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The calibra.tion of a polarimetric SAR system is achieved through use of radar reflectors with a significant crosspolarized reflection of known properties. Three such reflectors are the dihedral, the active radar calibrator (ARC), and the gridded trihedral. Each of thcse devices have been used as calibration radar reflectors Eo: the three frequency (I .25, 5.30 and 9.35 GHz) ERIM/NADC P-3 SAR System. T h e focus of this paper is on tlic chara.cteristics of the gridded trihedral. The gridded trihedral is a normal trihedral with one of the conducting faces replaced with a grid of closely spaced (relative to a wavelength) parallel conductors over a la.yer of microwave absorber. This grid muses the incident wave's polarization to changc and results in a calibration target with a significant cross-polarized reflection. The advantage of the gridded trihedral is t,liat it has a broad backscatter beamwidth (unlike the dihedral), ancl it is passive a,nd simple t o const,ruct (unlike the ARC). The performance characberistics of the gridded trihedral reflector will be reviewed using basic theoretical models and calibrated P-3 SAR imagery.

Ad van c ed C o 1 1 c e p t s D i v i s i o n E iivi r o ii 111 e ii t a1 Res ear ch Institute of Mi cli i gall P O Box 8618 Ami Arbor, MI 48107 USA Phone: (313) 994-1200 FAX: (313) 665-6559 AbstractRemote sensing and classification of terrain can be achieved with a radar by examining the spatial variations in radar scatter as well as the average intensity and the polarimetric properties of radar scatter. The spatial varicztion in SAR imagery of terrain has been modeled previously as the product of two terms: (1) a variation due to Rayleigh fading and (2) a variation due to spatial changes in thc density of scatterers. Work done previously by other researchers has shown using theoretical scattering models that the variation due to spatial changes in scatterer density is polarization independent under certain conditions. SAR data of natural terrain (grass, trees, etc.) typically satisfies these conditions. Scatter from cultural terrain (houses, cars, etc.) does not satisfy these conditions and the resultant image is more highly textured and this texture may be polarization dependent. Over the past two years polarimetric SAR data has been collected over various natural and cultural terrain with the three frequency (1.25, 5.30 and 9.35 GI-Iz), ERIM/NADC P-3 SAR System. The purpose of this paper is to review the texture characteristics of the imagery collected over the two terrain types and to evaluate its utility in distinguishing cultural and natura.1 terrain.