succeeded one another on the reference orbit, building the longest continuous climate record of global sea level measurements. Its continuity is ensured thanks to tandem flights between consecutive satellites. In this paper, we demonstrate that the first satellite of the Sentinel-6 series (Sentinel-6 Michael Freilich) has enabled the detection of a processing anomaly in the Jason-1/2/3 ground segment. An inconsistency in the altimeter range reconstruction has been identified, causing its underestimation by 3.65 mm. At certain latitudes, determined by the satellite’s orbital velocity, the altimeter range shows no effect from the anomaly. For the reference orbit, the range is not affected at the poles, around the equator and, for ascending tracks, at 40° S. All Jason Geophysical Data Record (GDR) versions prior to GDR-G are impacted by the described processing anomaly. While a full reprocessing of the Jason data with the GDR-G standard is pending, this paper presents a latitudinal empirical correction to be applied to Jason datasets generated with GDR-F and earlier ground segments. This correction, to be applied on the altimeter range, is derived from one month of patched Jason-3 data and is intended for reference orbit only. Additionally, SWOT Nadir ground processing is also affected by the same processing error and has been corrected from the GDR-S2 version onward. Finally, our analysis shows a negligible impact of this processing anomaly on Jason Level-2-derived products, models and metrics.

Over the last decade, there has been a burgeoning interest in altimetry measurements for inland waters, with a focus on comprehensive studies of water levels in lakes, reservoirs, and rivers on a global scale. This research is crucial for the hydrology community to accurately assess the Earth's freshwater resources. Significant advancements have been achieved in enhancing altimeters' capacity to obtain high-quality measurements over inland waters.The Open-Loop Tracking Command (OLTC) stands out as a noteworthy development in altimeter on-board tracking modes. Its effectiveness has been proven through successful implementation in previous missions and is now designated as the operational mode for current missions, including Sentinel-3, Sentinel-6, and SWOT nadir.Over the past decade, OLTC data, crucial in tracking inland water bodies from radar altimetry satellites, has undergone substantial refinement. Originally developed for Jason-2, new missions as Jason-3, Sentinel-3A&B, Sentinel-3B, Sentinel-6, SWOT nadir have been incorporated. Algorithms and procedures to compute location and elevation of inland waters targets (rivers, lakes, reservoirs) have also been largely improved. The number of hydrological targets has increased fivefold with an acquisition success rate which is now close to 90%. Presently, each mission tracks between 30,000 to 70,000 hydrological targets.Despite modifications to a software developed 15 years ago, ongoing advancements and the necessity for covering land ice surfaces in preparation of upcoming S3C&D missionshave prompted the development of a new software. In addition, the availability of new input data provided by the SWOT mission (water mask and elevation) required also to revise the current software to make their usage efficient. Work is currently underway to establish a new OLTC platform, named AltiGIS. The platform is designed with three primary objectives: facilitate collaboration, enhance data generation validity, and broaden dissemination through the use of DevOps practices. The presentation aims at harvesting new user needs but will also cover both the undergoing software development and roadmap.

