The microwave remote sensing technology can be used to acquire the qualitative and quantitative information related to earth’s surface from space or airborne plat­forms and is not influenced by the presence of clouds, light conditions and heat reaching towards it. The microwave systems are principally well suited for the assessment of woody biomass and other applications related to agriculture due to the fact that the signals of different wavelengths interact with particular part of the vegetation structure at different range of wavelengths. Thus interacting microwave radiation enables the rescue of vegetation structure parameters and related compo­nents of the standing woody biomass, rather than just the greenness of the top layer of a canopy which is depicted by the visible and infrared remote sensing technology (Koch et al. 2008; Woodhouse 2006a, b). The microwave radiation used for interac­tion with woody vegetation is categorized according to the applied parameters such as frequency, wavelength, reflection, refraction, diffraction, interference, polariza­tion and scattering. When compared to each other, these characteristics leads to the distortion of incident waves following the interaction in different forms of scattering such as reflection, diffraction and reflection by the elements present in biomass and this distortion is similar in size or less than for what can be observed during the change of wavelengths. However, the occurrence of reflection is due to the scatter­ing of waves from vegetation surfaces with specific features that are much smaller than the wavelength scale and similarly, the generation of diffraction signal are due to the scattering of incident waves at distinct boundaries. The radar remote sensing technology utilizes the backscattering signal, i. e. the intensity of signal which is reflected by the target and is received by the antenna. For point that is coherent tar­gets, the radar equation provides the estimation for magnitude of received power and is shown in Eq. 12.12 (Woodhouse 2006a).

Fig. 12.1 Schematic representation of Fresnel reflection onto a natural surface

where Pr and Pt corresponds to the received power and transmitted power, respectively; R is the distance between radar site and the location of target; G is the signal gain by the antenna; Ae is the effective area of the antenna; and a is the radar cross section of the object. The radar cross section (a) is the measure of radar reflec­tivity which indicates the strength of radar signal reflected from unit area of the target (Boyd and Danson 2005). When dealing with distributed targets especially the incoherent targets, a is replaced by sigma nought (a°)10 which is defined as the radar cross section per unit area (Woodhouse 2006a; Raney 1998).