Copernicus is the European Union’s Earth Observation Environment monitoring programme. It is implemented by a comprehensive integrated system of operational Sentinel satellites and Copernicus Services that respond to the needs of users both directly through products and applications, and indirectly through social, economic and environmental benefits. The Copernicus Sentinel-6/Jason-Continuity of Service (Jason-CS) is a cooperative mission, implemented by ESA, EUMETSAT, NASA and NOAA. The Sentinel-6/Jason-CS primary mission objective is to provide continuity of ocean topography measurements beyond TOPEX (Topography Experiment)/Poseidon (T/P), Jason-1, Jason-2, and Jason-3 cooperative missions, for determining sea level and sea state parameters. Importantly, the mission acts as a reference mission in the CEOS Ocean Surface Topography Constellation. Accordingly, the Sentinel-6/Jason-CS Mission will utilise the legacy T/P precision altimetry orbit. This presentation provides the scientific basis and operational applications for the Sentinel-6 Mission.

The Sentinel-6/Jason-CS mission of two satellites will contribute to high performance ocean altimetry (HPOA) within the Copernicus Programme with the first satellite (Sentinel-6A) being the continuity of the TOPEX/Poseïdon and Jason (1,2 & 3) series of reference missions. The primary objective is to continue the long term climate data record of Global Mean Sea Level (GMSL) measurements. The Sentinel-6A satellite is expected to be launched in late 2020 in the same reference orbit as the current Jason-3 mission. The primary EU payload consists of the Poseïdon 4 Radar Altimeter that will allow retrieval of precision sea level, wind speed and wave height measurements in near real time. Poseidon 4 is a state of the art altimeter having evolved from the SRAL instrument of the Sentinel 3 mission. The instrument operates simultaneously in both Low and High resolution modes (LRM and SAR) and has enhanced performance with respect to previous altimetry missions. The altimeter is supplemented by a European GNSS-POD receiver with GPS and GALILEO capability and a DORIS instrument, enabling precise orbit determination (POD). The EU payload is complemented with the US payload, namely the climate quality Advanced Microwave Radiometer (AMR-C) and the Laser Retro Reflector Array (LRA) used in Satellite Laser Ranging.

We present the Poseidon-4, current expected performances at instrument level as well as a summary of the expected performance of key geophysical retrievals (sea surface height, significant wave height and wind speed).

To support the planning and evaluation of future validation activities; a comparison between different data modes is performed on simulated data. To reduce data volume by a factor 2 (and open up the potential to cover inland water areas), the Poseidon 4 altimeter can generate data in a special mode known as SAR-RMC (Range Migration Compensation) compared with SAR-RAW (like CryoSat or Sentinel-3). Relative performances between SAR RAW data and those undergoing the on-board SAR-RMC function are explained for nominal and non-nominal ocean conditions. We also present results from an analysis of the expected pulse-to-pulse correlation and possible effects on performance of geophysical retrieval algorithms, as well as a comparison with current missions.

Additionally a summary of the analysis of the expected performance of the precise orbit determination from DORIS and the GNSS-POD receiver is presented.

SENTINEL-6 (JASON-CS) is an operational oceanography programme of two satellites that will ensure continuity to the JASON series of operational missions. Poseidon 4 is a new generation of altimeter based on the SRAL instrument of the Sentinel 3 mission, operating in both SAR and Low Resolution mode continuously (interleaved mode). To reduce the large SAR data volume produced on-board, an on-ground-reversible RMC (Range Migration Correction) is implemented. The useful range is reduced to about half (as a consequence data volume is reduced by a factor of 2). To allow for truncation without loss of relevant data, the SAR RAW data is aligned on board and corrected for range migration, then truncated and send to the ground. The on-ground processing is reversing this process and providing SAR RMC data, allowing for the usual SAR ground processing. The RMC processor has been designed by ESA and TAS to ensure no loss of information and performance degradation. RMC performances assessment has been already demonstrated using the Sentinel-6 System Performances Simulator. Here, in this paper, we propose to demonstrate the RMC performances with a different look angle. We apply a proxy of the Sentinel-6 RMC on-board processor to real Sentinel-3 measurements to compress the data volume. Then, S3-RMC-Compressed data are reversed and an usual SAR processing is applied. One cycle of measurements has been processed, assessed and compared to raw S3 SAR data over all surfaces (open ocean, coastal regions and hydro). This analysis consolidates the assessment provided by ESTEC and confirms the quality of RMC processor.

The Sentinel-6 (S6) Mission will provide continuity of ocean topography measurements beyond TOPEX-Poseidon (launched in 1992), Jason-1 (2001), OSTM/Jason-2 (2008), and Jason-3 (2016) for determining ocean circulation, climate change and sea level rise. The S6 mission consists of two satellites to be launched approximately 5 years apart to extend measurement continuity for at least another decade. This mission will serve both the operational user community and also enable the continuation of multi-decadal ocean topography measurements for ocean circulation and climate studies. The first S6 satellite is planned to launch by the end of 2020 with a suite instruments similar to the prior Jason series missions.

