Abstact
The aim of this paper is to describe and analyses the basic scattering characteristics for five selected classes such as water, forest, grass, residential and high density urban areas and how they vary with wavelength, polarization, and incident angle. For this purpose, quad-polarised images of Syney area taken from airborne SAR have been used. The results are discussed and some explanations on findings are given.

Introduction
Generally, the radar backscatter received from a surface is determined by incident angle, wavelength, polarization, surface roughness and dietectric properties of the surface. At radar wavelengths, 3 types of scattering such as surface scattering, volume scattering, and corner reflector-like scattering can occur. If the surface is homogeneous then surface scattering will occur and it can be either specula or diffuse, or intermediate depending on the wavelength and surface roughness. If the surface is dielectrically inhomogeneous then from the underlying materials will occur. The depth to which the radiation penetrates is dependent on the wavelength and the water content of the volume material. In volume scattering when the density of scatterers is low, the dependence of the backscattering coefficient is only slight and as the average dielectric constant increases, the dependence on incident angle increases. Corner reflector-like scattering occurs in result of right angles formed between natural and artificial objects.

Polarisation can be reflected in various ways from the natural objects. If the surface is sufficiently rough then both like and cross polarized fields can be received. HH polarized image will be dominated by reflectance coming from surface scattering mechanisms. HV and VV polarized data will display a greater component of volume scattering as a result of the signal penetrating to some depth below the surface. In general, the brighter the return on HV or VV images, the more likelihood the backscatter is coming from a three dimensional or heterogeneous layer [1,4]. Radar with its side viewing mode and which introduce different distortions and require various procedures for their correction. The aim of this paper is make judgment on the basis of interpretation of backscatter values and geometric problems will not be discussed.

For the analysis, five classes such as water, forest, grass, residential and high density urban areas of Sydney area have been selected. The classes are compared using individual and group pixels. For analysis of individual pixels, the backscatter values of pixels selected from different parts of the image were used, whereas for group analysis, 101-159 contextually dependent pixels representing the selected classes have been selected and compared on the basis of mean value and standard deviation (SD).

The Basic Theory of Scattering Mechanisms for the Selected Classes
The selected classes have different backscattering properties. The following describes the basic theory of scattering mechanisms of each class [2,3,4,5].

The aim of radar RS at the water surface is to determine wave amplitudes, surface wind velocities and directions. A perfectly flat sea will behave as a specular reflector and will appear black in imagery for all incident angles except 0. To obtain some backscatter from a sea surface, it must by some mechanism, be made rough and the principal mechanism for the roughening the surface of the sea is the generation of waves. Different types of waves having different means for generation and characteristics are given in Richards et al. (1987). Data at P(75cm), L(18.7cm), C(6.3cm) and X(3.3cm) reported by Guinard and Daley in ( Manual of RS : 1391) appear to saturate for wind velocities in excess of 10knots; however, data recorded at the higher frequencies varied more with wind speed than did the lower frequency data.

Open grass will act as a mixture of grass and soil and the backscatter will depend on the volume of either of them. Plant geometry, density and the water content are the main factors influencing on the backscatter coming from the vegetation cover. Most probably, grass and extended vegetated surfaces could have components of all (ie, diffuse, specular and intermediate), reflection depending on the wavelength and incident angle. At angles close to nadir and frequencies below about 8GHz (ie, C,L,P bands) the presence of vegetation cover (crops and grass) has a minor influence on the backscatter (Manual of RS). The backscattering of soil will depend on the surface roughness and incident angle. The presence of water strongly affects the microwave emissivity and reflectivity of a soil layer. At low moisture levels there is a low increase in the dielectric constant. Above a critical amount the dielectric constant rises rapidely. This increase occurs moisture is directly related to the texture and structure of the soil. Therefore, the backscatter values from open grass will not be as high as in the case of pure volume and corner reflector-like scattering.

In case of forest areas trunk-ground double bounce scattering, branch-ground double bounce and branch-direct backscattering, crown volume backscattering and crown volume attenuation and ground backscattering can occur, ie, backscatterfrom forest will be volume scattering derived from multiple-path reflections from leaves, twigs and trunks. Considering a mixture of different grass and forest types as vegetation, for its mapping Ulaby (1982) recommended the use of frequencies greater than 8GHz and moderate depression angles. This is probably based on the increasing penetration and sensitivity to soil properties underlying underlying vegetation with lower frequency and near nadir incident angles. Thus, short wavelength can be used for top layer study, whereas long wavelength for lower layer study. When incident angle increases in the far range, more volume scattering should be expected due to path difference of radiation.

The blackscatter from urban areas will contain information about street alignment, building size, density, roofing material, its orientation, vegetation and soil resulting in all kinds of scattering. Roads and buildings in urban areas can reflect a larger component of radiation if they are aligned at right angles to the incident radiation. Here the intersection of a road and a building tends to act as a corner reflector. The amount of blackscatter is very sensitive to street alignment. The variation in return between neighbouring areas of streets and buildings aligned at right angles to the incident radiation will have a saturated very bright appearance and non-aligned areas will have a more speckled, bright/dark appearance in the resulting image. Volume and surface scattering will also play an important role in the response from urban areas. Using L (23.5) band and Rayleigh's criterion of surface roughness the boundary between diffuse and specular relection can be determined at about 3cm when incident angle is 60 digress. Many urban surfaces have variations that approximate, are greater than or fall below these values. For example, bitumen and concrete surface would always have variations of less than 3cm and would generate a specular response, and appear dark at any incident angle, while roofing material, grass broken soil and extended vegetated surfaces could have both diffuse and specular reflection depending on the incident angle [4].