Electromagnetic Scattering from Foliage Camouflaged Targets

Mojtaba Dehmollaian and Kamal Sarabandi

Characterization of scattering from camouflaged targets is one of the most challenging problems in the area of remote sensing. Fig.1 describes such a problem scenario, where a complex target is reradiated by myriads of nearby foliage particles. In order to solve the forward problem, the scattering problem is divided into three parts: 1) modeling of propagation through and scattering from the forest, and 2) calculation of scattering from the hard targets, illuminated by an arbitrary field, and 3) computation of scattering interaction between the hard target and foliage around it.

The first part of the problem is solved by employing a single scattering foliage model. This is a coherent model that preserves the geometry of tree structures using a statistical Lindermayer system. Tree constituents are modeled by dielectric cylinders and disks, whose scatterings are analytically derived using high- and low-frequency techniques which are valid in both the near-field and far-field regions of the foliage scatterers. The scattered field from all tree components and the attenuated incident field are used as the excitation for the hard target. The second part, computation of the scattered field form an isolated target, can be solved using exact or approximate numerical methods. For relatively low frequencies, we used the finite difference time domain (FDTD) technique to compute the scattered field from the hard target. For higher frequencies on the other hand we employed the iterative physical optics (IPO) approximation. The connection between these two models, that accounts for the interaction between the foliage scatterers and the target and vice versa, is accomplished through the application of the reciprocity theorem.

As an example the backscattered RCS of a camouflaged metallic tank inside a pine stand forest as a function of incidence angle is shown in Fig.2. In this simulation frequency of operation is 2 GHz and the scattering effect of two adjacent trees is taken into account. Pine trees are generated by the statistical L-system having an average height of 15 m, crown radius of 3 m, crown height of 10 m, trunk radius of 10 cm. Each tree consists of about 5000 scatterers.

Fig.1 Fig.2