The first passive microwave images of the earth were obtained by the electrically scanned ESMR instrument on the Nimbus 5 spacecraft; it scanned a 3000-km swath at 19.3 GHz with a resolution at nadir of 25 km. A dual-polarized version observing near 37 GHz was launched in 1975 on Nimbus 6. The resulting images of humidity and rain (Wilheit et al., 1977) over ocean, and of the extent of snow and ice cover on land and at sea further stimulated the development of a series of more sophisticated spacecraft instruments.
In the United States, two thrusts of technical development have subsequently dominated: one involves instruments yielding atmospheric data to drive numerical weather prediction models (following NEMS and SCAMS), and the other (following ESMR) involves imagery of rain, sea ice, and other parameters for a different group of users. The first operational use of passive microwave soundings for numerical weather prediction began with the 4-channel MSU instrument on operational low-earth-orbiting NOAA satellites, beginning in 1979. A 7 -channel military version, the Special Sensor Microwave/Imager (SSM/I) was launched in the mid-1980's to measure four important meteorological parameters over the ocean: near-surface wind speed, total columnar water vapor, total columnar cloud liquid water (liquid water path) and precipitation. More recently, the Advanced Microwave Sounding Unit (first launched in 1998) and the Advanced Technology Microwave Sounder (first launched in 2011) have introduced improved capabilities for atmospheric sounding to improve numerical weather prediction.
In this paper, we review the history of microwave sounders culminating in the state-of-the-art ATMS flying today on Suomi NPP (and soon to fly on JPSS-1).