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In times of ever decreasing amount of in-situ data for hydrology, satellite altimetry has become key to provide global and continuous datasets of water surface height. Indeed, studying lakes, reservoirs and rivers water level at global scale is of prime importance for the hydrology community to assess the Earth’s global resources of fresh water. Much progress has been made in the altimeters’ capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC) now represents an essential feature of the tracking function. This tracking mode’s efficiency has been proven on past missions and it is now stated as operational mode for current Sentinel-3 and Sentinel-6 missions. It has benefited from iterative improvements brought to onboard tables contents repeatedly since 2017. In 2022, new updates will be performed on the onboard OLTC tables of the Sentinel-3A and Sentinel-3B missions, as well as Sentinel-6A and Jason-3 following their successful Tandem Phase. The number of hydrological targets used to define the tracking command currently reaches an unprecedented number of targets of almost 100,000 for each Sentinel-3 and about 30,000 for Sentinel-6A. We expect to define a similar number of targets in the interleaved orbit of Jason-3, previously flown by Jason-2, although mostly in Closed-Loop Mode. These major improvements over the last few years have been made possible by the analysis and merging of the most up-to-date digital elevation models (SRTM, MERIT and ALOS/PalSAR) and water bodies databases (HydroLakes, GRaND v1.3, SWBD, GSW, SWORD). In addition, special effort is put into introducing the most recent reservoir databases. This methodology ensures coherency and consistent standards between all nadir altimetry missions and types of hydrological targets. Finally, additional efforts have been carried out to define a relevant tracking command outside of hydrological areas, in order to keep track of the continental surface and enabling potential other land applications, while optimizing the OLTC onboard memory. The OLTC function of nadir altimeters constitutes a great asset for building a valuable and continuous record of the water surface height of worldwide lakes, rivers, reservoirs, wetlands and even a few continental glaciers. This work is essential at institutional and scientific levels, to make the most of current altimeters coverage over land and to prepare for the upcoming calibration and validation of the Surface Water and Ocean Topography (SWOT) mission. In this context, we will show an overview of OLTC achievements and perspectives for future altimetry missions.

Over the last few years, satellite radar altimetry measurements have become more and more numerous over inland waters, thanks to improvements in the tracking function as well as enhanced measurement modes (from pulse-limited low resolution mode to high resolution synthetic aperture radar). This higher quality and abundance of measurements also raises the question of their processing: from historical retracking algorithms (e.g. the Offset Centre Of Gravity - OCOG retracking) to most recent innovative ones (e.g. Fully-Focused SAR). In particular, it is of common knowledge in the hydrology community that current ground segments using the OCOG algorithm provide, on frequent occasions, biased water surface height estimates and is not reliable at global scale. The need for more reliable and robust processing methods is therefore one of the biggest challenges for radar altimetry over land. The main challenge is that contrary to ocean altimetry, the radar signal over inland waters is highly variable both in space and time, as it depends on the nature and size of the water body observed (lakes, rivers, flood plains, etc.) as well as on surface conditions. In this study, we focus specifically on rivers where various types of waveforms can be acquired: sinc²-like peak (the most frequent ones), asymmetric peak, multiple peaks, distorted Brown-like waveform. We address the representativeness of the signal’s specularity, as it has been documented in the literature that the radar backscattered signal over rivers is often highly specular and can be modeled as a squared cardinal sine function. We use two parallel approaches: simulation of radar signals over a specular surface, and analysis of real data acquired by current Sentinel-3 and Sentinel-6 Ku-Band missions over rivers. In the theoretical approach, we use simulations of various specular surfaces and analyze both the amplitude and phase behaviors. It is interesting to analyze the relative impact of the observing system configuration (e.g. radar bandwidth, altitude, sampling) and the nature of the surface (e.g. geometry of the observed scene, backscatter coefficient) on the simulated radar signal. In the data analysis approach, we use a unique dataset of one hundred rivers worldwide to perform a statistical analysis of the specular nature of the signal and other characteristics in function of the scene configuration. River width is one but not the only parameter impacting the measured signal. With this study, we aim at understanding the most significant factors controlling the measured radar signal over rivers in order to build a robust and universal processing algorithm capable of providing reliable water surface height estimates to inland waters users. In the perspective of the Surface Water and Ocean Topography (SWOT) mission, nadir altimetry more than ever stands as an important asset for calibration and validation of this mission as well as for the design of future altimetry missions such as the Copernicus Sentinel-3 Next Generation: Topography mission, which will necessarily address the question of processing performance over inland waters.

