A new satellite instrument, GOMOS, will be launched on ESA's ENVISAT in year 2000 and will measure ozone with unprecedented long term accuracy and vertical resolution. GOMOS observes stellar occultations, and determines ozone, NO2, NO3, aerosol, O2, H2O and possibly OClO concentrations from the ratio of two stellar spectra, one outside the atmosphere, one through the atmosphere. The first step of the processing, Level 1b, consists of various instrumental corrections to obtain the measured atmospheric spectral transmission as a function of altitude/geolocation. The second step (Level 2) consists of one spectral inversion for each line of sight (retrieval of integrated line densities of various constituents) and one vertical inversion for the whole occultation. The analysis of two fast photometers (1kHz) data will produce a high resolution profile for the temperature profile. Higher data products will be constructed with the sequential assimilation of GOMOS data into a CTM model.