In-orbit performance of the ozone monitoring instrument

Abstract

In July 2004 Nasa's AURA satellite was launched carrying the Dutch-Finnish Ozone Monitoring Instrument and since then it is producing high quality trace gas measurements of a.o. ozone and NO₂. The OMI is a non-scanning nadir viewing spectrograph with a wavelength coverage of 270 to 500 nm and a spectral resolution of 0.4 to 0.7 nm. It has a large spatial field-of-view of 114 degrees perpendicular to the flight direction and uses the resulting swath of 2600 km to measure the complete globe in a single day with ground pixels of nominally 13 km × 24 km. After a brief instrument overview, this paper discusses a number of in-flight performance issues, such as the wavelength calibration and the stray light correction. OMI's wavelength calibration is based on fitting the sun's Fraunhofer structures, both on sun irradiance spectra and Earth radiance spectra. For the latter the cloud structures impact the wavelength results via inhomogeneous illumination of the spectrometer slit. This is explained together with the basics of a correction algorithm. OMI has a carousel with three on-board sun diffusers. Measurements with the quartz volume diffuser will be used to show remaining diffuser features in the data. The measured irradiances are compared to the results obtained by convolving the high-resolution solar reference spectrum with the accurately calibrated spectral slit functions. In the in-flight measurement data in the wavelength range below 300 nm spatial stray light features are observed, resulting from clouds observed at wavelengths above 300 nm. These features are shown together with an explanation of the means to analyze the in-orbit stray light performance.