Sentinel 6

The Sentinel-6/Jason-CS mission will consist of 2 spacecraft and will be the latest in a series of ocean surface topography missions that will span nearly three decades. They follow the altimeters on- board TOPEX/Poseidon through to Jason-3 (expected March 2015). Jason-CS will continue to fulfil objectives of the reference series whilst introducing a major enhancement in capability providing the operational and science oceanographic community with the state of the art in terms of platform, measurement instrumentation design thus securing optimal operational and science data return.

The programme is a part of the EC Copernicus initiative, whose objective is to support Europe’s goals regarding sustainable development and global governance of the environment by providing timely and quality data, information, services and knowledge.

The programme brings together:

ESA for development, procurement & early orbit activities;

EUMETSAT for mission management, ground segment, flight ops, contributing funding of the 1st satellite and participation in funding for the 2nd satellite;

NASA for the US payload and launcher procurement in addition to funding US science opportunities;

EC for funding the operations and participation in funding (with EUMETSAT) for the 2nd satellite;

NOAA are expected to provide US ground stations & operations services;

CNES for mission expertise and provision of the POD service.

The consortium plan to procure 2 satellites with the 1st planned for launch readiness in the 1st half of 2020 with the 2nd satellite 5 years later.

The first major commitment to funding was given by the ESA member states that approved the programme in June 2014 and in addition the European Commission funding is also fully secure.

The design is based on a platform derived from CryoSat-2 but adjusted to the specific requirements of the higher orbit. The principle payload instrument is the state of the art Poseidon-4, a Ku/C band radar altimeter with capability to retrieve geophysical parameters such as surface elevation, wind speed and SWH to a higher precision than previous missions. Supporting measurements to complement that of the altimeter are acquired from the GNSS-POD (Galileo and GPS compatible) and DORIS receivers for Precise Orbit Determination; The Climate Quality Advanced Microwave Radiometer (AMR-C) supplied by NASA-JPL provides high long-term stability path delay corrections for the altimeter.

An additional NASA-JPL GPS receiver, GNSS-RO, forms a secondary mission complementary to other satellites (Metop, Metop-SG and Cosmic, for example) dedicated to radio-occultation measurements allowing retrieval profiles of pressure, temperature and humidity as a function of altitude.

This paper describes the main mission objectives, requirements and how the satellite, its payload and prototype processors have been developed to meet those mission requirements.

The Sentinel-6/Jason-CS mission is the latest successor mission of the Jason altimetry mission series and part of the ESA Copernicus Programme. Its main objective is to continue the long term climate data record of sea level measurements that started in 1992 with the Topex/Poseidon mission. Sentinel-6 will operate from 2020 onwards in the same reference orbit as the current Jason-2 mission and its altimeter Poseidon 4 will retrieve sea level, wind speed and wave height measurements in near real time. Poseidon 4 is a new generation of altimeter based on the SARL instrument of the Sentinel 3 mission, operating in both SAR and low resolution mode continuously.

To ensure reliability and precision of wave height estimates for future satellite altimetry missions such as Sentinel 6, reliable parameter retrieval algorithms, that can extract significant wave heights up to 20 m have to be established. The retrieved parameters, i.e. the retrieval methods need to be validated extensively on a wide range of possible significant wave heights. Although current missions require wave height retrievals up to 20 m, there is little evidence of systematic validation of parameter retrieval methods for sea states with wave heights above about 8 m.

The usual significant wave height histogram has a peak around 2 m and there is less than 5% of data with significant wave heights above 5 m [1]. There have been rare events like Hurricane Katrina in 2005 [2] or the North Atlantic Storm Quirin in 2011 [3] during which significant wave heights reached up to 10 and 14 m respectively. These measurements have been evaluated for crossover consistency and compared to in-situ observations and numerical models using wind data [3]. Considering the rare occurrence of storms that produce the required conditions, it is necessary to establish ways of validating (SAR-) altimetry data, which do not rely only on available satellite data of previous missions or in-situ measurements for comparison.

This paper provides a definition of a set of simulated sea states with significant wave height up to 20 m, that allow simulation of radar altimeter response echoes for extreme sea states in SAR and low resolution mode. The simulated radar responses are used to derive significant wave height estimates, which can be compared with the initial models, allowing precision estimations of the applied parameter retrieval methods. Thus we establish a validation method for significant wave height retrieval for sea states causing high significant wave heights, to allow improved understanding and planning of future satellite altimetry mission validation.

[1] - P. C. Chu, G. Galanis and Y.-H. Kuo, "Statistical Structure of Global Significant Wave Height," in 20th Conference on Probability and Statistics in Atmospheric Science, Atlanta, 2010.

[2] - R. Scharroo, W. H. Smith and J. L. Lillibridge, "Satellite altimetry and the intensification of Hurricane Katrina," Earth and Space Science News, vol. 86, no. 40, p. 366, 2005.

[3] - J. A. Hanafin, Y. Quilfen, F. Ardhuin, J. Sienkiewicz, P. Queffeulou, M. Obrebski, B. Chapron, N. Reul, F. Collard, D. Corman, E. B. De Azevedo, D. Vandemark and E. Stutzmann, "Phenomenal sea states and swell radiation: a comprehensive analysis of the 12-16 February 2011 North Atlantic storms," Bulletin American Meterological Society, pp. 1825-1832, 01/2012.

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 Space Segment development along with Astrium GmbH as a prime contractor.

