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Leads between ice floes are key elements to determine the sea surface height in sea ice scenarios since they provide a sea-level reference for the freeboard height measurements. Satellite-based techniques for lead-detection have traditionally required conventional delay-Doppler processing (DDP), where all echoes of a burst are coherently combined within the burst but then incoherently averaged with the other bursts received during the illumination time. DDP has a typical spatial resolution of 300m along-track, which in practice limits the type (orientation, width) of leads that can be detected. Fully-Focused SAR processing technique (Egido & Smith 2017) allows reductions of spatial resolution down to approximately 0.5m by coherently integrating all the echoes received within the illumination time, yielding significant improvements in terms of lead detection among other applications. Future satellite altimeters such as Sentinel-6 and eventually the Copenicus polaR Ice and Snow Topography Altimeter (CRISTAL) mission will be operated at open bursts (or interleaved) and will provide fully-focused SAR echoes with high spatial resolution and unambiguous along-track backscattered echoes. Currently flying altimeters such as CryoSat-2 and Sentinel-3 are not optimized for fully-focused SAR processing due to their close burst operation. They can still be used for lead detection using FFSAR applied together with additional steps to mitigate the ambiguities caused by not sampling the surface uniformly. They are expected to be particularly useful for the detection of small-sized leads down to ~10m in scenarios with leads properly separated some hundreds of meters. In this contribution we present the current status of the fully-focused SAR analysis over Antarctica sea ice, combining data from current altimetry missions and validating it with satellite images. This work is part of our contribution to the "CryoSat+ Antarctica: Improved Sea Ice Thickness and Ocean Observations" project.

The CryoSat-2 mission is designed to determine fluctuations in the mass of the Earth’s land and the marine ice fields. Its primary payload is a radar altimeter that operates in different modes optimised depending on the kind of surface: Low resolution mode (LRM), SAR mode (SAR) and SAR interferometric mode (SARIn). This instrument is named SIRAL: SAR Interferometric Radar Altimeter. Transponders are commonly used to calibrate absolute range from conventional altimeter waveforms because of its characteristic point target radar reflection. The waveforms corresponding to the transponder distinguish themselves from the other waveforms resulting from natural targets, in power and shape. ESA has deployed a transponder available for the CryoSat-2 project (a refurbished ESA transponder developed for the ERS-1 altimeter calibration). It is deployed at the KSAT Svalbard station: SvalSAT. Another transponder was deployed in Greece Technical University of Crete for the Sentinel-3 calibration, and later moved to West Crete in a permanent position. A new transponder is located in Gavdos, in another island close to Crete, and was deployed during the Commissioning phase of Sentinel-6. We are using these transponders to calibrate SIRAL’s range, datation, and interferometric baseline (or angle of arrival) to meet the mission requirements. In these calibrations, we are using three different types of data: raw Full Bit Rate data, stack beams before they are multi-looked (stack data) in the Level 1B processor, and the Level 1B data itself. Ideally the comparison between (a) the theoretical value provided by the well-known target, and (b) the measurement by the instrument to be calibrated provides us with the error that the instrument is introducing when performing its measurement. When this error can be assumed to be constant regardless the conditions, it will provide the bias of the instrument. And if measurements can be repeated after a certain period of time, an indication of the instrument drift can also be provided.

Radar altimeter processors require knowledge of platform and radar antenna attitude to provide accurate range and datation measurements. Although the attitude solution generated at real-time by the on-board computer may be accurate enough for ground applications, it is actually of interest to reprocess the star-trackers raw measurements on-ground with consolidated orbit data for validation purposes. Additionally, having more than one star-tracker on-board allows to derive independent Roll-Pitch-Yaw attitude estimates as observed from each star-tracker, what is of interest to identify and characterise eventual mechanical distortions between the star-trackers and the platform, such as thermo-elastic distortion due to the long-term variable thermal loads. In this contribution we present the star-tracker processor that has been implemented by isardSAT for the Sentinel-6 mission as part of the ESA Sentinel-6 Poseidon-4 (P4) L1 Ground Prototype Processor (GPP) to address the points raised before. We describe and discuss the overall performance of the star-tracker processor implemented. Then, we provide a comparison of the pitch measured by the star-trackers with an independent estimate provided by the P4 L1 GPP, allowing to determine the residual bias in pitch between the star-trackers measurements and the actual radar antenna pointing. Finally, we present a thermal analysis addressing how this residual bias evolves during the long-term thermal variation of the spacecraft.

