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| SSM/I | |
|---|---|
| Name | Special Sensor Microwave/Imager |
| Acronym | SSM/I |
| Operator | United States Department of Defense / Defense Meteorological Satellite Program |
| Manufacturer | Hughes Aircraft Company |
| Type | passive microwave radiometer |
| Wavelength | 1.5–11.2 cm (19–85 GHz) |
| Launch first | DMSP F6 |
| Status | retired (replaced by successor sensors) |
SSM/I
The Special Sensor Microwave/Imager (SSM/I) was a spaceborne passive microwave radiometer deployed on Defense Meteorological Satellite Program satellites to observe atmospheric and surface parameters. The instrument provided global, all-weather measurements of brightness temperature across multiple microwave frequencies, enabling retrievals of precipitation intensity, sea ice concentration, soil moisture proxies, and ocean surface wind signatures. Its long operational record established foundational datasets used by agencies such as the National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, and European Centre for Medium-Range Weather Forecasts.
SSM/I operated as a conical-scanning, radiometric instrument on sun-synchronous polar orbit platforms including DMSP F6, DMSP F7, and later spacecraft. It measured emission in four frequency bands (approximately 19, 22, 37, and 85 GHz) with dual polarization channels, enabling discrimination among hydrometeors, frozen hydrometeors, and surface emissivity differences. The sensor’s design targeted diurnal stability and global coverage suitable for operational applications by organizations like Fleet Numerical Meteorology and Oceanography Center, US Air Force Weather Agency, and research centers including Jet Propulsion Laboratory. SSM/I datasets became integral to reanalyses such as ERA-Interim and NCEP/NCAR Reanalysis.
SSM/I was built by Hughes Aircraft Company as a scanning microwave radiometer with a 1.6° to 7.2° instantaneous field of view depending on frequency. A rotating antenna provided a constant incidence angle near 53°, producing swaths of approximately 1400 km. The four operational frequencies—~19.35 GHz, 22.235 GHz, 37.0 GHz, and 85.5 GHz—each had vertical and horizontal polarization channels except the 22 GHz channel, which focused on atmospheric water vapor sounding. Radiometric sensitivity (NEΔT) and calibration stability were engineered to meet requirements from agencies such as Naval Research Laboratory and Defense Advanced Research Projects Agency programs. Thermal control and on-board calibration loads allowed cross-comparison with contemporaneous sensors like the Special Sensor Microwave Imager/Sounder and later Advanced Microwave Scanning Radiometer instruments.
Raw SSM/I brightness temperatures were processed into geophysical products by operational centers including NESDIS, DMSP ground stations, and research groups at University of Washington and Woods Hole Oceanographic Institution. Standard products included vertically integrated water vapor, cloud liquid water path, surface precipitation rate, sea ice concentration, and wind speed retrievals using empirical and physically based algorithms. Data processing chains involved antenna pattern correction, radiometric calibration, geolocation, and gridding into swath and gridded products used by datasets such as Global Precipitation Climatology Project and regional analyses like North Atlantic Oscillation studies. Intercomparison projects with TRMM and GPM helped refine algorithms and harmonize time series.
SSM/I enabled breakthroughs across meteorology, cryospheric science, and oceanography. Long-term sea ice time series informed studies involving the Arctic Council and climate assessments by the Intergovernmental Panel on Climate Change. Precipitation estimates contributed to flood forecasting used by Federal Emergency Management Agency and improved tropical cyclone surface rainfall analysis in combination with National Hurricane Center advisories. Soil moisture proxy products aided agricultural monitoring in regions covered by Food and Agriculture Organization programs and supported hydrological modeling in projects with World Meteorological Organization. SSM/I-derived wind speed fields were assimilated into numerical weather prediction systems at Met Office and ECMWF, and records supported trend analysis studies cited in literature from institutions such as Scripps Institution of Oceanography.
Calibration methodologies combined on-board hot/cold reference loads, vicarious cold-space views, and matchups with in situ networks including TAO/TRITON buoys, ARGOS drifters, and ICESS sea-ice field campaigns. Validation campaigns coordinated with the National Snow and Ice Data Center and research cruises run by NOAA Ship Ronald H. Brown. Cross-calibration with contemporaneous sensors like SSMIS and AMSR-E reduced inter-sensor biases through intercalibration efforts led by Global Climate Observing System working groups. Error budgets accounted for antenna pattern sidelobes, radio-frequency interference mitigation in coordination with International Telecommunication Union recommendations, and geolocation errors corrected using orbital ephemerides from the United States Space Force archives.
SSM/I flew on multiple DMSP satellites beginning in the 1980s, with successive instruments designated to maintain continuity through the 1990s and 2000s. Primary operators included United States Air Force weather units, NESDIS, and the Naval Research Laboratory. International partners and data users encompassed Canadian Ice Service, Japan Meteorological Agency, and research groups at Max Planck Institute for Meteorology and Lamont–Doherty Earth Observatory. Over its operational lifetime, the sensor supported numerous field campaigns such as the Arctic Ocean Experiment and tropical studies coordinated with Tropical Rainfall Measuring Mission investigators.
Limitations of SSM/I included coarse spatial resolution at lower frequencies, difficulties detecting light precipitation over land, and ambiguity over mixed-phase snow/ice surfaces. Radio-frequency interference and changes in orbital local time introduced challenges for trend analysis addressed through homogenization efforts in climate records. Successor instruments—such as SSMIS, AMSR-E, GMI—and constellation approaches aim to improve spatial resolution, radiometric sensitivity, and spectral coverage. Ongoing developments in radiative transfer modeling at institutions like NOAA and NCAR and synergistic retrievals combining microwave with visible/infrared sensors from MODIS and VIIRS continue to extend the legacy established by SSM/I.
Category:Remote sensing instruments