Polarimetric SAR characterization of man-made structures in urban areas using normalized circular-pol correlation coefficients

Polarimetric Synthetic Aperture Radar (SAR) backscatter from man-made structures in urban areas is quite different than backscatter from predominantly natural areas. Backscatter from natural areas is often reflection symmetric; i.e., characterized by near zero values for covariance matrix off-diagonal terms of the form 〈SHVSHH⁎〉, 〈SHVSVV⁎〉 and their conjugates. A new approach is proposed to detect scattering from non-reflection symmetric structures using circular-pol, RR-LL, correlation coefficients, |ρ|. This method creates a normalization term, |ρ0|, and then forms a ratio, |ρ|/|ρ0|. The normalization term, |ρ0|, contains the same diagonal terms of the covariance matrix. The 〈SHVSHH⁎〉 and 〈SHVSVV⁎〉 off-diagonal terms and their conjugates are purposely set to zero. The ratio, |ρ|/|ρ0|, is rewritten as a product of separable helicity (τ) and orientation angle (θ) dependencies. The mathematical form of the τ dependence is a resonant singularity, or pole, term. This pole significantly enhances returns from man-made, high helicity, non-reflection symmetric structures. These structures have values of τ near the resonance value at τ = ± 1. Natural scatterers possess very strong RR / LL symmetry (τ ≈ 0) and the pole response for them is correspondingly weak. The dependence of |ρ|/|ρ0| on the orientation angle (θ) is known from previous studies to be useful for measuring urban building alignments (relative to the azimuth direction) and measuring surface topography. The ratio |ρ|/|ρ0| reduces much of the un-needed image detail of backscatter variations from natural areas of different surface roughness. This image simplification further facilitates detection of localized man-made targets.