Starlight eyes the ozone

Engineers are in the final stages of assembling a unique satellite instrument which will keep an extra-sharp eye on the Earth's fragile ozone layer.

Developed and built by Matra Marconi Space at Filton, near Bristol, the GOMOS device will use starlight for the first time to measure the thinning rate of the atmospheric layer which protects against harmful ultraviolet radiation.

It is one of 10 sophisticated instruments for the European Space Agency's Envisat mission scheduled for a 1999 launch.

GOMOS will routinely find and track around 350 stars daily in a four- year mission. The stars form fixed point light sources and offer unprecedented measuring accuracy in a complex process which determines the vertical profile of ozone in the atmosphere.

The device works by taking a first reading of a star while it appears above the Earth's atmosphere. A second reading is taken as the star "sets" through the Earth's atmosphere.

The difference in reading provides an extremely accurate measurement of atmospheric ozone levels.

Gomos will produce a global map of vertical ozone distribution which will be beamed daily to ground stations. Changes in concentration levels of less then 0.1% will be detectable. Previous missions have used the setting sun as the light source for measurement.

A key element of GOMOS is an extremely precise flat steerable mirror measuring 22 inches by 25 inches and costing pounds 5million. At its heart is a mechanism which finds and locks on to the target stars.

The mirror reflects their starlight through a telescope to a series of ultra-sensitive detectors and recorders

Matra Marconi Space is responsible for the key front-end package totalling pounds 9million. It includes the mirror, steering mechanism and electronics.

It will go to Toulouse for integration with the main GOMOS structure for final flight testing early next year.

Concern over the ozone layer grew more than 10 years ago when variations were first noted above Antarctica. The so-called ozone "hole" now covers thousands of square kilometres.

GOMOS will enable scientists to keep a close watch on long term trends and help assess the extent of possible environmental risks. It derives its name from its function - Global Ozone Monitoring by Occultation of Stars.

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