The primary US payload is provided by NASA and developed by the Jet Propulsion Laboratory (JPL). It consists of the Advanced Microwave Radiometer for Climate (AMR-C) instrument that includes a new on-board absolute Supplemental Calibration Subsystem (SCS). The SCS is a key evolution of the radiometer instrument that will enable a significant improvement in the sea surface height measurement stability. The AMR-C provides the water vapor correction to the ocean height measurement from the Poseïdon 4 Radar Altimeter which is one of the prime European payloads. The AMR-C is enhanced by an experimental High-Resolution Microwave Radiometer (HRMR) that demonstrates the capability of high frequency (90, 130, 166 GHz) radiometer channels for extending the wet path delay measurements into the coastal zone. The US payload also contains a Laser Retroflector Array (LRA) that supports ground-based laser tracking for precise orbit determination validation. The US payload is complemented with the EU payload (presented in this session as a separate paper).

As a secondary mission objective, S6 will also characterize atmospheric temperature and humidity profiles by measuring bending angles of GNSS signals occulted by the Earth’s atmosphere. These measurement products will be ground-processed within three hours of acquisition on board the satellite and made available for ingestion into national weather service models to support weather forecasting capabilities. This measurement is provided by the Global Navigation Satellite System for Radio Occultation (GNSS-RO) instrument.

We present a description of the US payload architecture, development status and (pre and post) launch expected performances.

The Sentinel-6 (Jason-CS) mission will follow TOPEX and the Jason-series of “reference altimeter missions”. But it is in many ways a totally new type of mission, a different platform (similar to CryoSat) and a different altimeter (dissimilar from any of the previous altimeters). Not only will it be the first Synthetic Aperture Radar (SAR) altimeter used on one of the reference missions, it will also be the first altimeter that operates in a continuous high-rate pulse mode, 100% of the time. This particular operating mode allows simultaneous production of Low Resolution (LR) mode measurements on-board as well as the processing of SAR echoes (High Resolution, HR, processing) on-ground. Both types of measurements will be provided in (separate) Sentinel-6 altimeter data products.

Sentinel-6 will bring some unique opportunities for cross-calibrating and cross-validating LR and HR altimetry, housed on the same platform, working from the same altimeter echoes, just using different processing techniques. Also, it will be the first time that we will be able to fully process on-ground 100% of the echoes that would otherwise be averaged on-board. This presentation will show how this is reflected in the Sentinel-6 products, how these compare with the products of the Jason-series and of Sentinel-3 and how continuity is ensured. Particular emphasis will be on highlighting the Level 1A (L1A) and Level 1B (L1B) product content.

Level 1 products will be made available containing all the individual echoes in the time domain (L1A) or the measurement data and waveforms without geophysical corrections (L1B). A L1B-S product (with the individual waveforms stacked and geo-located such as is available for Sentinel-3) can be derived from the L1A after performing Delay Doppler processing. Although L1B-S is not part of the product line, a tool will be provided to the users so that they can process L1A to L1B-S.

Level 2 (L2) products will contain the geophysical measurements of sea level, wind speed, and significant wave height, at 20-Hz and 1-Hz, from both LRM and SAR altimetry. They will also contain an appropriate set of geophysical corrections, outlined in this presentation, aimed at providing sea level measurements at the cutting edge of what is feasible. In numerous cases alternatives are provided to support various applications and error assessment. The L2 products can be easily aligned with the L1B products (and vice versa) in order to combine waveforms and geophysical corrections and retrievals.

Sentinel-6/Jason-CS will fly along the same orbit as TOPEX/Poseidon and its successors, Jason-1, Jason-2, and Jason-3, to ensure continuity of service and long-term sea-surface height observations. Its new Poseidon-4 altimeter aligns with the technology of Sentinel-3 SRAL altimeter and will operate not only in low-resolution mode but also in Synthetic Aperture Radar mode. Specifications of this generation of altimeter are more than demanding and challenging: The instrumental range noise is set, for example, to be less than 1cm, and lower than those errors of the previous altimeters of Jason-3 and Sentinenl-3.

This work describes the operational plan to be implemented at the Cal/Val facilities in west Crete, Greece for this demanding absolute calibration of the upcoming Sentinel-6/Jason-CS. Both its ascending (Pass No.109) and descending (Pass No.018) passes are to be calibrated with sea-surface and transponder techniques. The newly introduced standard of Fiducial Reference Measurements for altimetry is to be put into action for this altimeter calibration of the Sentinel-6/Jason-CS. Also, relative calibration with respect to other altimetry missions is to be carried out at sea and on land.

The Permanent Facility for Altimetry Calibration established in Gavdos and Crete islands, Greece, has been providing calibration services for all international altimeters for more than 15 years. A network of five permanent Cal/Val sites have already been established and distributed along the ground tracks of several satellite altimeters in Gavdos and West Crete. Diverse calibration techniques are being implemented for the same satellite passes, maximizing security, robustness and confidence upon the produced Cal/Val results.

This work will present all the necessary preparations for calibrating the upcoming Sentinel-6/Jason-CS. These include (1) Presentation of the essential observations & instruments to support the Cal/Val of Sentinel-6/Jason-CS; (2) Definition of the “benchmark calibrating parameters”, (3) setting of the procedures for the characterization and internal calibration of the field instrumentation before they are put into use in the field; (4) Putting into operation the fundamental metrology standards to build trust in and reach the FRM standards in the Cal/Val results; (5) Definition of all error constituents contributing into the final Cal/Val error budget, (6) Documentation procedures for reporting uncertainty budgets for Cal/Val; and (7) Description of standards, protocols and characterization procedures for this FRM altimeter calibration.

In support of the above lines, this work will present the planned new Cal/Val sites to be established in West Crete and Gavdos as well as new calibrating instrumentation for the satellite altimeter and its radiometer.