Remote sensing techniques are crucial for sustaining a continuous and global climate monitoring of inland waters. In particular, recent progress in satellite radar altimetry has enabled the observation of an increasing number of small and medium size lakes and reservoirs, even in complex topography. The arrival of nadir radar altimeters operating in Synthetic Aperture Radar (SAR) mode has considerably improved the resolution of the observations in the along-track direction, passing from several kilometers in conventional limited-pulse altimeters, to hundreds of meters in close-burst altimeters when applying unfocused SAR (UFSAR) processing and even to the theoretical limit of half the along track antenna length in open-burst altimeters that can totally exploit the Fully-Focused SAR (FFSAR) processing technique. Sentinel-6 is the first operational mission to operate in open-burst mode allowing this enhanced performance over inland waters [1]. Complementary to nadir radar altimetry, SWOT mission provides since the beginning of 2023 radar interferometry observations over wide-swaths that could entail great advances in hydrology [2]. The inversion methods to estimate geophysical parameters, such as Lake Water Level (LWL), from the backscattered altimetry signal are commonly called retrackers. These retrackers can be empirical, such as the widely used OCOG method or physically-based, that is to say, a background waveform model is derived from the theoretical knowledge of the microwave scattering process and then fitted to the real backscattered signal received on-board. Several retrackers of the second type have been developed for processing conventional pulse-limited radar observations, like the Brown-like models, and also for UFSAR observations in the case, for example, of the SAMOSA model. Nevertheless, no specific retracker for FFSAR observations has been developed yet. One of the limitations of analytical and numerical physical-based retrackers concerns the assumption that the radar footprint is completely covered by water. This assumption, that holds for large lakes, begins to degrade the accuracy on the retrieved geophysical parameters when monitoring smaller water bodies. For this reason, a retracker based on numerical simulations was proposed in 2021 adapted to UFSAR observations [3]. This latter model has the advantage of taking into account a prior knowledge of the lake contour and, in this way, only in-water areas of the radar footprint contributes to the simulated backscattered waveform. In this work, the derivation of a similar retracker taking into account the FFSAR processing particularities is presented. This results in the first retracking model specifically developed for FFSAR observations. Preliminary performance is assessed with a variety of lakes for which in-situ observations of LWL are available. Furthermore, a comparison with the recently delivered first products of the SWOT mission over lakes will be presented. [1] Donlon, C.J., et al, 2021. The Copernicus Sentinel-6 mission: Enhanced continuity of satellite sea level measurements from space. Remote Sensing of Environment, 258, p.112395. [2] Biancamaria, S., et al, 2016. The SWOT mission and its capabilities for land hydrology. Remote sensing and water resources, 117-147. [3] Boy, F., et al, 2021. Improving Sentinel-3 SAR mode processing over lake using numerical simulations. IEEE Transactions on Geoscience and Remote Sensing, 60, pp.1-18.

The observation of sea level variability on very small time scales ranging from less than an hour to a few days is currently very limited with the constellation of nadir altimeter satellites. Calculating sea surface height at crossovers between a single mission or several nadir altimeter missions makes it possible to analyse the sea level variability on these very short timescales for a small number of measurements (a few hundred). Moreover, these observations are spatially sparsely distributed, most often at very high latitudes for crossovers of less than a few hours of time difference. Thanks to the launch of the joint CNES/NASA SWOT mission in December 2022 with swath measurements, a new paradigm for observing high-frequency temporal variability in sea level is now possible. SWOT KaRIn instrument offers 2D observations of the oceans with an unprecedented coverage and resolution. We take advantage of this new high quality dataset to estimate the sea level variability over short time scales. The crossovers of SWOT KaRIn during the 1-day orbit phase with itself and with Sentinel-3A/B nadir observations less than 1 hour to 72 hours of time difference from 17/04/2023 to 07/07/2023 are analysed. The crossovers with Sentinel-3A/B offer a very good spatial sampling of the oceans which cannot be reached with SWOT crossovers only. We analyse the variance of the sea surface height differences at the crossovers, as a function of time difference and of latitude. The variability of the sea surface height differences at the crossovers contains both the sea level variability and the instrumental errors. We attempt to disentangle the errors from the sea level variability. Sea surface height differences variance with time differences tending to 0 hours, hence free of oceanic variability, amount to ~3.2-3.5 cm. This value increases fastly with time difference up to about 4 hours as the contribution of sea level variability and errors increase. With time differences of 4 hours, the sea surface height differences variance reaches 4 cm. Then, the increase of the sea surface height differences variance slows down, as some phenomena are not correlated anymore, and roughly linearly increases to reach 5 cm for time differences of 72 hours.

