Sentinel-6 (Jason-CS) is an operational oceanography programme of two satellites that will ensure continuity to the Jason series of operational missions. The mission is being developed by a multi Agency partnership consisting of ESA, EUMETSAT, NOAA, CNES and NASA-JPL. ESA is responsible for the Sentinel-6 (Jason-CS) Space Segment development along with Airbus GmbH as a prime contractor.

The main payload of the Sentinel-6 satellite is the Poseidon-4 radar altimeter, an evolution from the altimeters on-board the Jason satellites. Poseidon-4 also inherits the Synthetic Aperture Radar (SAR) High Resolution Altimeter mode from CryoSat-2 SIRAL and Sentinel-3 SRAL.

isardSAT is responsible for the development of the Altimeter Level-1 Product Generation Function (ALT L1 PGF), within the S6 Payload Data Acquisition and Processing (S6 PDAP) project, which is part of the Sentinel-6 Overall Ground Segment (S6 OGS). This processing function is divided in different chains: LR (Low Resolution), HR (High Resolution), CAL1 LR, CAL1 HR, Echo CAL and CAL2. Each chain processes the corresponding Level-0 data and produces Level-1B and Level-1A data (only the HR processor). This data will be archived and disseminated to the users, besides being input to the Level-2 Product Generation Function.

The L1 data will be generated following the different timeliness services: Near Real Time (NRT), Short Time Critical (STC) and Non-Time Critical (NTC). The availability of different auxiliary data (Orbit State Vector and Attitude data) will trigger the processing for the different services. This poster will present the different timeliness services, which files trigger each service, and their time availability. Also, the difference between the L0 data that is processed in each service, such as granules in NRT or pole-to-pole data in STC/NTC.

This poster will also review the ALT Level-1 PGF in terms of its architecture and the different processing modes available, and how they relate to each other.

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Sentinel-6 (Jason-CS) is an operational oceanography programme of two satellites that will ensure continuity to the Jason series of operational missions. The mission is being developed by a multi Agency partnership consisting of ESA, EUMETSAT, NOAA, CNES and NASA-JPL. ESA is responsible for the Sentinel-6 (Jason-CS) Space Segment development along with Airbus GmbH as a prime contractor.

The main payload of the Sentinel-6 satellite is the Poseidon-4 radar altimeter, an evolution from the altimeters on-board the Jason satellites. Poseidon-4 also inherits the Synthetic Aperture Radar (SAR) High Resolution Altimeter mode from CryoSat-2 SIRAL and Sentinel-3 SRAL, with a technological evolution characterised by:
* Improved radio frequency hardware and an on-board digital architecture (matched filter digital operation instead of analogue-based de-ramping).
* Interleaved Ku-band mode operation: near continuous transmission of Ku-band pulses. It allows the simultaneous operation of the low resolution mode (LR or LRM) and high resolution (HR or SAR).
* On-board processing, range migration correction (RMC), is performed on the SAR Raw data to reduce the amount of data to download, with the same rate, thus producing the SAR RMC data.
* A calibration pulse is included in the tracking cycle to monitor instrumental variations around the orbit.

isardSAT is responsible for the development of the Level-1 Ground Prototype Processor (GPP) for the Sentinel-6 Poseidon-4. This prototype processes all the chains starting from the Instrument Source Packets (ISPs, also known as Level-0 data), and producing Level-1A and Level-1B-S (only for HR processing) and Level-1B (all modes). The prototype has been verified using simulated data generated by the ESA Sentinel-6 mission performance simulator provided by ESA and also using in-orbit CryoSat adapted data.
The processing chains of the GPP are divided into the scientific modes, Low Resolution (LR), High Resolution (HR), Low Resolution Over-sampled (LR-OS), Low Resolution from RMC (LR-RMC) that produce geophysical results and the calibration modes, CAL1 LRM, CAL1 SAR and CAL Pulse, that produce calibration results for the scientific modes. Besides, the HR and LR-OS processing chains can ingest two different Level-0 data: SAR Raw data and RMC data.

This poster will review the Level-1 GPP in terms of its architecture and the different processing modes available, explaining the differences between them and evaluating the use case for each mode: some modes will be used depending on the surface type (e.g. HR from SAR / HR from RMC), other modes will be used always (e.g. LR), some modes can coexist with other modes, thus allowing cross-validation of the different modes (e.g. LR and HR, HR from SAR and HR from RMC).

