Spinning Truths

A spacecraft that could service Earth observation satellites in orbit, ensuring their data is accurate, is being proposed by UK researchers.

Information produced by these satellites is used to determine the causes of global warming and assess the threat from climate change. They measure solar radiation hitting and being reflected off the Earth to produce a diagnosis of the planet’s health.

But researchers cannot be sure that the instruments used to gather this data are accurate enough once the satellites have gone through a violent launch and been in space for some time.

This uncertainty allows climate change sceptics to dispute the data produced, said Dr Nigel Fox, head of science in the optical radiation measurement group at the National Physical Laboratory. Accurate data would provide unequivocal evidence to support national and international legislation, he said.

As the satellites cannot be brought back to Earth, Fox is proposing to build a calibration laboratory that can be launched into orbit, against which all other satellites can be tested. As it is in orbit, the device, called Truths (Traceable Radiometry Underpinning Terrestrial and Helio Studies) would view the sun and Earth through the same atmospheric conditions as the instruments on the satellites.

‘We use the method you would use on the ground to establish calibration, and take it into orbit to do all the tests in space,’ said Fox.

The device is based on a process called electrical substitution radiometry, in which an absorbing surface is used to compare the heating effect of optical power with electrical power. Optical radiation is applied to the surface and is absorbed by it, raising its temperature. This process is then repeated using electrical power, until the surface reaches the same temperature as under the optical radiation.

This provides an accurate measurement of the optical power. The process is carried out at cryogenic temperatures to increase the specific heat capacity of the material.

This method would be used to regularly calibrate an optical imager in space, by measuring the power of a monochromatic beam of solar radiation and then measuring the detector’s response to that power.

‘Once we have calibrated the imager, we can use it to view spatially uniform desert sites on Earth, which can be viewed by other satellites. We can then transfer the calibration to those satellites,’ said Fox. This means satellites would not need their own calibration and monitoring tools, reducing costs.

The NPL team hopes to secure funding for the e100m (£70m) proposed mission.