The TIROS Operational Vertical Sounder
(TOVS)
equipped aboard NOAA's TIROS series of polar orbiting satellites
consists of three instruments: the High Resolution Infrared
Radiation Sounder (HIRS),
the Microwave Sounding Unit (MSU)
and the Stratospheric Sounding Unit (SSU).
The MSU and SSU have been replaced with improved instruments,
the AMSU-A
and AMSU-B,
on the newer satellites.