On the use of dual polarized interferometry to measure scattering from vegetated areas

Several polarimetric scattering models for vegetated surfaces have been developed over the past few years. Most of these models predict scattering from cross-polarized returns to come mostly from the randomly oriented branches, twigs and leaves, while the co-polarized return comes mainly from lower down in the canopy. Few experiments have actually measured the position in the canopy from which the scattering originates. During the 1993 night campaign, the AIRSAR instrument was reconfigured to measure interferograms at both VV and HV polarizations at C-band. This was achieved by transmitting out of the AIRSAR C-band polarimetric antenna, and transmitting alternately horizontal and vertical polarizations. The scattered signals were then measured simultaneously using two vertically polarized antennas, separated by about 2.4 meters on the fuselage of the DC-8 aircraft. Since the phase in the interferograms is related to the elevation of each pixel, one can measure the difference in penetration into the vegetation by comparing the difference in elevation derived from the co-polarized and cross-polarized interferograms. This is a simple and effective way to actually measure the relative positions of the scattering centers for the co- and cross-polarized returns. Data were acquired over a uniform tropical rain forest in French Guyana. The scene analyzed contained mostly rain forest, but a large river also runs through the image, allowing one to calibrate the relative elevations in the VV and HV elevation maps