267.

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Page 39

antenna, and 0 and μ0 are the permittivity and permeability of free space (i.e. transmission medium), respectively. Note that both permittivity 0 and permeability μ0 determine the propagation velocity v (or phase velocity) of the wave; the relationship can be expressed as

Polarisation synthesis can also be expressed in terms of either the Stokes matrix (van Zyl et al., 1987) or the covariance matrix (Swartz et al., 1988). Both of these representations consist of linear combinations of the cross-products of the four basic elements of the scattering matrix. The entries of these matrices can also provide a variety of features for classification purposes.

1.7.3 Polarisation signatures

A particular graphical representation of the variation of received power (or cross-section σ) as a function of polarisation is called the polarisation signature of an object (Zebker et al., 1987; van Zyl et al., 1987). A polarisation signature is a three-dimensional representation consisting of a plot of synthesised scattering power as a function of the ellipticity and orientation angles (i.e. χ and Ψ in Figure 1.21) of the transmitted and received wave. Normally, analyses are based on only two types of polarisation signatures, namely co-polarisation and cross-polarisation.

Figure 1.23 illustrates the polarisation signature based on a theoretical model of a large conducting sphere (Evans et al., 1988). Note that at the ellipticity angle χ=0° the co-polarisation signature reaches its highest value of cross-section, while the cross-polarisation signature shows the lowest value at the same point. There is a measurement of surface roughness in accordance with polarisation signature called the coefficient of variation (CoV) defined as:

(1.34)

Figure 1.23 Polarisation signature based on a theoretical model of a large conducting sphere.

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Classification Methods for Remotely Sensed Data
Classification Methods for Remotely Sensed Data, Second Edition
ISBN: 1420090720
EAN: 2147483647
Year: 2001
Pages: 354

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