The UARS and EOS microwave limb sounder (MLS) experiments

The Microwave Limb Sounder (MLS) experiments obtain measurements of atmosph eric composition, temperature, and pressure by observations of millimeter- and submillimeter-wavelength thermal emission as the instrument field of vi ew is scanned through the atmospheric limb. Features of the measurement tec hnique include the ability to measure many atmospheric gases as well as tem perature and pressure, to obtain measurements even in the presence of dense aerosol and cirrus, and to provide near-global coverage on a daily basis a t all times of day and night from an orbiting platform. The composition mea surements are relatively insensitive to uncertainties in atmospheric temper ature. An accurate spectroscopic database is available, and the instrument calibration is also very accurate and stable. The first MLS experiment in s pace, launched on the (NASA) Upper Atmosphere Research Satellite (UARS) in September 1991, was designed primarily to measure stratospheric profiles of ClO, O-3, H2O, and atmospheric pressure as a vertical reference. Global me asurement of ClO, the predominant radical in chlorine destruction of ozone, was an especially important objective of UARS MLS. Ail objectives of UARS MLS have been accomplished and additional geophysical products beyond those for which the experiment was designed have been obtained, including measur ement of upper-tropospheric water vapor, which is important for climate cha nge studies. A follow-on MLS experiment is being developed for NASA's Earth Observing System (EOS) and is scheduled to be launched on the EOS CHEMISTR Y platform in late 2002. EOS MLS is designed for many stratospheric measure ments, including HO, radicals, which could not be measured by UARS because adequate technology was not available, and better and more extensive upper- tropospheric and lower-stratospheric measurements.