A radiometer sensing the emission of radiation at microwave frequencies from a medium. Frequencies from about 5 ghz up to 37 ghz are used in the study of quantities related to mass balance with passive-microwave sensors. The intensity of emission depends on the temperature of the medium and its emissivity. See brightness temperature. Microwave radiometers in orbit have resolutions of a few to a few tens of kilometres, so that they are best suited to monitoring of extensive ice and snow covers. All are in polar, sun-synchronous orbits, and offer daily near-global coverage. At high latitudes, coverage is available at least twice daily, that is, from an ascending (south to north) pass, typically in the afternoon or evening, and a descending (north to south) pass, typically in the morning. SMMR, the Scanning Multi-channel Microwave Radiometer, operated from 1978 to 1987. SSM/I, the Special Sensor Microwave Imager, was first launched in 1987 and has operated on several different satellites since. AMSR-E, the Advanced Microwave Scanning Radiometer for the Earth Observing System, has operated since 2002. In snow hydrology, passive-microwave radiometers are operational tools for the estimation of snow water equivalent ('SWE').