The threat of sea level rise to coastal communities is an area of significant concern to the well-being and security of future generations. Environmental policy actions and decisions affecting coastal states are being made now. Given the considerable range of applications, sustained altimetry satellite missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to address the needs of the European Copernicus programme for precision sea level, near-real-time measurements of sea surface height, significant wave height, and other products tailored to operational services in the climate, ocean, meteorology, and hydrology domains. It is designed to provide enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon (T/P) mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU), with additional technical support provided by CNES. It includes two satellites that will fly sequentially (separated in time by about 5 years). The first satellite, named Sentinel-6 Michael Freilich, launched from Vandenberg Air Force Base, USA on 21 November 2020. The main payload is the Poseidon-4 dual frequency (Ku/C-band) nadir-pointing radar altimeter providing synthetic aperture radar (SAR) processing in Ku-band to improve the signal through better along-track sampling and reduced measurement noise. The altimeter has an innovative interleaved mode enabling radar data processing on two parallel chains, one with the SAR enhancements and the other furnishing a "Low Resolution Mode" that is fully backward-compatible with the historical T/P and Jason measurements, so that complete inter-calibration between the state-of-the-art data and the historical record can be assured. A three-channel Advanced Microwave Radiometer-Climate Quality (AMR-C) developed by NASA JPL provides measurements of atmospheric water vapour that would otherwise degrade the radar altimeter measurements. An experimental High Resolution Microwave Radiometer (HRMR) is also included in the AMR-C design to support improved performance in coastal areas. Additional sensors are included in the payload to provide Precise Orbit Determination, atmospheric sounding via GNSS-Radio Occultation and radiation monitoring around the spacecraft.

This presentation provides an overview of the Sentinel-6 Michael Freilich Mission highlighting the current status of the mission, satellite, ground segment and products, and includes an outlook for the mission.

The main payload of the Sentinel-6 Michael Freilich (S6-MF) is the Ku-band Poseidon-4 Radar Altimeter (Poseidon-4) that has been in orbit since Nov 2020 and was developed based on the in-orbit performances of the Jason-2 mission in addition to, for the first time, requirements concerning Global Mean Sea Level (GMSL) drift. The design also took into account the in-orbit performance of the CryoSat-2 mission and the CryoSat-2/Sentinel-3 altimeter definitions, and explored further enhancements that now demonstrate the capability for both unfocussed and focussed processing capabilities for future applications. The Poseidon-4 design and its performances has provided the basis for future missions, such as CRISTAL and potentially for the Sentinel-3 Next Generation satellites.

In order to meet the requirements on drift the Poseidon-4 has in its design and operations the capability to calibrate the Level 1b product at 20 Hz in terms of power and delay, power and phase at the burst level and provide the capability to adapt the on-board chirp in order to counter drifts of the instrument range response shape (in particular, the main-lobe width, for example). The design has established the functionality to expand on the full control transponder commanding capability for use also with corner reflectors which will be a game change for global external calibration enhancement once fully demonstrated and characterised. Furthermore, the altimeter embarks a mode that reduces data volume by a factor 2, labelled Range Migration Compensation (RMC) that has now been fully demonstrated in orbit.

The presentation will show the high level design and establish the in-orbit characterisation of both the nominal and redundant and the in-orbit internal and external calibration and some geophysical retrievals that demonstrate the performance of the RMC, noting how both unfocussed and focussed SAR has been demonstrated, opening up a new range of applications.

The Sentinel-6 B satellite is due for launch 2025 (TBC) and is undergoing final testing before being placed in storage in summer 2022. The presentation will highlight any key results from the on-ground testing.

The success of the S6 concept and development, going back to original concept of 2007, and its international payload demonstrates the excellent partnership, collaboration and team spirit of CNES, EUMETSAT, ESA, NASA and NOAA and European satellite, payload and ground segment industries.

Acknowledgement

The authors wish to acknowledge the support of the teams of Aresys, isardSAT and CLS, developing the specifications for the ground processor my means of prototyping, European industry supplying the satellite and payload, Airbus (Friedrichsafen, Germany), Thales Alenia Space (Toulouse, France) , Ruag (Vienna, Austria), TSA (Elancourt, France) and partner Agencies CNES, EUMETSAT, NASA-JPL and NOAA providing the system and mission performance expertise under the Sentinel-6 Mission Performance Working Group (MPWG).

