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Climate change study searches for the TRUTHS

 SSTL is about to embark on a pioneering study into acquiring more accurate climate data from satellites, that will help us better understand the impact of climate change.

SSTL is about to embark on a pioneering study into acquiring more accurate climate data from satellites, that will help us better understand the impact of climate change.

The study, funded by the UK Space Agency through its Centre for Earth Observation Instrumentation (CEOI) and conducted with partners from Imperial College London, The National Physical Laboratory and Rutherford Appleton Laboratory, will look into the feasibility of a mission called TRUTHS (Traceable Radiometry Underpinning Terrestrial and Helio-Studies).

TRUTHS and its US sister mission CLARREO (Climate Absolute Radiance and REfractivity Observatory) are both aiming to address the need for accurate and scientifically traceable measurements to get an idea of the extent and potential impact of climate change. The only way to ensure that measurements are accurate, and stay accurate is by performing the best possible calibration. Currently calibration is carried out using several techniques:

  • Illumination of the optical system using calibration lamps (typically tungsten).

  • Illumination of the optical system using sunlight reflected off diffusers.

  • Vicarious calibration involving imaging a test area of known solar brightness that can be measured locally.

The first two approaches are either fragile or prone to contamination that results in signal changes over long periods of time. The third approach is not particularly convenient and has limited accuracy due to uncertainties in the atmospheric absorption and observation geometries. This means that our current estimates of global temperature rises differ significantly, presenting difficulties in mitigating climate change and planning for the future.

Image courtesy of NASA.
Image courtesy of NASA.
What makes TRUTHS different is that it would use a black cavity, operating at cryogenic temperatures, and acting as a highly sensitive thermometer that can measure sunlight that would be incident on the optical system. The sunlight incident on the cavity will cause the temperature of the black cavity (or black body-an ideal absorber and emitter) to rise. This rise in temperature is then compared to the amount of electrical current that is required to raise the temperature of the cavity by the same amount. This allows one to work out the brightness of the sunlight incident on the cavity to high accuracy. Dispersing the sunlight into spectral bands (colours) allows detailed calibration of a wide range of instruments including Earth viewing imaging cameras. The cavity is much less sensitive to contamination than conventional solar diffusers and much more robust than calibration lamps and so it works better and for longer.

TRUTHS may also be able to upgrade the measurements of satellites already in orbit by acting as a third-party calibration tool itself. The calibration satellite and existing satellite would ‘look’ at the same point on Earth. The calibration satellite would then take a measurement and recalibrate the existing satellite so that its measurement matches. This would mean that scientists could use the calibrated satellites together and compare their measurements to provide even more accurate climate change data.

SSTL’s Optical Payload Group will assess the requirements for the proposed system and the criticality of the technologies needed to meet the scientific objectives of the TRUTHS mission. It will also provide recommendations as to which of these technologies are most in need of demonstration in orbit on a scaled down ‘TRUTHS-Lite’ mission to secure funding for the full mission in the future. TRUTHS-Lite would be a low-cost and rapid development mission to provide baseline measurements and provide confidence and in-orbit validation of the key technologies, much like as is undertaken on SSTL’s TechDemoSat-1.

SSTL’s expertise in developing state-of-the-art optical systems means that it is well equipped to participate in the study. SSTL’s CHRIS imager, for example, provides the highest spatial resolution of any hyperspectral system. SSTL also built the Disaster Monitoring Constellation and has extensive experience building, integrating and launching technology demonstration missions.






25 October 20120 Comments1 Comment

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