The infrared and microwave sounding instruments were implemented in operational form on the TIROS-N (Television and Infrared Observation Satellite) spacecraft in 1978, as the HIRS and the Microwave Sounding Unit (MSU). The channels were carefully selected to cover the depth of the atmosphere. Infrared soundings of 20-40 km resolution horizontally were supplemented with microwave soundings of 150-300 km resolution horizontally (Smith et al. 1979).

Regardless of cloud cover, the complement of infrared and microwave instruments aboard each of the polar orbiting spacecraft provided complete global coverage of vertical temperature and moisture profile data every 12 hours at 250-km spacing (Smith et al. 1981a).