THE Ocean Surface Topography Jason-CS Mission Payload Design and Development
Cullen, Robert; Francis, Richard
ESA-ESTEC, NETHERLANDS

This paper describes the ocean surface topography Jason-CS mission in terms of its payload definition, current available options, trade-offs and expected performances meeting mission objectives.
TOPEX/Poseidon (NASA/CNES) launched in 1992, was the first of a series of ocean surface topography missions. It was followed by Jason-1 (NASA/CNES) launched in 2001 and the operational Jason-2 (EUMETSAT & NOAA with development support from CNES and NOAA), launched in 2008. Jason-2 will be succeeded by Jason-3 which has the same design as Jason-1 and -2, and is under procurement with a launch scheduled in 2014. Jason-3 will be the last of this sequence of operational ocean surface topography missions. Jason-CS (Continuity Service) will continue to fulfil the mission objectives of this successful series of missions but represents a change in design and capabilities that will provide the operational and science oceanographic community with the state of the art in terms of platform, measurement instrumentation design securing optimal operational and science data return.
The Jason-CS mission is foreseen to be part of the Global Monitoring for Environment and Security (GMES) initiative, whose overall objective is to support Europes goals regarding sustainable development and global governance of the environment by providing timely and quality data, information, services and knowledge.
The Jason-CS development, about to enter its phase B2 study stage, is based on the CryoSat-2 platform and builds on the heritage of the Jason-2 and -3 payloads and those of CryoSat and Sentinel-3. The development brings together several organizations and agencies: ESA will be responsible for development, procurement and early orbit activities, while EUMETSAT and NOAA will be responsible for operations. CNES and NASA are heavily involved in the development and it is foreseen that, like the other GMES satellites (the Sentinel series), the European Commission will fund the operations. The consortium of agencies plan to procure two Jason-CS satellites, with the first of these, Jason-CS A, planned for launch in 2018 with the second, Jason-CS B, planned for launch in 2023 ensuring continuity of the long-term ocean surface topography climate data record until the late 2020s.
The Jason-CS spacecraft will be based on a platform derived from CryoSat-2 but adjusted to the specific requirements of the higher orbit. The principle payload instrument is a radar altimeter (based on the Sentinel-3 SRAL) with a number of design and performance improvements.
The radar altimeter, named Poseidon-4, is a normal incidence Ku and C band pulse-width limited radar altimeter with the capability of acquiring measurements of a surface allowing synthetic-aperture processing that improves along-track resolution and reducing range and Significant Wave Height (SWH) noise as a function of SWH. Now with the improvements of the SAR method over pulse-width limited processing clearly demonstrated, see [1], [2] and [3], for example, industry have investigated the possibility of operating both SAR and pulse-width limited modes at the same time over all open ocean improving science return. With this facility the reference mission is secured operationally whilst providing science users with a unique global data set with reduced uncertainties that maybe assimilated into operational modelling in time for the second mission; if not before. In addition, the instrument design will rely on state of the art digital technology improving on-board calibration strategy whilst reducing the time necessary to tune recurrent radio frequency units (RFU) during instrument assembly. Retrieval of the key geophysical parameters (surface elevation, SWH and wind speed) from the altimeter requires a number of other sources of information that is provided by supporting instruments:
1. a DORIS receiver (recurrent from CryoSat-2) will provide data to enable precise orbit determination.
2. A Microwave Radiometer is needed to provide the retrieval of total water vapour content necessary to correct the altimeter data. With regard to the latter, NOAA is expected to provide the climate quality Advanced Microwave Radiometer (AMR-C) which is a 3-frequency radiometer developed by NASA-JPL based on the AMR embarked on Jason-2 and -3 though improved to reduce measurement drift.
3. A recent addition to the payload complement is the development of a High Resolution Microwave Radiometer (HRMR). Currently under feasibility assessment, the instrument is expected to operate at centre frequencies of 89.9, 150 and 183.31 GHz in order to allow retrieval of the altimeter wet tropospheric correction closer to the coast that previously possible.
4. Orbit tracking data are also provided by a GPS receiver (recurrent from Sentinel-3b), and a Laser Reflector supporting POD.
5. Star trackers are required not only for attitude control but to improve the knowledge of platform pointing for which the altimeter SAR data is sensitive.
6. An additional GPS receiver provided by NOAA (developed by NASA/JPL) is also expected to be embarked, and is being developed to provide additional radio-occultation measurements [4].

The Jason-CS series of missions is presented and the international partner agencies in addition to European and US industry are converging on a platform design that will improve on the existing reference mission whilst securing the long-term climate data record.

References:
[1] L. Phalippou and F. Demeestere, 'Optimal re-tracking of SAR altimeter echoes over open ocean: from theory to results for SIRAL2', OSTST 2011 meeting, San Diego, 19th-21st October 2011. Available from www.aviso.oceanobs.com.

[2] C. Gommeninger, 'Improved altimetric accuracy of SAR altimeters over the ocean: observational evidence from Cryosat-2 SAR and Jason-2', OSTST 2011 meeting, San Diego, 19th-21st October 2011. Available from www.aviso.oceanobs.com.

[3] K. A. Giles, D. J. Wingham, N Galin, R. Cullen and W. H. F. Smith, 'Precise Estimates of Ocean Surface Parameters from the CryoSat-2 altimeter 'SAR' and Pulse-limited Echoes', in preparation.

[4] Kursinski et al. 1997. Observing the Earth's atmosphere with radio occultation measurements using the Global Positioning System. J. Geophys. Res. 102:23.429-23.465