VEGETATION CHARACTERISTICS AND UNDERLYING TOPOGRAPHY FROM INTERFEROMETRIC RADAR

This paper formulates and demonstrates methods for extracting vegetati on characteristics and underlying ground surface topography from inter ferometric synthetic aperture radar (INSAR) data. The electromagnetic scattering and radar processing, which produce the INSAR observations, are modeled, vegetation and topographic parameters are identified for estimation, the parameter errors are assessed in terms of INSAR instr umental performance, and the parameter estimation is demonstrated on I NSAR data and compared to ground truth. The fundamental observations f rom which vegetation and surface topographic parameters are estimated are (1) the cross-correlation amplitude, (2) the cross-correlation pha se, and (3) the synthetic aperture radar (SAR) backscattered power. A calculation based on scattering from vegetation treated as a random me dium, including the effects of refractivity and absorption in the vege tation, yields expressions for the complex cross correlation and backs cattered power in terms of vegetation characteristics. These expressio ns lead to the identification of a minimal set of four parameters desc ribing the vegetation and surface topography: (1) the vegetation layer depth, (2) the vegetation extinction coefficient (power loss per unit length), (3) a parameter involving the product of the average backsca ttering amplitude and scatterer number density, and (4) the height of the underlying ground surface. The accuracy of vegetation and ground s urface parameters, as a function of INSAR observation accuracy, is eva luated for aircraft INSAR, which is characterized by a 2.5-m baseline, an altitude of about 8 km, and a wavelength of 5.6 cm. It is found th at for approximate to 0.5% accuracy in the INSAR normalized cross-corr elation amplitude and approximate to 5 degrees accuracy in the interfe rometric phase, few-meter vegetation layer depths and ground surface h eights can be determined from INSAR for many types of vegetation layer s. With the same observational accuracies, extinction coefficients can be estimated at the 0.1-dB/m level. Because the number of parameters exceeds the number of observations for current INSAR data sets, extern al extinction coefficient data are used to demonstrate the estimation of the vegetation layer depth and ground surface height from INSAR dat a taken at the Bonanza Creek Experimental Forest in Alaska. This demon stration shows approximately 5-m average ground truth agreement for ve getation layer depths and ground-surface heights, with a clear depende nce of error on stand height. These errors suggest refinements in INSA R data acquisition and analysis techniques which will potentially yiel d few-meter accuracies. The information in the INSAR parameters is app licable to a variety of ecological modeling issues including the succe ssional modeling of forested ecosystems.