Microwave Radiometer

A microwave radiometer is a passive sensor that measures the natural brightness temperature emitted by the surface and atmosphere across microwave bands. The signal varies with sea surface temperature, salinity, wind-roughened emissivity, water vapor, cloud liquid, and sea ice, so different frequencies retrieve different geophysical fields: L-band near 1.4 GHz for salinity and soil moisture on SMAP and SMOS, C and X-band for sea surface temperature, K-band for the altimeter wet-troposphere correction. Unlike radar it transmits nothing, working day and night through cloud. AMSR-class radiometers map sea ice concentration and SST under conditions that defeat infrared sensors.