On the Problem of Reciprocity of Radar Images of Forests at HV and VH Polarizations

In the scientific literature on radar, it is a priori assumed that reflections from the Earth’s underlying surfaces at cross-polarizations are mutual (reciprocal), i.e., complex values of reflected signals at HV and VH are the same. Compliance with the principle of reciprocity is considered as an important point in the study of reflective properties of the Earth’s surfaces by means of space long-wave polarimetric radars with synthesized aperture (SAR). However, the authors are not aware of studies on the validity of reciprocity principle for reflection of signals at cross-polarizations from a rough surface. As a mathematical value which characterizes the reciprocity, we propose to use the coefficient of mutual correlation of reflections at VH and HV . Preliminary studies of forest areas using polarimetric radar of P-band (λ∼ 70cm) have shown that deviations from the reciprocity principle are observed. Herewith, the results of analysis also reveal features which are difficult to explain.