4.6 Characteristics of Radar Image

4.6 Characteristics of Radar Image

The main objective of microwave remote sensing is to estimate the property of objects by interpreting the features of the radar image. Typical objects to be measured by microwave remote sensing are mountainous land forms, subsurface geology, sea wind and waves etc. In order to estimate these properties, it is very important to understand the effects of microwave backscattering on the objects.

Two factors of microwave characteristics are of importance; frequency (or wavelength) and polarization. In microwave remote sensing, various wavelengths (or frequency) such as L band, C band, X band, P band etc. will be used ranging from millimeter wavelengths (1 mm - 1cm) up to about 30 cm. According to the wavelength or frequency, specular reflection will occur, so that the surface roughness can be detected if multi-frequency radar images are compared.

Figure 4.6.1 and 4.6.3 are radar images of P band, L band and C band respectively with HH polarization, in which the difference of wavelength or frequency will provide different images.

Polarization is defined as the oscillating direction involved in an electric field. Usually transmitted microwave and received microwave will have a choice between horizontal polarization and vertical polarization. Therefore four combinations; HH, HV, VH and VV can be used for SAR. The backscattering characteristics are also different with respect to polarization. Figure 4.6.2 and 4.6.4 are HH, HV, and VV combinations respectively with L band in which one can see different features.

In future, SAR systems with functions of multi-frequency and multi- polarization will be onboard earth observation satellites.