The main payload of the Sentinel-6 satellite is the Poseidon-4 radar altimeter that has evolved from the altimeters on-board the Jason satellites (Poseidon-2 of Jason-1, Poseidon-3A of Jason-2 and Poseidon-3B of Jason-3). Poseidon-4 also inherits the Synthetic Aperture Radar (SAR) High Resolution Altimeter mode of CryoSat-2 SIRAL and Sentinel-3 SRAL now proven to reduce errors in elevation and SWH retrieval over ocean. Furthermore, Poseidon-4 will be the first radar altimeter embarked on a satellite that includes improved digital and radio frequency hardware and, in particular, open burst Ku-band pulse transmission (an operation currently termed the interleaved mode), that performs a near continuous transmission of Ku-band pulses, that will allow the simultaneous processing of the measurements to obtain High Resolution along-track (HR or SAR) and Low Resolution along-track (LR or LRM) data. As previous satellite radar altimeters, the Poseidon-4 transmits C-band pulses in order to retrieve a correction for ionospheric path delay.

isardSAT is responsible for the development of the Ground Prototype Processor for the Poseidon-4. This prototype processes all the chains starting from the Instrument Source Packets, and producing L1A and L1B-S (only for HR processing) and Level 1B (both calibrated LR and HR data processing). The prototype has been verified using simulated data generated by the Sentinel-6 mission performance simulator and also using in-orbit CryoSat data adapted in format to Sentinel-6, provided by ESA.

In October 2014, ESA delivered a few P4 data produced by the ESTEC Sentinel-6 simulator over ocean, and processed by the P4 GPP, in the all L1 data types: L1A HR (or SAR), L1B-S HR (or SAR), L1B HR (or SAR), and L1B LR (or LRM).

This paper will present the Ground Processor Prototype developed for Sentinel-6 Poseidon-4, and the results obtained (including some post-processing as retracking) of the data delivered by ESA to users. Special attention will be given to the new features of the processing chain compared to previous altimeters, as:

assessment of performance improvement thanks to the interleaved mode;

un-correction of the Range Migration Correction (RMC) performed on-board in order to reduce the data rate;

Doppler ambiguity effects, due to the low PRF, onto the geophysical retrievals;

stack masking to remove clutter effects such as Doppler ambiguity, land contamination, aliasing, etc.;

weighting applied to the Doppler beams before the multi-looking to correct the different echo shapes as a function of the incidence angle;

Amplitude Compensation and Dilation Correction (ACDC) stacking algorithm;

reconstruction of the waveform scaling factor in order to be able to compute the surface backscatter. 

The Sentinel-6 mission is a multi-partner programme continuing the Jason satellite altimeter data services beyond the Jason-2 and Jason-3 missions.

The Jason-CS/Sentinel-6 mission programme consists of two identical satellites flying in sequence to extend the climate record of sea level measurements accumulated by the Topex-Poseidon, Jason-1, Jason-2 and Jason-3 missions beyond 2030.

The Sentinel-6 mission intends to maintain these services in a fully operational manner.

A key feature is the simultaneous pulse-limited and synthetic aperture radar processing allowing direct and continuous comparisons of the sea surface height measurements based on these processing methods and providing backward compatibility.

The Sentinel-6 mission will also include Radio Occultation user services.

The threat of sea level rise to coastal communities is an area of significant concern for Governments and policymakers on the global stage. The European flagship Copernicus programme has been established to provide environmental information to understand how our planet and its climate are changing, the role of human activities in these changes and how these will influence our daily lives.  The well-being and security of future generations depend on actions and decisions on environmental policies that are being made now. To ensure decision makers and policy makers have timely and easy access to the best information on aspects of societal relevance such as sea level rise, a world-leading Copernicus Earth Observation capability together with dedicated services generate relevant knowledge products is being established now. 

Within Copernicus and together with other satellite altimeter missions,  the twin satellite polar-orbiting Sentinel-3 and the Sentinel-6/Jason-CS reference missions will work together to monitor global sea level changes and ensure a complete record of sea level for the coming decades.  Sentinel-3A is due for launch in February 2016 (followed by a second satellite in 2018). Two Sentinel-6/Jason-CS cooperative missions (with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union) will be launched sequentially into the ‘Jason orbit’ (up 66^o latitude) to overlap and continue the services initiated by Topex/Posiedon and maintained by the Jason reference altimeter series. The launch of the A and B satellites are planned for 2020 and 2025, respectively.

This paper provides a review of how the Sentinel-3 and Sentinel-6 missions will work in synergy to monitor sea level changes.

The Sentinel-6 mission will carry a new radar altimeter, flying on the Jason-series 10 day repeat orbit, to provide high precision and timely measurements of the topography of the global ocean. The Sentinel-6 altimeter will operate a so-called “interleaved” mode, operating “Low Rate” and SAR mode at the same time, so offering perfect continuity with the measurements from the previous Jason missions, whilst also providing global high resolution and high precision coverage in SAR mode.

In this presentation we discuss the important new contribution that Sentinel-6 will make in terms of measurements of ocean winds and waves, for research and operational applications. From a research perspective the higher along track resolution will support investigations into small-scale spatial variability in ocean waves, especially important close to the coast where sea floor bathymetry, coastal topography and coastal currents all combine to generate significant variability in wave fields. The coastal zone is becoming increasingly important for operational applications, for the planning, building and maintenance of coastal defences, and also for offshore renewable energy installations, particularly offshore wind farms. Accurate and high-resolution real-time and climatological wave and wind information are needed to support these operations.

We also consider the impact that Sentinel-6 wave and wind data will have on a global scale, and review how large scale modelling and forecasting systems, such as those operated by ECMWF, are being developed so they will make best use of these new data sets when they become available.