Monday, 01 September 2014
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Hot topic: climate satellite mission seeks TRUTHS

A proposed satellite calibration system could improve the accuracy of climate data and projections about the impact of global warming. Ellie Zolfagharifard reports.

As this year’s unseasonal weather swept in, so too did the emotive debates on climate change. It seems that no other topic divides the scientific community quite like the implications of global warming.

The debate boils down to one critical issue: the accuracy of climate data. Existing forecasts have some uncertainty, with estimates of global temperature increases ranging from 2 to 5°C by 2100. This range could be the difference between the need for minor improvements in flood defences and major new developments to prevent large-scale land losses.

The problem is with the calibration of Earth Observation (EO) satellites used to develop models that advise policy makers. In most cases these satellites are not accurate enough to distinguish between the predictions of different environmental models, placing scientists at odds in their conclusions.

‘One of the challenges for any satellite instrument is losing the link to the primary standard,’ said Dave Smith, a technical consultant at RAL Space, a research group at the UK’s Rutherford Appleton Laboratory. ‘When you’re dealing with reflected sunlight, the calibration sources that are flown are mainly white tile diffusers [which have a known reflectivity used to calibrate sunlight measurements]. The challenge we’ve found is that the white tiles themselves are subject to a very harsh orbit environment, so their calibration changes after launch.’

For policy makers to act with confidence, they require a traceable and internationally accepted standard that has a much greater accuracy. This is the goal of a project being proposed by the National Physical Laboratory (NPL) with partners from Imperial College London, Surrey Satellite Technology (SSTL) and RAL Space.

The mission, called TRUTHS (Traceable Radiometry Underpinning Terrestrial and Helio-Studies), would effectively act as a measurement laboratory in space. The project, which was first proposed in 2002 but until now has struggled to attract funding, aims to measure the radiation of the sun, Earth and moon 10 times more accurately than existing techniques. If successful, it will also be the first satellite to calibrate itself in orbit. Ultimately, the mission hopes to put the issue of climate change to bed once and for all.

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TRUTHS aims to measure the radiation of the sun, Earth and moon 10 times more accurately than existing techniques.

Climate satellites are currently calibrated by illuminating the optical system with calibration lamps or using sunlight reflected off diffusers. However, this process is fragile and prone to contamination that results in signal changes over long periods of time. Calibration can also be carried out by imaging a test area of known solar brightness locally. This approach has limited accuracy due to uncertainties in the atmospheric absorption and observation geometries.

The TRUTHS project is proposing to use a black “cavity”, a cylinder that absorbs almost 100 per cent of light and acts as a sensitive thermometer that can measure sunlight. It is based on a principle similar to the way national standards laboratories take measurements of radiation on the ground.

‘The concept is revolutionary, but the technology is evolutionary,’ said NPL’s Nigel Fox, who is leading the project. ‘The beauty of it is we’re not trying to create new technologies, but to optimise the use of existing concepts… for instance, we are using the same technology but reducing the risk of failure by cutting the number of movements made during calibration from seven to three.’

The beauty of it is we’re not trying to create new technologies, but to optimise the use of existing concepts

Sunlight striking the black cavity causes its temperature to increase. Scientists can then determine the amount of electrical current needed to raise the cavity’s temperature by the same amount and use this to calibrate the system. This method of calibration has a very low risk of contamination compared with conventional solar diffusers, and operating the cavity at cryogenic temperatures should improve its sensitivity further.

‘Typically satellite systems with minimal effort will offer 10 per cent accuracy in measurement,’ said Mike Cutter, head of the optical payload group at SSTL. ‘If you start using vicarious calibration, you can get to something like three per cent. What we’re talking about here, to provide confidence to the level require by the scientists, is to get down to 0.3 per cent accuracy. That still means we need to take measurements for about 19 years before we can be confident on any definitive statement about climate change.’

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The prototype TRUTHS reference block will use four black cavities and two radiant power detectors to help calibrate satellite readings

TRUTHS could also upgrade the measurements of satellites already in orbit by acting as a third-party calibration tool. The calibration satellite and existing satellite would observe the same point on Earth. The calibration satellite would then take a measurement and recalibrate the existing satellite to match. This would mean that scientists could use the calibrated satellites together and compare their measurements to provide more accurate climate change data over a longer period of time.

The group is currently assessing the criticality of the technologies needed to meet the objectives of the TRUTHS mission. ‘We’re trying to do what NPL currently does in a lab and translate that into something that would work in orbit,’ said Smith. ‘Once you launch it, you can’t go back and fix it. You can’t throw up half a tonne of equipment; you’ve got to turn that into a few kilograms. The mass and the power have to come right down. You have to try to fit several metres of lab, into a metre cubed of satellite.’

The cost for a full-scale mission would be somewhere in the tens of millions of pounds and the project has yet to secure this funding. But the UK Space Agency has now stepped in to fund a study of a potential scaled-down ‘TRUTHS-Lite’ mission, which could be undertaken on a small satellite platform such as SSTL’s TechDemoSat. If it goes ahead, it would not only provide some of the necessary baseline measurements for climate change, but also serve as a showcase for the UK commercial space sector. Fox hopes TRUTHS-Lite could attract enough funding to launch as early as 2016.

‘This is an opportunity for the UK to shine,’ said Fox. ‘We have the chance to show you can do very high-performance, state-of-the art measurements from space at a fraction of the cost. We can do it by using novel, low-cost solutions and achieve better performance than has ever been done before. It doesn’t have to cost the earth.’

In depth: TRUTHS technical specifications

• Accuracy 320-2,450nm < 0.3 per cent
• Lifetime up to five years
• Power consumption < 200W
• Mass < 175kg
• Designs demonstrated terrestrially
• Available for flight in < 3 years


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