We wish to present the CNES contribution to the Sentinel-3 Next Generation - Topography (S3NG-T) project. In the wake of the SWOT mission, which pioneered the use of SAR interferometry for surface water altimetry, ESA is considering using this new approach for the successor to its current operational mission Sentinel-3 (S3), S3NG. Such so-called “swath altimetry” enables the access to two-dimensional features on water surfaces, much more directly than traditional Nadir altimetry (such as that used aboard S3) does, but it must also stand the test of accuracy requirements.Scheduled to take flight in 2033, the altimetry component of S3NG, called S3NG-T, is wrapping up its development phase B1 wherein two consortia designed their proposal of a swath altimetry mission, and during which SWOT’s very promising first data released. A Mission Gate Review in early 2024 should lead to the definitive decision whether to adopt this new measurement technique for S3NG-T or not. Rich with the heritage of their contribution to SWOT and convinced of the potential of swath altimetry, the CNES teams bring a technical expertise to the S3NG table.As such, we developed evolutions for Radarspy, our in-house simulator of swath altimeter data, in order to assess S3NG’s performances over oceans and inland waters. The swath altimetry instrument aboard S3NG-T, called SAOOH, differs from SWOT’s instrument mainly in its 3-meter baseline, its multiple receptors (four per swath – left or right – in order to flatten the gain pattern), and its interleaved observation pattern, where bursts of 128 Radar pulses are sent alternatively left and right. We wish to present the results of our simulations, which test SAOOH over scenes of various reflectivity, water content and topography. These simulations yield encouraging first results and let us see how some choices made in its on-board processing algorithm affect the random noise, the water detection performance and the point-target response.

In this study, we present a facet-based numerical model dedicated to ice sheet radar altimetry. The model simulates Sentinel-3 UnFocused-Synthetic Aperture Radar (UF-SAR) waveforms by calculating the backscattered radar signal over the 10 m facets of the Reference Elevation Model of Antarctica (REMA). The simulation is exploited to determine the coordinates of the impact point on the ground, where the surface elevation is estimated. The complete processing chain, named the “Altimeter data Modelling and Processing for Land Ice” (AMPLI), provides topography estimations posted at ∼330 m along the satellite track. Using ICESat-2 as a reference mission, we evaluated the performance of the AMPLI software over the Antarctic ice sheet. The median bias between Sentinel-3 AMPLI and ICESat-2 ATL06 nearly co-located measurements is estimated at +12 cm on average over the Antarctic ice sheet. This surface height difference exhibits spatial variations over the Antarctic ice sheet, of the order of few decimetres. These divergences are most likely induced by the terrain characteristics (slope and roughness) and snow volume scattering affecting Ku-band altimetry measurements. The performance improvement is substantial compared to the ESA level-2 products, in particular over the ice sheet margins. For example, where the surface slope is greater than 0.5°, the median bias and the median absolute deviation relative to ICESat-2 ATL06 are reduced by about 83 % and 90 %, respectively. We also assessed the capability of Sentinel-3 to monitor surface elevation change (SEC) over the Antarctic ice sheet. The comparison between SEC maps from Sentinel-3 AMPLI and ICESat-2 ATL15, calculated over the 2019–2022 period, shows a Pearson correlation of 0.92. The study highlights the benefit of radar signal modelling, in synergy with high-resolution digital elevation models (DEMs), for reducing the slope-induced errors over ice sheets. The results emphasise the potential of the Sentinel-3 constellation for ice sheet mass balance studies.

