258.

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soil contains large amount of moisture, the signal does not penetrate the soil surface and is reflected back to the radar antenna. If the soil is dry, then the radar signal can penetrate more deeply into the soil surface layer. Wavelength is also another control on the depth of penetration. Lakes and other water bodies might be expected to exhibit high backscatter, but in fact the surfaces of rivers and lakes are generally smooth relative to radar wavelengths and act as specular reflectors. The ocean surface is generally rougher, and therefore the magnitude of backscatter depends on sea state as well as on wavelength and depression angle.

1.6.3.3 Parameters of the radar equation

The parameters of the radar equation (van Zyl et al., 1993) are fundamental factors that influence the level of the returned signal. The radar equation is expressed as:

(1.25)

P_t is the transmitted power from the antenna, λ is the transmitted wavelength, R is the distance to imaging area, γ is the radar look angle, A is the area on the ground responsible for scattering, G_t and G_r are the transmitted and received antenna gains (describing the system’s ability to focus the transmitted microwave energy) at look angle γ, and σ° is the radar backscatter coefficient measured in decibels (dB). All of parameters in Equation (1.25) affect the received power P_r. However, only σ° is related to the properties of the illuminated surface. Thus, the quantised pixel values (0–255) in a radar image are sometimes converted to σ° before being interpreted. The received power P_r in Equation (1.25) can also be characterised in terms of other parameters such as the scattering matrix (Equation 1.30) and the coefficient of variation (Equation 1.34), which also relate to the properties of surface objects, and can be used for classification purposes. A discussion of the scattering matrix is presented in the next section.

1.7 Imaging radar polarimetry

The polarimetry theory presented in this section is mainly derived from Evans et al. (1988), Tomiyasu (1978), Zebker and van Zyl (1991), Zebker et al. (1987, 1991), van Zyl (1985, 1989), and van Zyl et al. (1987, 1993). Knowledge of radar polarimetry enables us to use a variety of features (such as complex format data, the elements of the scattering matrix, and the coefficient of variation of polarisation signature) to perform image interpretation. Some basic concepts are described first.

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