Passive microwave radiometers provide a powerful, nearly all-weather technique for retrieval of a wide variety of ocean, sea ice and atmospheric geophysical variables. In contrast to the lenses and mirrors used in the VIR, antennas are used with passive microwave radiometers to receive the Earth-emitted radiances, and with radars to transmit and receive energy pulses. The present chapter describes the radiometer antennas and their terminology, discusses how radiometers work and summarizes the properties of the past, present and near-future operational passive microwave imagers. The next chapter describes the atmospheric transmissivity in the microwave, discusses the dependence of the surface emissivity on a variety of geophysical variables, gives examples of the retrieval algorithms and concludes with examples.
Microwaves occupy that part of the electromagnetic spectrum between 1 and 500 GHz in frequency, or between 0.3 m and 1 mm in wavelength. The microwave band is bounded at low frequencies by the television broadcast bands, the presence of which makes Earth observations difficult, and at high frequencies by the far-infrared wavelengths where alternative methods exist for detection. The frequency dependence of the atmospheric transmissivity and interference from other users restricts the frequencies used in retrieval of oceanic variables to specific windows in the range 1–90 GHz. The importance of microwave observations is that the ocean surface emissivities and atmospheric transmissivities depend both on frequency and on geophysical variables such as atmospheric water vapor and liquid water, rain rate, sea surface temperature and salinity, wind speed, and sea ice type and extent. As Chapter 9 discusses in detail, to retrieve these variables, the instruments must observe the ocean at several different frequencies.