The Copernicus Sentinel-3 Surface Topography Mission (STM), based on a constellation of two satellites, provides extremely valuable surface elevation information over inland waters, sea ice and land ice, thanks to its SAR altimeter which retrieves high-resolution along-track elevation measurements, and to its orbit that covers high-latitude polar regions. To ensure that these measurements can be used with confidence, and to maximize the return on investment of the Copernicus Sentinel-3 STM, adequate validation of the geophysical retrieval methods, processing algorithms and corrections must be performed, considering independent observations as Reference Measurements. St3TART is an EU and ESA funded project led by NOVELTIS with a consortium of 17 European partners (CNES, DTU, NPI, vorteX.io, LEGOS, Hydro Matters, CLS, LOCEAN, IGE, SERTIT, GIS, CNR-IRPI, NPL, DT/INSU, IRD, M2C, SYRTE). It aims to generalize the concept of Fiducial Reference Measurements (FRM) for the Copernicus Sentinel-3 STM and to collect and distribute FRM data for the validation of the satellite mission over inland waters, sea ice and land ice. The provision of FRM will serve the validation activities of the ESA S3 Land Altimetry products, currently performed by the Sentinel-3 Mission Performance Centre, with the support of the Sentinel-3 Validation Team, for the sake of performance assessment and fitness-for-purpose of the Sentinel-3 STM Land core products. Those products cover inland waters, land-ice and sea-ice areas. The objective of the St3TART project is not only to collect existing data or measure new observations during field campaigns, but to ensure that these observations meet the criteria of FRM standards and can be used in an operational way for the validation of the Sentinel-3 Land topography mission. Within the project, field campaigns are means to provide operational observations for the validation of the Sentinel-3 data. One of the tasks of the project is the development of a Super Cal/Val Site Identifier Tool (SCalSIT). This tool will support the Cal/Val community and scientists in identifying potential in-situ Cal/Val sites over Inland Water Surfaces, by an appropriate cross-processing of the satellite orbits versus water mask ground data. The tool is configurable, to take into account a specific geographic area, a list of satellite orbits (theorical or user orbit products), a land water mask, the percentage of surface water occurrence, a buffer around the satellite tracks, and the minimum number of passes over a crossover. The output vector files can be in KML, CSV or GeoJSON formats. It has been developed for the needs of the Sentinel-3 Surface Topography Mission constellation, but its generic implementation makes it compatible with other missions such as CryoSat-2, Sentinel-6 or SWOT. The SCalSIT software is an open-source software, developed as a QGIS plugin, which makes it very easy to use and platform-independent, and it can also be used via a Command Line Interface. The poster will present the main characteristics of the tool and an example of outputs.

AlTiS (Altimetry Time Series) is a software to visualize and process radar altimetry data. It was developed to analyse radar altimetry data over small areas like river, lakes and wetlands, located in the altimetry groundtracks. Its major goal is the creation of time-series of water levels from the height derive from the altimeter measurement but it can also be used to generate time series of any altimetry-based parameters (e.g., corrections applied to the range, backscattering coefficients, or brithness temperatures). Through a Graphical User Interface (GUI), without any skills in data processing, the user can handle altimetry data in order to : • Display several parameters of altimetric measurement like surface height, altimetric range, atmospheric corrections applied to the range to take into account the propagation delay of the electromagnetic wave through the earth atmosphere (ionospheric correction and wet and dry tropospheric corrections) and also to display some characteristic parameters of the waveform like the backscatter coefficient, and peakiness. • Graphically select altimetric measurements to remove outliers and easily done owing to Landsat background image. • Generate water height time series from the valid altimetry data previously selected AlTiS accepts CTOH altimetry products (Level 2 GDR supplied by the CTOH). CTOH GDR data have been specifically conditioned to optimize the data size by making a geographical selection and includes the right altimetry parameters for hydrological studies. AlTiS can process several altimetric data products from followed missions : Jason-1/2/3, ERS-2, ENVISAT, SARAL, Sentinel-3A/B, and soon, Sentinel-6/Jason-CS and the nadir altimeter onboard SWOT. They are supplied for free through a web request form on the CTOH website (http://ctoh.legos.obs-mip.fr/applications/land_surfaces/altimetric_data/altis). AlTiS is mainly employed for hydrological applications and can be used for training courses on radar altimetry at bachelor or master levels. It is also a very convenient tool to analyse the radar altimetry data contained in the GDR over any type of land surfaces. AlTiS is a software developed by CTOH as part of its activities as a National Observation Service. AlTiS is a free software and it is released as an open source under the CeCill License. Altis is working under python3 environment and tested for GNU/Linux, Windows 10. AlTiS is available on GitLab : https://gitlab.com/ctoh/altis