Satellite radar altimetry has been used for over 30 years to measure sea surface height (SSH) variations to build records of essential climate variables like the mean sea level (MSL) for a robust assessment of climate change. A key step of the data processing toward this goal is the retracking, that is, the statistical analysis of the radar waveforms to estimate the geophysical parameters. For a robust and optimal estimation, retracking algorithms should account for a reliable waveform model, the time-varying instrumental point target response (PTR), and an accurate description of the waveform noise. However, this is not the case for the operational retracking solutions implemented in the ground segments, which specifically make use of an unweighted estimator, therefore not accounting for the waveform speckle noise. In this article, we present a novel computationally efficient retracking solution for Ku-band low resolution mode (LRM) data that accounts for both the in-flight PTR and a realistic waveform noise through a weighted estimator defined to be statistically equivalent to a maximum likelihood estimator. We consider two waveform models: the Adaptive model and the numerical Brown model, for a consistent comparison with operational retracking solutions. We focus on the current reference mission, Sentinel-6 MF, for which an accurate noise characterization is crucial to account for the pulse-to-pulse correlations resulting from the higher pulse repetition frequency (PRF) compared to conventional configurations. We demonstrate that the novel retracking solution is optimal, providing parameter uncertainties compatible with the Cramer-Rao bounds of minimum variance, and unbiased, while a bias up to 1 cm in the epoch estimation and suboptimality for all parameters is found for the unweighted solutions. We validated the algorithm on realistic simulations and applied it to one cycle of Sentinel-6MF LR 20-Hz data, demonstrating significant improvements in the precision of estimated parameter: similar to 60% for significant wave height (SWH), similar to 12% for the epoch, and similar to 50% for sigma(0) compared to current operational solutions. We report for the first time, geophysical parameter uncertainties consistently computed at 20 Hz as output of the retracker. We also introduce an innovative Bayesian approach for analyzing waveform data to complement current solutions, providing a robust method for the estimation of the parameter uncertainties and correlations. Finally, we perform a comprehensive analysis of parameter correlations on simulations and data, compared to theoretical expectations based on Fisher matrix analysis, demonstrating the importance of optimality for a correct estimation. Specifically, we show that the unweighted, suboptimal retracking solution significantly underestimates the epoch-SWH correlation by a factor of similar to 1.5 in the correlation coefficients, compared to the optimal solution and theoretical expectations. This could significantly impact the estimation of corrections such as sea state bias (SSB) and high-frequency adjustment (HFA), warranting further assessment. Overall, optimal retracking solutions should be considered to derive robust long-term sea-level records for climate research using past, current, and future altimetry missions.

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.

In this study, the full-focusing (FF) algorithm is reviewed with the objective of optimizing it for processing data from different types of surfaces probed in altimetry. In particular, this work aims to provide a set of optimal FF processing parameters for the Sentinel-6 Michael Freilich (S6-MF) mission. The S6-MF satellite carries an advanced radar altimeter offering a wide range of potential FF-based applications which are just beginning to be explored and require prior optimization of this processing. In S6-MF, the Synthetic Aperture Radar (SAR) altimeter acquisitions are known to be aliased in the along-track direction. Depending on the target, aliasing can be tolerated or may be a severe impairment to provide the level of performance expected from FF processing. Another key aspect to consider in this optimization study is the unprecedented resolution of the FF processing, which results in a higher posting rate than the standard SAR processing. This work investigates the relationship between posting rate and noise levels and provides recommendations for optimal algorithm configurations in various scenarios, including transponder, open ocean, and specular targets like sea-ice and inland water scenes. The Omega–Kappa (WK) algorithm, which has demonstrated superior CPU efficiency compared to the back-projection (BP) algorithm, is considered for this study. But, unlike BP, it operates in the Doppler frequency domain, necessitating further precise spectral and time domain settings. Based on the results of this work, real case studies using S6-MF acquisitions are presented. We first compare S6-MF FF radargrams with Sentinel-1 (S1) images to showcase the potential of optimally configured FF processing. For highly specular surfaces such as sea-ice, distinct techniques are employed for lead signature identification. S1 relies on image-based lineic reconstruction, while S6-MF utilizes phase coherency of focalized pulses for lead detection. The study also delves into two-dimensional wave spectra derived from the amplitude modulation of image/radargrams, with a focus on a coastal example. This case is especially intriguing, as it vividly illustrates different sea states characterized by varying spectral peak positions over time.

Two microwave transponders have been operating in west Crete and Gavdos to calibrate international satellite radar altimeters at the Ku-band. One has been continuously operating for about 8 years at the CDN1 Cal/Val site in the mountains of Crete, and the other at the GVD1 Cal/Val site on Gavdos since 11 October 2021. This ground infrastructure is also supported at present by four sea-surface Cal/Val sites operating, some of them for over 20 years, while two additional such Cal/Val sites are under construction. This ground infrastructure is part of the European Space Agency Permanent Facility for Altimetry Calibration (PFAC), and as of 2015, it has been producing continuously a time series of range biases for Sentinel-3A, Sentinel-3B, Sentinel-6 MF, Jason-2, Jason-3, and CryoSat-2. This work presents a thorough examination of the transponder Cal/Val responses to understand and determine absolute biases for all satellite altimeters overflying this ground infrastructure. The latest calibration results for the Jason-3, Copernicus Sentinel-3A and -3B, Sentinel-6 MF, and CryoSat-2 radar altimeters are described based on four sea-surface and two transponder Cal/Val sites of the PFAC in west Crete, Greece. Absolute biases for Jason-3, Sentinel-6 MF, Sentinel-3A, Sentinel-3B, and CryoSat-2 are close to a few mm, determined using various techniques, infrastructure, and settings.

