Advances of Techniques for Utilizing

The scattering properties of a radar target can only be fully characterized by measuring a scattering matrix in which the elements represent the combinations of transmit and receive polarization states. The amount of information gathered by a traditional single polarization radar may therefore be even very limited, for example when the single polarization response is near zero while the cross polarized return is at a maximum. By polarimetric processing, different scatterers, i.e., scattering mechanisms, can be completely and coherently separated, even if they are located within the same resolution cell of the radar image. This is particularly important in relation to non-cooperative target identification and recognition. Nevertheless, the practical utilization of polarization for optimizing information content has not gained more widespread interest until very recently. This is mainly due to a lack of proper technology in the past, but also due to negative conclusions drawn from early experiments with polarimetric radar. At the moment, significant efforts are put into the development of techniques for efficiently and conveniently handling polarimetric data. Such techniques differ considerably from techniques known from traditional single polarization target imaging, since one has to deal with a matrix for each resolution cell instead of just a single scalar. In this paper, some of the advances and advantages of polarimetric radar techniques will be reviewed and presented in an application-oriented perspective, emphasizing the potential of utilizing the information contained in the polarization transforming properties of radar targets. INTRODUCTION The vectorial nature of the electromagnetic fields, on which any radar system is based, implies that the full amount of potentially available information can only be exploited by employing polarimetric radar systems, i.e., systems capable of measuring all four combinations of two orthogonal polarizations. For simplicity and cost-effectiveness, however, traditional operational radar systems have been singlepolarization systems employing one and the same polarization on both transmission and reception. Such systems have obviously served many needs satisfactorily, but for modern high resolution applications, such as target classification, identification and recognition, the properties and capabilities of polarimetric systems cannot be ignored without a severe loss of useful target information. This was realized rather early in civilian remote sensing applications using high-resolution SAR (Synthetic Aperture Radar) systems, and single-polarization systems would simply not be able to produce the classification results that are delivered by modern polarimetric SAR systems. In the military community, however, the issue of polarimetric radar has been surrounded by skeptics and opposition, largely based on claims that the money could be spent more efficiently on other performance parameters, like range resolution. One reason for this, of course, is the obvious fact that the polarimetric techniques could not readily be implemented as upgrades to existing radars. Nevertheless, with the technological developments that have taken place in recent years, the implementation of full polarimetric Paper presented at the RTO SET Symposium on “Target Identification and Recognition Using RF Systems”, held in Oslo, Norway, 11-13 October 2004, and published in RTO-MP-SET-080.