Microwave sounding unit
From Glossary of Meteorology
Microwave Sounding Unit
(Abbreviated MSU.) One of three sounders that comprise the [[<ITALIC>TIROS-N</ITALIC> Operational Vertical Sounder|TIROS- N Operational Vertical Sounder]] package on NOAA polar orbiter satellites.
The primary function is to make temperature soundings in the presence of clouds.