Remote sensing products provided by satellite missions, airborne and unmanned aerial vehicle (UAV) campaigns have tremendously developed over the last decade. They undoubtedly offer opportunities to improve our ability to monitor and forecast flooding. The observation of inland waters benefits from several altimetry missions that provide along-track water surface elevation observation from nadir (e.g. TOPEX/Poseidon, Jason, SARAL/AltiKa, Sentinel-6) or large-swath altimeters (e.g. SWOT launched in December 2022), as well as other radar/optical missions (Sentinel-1, Sentinel-2) that provide high-resolution water masks. The limitations of each type of sensor are potentially circumvented when data from different satellite sensors are combined; the fusion of multi-source data has thus become one of the mainstream research topics in the remote-sensing community nowadays. Such fusion can be achieved with data assimilation algorithms applied to hydrodynamics models, namely MASCARET-1D and TELEMAC-2D. The present work focuses on the validation of water surface height (WSH) data from Sentinel-6MF with respect to in-situ gauge data, UAV and 1D/2D-hydrodynamics model outputs as shown in Figure 1. This work participates in a global effort that aims at combining various remote sensing products to represent and forecast flooding. The study is carried out over a dry period in June 2022 and over a flood event that occurred in December 2021-January 2022 over the Garonne catchment near Marmande, in the southwest of France. The WSH of the river is retrieved from Sentinel-6MF high-resolution fully-focused SAR data with an algorithm that relies on the estimation of the river width and the positioning of the river center line. The impact of these a priori data is investigated and the Sentinel-6MF-derived WSH observations are compared to the WSH simulated with TELEMAC-2D. It should be noted that due to the defection of Sentinel-1B (one of the two satellites in the Sentinel-1 constellation) in mid-December 2021, this flood event is only partly observed by Sentinel-1A and that the additional data from Sentinel-6MF with a 10-day revisit period are of great use. This study shows that hydrodynamic simulations and satellite altimetry time-series compare particularly well during the dry period whereas flooding events are often underestimated by satellite altimetry data. We believe that the combined use of satellite altimetry, hydrodynamics model simulations and independent UAV-borne data is key to a better understanding of the processes involved in the Garonne river flow dynamics and flooding events. We also take advantage of this study to improve our Sentinel-6MF data processing techniques. Figure 1- Representation of water level height and flood extent with remote sensing data and hydrodynamic models.

Satellite-based radar altimetry enables the long-term and global monitoring of inland water bodies. Recent progress of these altimeters concerning the tracking and acquisition mode allows to monitor an increasing number of water bodies, including lakes of small size or located in a complex orograhpy environment. As a consequence of these more demanding situations, the current operational radar processing techniques need to get improved in order to provide accurate water level retrievals. A numerical processing technique that simulates the satellite-observed scene was proposed in [1], taking into account all these challenges. In this work, the performance of such processing technique is assessed from Sentinel-3 mission data. It concerns 1000 lakes and reservoirs worldwide with a great variety of situations, looking for the current limits and potential future improvements, including about thirty water bodies with publicly available in-situ measurements allowing to determine the current accuracy attained.

The Copernicus Sentinel-3 Surface Topography Mission (STM) Land Altimetry provides valuable surface elevation information over inland waters, sea ice, and land ice, thanks to its synthetic aperture radar (SAR) altimeter and its orbit that covers high-latitude polar regions. To ensure that these measurements are reliable and to maximise the return on investment, adequate validation of the geophysical retrieval methods, processing algorithms, and corrections must be performed using independent observations. The EU-ESA project St3TART (started July 2021) aims to generalise the concept of Fiducial Reference Measurements (FRMs) for the Copernicus Sentinel-3 STM. This work has gathered existing data, made new observations during field campaigns, and ensured that these observations meet the criteria of FRM standards so that they can be used to validate Sentinel-3 STM Land Altimetry products operationally. A roadmap for the operational provision of the FRM, including the definition, consolidation, and identification of the most relevant and cost-effective methods and protocols to be maintained, supported, or implemented, has been developed. The roadmap includes guidelines for SI traceability, definitions of FRM measurement procedures, processing methods, and uncertainty budget estimations.

