A Microwave Sounding Unit (“MSU”) is a device that has been installed on polar orbiting satellites to measure, from space, the intensity of microwave radiation emitted by earth’s atmosphere. Different “channels” of the MSU measure different frequencies of radiation which can, in turn, be related to temperature averages of the atmosphere over different vertical regions. Channel 2 measurements provide a vertically-weighted temperature estimate that emphasizes the mid-troposphere (with small contributions from the stratosphere), while Channel 4 largely measures temperatures in the lower stratosphere. Information from MSUs have been used to generate the “MSU Temperature Record“. More information on MSU can be found here.