TRUTHS: a benchmark mission for decadal climate – Facilitating its urgent implementation

National Physical Laboratory with Surrey Satellite Technologies Ltd (SSTL)

Project Funded in the CEOI-ST 7th Call for EO Technologies.

TRUTHS Overview The establishment of an observational climate benchmark data set of sufficient accuracy to enable the unequivocal detection of climate change with the ability to constrain and test climate forecast models on a decadal time scale is one of the key challenges laid down by the international climate science community.  The UK led TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) and its US sister, CLARREO (Climate Absolute Reflectance and Refractivity Observatory) are mission concepts proposed to address this exacting issue. TRUTHS’ primary goal is to provide benchmark measurements of both incoming (solar) and outgoing (reflected solar) radiation with sufficient spectral resolution and accuracy to detect the subtle changes in as short a timescale as possible (~12 yrs) - limited by natural variability of the climate system. In summary, measuring global spectrally resolved (10 nm) Earth radiances, continuously sampled (spectrally and spatially) with GIFOV of ~<100 m from 340 to 2340 nm and the corresponding solar spectral irradiances both with uncertainties to SI units of <0.3%.

Although not selected in the ESA EE8 call, TRUTHS received a strong recommendation of support and encouragement that an early implementation should be explored as part of a wider international collaborative effort.  In support of that goal this project will look towards developing optimised baseline designs for the missions core elements: Imaging spectrometer (Earth and Sun view) and the on-board calibration system together with key aspects of the satellite bus impacting the observational requirements, with the aim to establish a set of baseline building blocks with outline designs to facilitate the establishment of international partnerships for early implementation of TRUTHS

The design study will trade-off complexity/risk/cost against science drivers ensuring that the core objectives, climate benchmark are achieved whilst maximising the opportunity for secondary objectives.  The study will also evaluate the performance advantages achievable from using TRUTHS as a reference calibration satellite for sensors with Sentinel 2 & 3 like characteristics and also as a ‘calibration mother ship’ for low cost constellations.

TRUTHS: Increasing TRL of the Cryogenic Solar Absolute Radiometer (CSAR) and the in flight calibration system to level 5/6

NPL Management Ltd with Airbus DS Ltd

Project Funded in the CEOI-ST 8th Call for EO Technologies

Flagship Project

To enable CSAR to provide the radiometric calibration of the Earth & Sun viewing imaging spectrometer, a novel calibration procedure utilising monochromatic radiation has been devised, mimicking that used terrestrially at National Metrology Institutes such as NPL. This full calibration system will be designed and built to space consistent constraints: scales, materials, mechanism types etc and fully tested in vacuum with the new HPSC cooled CSAR. This calibration system will look to demonstrate calibration uncertainties of <0.3% for spectral radiance and irradiance over the Visible to SWIR spectral range, greater than a factor ten better than has been achieved for any previous EO mission (although downgraded from that achievable using optimised terrestrial technologies (0.02%)) resulting in a TRL of 5/6.

The conclusion of this project will mean that all key technologies and concepts needed for TRUTHS will be at TRL 5/6 with little risk to implementation remaining. In addition key sub-systems like the HPSC and VANTA black coatings will have a wider range of applications in the space sector, Earth Observation and space science leading to economic return to the UK. The HPSC, will be able to continue and expand the market lead of its predecessor the 50-80K cooler, emphasising its strengths over competing technologies such as pulse tubes from elsewhere in Europe.