The Microwave Limb Sounder (MLS) is a satellite-borne instrument making limb sounding measurements of atmospheric composition at microwave frequencies. It was built by an instrument team based at the Jet Propulsion Laboratory and launched on the Upper Atmosphere Research Satellite (UARS) on the 12th September 1991. It began collecting data on 19th September 1991. The PI was Joe Waters.

The instrument measures thermal emission from the Earth's limb, from which vertical profiles of chlorine monoxide (ClO), ozone (O3), water vapour (H2O) sulphur dioxide (SO2) and atmospheric temperature are derived. HNO3 profiles are also available for some periods in the Version 4 data.

The data coverage extends from 80°S to 80°N, but at any one time this is restricted to 34°S to 80°N or 34°N to 80°S. In the vertical, measurements are made between approximately 10 and 60 km, from October 1991 to June 1997.

Also available as part of this archive are the prototype water vapour profiles from the Microwave Limb Sounder (MLS) data, retrieved at the University of Edinburgh (NERC funded project). Data generally cover wider altitude range than "official" MLS version 4 retrievals and are available at a higher vertical resolution. Coverage is from September 1991 to April 1993.