Polarization Signatures Of Frozen And Thawed Forests Of Varying Biomass

Abstract

P O L A R I Z A T I O N S I G N A T U R E S OF F R O Z E N AND T H A W E D F O R E S T S OF V A R Y I N G B I O M A S S Ronald Kwok, JoBea Way, Eric Rignot, Anthony Freeman and J. Holt Jet Propulsion Laboratory California Institute of Technology Pasadena, CA 91109 Telex 675-429 FAX (818) 393-6943 P h (818) 354-5614 Abstract During the two different overflights (by the J P L radar po- larimeter) of the Bonanza Creek Experimental Forest, the environmental conditions changed significantly with tem- peratures ranging from unseasonably warm (1 to 9°C) to well below freezing (-8 to -15°C) and the moisture content of the snow and trees changed from a liquid to frozen state. Preliminary investigations have focused on the characteri- zation of the radiometric as well as polarimetric signatures of the data. Significant changes (up to 6dB in certain forest stands) were observed in the L-band radar cross-sections. Features extracted from the Stokes matrices of the same stands from the thawed and frozen days also suggest the relative contribution of the different scattering mechanisms to the radar return. Comparison of the diffuse component and variance of the H-V phase (white spruce, for example) indicate a relatively higher contribution from diffuse scat- terers on the thawed day than on the frozen day. These results indicate that the contributions of scattering from the crown is responsible for this appreciable increase in backscatter. Some of these preliminary L-band and P-band observations will be presented here. Keywords: Polarimetry, H- V phase difference, Fractional Polarization 1. I n t r o d u c t i o n In March 1988, the NASA/JPL Airborne Synthetic Aper- ture Radar (SAR) acquired a series of S,4R datasets over the Bonanza Creek Experimental Forest near Fairbanks, Alaska. The P-, L- and C-band polarimetric dataset con- sists of data collected on five different days over a period of two weeks. During that period, the environmental condi- tions changed significantly with temperatures ranging from unseasonably warm (1 to 9°C) to well below freezing (-8 to -15°C) and the moisture content of the snow and trees changed from a liquid to frozen state. This significant change in environmental conditions during t h e two data ac- quisition flights (March 13 and 19) over the Bonanza Creek Experimental Forest offered an excellent opportunity for comparison of polarimetric characteristics of forest stands in their frozen and thawed states. The synthesized total power (L-band) images for both days are shown in Fig. 1. Calibration of the L-band dataset was performed at JPL using in-scene corner reflectors (in an adjacent scene - Fairbanks) which were deployed during the March cam- paign. Dramatic changes in the scattering signature have been observed in a preliminary paper by Cimino et. al [I]. The purpose of this paper is to present a subset of the quantitative polarimetric measurements based on ra- diometrically and polarimetrically calibrated data. These data were calibrated using procedures developed at JPL. The measurements and observations presented herein have been compared with model results. The calibration and modelling efforts are present in companion papers by Holt and Freeman [2] and Dobson et. a1[3], respectively. 2. Data C a l i b r a t i o n C-Band. The calibration of the C-band data is in progress and the results will not be presented here. L-Band. The calibration procedure for this polarimetric radar dataset was reported in a paper by Holt and Free- man [3]. Briefly, the procedure first removes crosstalk be- tween the radar channels, the relative amplitude balance between the channels and then it adjusts the absolute gain of the radar data. The relative and absolute calibration of this data is accomplished by the exploiting the scatter- ing properties of distributed targets and the known radar cross-sections of corner reflectors in radar data. P-Band. This data was phase calibrated without invok- ing the above procedure. A target with known H-V phase difference was selected as the reference and the data com- pensated based this expected value. In this case, the bog area above the forested area near the Tanana River was se- lected as the reference. As a check, the phase difference at CH2825-8/90/0000-0337/$01 .OO