High resolution polarimetric imaging of biophysical objects using synthetic aperture radar.

A synthetic aperture microwave near-field system is used to image biophysical objects
in order to investigate the nature of radar-target interaction.
Two different imaging algorithms for focusing data collected over a two-dimensional
planar aperture are investigated. The first of these is the single frequency backward
propagation technique which is mathematically simple to implement and provides a
high degree of resolution. Secondly, a multifrequency development of the backward
propagation algorithm is presented and derived from two separate perspectives. This
latter algorithm, known as the auto-focusing algorithm, requires no information about
the range of the target from the aperture. Full characterisation by simulation of both
algorithms is carried out and different filtering techniques are investigated.
The backward propagation algorithm is applied to the polarimetric imaging of three
different leafless trees and a sugar beet plant at the X-band frequency of 10GHz. The
images so produced demonstrate that the backscattered signal is dependent on the
orientation of individual tree elements with respect to the polarisation. Furthermore,
multiple scattering terms can be identified within the structure of the tree.
The auto-focusing algorithm is applied to the polarimetric imaging of two trees at
10GHz and repeat measurements are made over several months. As with the single
frequency measurements, the backscattered signal is dependent on the orientation of
individual tree elements relative to the polarisation. The relative contributions from
the leaves and branches of the trees to the backscattered signal are assessed and found
to be seasonally dependent.
Measurements are also carried out to investigate the variation of backscatter from a
beech tree with varying incidence angle. It is demonstrated that at small angles of
incidence, the leaves are the dominant source of backscatter but at large incidence
angles, the branches and trunk of the tree have the greatest contrbution.