There are two types of microwave remote sensing; active and passive. The active type receives the backscattering which is reflected from the transmitted microwave which is incident on the ground surface.
Synthetic aperture radar (SAR), microwave scatterometers, radar altimeters etc. are active microwave sensors. The passive type receives the microwave radiation emitted from objects on the ground. The microwave radiometer is one of the passive microwave sensors. The process used by the active type, from the transmission by an antenna, to the reception by the antenna is theoretically explained by the radar equation as described in Figure 3.1.1.
The process of the passive type is explained using the theory of radiative transfer based on the law of Rayleigh Jeans as explained in Figure 3.1.2 (see 1.7, 1.12 and 3.2) In both active and passive types, the sensor may be designed considering the optimum frequency needed for the objects to be observed. (see 4.1)
In active microwave remote sensing, the characteristics of scattering can be derived from the radar cross section calculated from received power Pr and antenna parameters (At , Pt , Gt ) and the relationship between them, and the physical characteristics of an object. For example, rainfall can be measured from the relationship between the size of water drops and the intensity of rainfall.
In passive microwave remote sensing, the characteristics of an object can be detected from the relationship between the received power and the physical characteristics of the object such as attenuation and/or radiation characteristics. (see 3.2 and 3.3)
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