Relating Polaization Phase Difference of SAR Signals to Scene Properties

This paper examines the statistical behavior of the phase difference AO between the HH-polarized and VV-polarized backscattered signals recorded by an L-band SAR over an agricultural test site in Illinois. Polarization-phase difference (¿¿) distributions were generated for about 200 agricultural fields for which ground information had been acquired in conjunction with the SAR mission. For the over-whelming majority of cases, the AX distribution is symmetrical and has a single major lobe centered at the mean value of the disstribution ¿¿ Whereas the mean AX was found to be close to zero degrees for bare soil, cut vegetation, alfalfa, soybeans, and clover, a different pattern was observed for the corn fields; the mean ¿¿ increased with increasing incidence angle 0 from about zero at 0 = 150 (near-range of the image) to about 140° at 0 = 35°. The explanation proposed for this variation is that the corn canopy, most of whose mass is contained in its vertical stalks, acts like a uniaxial crystal characterized by different velocities of propogation for waves with horizontal and vertical polarization. Thus, it is hypothesized that the observed backscatter is contributed by a combination of propagation delay, forward scatter by the soil surface, and specular bistatic reflection by the stalks. Model calculations based on this assumption were found to be in general agreement with the phase observations.