FDTD simulations of the Polarization dependency of terrain vehicles in VHF-band SAR images

The objective of the report is to discuss the relative merits of using HH- or VV-polarization for target detection using synthetic-aperture radar (SAR) in the 20-90 MHz band. The report shows results of computing electromagnetic scattering amplitudes and simulating SAR images of a military terrain vehicle on dielectric ground. Incidence angles studied are 60°, 73° and 85°. The electromagnetic computations are performed using the FDTD method and the SAR simulations are performed using a plane-wave model. The radar-cross section in the low-frequency limit is generally higher for VV- compared to HH-polarization. This is a consequence of the higher total incident field for VV-polarization. However, the scattering mechanism is more complicated in the studied range of parameters and includes both low- and high-frequency properties. The main conclusion is that high-frequency mechanisms dominate for incidence angles 60° and 73°, i.e. the scattering comes mainly from the dihedral formed between the vehicle side and the ground. This gives higher radar-cross section for HH-polarization. VV-polarization is slightly higher at the highest incidence angle of 85° due to the angular dependency of the Fresnel reflection coefficients. For foliage-penetration applications, however, it is noted that both forest clutter and attenuation are lower for HH-polarization so that detection performance using HH-polarization will generally be superior to VV-polarization in the studied range of parameters.