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Extant techniques for remotely sensing the earth with instruments operating at microwave frequencies are surveyed. Microwave sensors can provide day-night operation and almost an all-weather capability due to higher transmission through clouds at microwave than at visible or infrared wavelengths. Passive (radiometers), active (radars, altimeters, and scatterometers), and composite (passive-active) microwave sensors are in use or planned for such diverse applications as measuring ocean surface dynamics, Ocean salinity, soft moisture content, atmospheric temperature and atmospheric constituents; detecting sea ice, oil slicks, and storm cells; and identifying agricultural crops. These measurements have been taken with sensors located in towers, aircraft, and most recently satellites.