ENVISAT, the European Space Agency鈥檚 environmental monitoring satellite, last week started beaming back striking images of Earth. But the spacecraft may not live up to its promise, say scientists at Britain鈥檚 National Physical Laboratory in Teddington, Middlesex. They claim the optical sensors on the craft are not accurate enough to provide definitive data on climate change. NPL is suggesting a new space mission to correct what it sees as Envisat鈥檚 shortcomings.
Envisat has a battery of sensors designed to capture data on ocean colour, sea surface temperatures and wave heights, for example. But Nigel Fox, a measurement scientist at NPL, says some instruments may not have been calibrated well enough. 鈥淢ost if not all optical instruments degrade in storage鈥攁nd sometimes that鈥檚 over two or three years before lift-off,鈥� he says. And blast-off makes matters worse. Sensors on NASA鈥檚 TERRA probe, for instance, had measurement errors that grew from 3 per cent on the ground to 10 per cent in orbit.
Such errors are grist to the mill of greenhouse sceptics, and feed political arguments over the extent of climate change. NPL鈥檚 answer is for ESA to launch a new space probe that will let Envisat calibrate its instruments accurately. Called TRUTHS, it would carry a cryogenic radiometer鈥攖he standard instrument for measuring radiant power鈥攚hich Envisat lacks. Once in orbit, TRUTHS鈥� measurements of radiance from reference spots on Earth, such as baking deserts, can be compared with data from Envisat to correct its readings. This would be crucial for measuring light reflected from the oceans鈥攁 measure of CO2 absorption. Fox says TRUTHS could reduce Envisat鈥檚 error margin by a factor of 10.
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But ESA bridles at the suggestion that Envisat might not be up to its task. Instead of critisising Envisat, spokesman Evert Attema says NPL should be working with ESA to improve future missions. While TRUTHS is being considered for an upcoming ESA mission, Attema says it faces stiff opposition from 27 other possible projects.