CALIBRATION OF THE GOME INSTRUMENT FOR ERS-2

The Global Ozone Monitoring Experiment (GOME) is an optical spectromet er to be flown on the second European Remote-Sensing Satellite, ERS-2( 1). Its main task will be to measure the ozone content of the atmosphe re and to monitor its long-term variation. Ozone is a trace gas of vit al importance to mankind, as it absorbs the harmful ultraviolet radiat ion emitted by the Sun and prevents it from reaching the Earth's surfa ce. This absorption occurs in an ozone layer concentrated in the strat osphere, at altitudes of between 18 and 40 km, which is threatened by human activities, in particular by the release of Chloro-Fluoro-Carbon s (CFCs) used in spray cans, refrigerators and foams, and as a cleanin g agent. Ozone depletion is believed to be taking place at a rate of a bout 0.3% per year. The reliable measurement of such a small global tr end against the background of normal spatial and temporal variability, places high demands on the instrument's accuracy and stability. To me et with these requirements, the instrument needs thorough calibration and characterisation, both on the ground and in orbit. The breadboard model of the GOME instrument - which is functionally fully representat ive including the on-board means of calibration - has been subjected t o a dry calibration run on the ground, and the test results are report ed here.