Hipparcos as Input Catalogue for the Gomos Envisat Instrument

GOMOS is being developed as one of the ESA instruments to be own on-board the ENVISAT environmental satellite. This instrument is based on the star occultation principle. Using a medium resolution spectrometer, GOMOS will measure the concentration prooles of, and monitor the trends in, Ozone and other trace gases. The input catalogue of GOMOS, which could include some 1000 stars, is currently being prepared using the Hipparcos and Tycho Catalogues as reference. 1. THE GOMOS MISSION In response to the growing concern about the ozone equilibrium in the stratosphere, the GOMOS instrument (Global Ozone Monitoring by Occultation of Stars) has been included in the ENVISAT-1 mission of the European Space Agency. In order to ensure a highly accurate and stable measurement over the 4 years of the ENVISAT mission (which is a must for a ne analysis of the trends in stratospheric ozone), a method based on star occulta-tions (Figure 1) has been proposed by a team of Euro-pean Scientists led by Service d'A eronomie (CNRS) and the Finnish Meteorological Institute (Bertaux et al. 1991). From its polar orbit, GOMOS is pointing at various stars whose lines-of-sight are tangential to the Earth limb (Figure 1). For each individual star, the spectrum measured outside the atmosphere is compared to the spectrum seen through the atmosphere when the star approaches the limb. The wide spectral range of the instrument (250 ? 952 nm) allows absorption lines of a number of trace gases, such as O 3 , NO 2 , NO 3 , to be recorded as well as the water vapour content. In addition GOMOS will provide information on aerosols and temperature distributions in the stratosphere as well as an analysis of the atmospheric turbulence (scintillation). The main feature of GOMOS is that it is a self-calibrated instrument, as it allows to derive an absolute concentration of atmospheric molecules from relative measurements and is therefore protected against instrumental radiometric drifts. The method has been already demonstrated in space using the Sun as light source; but the speciic advantage of GOMOS, related to the large number of observable stars, is that it combines the advantages of high vertical resolution in the atmosphere (1.7 km) and good Earth coverage. 2. THE INSTRUMENT DESIGN The GOMOS block diagram is shown in Figure 2. It is based on a 20 30 cm Cassegrain telescope which simultaneously feeds, through an optical beam dispatcher placed at its …