The Sentinel-6 Michael Freilich is the newest reference satellite altimetry mission. It includes a radar altimeter, a precision orbit determination suite and a radiometer to measure wet tropospheric path delay. The AMR-C radiometer on Sentinel-6 includes two new innovations compared to the prior Jason-series missions. The first innovation is the inclusion of a secondary calibration system external to the radiometer that is used to stabilize the wet path delay measurement to 0.7mm/yr over 5+ years. The second is a high-frequency radiometer (termed HRMR) with less than 5km spatial resolution that improves the measurement near land and sea ice boundaries. We will present the performance of the AMR-C and HRMR radiometer systems over the first 1.5 years of in-flight operation. We show that the secondary calibration system provides an unprecedented level of stability and is a new standard for calibration stability for altimeter missions. We also show that the HRMR is providing path delay with less than 1cm uncertainty to within 5km from land, a dramatic improvement from prior altimeter missions.

Sentinel-6 PDAP products assessment over ocean


Launched on 21 November 2020, Sentinel-6 Michael Freilich is a Copernicus satellite designed to ensure the continuity to the mean sea level climate time series measured by the TOPEX/Poseidon and Jason satellites since 1992. The main payload carried on-board Sentinel-6 is the Poseidon-4 (POS4) dual frequency radar altimeter. POS4 uses a 9 kHz Pulse Repetition Frequency and an innovative interleaved chronogram which allows the optimization of the number of measurements acquired. Like this interleaved mode enables the acquisition of two modes in parallel: a Low Resolution Mode (LR) which aims in extending the legacy of the mean sea level record, and a High Resolution Mode (HR) or Synthetic Aperture Radar Mode (SAR) that significantly improves the along-track spatial resolution and reduces measurement noise. Downlink of both LR and HR data is enabled thanks to the on-board Range Migration Correction (RMC) algorithm which allows HR data volume reduction.

Started right after the Launch and Early Orbit Phase, the Commissioning Phase aimed in calibrating and validating Sentinel-6 data after they are processed on-ground. It ended on 29 November 2021 with the dissemination of all Sentinel-6 products to the operational users after a successful In-Orbit Commissioning Review.

In this presentation we focus on the calibrated and validated altimetry products processed and disseminated by the EUMETSAT PDAP ground segment. All the NRT/STC/NTC timeliness products are assessed, and particular attention is given for NTC product quality assessment. Both LR and HR performances are addressed.

POS4 instrument performances from the PDAP Long Term Monitoring are briefly summarized and it is shown that even if POS4 presents a sensitivity to temperature all the instrument specification requirements are met. For all latencies, it is shown that the LR requirements are fulfilled for all latencies and that data products show good continuity with Jason-3. GMSL inter-mission bias is well characterised after about 6 months and GMSL continuity with Jason-3 is expected, although a longer time series is needed to fully demonstrate this. It is also shown that HR requirements are met for all latencies, except for SWH that are overestimated at high wave heights due vertical wave velocity impacts (a known issue in SAR altimetry). HR RMC is also assessed and presents performances very similar to the HR RAW data performances over open ocean and in coastal regions.

This work presents the first calibration results for the Sentinel-6 using the permanent CDN1 transponder Cal/Val facility during its tandem phase with Jason-3. Evaluation has started on 18-Nov-2020 and continues without interruption till today. The CDN1 Cal/Val is part of the ESA Permanent facility for Altimeter Calibration and serves as a dedicated external calibration source for multi-mission (S3A, S3B, Jason-3, CryoSat-2) satellite altimetry.
At the time of writing, Sentinel-6 has passed over the CDN1 Crete transponder more than thirty times. For these passes, the Sentinel-6 high-resolution Short Time Critical and Non Time Critical products have been analyzed by an international working group headed by ESA to assess performance of the Poseidon-4 altimeter. In particular, the instrument performance in the sense of absolute range bias and datation bias were assessed together with the instrument capabilities in terms of achievable resolution in both the along- and the across-track direction. Diverse processing methodologies and algorithms were implemented within this group to cross-validate results.
The performance of Poseidon-4 altimeter is here presented, considering both the nominal and the redundant instrument. The performance of Poseidon-4 altimeter is also evaluated against the reference Jason-3 Poseidon-3 altimeter, taking full advantage of this tandem mission for both satellites. These preliminary results show that the Sentinel-6 altimeter performance is within mission requirements .