The Determination of a Time and Wavelength Dependent Spectral Point Spread Function for the OSIRIS Satellite Instrument

The Canadian designed and built OSIRIS (Optical Spectrograph and Infrared Imaging System) satellite instrument measures the spectrum of sunlight scattered from the Earth's atmosphere at wavelengths from the ultraviolet to the near infrared, employing a limb scattering technique to collect information about the vertical profile of atmospheric composition. OSIRIS has been in operation on the Swedish satellite Odin since 2001 and as such the data it has collected have become extremely useful for studying trends in the atmosphere. Due to the sun-synchronous, dusk-dawn orbit of Odin the OSIRIS instrument is almost always exposed to direct sunlight except for a few months of the year. During this eclipse period that occurs within the northern hemisphere spring and summer months the loss of direct sunlight for part of the orbit causes temperature changes within OSIRIS that lead to slight changes in its spectral point spread function. The current OSIRIS ozone retrieval scheme does not properly accommodate this change in point spread function and this is believed to cause systematic errors in the ozone retrieval of up to seven per cent in altitude range from 30 to 40 km during the northern hemisphere summertime. This paper presents a technique used to determine the time and wavelength dependent OSIRIS spectral point spread function as well as the results of a study to determine the expected improvement this knowledge will have on the OSIRIS ozone data product.