The Copernicus Sentinel-3 Surface Topography Mission (STM) provides extremely valuable surface elevation information over inland waters, sea ice and land ice, thanks to its SAR altimeter which retrieves high-resolution along-track elevation measurements, and to its orbit that covers high-latitude polar regions. To ensure that these measurements can be used with confidence, and to maximize the return on investment of the Copernicus Sentinel-3 STM, adequate validation of the geophysical retrieval methods, processing algorithms and corrections must be performed, considering independent observations as Reference Measurements. St3TART is an EU and ESA funded project led by NOVELTIS with a consortium of 17 European partners (CNES, DTU, NPI, vorteX.io, LEGOS, Hydro Matters, CLS, LOCEAN, IGE, SERTIT, GIS, CNR-IRPI, NPL, DT/INSU, IRD, M2C, SYRTE). It aims to generalize the concept of Fiducial Reference Measurements (FRM) for the Copernicus Sentinel-3 STM and to collect and distribute FRM data for the validation of the satellite mission over inland waters, sea ice and land ice. The provision of FRM will serve the validation activities of the ESA S3 Land Altimetry products, currently performed by the Sentinel-3 Mission Performance Centre, with the support of the Sentinel-3 Validation Team, for the sake of performance assessment and fitness-for-purpose of the Sentinel-3 STM Land core products. Those products cover inland waters, land-ice and sea-ice areas. The objective of the St3TART project is not only to collect existing data or measure new observations during field campaigns, but to ensure that these observations meet the criteria of FRM standards and can be used in an operational way for the validation of the Sentinel-3 Land topography mission. The FRM Data Hub will provide a centralized access to these FRM measurements. It aims to federate the Cal/Val community in sharing these reference observations in a free and accessible manner, with fully characterized and documented FRM processing and measurements. The poster will present the architecture of the tool, its interface, the first available datasets and the format specification.

The Center for Topography of the Oceans and Hydrosphere (CTOH) is a French Observation Service created in 1989 and dedicated to satellite altimetry studies. It focuses on the development and promotion of new processing approaches of the altimetric data for emerging research domains (coastal ocean, oceanic sub-mesoscale phenomenons, continental surface water, sea ice, polar caps). It works in close relationship with space agencies (mainly CNES and ESA) at different levels for satellite altimetry missions: preparation of new missions, definition of the user’s needs, CAL/VAL studies, signal analysis & data reprocessing, development of thematic products, teaching and outreach. In terms of data distribution, the CTOH maintains a global GDR data base for almost all altimetry missions since Topex/Poseidon. All the products are made homogeneous (addition of the most recent parameters / corrections for the old missions) and provided in netcdf format. A new version of ERS-2 data, reprocessed by the CTOH for hydrological applications is also available (it includes ICE-1 and ICE-2 retrackers). Both 1Hz and 10/20/40Hz data are available globally. For some products, the CTOH database contains also L1 products (waveforms). In addition, the CTOH develops new altimetry products: • Fine-resolution ocean products: position of the main Southern Ocean polar fronts and global climatology of near-inertial current characteristics. • Coastal products : X-TRACK along-track SLA time series and tidal harmonic constants, reprocessed with a software designed for coastal altimetry, but also the recent high-resolution (20-Hz) X-TRACK/ALES sea level product, now reaching a distance of 3-4 km from the coast on average (see Leger et al., OSTST 2022). These products are distributed by AVISO+ and by ESA, respectively. • Continental hydrology products : including Topex/Poseidon reprocessed by the CASH project and water level maps developed for 5 rivers: the Amazon, Orenoque, Gange-Bramhapoutre, Congo and Mekong. It also contributes to the development of sea-ice products distributed by AVISO+ (Arctic Sea Ice Thickness and Snow depth monthly maps over sea ice) and to the “Hydroweb” data base for monitoring river and lake levels.

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.