Sentinel-6 is an operational oceanography programme consisting on two satellites that will map the sea surface levels with great precision. Sentinel-6 includes two identical satellites with the first launched on November 21, 2020 (Sentinel-6 Michael Freilich) and the second scheduled for launch in 2025 (satellite B). Following its launch, the Sentinel-6 Validation Team (S6VT) has carried out validation activities with Sentinel-6 Michael Freilich data for more than a year to ensure the best possible outcome of the mission. Thanks to the transmission pattern of Sentinel-6 Poseidon-4 (continuous transmission at 9kHz PRF, Open Burst mode), the data can be processed using Fully-Focussed SAR techniques, which enables a much better along track resolution and also better scene focussing. This is highly beneficial over sea ice areas, where the coherence of the scene is kept during the illumination time and very detailed features, such as small leads and icebergs, can be precisely acquired. This work will show how we can use Sentinel-6 Poseidon-4 radar altimeter data over sea ice waters to detect the presence of small icebergs and extract useful parameters from them. Furthermore, we will show the improvement obtained in resolution when analysing the data using Fully-Focussed techniques over conventional unfocussed SAR processing. Nevertheless, carrying a single radar antenna (as it happens in the Sentinel-6 Poseidon-4 radar altimeter) severely reduces the number of parameters we can extract from the detected icebergs using this technique, as well as not allowing us to locate them with precision. The future Copernicus Polar Ice and Snow Topography Altimeter mission (CRISTAL), with an expected launch on 2027, will carry on-board as its main payload the IRIS altimeter. Amongst other improvements with respect to previous missions, a key feature from the IRIS altimeter will be the use of 2 antennas in the Ku band allowing the determination of the across track location using the SAR interferometric (SARIn) information. The use of an interferometric mode greatly improves the information we can extract from the small icebergs detected, as well as to locate them with great precision with respect to the satellite track. To finalise this work, we will show which are the future prospects regarding iceberg detection for CRISTAL, and how it will allow us to combine the Fully Focussed analysis techniques used during the S6VT activities with the SARIn mode to maximise the information and resolution we can obtain detecting icebergs.

The Sentinel-6A satellite, launched in November 2020, carries the first radar altimeter operating in open burst with a PRF high enough (~9kHz) to perform the focussing of the whole target observation echoes in a fully coherent way with practically negligible impact from along-track replicas. Furthermore, such a feature allows to improve the along-track resolution down to the theoretical limit around 0.5 m when processing the data with a Fully-Focussed SAR (FFSAR) algorithm. This resolution increment actually represents a revolutionary step with respect to the ~300 m along-track resolution provided by current operational processors based on Unfocussed SAR algorithms, commonly used in previous radar altimeters with a closed burst chronogram, such as CryoSat-2 and Sentinel-3. In this contribution, we explore new applications over inland water surfaces derived from such new Sentinel-6 FFSAR products. Indeed, the FFSAR Ground Prototype Processor (GPP), developed by isardSAT and based on the backprojection algorithm [1], has been used to process data over different types of inland water targets with the following objectives: (1) validate range measurements with in-situ water height data in case of nadir targets and (2) monitor water extent for off-nadir targets located within certain observation constraints. In this contribution we present results for both activities. First, we provide statistics of long-term monitoring of a series of reservoirs, which have reported global biases and standard deviations well below 10 cm. Secondly, we present a methodology to estimate water extent of small targets located on unambiguous across-track targets. We have analysed targets that present strong seasonal variability in terms of area, and validated the method by comparing water extent measurements derived from Sentinel-6 with the ones derived from optical, SAR imagery and in-situ observations. The overall work is part of the VALERIA (Validating Algorithms Levels 1A and 2 in Ebre RIver Area) project developed within the Sentinel-6 Validation Team using data from the satellite commissioning phase. [1] Egido, Alejandro and Walter H. F. Smith. “Fully Focused SAR Altimetry: Theory and Applications.” IEEE Transactions on Geoscience and Remote Sensing 55 (2017): 392-406.

Swath mode processing of CryoSat-2 Synthetic Aperture Radar Interferometric (SARIn) mode has been used to monitor elevation of areas with complex topography such as over ice sheet and ice cap margins. Swath processing relies on an accurate measure of the angle of arrival of the measured echo and, therefore, requires custom strategies in order to resolve the ambiguous phase measurement. In mountainous regions of complex terrain, it may be necessary to apply different phase ambiguities across a waveform record when returns come from different scatters distributed perpendicularly to the CryoSat-2 ground tracks. In this letter, we present modifications to the conventional swath processing method whereby a multisurface retracker is first applied to the record in order to identify potential different scattering surfaces. Phase ambiguity is then independently resolved for each of these subsurfaces. The improvements with this new method over the Karakoram glaciers are a 10% increase in the number of measurements with improvements of almost 50% for individual glaciers and a reduction in the median absolute deviation of the elevations from 20.18 to 14.69 m.