<p>The soon-to-be available swath altimetry observations from the SWOT mission are expected to drastically improve our capacity to observe fine-scale ocean processes (wavelengths shorter than 100 km). With its increased observing capacity and lower signal-to-noise (SNR) ratio in comparison to conventional nadir altimetry, this new technology is expected to deliver unprecedented high-resolution two-dimensional observations of the ocean surface circulation and surface water bodies. Benefiting from the technological maturity acquired during the preparation for SWOT, a new constellation concept composed of swath altimeters has been proposed to carry on the European operational observing system towards the end of the decade. In the present work, we focus on evaluating the ocean observing capabilities of a novel swath altimeter concept (WiSA – Wide Swath Altimeter). Using the observed surface wave field (SWH – Significant Wave Height) and the instrumental characteristics, we compute global estimates of the SNR. We observe an average global observability around 40 km wavelength over 50% of the global ocean, and 47 km on average over 80% of the globe. Little or no seasonality is observed in the SNR, related to the seasonal compensation of the two competing factors that contribute to the observability, namely the instrumental noise levels and the observed spectral slopes. The performance of recently developed data-driven filtering techniques is also evaluated, considerably increasing the purely instrumental observing capabilities. The results are also discussed from an operational perspective, considering the contribution of a constellation of swath altimeters over a mono-satellite mission.</p>

A microwave range transponder has been operating at the CDN1 Cal/Val site on the mountains of Crete for about 6 years, to cal-ibrate international satellite radar altimeters in the Ku-band. This transponder is part of the European Space Agency Permanent Facility for Altimetry Calibration, and has been producing a continuous time series of range biases for Sentinel-3A, Sentinel-3B, Jason-2, Jason-3 and CryoSat-2 since 2015. As of 18-Dec-2020, the CDN1 transponder has allowed calibration of the new operational altimeter of Sentinel-6 MF satellite as it flies in tandem with Jason-3. This work investigates range biases derived from the long time series of Jason-3 (and subsequently that of Sentinel-6 MF since both follow the same orbit) and tries to isolate systematic and random con-stituents in the produced calibration results of the transponder. Systematic components in the dispersion of transponder biases are iden-tified as of internal origin, coming from irregularities in the transponder instrument itself and its setting, and of external cause arising from the altimeter, satellite orbit, Earth's position in space, geodynamic effects and others. Draconic harmonics, principally the 58-day period, play a significant role in the transponder results and create cyclic trends of maximum amplitude of 17 mm at times in the cal-ibration results. The attitude of the satellite body as it changes for solar panel orientation contributes an offset of about 7 mm when yaw rotation is off its central position, and the atmospheric, water mass and non-tidal ocean loadings are responsible for an annual sys-tematic signal of 10 mm. At the time of writing, all other constituents of uncertainty seem random in nature and not significantly influ-ential, although humidity requires further investigation in relation to the final transponder calibration results.(c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

Poseidon-4 is a dual-frequency redundant radar altimeter, embarked onboard the Sentinel-6 Michael Freilich (MF) European Commission Copernicus Programme satellite. In this article, 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 an external calibration analysis over a transponder. The instrumental performances of the radar altimeter are excellent for both radar chain sides: Poseidon-4 delivers a range/azimuth instrument impulse response (IR) with the highest quality and fidelity in the era of space-borne radar altimetry, and its thermal noise response is almost just random noise. A power decay of the level of the transmitted power in the Ku-band has been detected both for the nominal and redundant sides, which is larger than expected, though it will not violate the requirement of the minimum signal-to-noise ratio over the ocean at the end of the satellite design lifetime. The innovative CAL1 ECHO CAL calibration mode allows for very precisely characterizing the sensitivity of the instrument IR to the in-orbit temperature variations and, thus, correcting for it in the science data as standard practice.