Electric Field ratio Formalism in Radar Polarimetry
Laboratory of Space Technology,
Otakaari 5 A,
Both the polarization state and power of a synthetic aperture radar backscatter vary as a function of the transmit polarization of the radar and environmental properties of the illuminated target. With a polarimetric radar, this variation can be registered and, in the best case, attributed to the target’s bio- and geophysical parameters. In spite of the fact, that radar is practically an all-weather instrument, radar imagery may suffer from precipitation and other environmental factors. Moreover, the radar imagery is often noisy and not of visual quality comparable to high- resolution optical imagery. However, radar polarimetry allows us not only to monitor cloud- covered areas day and night, but to enhance the SAR data in order to improve target to background contrast or target to target separation, to diminish unwanted signals like speckle, topographic and scatterer orientation effects, or, to derive more detailed digital elevation models, to mention a few examples. This means better usability for remote sensing. Much effort has been put on developing the theoretical foundations and practical applications for radar polarimetry. Also in this presentation both the theory and applications will be addressed. Traditionally, the polarization response has been represented as a function of both the receiver and transmitter polarization states. This may unnecessarily complicate a fully polarimetric representation. Actually, the properties of a receiving antenna should not be of any interest as soon as the properties of the backscattered wave are known through the available scattering matrix. Thus, a substantially simpler, but nevertheless fully polarimetric polarization response will be discussed. Since the complex-valued scattering matrix contains all the backscattering information of a single target, it would be sufficient to concentrate in studying the properties of this matrix alone. However, polarization is the key term in understanding the scattering matrix and many studies on radar polarimetry are still being done using only the backscattered power at some standard combinations of linear polarizations. Using these standard polarizations is not an optimal approach, because targets are not only horizontally or vertically oriented, for example, which may lead to contradictory research results. A better approach is to use polarization optimization. Evidently, the optimization result will not be invariant - as the nature is not - which means that an optimum polarization cannot be selected in advance, and instead of a single polarization state many more should be used in one image. An approach to polarization optimization using complex scattering matrices will be discussed. To accomplish these tasks, a not so often used theoretical definition for the polarization will be redeveloped. It will be shown, that an Electric Field Ratio representation is not only simple, but substantially better definition for polarization than the commonly used Polarization Ellipse in many types of calculations. In the electric field ratio formalism, the Poincaré sphere will reduce onto a two-dimensional polarization circle, which is a great advantage. Moreover, a quite practical Polarimetric Radar Equation pair can be derived by using this formalism.