A 3-DIMENSIONAL RADAR BACKSCATTER MODEL OF FOREST CANOPIES

A three-dimensional forest backscatter model, which takes full account of spatial position of trees in a forest stand is described. A forest stand was divided into cells according to arbitrary spatial resolutio n. The cells may include ''crown,'' ''trunk'', and ''gap'' components, determined by the shape, size and position of the trees, The forest f loor is represented by a layer of ''ground'' cells. A ray tracing meth od was used to calculate backscattering components of 1) direct crown backscatter, 2) direct backscattering from ground, 3) direct backscatt ering from trunk, 4) crown-ground scattering, and 5) trunk-ground scat tering. Both the attenuation and time-delay of microwave signals withi n cells other than ''gap'' were also calculated from ray tracing. The backscattering Mueller matrices of these components within the same ra nge intervals were incoherently added to yield the total backscatterin g of an image pixel. By assuming a zero-mean, multiplicative Gaussian noise for image speckle, the high-resolution images were aggregated to simulate a SAR image with a given spatial resolution and number of in dependent samples (looks). A well-characterized 150 m x 200 m forest s tand in Maine, USA, was used to parameterize the model. The simulated radar backscatter coefficients were compared with actual JPL SAR data. The model gives reasonable prediction of backscattering coefficients averaged over the entire stand with agreement between model and data w ithin 1.35 dB for all channels, The correlations between simulated ima ges and SAR data (10 by 15 pixels) were positive and significant at th e 0.001 level for all frequencies (P, L, and C bands) and polarization s (HH, HV, and VV).