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The Sentinel-6 mission, also called Jason-CS, is part of the Copernicus programme, which aims at continuing the Jason series of operational oceanography missions for the period 2020-2030. The main payload instrument of the mission is a dual-frequency Ku/C radar altimeter (Poseidon-4), evolving significantly from its predecessors (Poseidon-3A/3B carried on-board Jason-2 and Jason-3 satellites respectively). Poseidon-4 will operate in open burst mode which continuously transmits pulses, interleaved with receive echoes, at relatively high repetition rate (9kHz) such that the pulse-to-pulse coherence is achieved. That way, Low Resolution Mode (LRM) and Synthetic Aperture Radar (SAR) mode data can be generated simultaneously, thus giving us the opportunity to perform direct in-flight comparison between SAR altimetry and conventional data. In addition, as the number of independent looks is higher than for closed-burst SAR altimeters, the measurement noise of the Sentinel-6 altimeter is expected to be significantly reduced. For the sake of continuity with preceding missions, low- and high-resolution products will be generated from usual data processing, called baseline processors in LR and HR modes. Beside that, the altimeter mission will experiment new products and process developments (called experimental processors: LR-RMC and LR-OS) whose benefits are known by the altimeter community.
In the past two years, CLS has conducted the development of the level-2 prototype processor (GPP) under ESA contract. The level-2 GPP consists of four processors (two baseline and two experimental processors), each of them enabling to generate geo-located geophysical retrievals from high- or low-resolution waveforms. Furthermore, the level-2 GPP contains an innovative retracker in LR mode, ensuring high-performances and notable benefits in comparison with conventional one. The level-2 GPP has been tested and validated using simulation data, and fully assessed with reconditioned Sentinel-3 data, thus demonstrating the high-reliability and robustness of the software, and its capability to be used as a good reference for future comparisons with operational processing.
The level-2 GPP can also be interfaced with the CLS/CNES Sentinel-6 altimeter simulator to form an end-to-end simulator of the Sentinel-6 Poseidon-4 instrument. These tools, when combined, provide an opportunity to conduct performance evaluations of the instrument system over different types of surface (open ocean, coastal zones, river and lakes and other land areas). They also permit to better understand the various specificities of the altimeter processing (Doppler ambiguities, on board Range Migration Correction, unequal range bin sampling and resolution) and design and assess any innovative processing. Besides its large list of possible activities and interests, the major asset of the Sentinel-6 simulator is its capability to significantly reduce the computation time of simulation, thanks to the use of innovative and powerful simulation processes, and to process data segment longer than 1000km in quite short run time.
This poster describes the different units of the level-2 Sentinel-6 Ground Prototype Processor and its major characteristics. Validation results demonstrating the potential of this software are also provided.

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During the last decade the radar altimetry has entered in its golden age as demonstrated by the different number of missions (Jason-2/-3, CryoSat-2, Saral/Altika, Sentinel-3) currently operating and the forthcoming ones (Sentinel-6). The relatively new operational synthetic aperture radar (SAR) mode in CryoSat-2 and Sentinel-3 missions, opens a new paradigm in the capabilities that can offer an altimetric radar mission. In this line, lessons learned from classical 2-D SAR focusing [Curlander1991] are exploited to evaluate the imaging-like capability of delay-Doppler altimetric radar mounted on future Sentinel-6 over coastal areas. In this way, the altimetric product gets closer to the conventional SAR imaging data, but in the altimeter case a “strip-like” image is obtained compared to the classical 2D SAR image.

Conventional delay-Doppler processor (DDP) coherently integrates a series of pulses to provide specific Doppler beams focused to a specific location, which after being correctly aligned (compensating for the slant-range variation, among others) provide several looks that can be incoherently averaged, increasing the performance in terms of geophysical retrieval (increasing the signal-to-noise ratio-SNR), and the spatial resolution. The fully focused DDP (FF-DDP) moves one step ahead and intends to coherently integrate such information to get an even higher along-track resolution with an improved SNR and the available number of looks [Egido2017].

In order to achieve such imaging capability, the azimuth or along-track phase modulation needs to be compensated for. The relative movement between the scene and the satellite creates a chirp-like modulation in the along-track dimension (quadratic phase response), and so an azimuth compression needs to be performed (once range migration has been compensated) to obtain a fully focused SAR strip, analogous to the well-known range compression (where a specific chirp pulse is compressed).

The main objective of this presentation is to comparatively evaluate the FF-DDP when exploiting Sentinel-6 RAW and RMC modes in order to show that RMC doesn’t produce any significant performance degradation. The operation of this processing chain will be demonstrated with simulated data provided by ESA-ESTEC considering both input data from RAW and RMC modes (undoing the on-board RMC processing). Transponder as well as ocean data sets will be considered in the comparison.

References:
[Curlander1991] Curlander, John C., and Robert N. McDonough. Synthetic aperture radar. New York, NY, USA: John Wiley & Sons, 1991.
[Egido2017] A. Egido and W. H. F. Smith, "Fully Focused SAR Altimetry: Theory and Applications," in IEEE Transactions on Geoscience and Remote Sensing, vol. 55, no. 1, pp. 392-406, Jan. 2017
[Makhoul2018] E. Makhoul et al., “S6 P4 GPP: Fully Focused Delay-Doppler Processing - Preliminary results”, Ocean Surface Topography Science Team Meeting 2018, September 2018.

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