SUPPLEMENT CALIBRATION
Calibration in space: the key to accurate climate models
Climate models are used to indicate how global temperatures are likely to change, however predictions can vary widely. A new satellite mission has, however, been designed where the instruments are calibrated directly against an on-board primary standard, making measurements more accurate and improving the overall quality of climate data from space. Dr Nigel Fox from the National Physical Laboratory explains
To help address this, a team of
A team of international scientists led by the National Physical Laboratory, UK, have designed a satellite mission with the ability to make measurements ten times more accurate than currently possible
A
s the only method we have of indicating how global temperatures
are likely to change in the future, climate models are vital to our ability to mitigate and adapt to climate change. Despite their importance, however, the measurements that underpin and constrain these models have significant uncertainties that limit their usefulness in developing effective adaptation strategies. The models rely on data acquired
through a range of complex measurements – such as ice cover, cloud cover, sea levels and temperature, chlorophyll and the radiation balance (incoming to outgoing energy). While satellites can measure these variables regularly and on a global scale, there are two major problems. Firstly, we have to detect small changes
in the levels of radiation or reflection from a background that naturally fluctuates. This means measurements must be made spanning decades to identify a trend, which is beyond the life of any single satellite mission, meaning truly consistent monitoring cannot easily be achieved. Secondly, satellites tend to lose their calibration during launch, and this drifts further over time. Whilst ground based instruments would be regularly calibrated to ensure confidence in the measurements, this
S4 MARCH 2016 | INSTRUMENTATION: SUPPLEMENT is much harder in space.
THE SATELLITE MISSION The margin of uncertainty in our measurements, coupled with uncertainty in assessing how much greenhouse gas will be emitted/sequested, means
international scientists led by the National Physical Laboratory, UK, have designed a satellite mission with the ability to make measurements ten times more accurate than currently possible. The instruments on the TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) satellite will be calibrated directly against an on-board primary standard – an instrument called a CSAR (Cryogenic Solar Absolute Radiometer). This compares the heating effect of optical radiation with that of electrical power, transferring all the difficulties associated with existing space based optical measurements (such as drift) to more stable electrical SI units. In effect, this mimics the traceability chain that would be carried out on the ground. This calibration system not only
“The instruments on the TRUTHS (Traceable Radiometry
Underpinning Terrestrial- and Helio- Studies) satellite will be calibrated directly against an on-board primary standard... The calibration system not only makes TRUTHS own measurements
extremely accurate, but by comparing its measurements to other Earth observation missions, it can transfer its calibration accuracy to them”
that predictions of the extent of climate change can vary widely. The Intergovernmental Panel on Climate Change tries to get round this by modelling multiple ‘scenarios’ based on different mitigation strategies. Whilst demonstrating good scientific method, there’s a distinct lack of agreement in these projections that makes it difficult to assess the required mitigation actions to prevent the potentially devastating effects of global warming. Clearly, if we are to really tackle the issue of climate change, we need better ways of forecasting.
makes TRUTHS own measurements extremely accurate, but by comparing its measurements to other Earth Observation missions, it can transfer its calibration accuracy to them and improve the overall quality of climate data from space. The TRUTHS mission can therefore
upgrade the global Earth Observation system, making the measurements that underpin our climate models more accurate. This would help to improve our forecasts, allowing scientists to determine which models are ‘getting it right’ in forecasting the likely impact of
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