OSIRIS A Decade of Scattered Light

I t is the trace composition of the atmosphere that governs the chemistry of the lower and middle atmo- sphere and to a large extent the climate of our planet. Accurately determining the abundance of species such as ozone (~1 ppm), nitrogen dioxide (~1 ppb), or bromine monoxide (~1 ppt) presents a large challenge. Further determining how they vary in space and time on a global scale is even more daunting. A global view of the atmospheric trace gas and aerosol composition can only be achieved from space via remote sensing. Balloons, and later rockets, were the first Fig. 1. Illustration of the Odin satellite. The limb-scanning pattern is shown. Into year 11 of a 2-yr mission, OSIRIS is redefining how limb-scattered sunlight can be used to probe the atmosphere, even into the upper troposphere. platforms to provide remote sensing observations aloft. The 1960s and 1970s saw the first generation of satellite instruments measuring meteorological and composition parameters. Rapid advances in instru-ment technology and computing capabilities through the 1980s and 1990s led to an explosion in resolution, spectral range, and downlinking capability. For example, the advent of the array detector in the 1980s meant entire spectra could be measured simultane-ously. Capitalizing on these advances, the last decade has seen an unprecedented number of missions with observation of atmospheric composition as a primary goal. The Odin satellite represents one such example. Odin is a small satellite mission sponsored by Swede