Microwave sounding unit Wikipedia
The Microwave sounding unit was the predecessor to the Advanced Microwave Sounding Unit (AMSU).
The MSU was first launched aboard the TIROS-N satellite in late 1978 and provided global coverage (from Pole to Pole). It carries a 4-channel microwave radiometer, operating between 50 and 60 GHz. Spatial resolution on the ground was 2.5 deg in longitude and latitude (about 250 km circle). There were 9 different MSUs launched; the most recent one on NOAA-14. They provided measurements of the temperature of the troposphere and lower stratosphere until 1998, when the first AMSU was deployed. AMSU provides many more channels and finer resolution (about 50 km).