Water Content: Passive and Active Microwave Remote Sensing

Soil moisture measurement with in situ methods over large areas and at frequent time intervals is not practical for most purposes. Remote sensing from air- and space-borne platforms permits data collection from large areas and at frequent time intervals, matching the dynamic nature of soil moisture. Remote sensing with the microwave spectrum, wavelengths ranging from 1 mm to 1 m, is attractive as data can be collected day or night and through clouds. Terrestrial applications of microwave sensing typically use wavelengths from 3 to 75 cm, although the convention of microwave remote sensing is to refer to frequency rather than wavelength. The topic of microwave sensing is a broad one as it not only includes two general types of instruments, but also a variety of applications spanning fields such as agronomy, soil science, hydrology, meteorology, and astronomy.