Advances in Spaceborne Observations of the Middle Atmosphere with the Microwave Limb Sounding Technique

Spaceborne instruments employing the microwave limb sounding technique provide daily near-global observations of atmospheric composition and other parameters including temperature, geopotential height, and cloud properties from the upper troposphere through the mesosphere. Limb sounding provides high vertical resolution measurements with good sensitivity to trace species thanks to the long atmospheric path observed. Observations at microwave and submillimeter frequencies have the additional advantage of being able to measure in the presence of atmospheric aerosols and all but the thickest cloud. We review the fundamentals of the microwave limb sounding approach and the capabilities and accomplishments of previous such instruments. We describe how recent advances in technology, mainly driven by the communications industry, enable dramatic reductions in the mass and power needed for next-generation instruments. Various potential future instrument designs are described that provide a range of vertical, horizontal, and temporal resolutions to fit the needs of different mission profiles.