Approaching 50 Years of Earth Atmospheric Sensing from Space Using Passive Microwave Observations: A Retrospective (Invited Presentation)

Passive microwave remote sensing of planetary atmospheres was first accomplished by the Mariner 2 spacecraft as it flew by Venus in 1962 (Barath et al., 1964). Observing near 15.8 and 22.2 GHz, strong lapse rates and extremely high surface temperatures (near 700 K) were revealed. Cosmos 243 and 348 were the first satellites to monitor the earth with microwaves, in 1968 and 1970 respectively, measuring atmospheric water vapor, hydrometeors, sea temperature, snow cover and ice; observations in four frequencies from 3.5 to 37 GHz were made (Basharinov et al., 1969). The first passive microwave observations of the terrestrial temperature profile were obtained by the NEMS instrument launched on the Nimbus 5 spacecraft in 1972 (Staelin et al., 1973). It carried three channels observing near 60-GHz oxygen absorption lines for the purpose of monitoring temperature profiles from the surface to 18 km altitude. A scanning version called SCAMS was later launched on Nimbus 6 in 1975 (Staelin et al., 1977). The success of NEMS and SCAMS in measuring atmospheric temperature and humidity in the presence of most cloud types was a major stimulus in the subsequent adoption of this technology by the operational meteorological community (Staelin, 1989).

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).