Polarimetry for penetrating radar - Evaluation of polarimetric ISAR radar measurements

Authors:

  • Tommy Johansson
  • Ain Sume
  • Jonas Rahm
  • Stefan Nilsson
  • Anders Örbom

Publish date: 2011-10-25

Report number: FOI-R--3260--SE

Pages: 53

Written in: English

Keywords:

  • “See-through-the-wall”
  • radar
  • ISAR
  • polarization
  • classification
  • wall
  • penetration
  • human activity

Abstract

This report describes fully polarimetric through-the-wall (TTW) radar measurements with high spatial resolution, attained by ISAR (Inverse Synthetic Aperture Radar) imaging. Polarimetric methods have potential in this application since they may reduce the effects of the wall interaction and increase the contrast between humans and the background. This is especially the case with stationary target objects, where the highly sensitive Doppler technique is not applicable. The measurements were performed at FOI's test range Lilla Gåra in June 2010. The main scene was of a human sitting in a small wooden cabin. The cabin was placed on a turntable and rotated, to obtain ISAR imaging. By using separate, stationary but colocated antennas for transmitting and receiving, both capable of switching between vertical (V) and horizontal (H) polarization, four polarization combinations were obtained. Data were collected over four full 360° rotations of the turntable changing the combination between turns. The central feature in this measurement was that phase coherence was maintained through a whole measurement series over four turns. This enabled co-processing of the whole collected data set with coherent methods, where the information of the relative phase between received signals in the various polarization combinations could be used for, e.g., object classification. For the co-processing, a statistical description was used of the measured data with the so-called coherency matrix for the received signals. Accurate calibration of raw data formed an essential part of the work. Three separate calibration reflector measurements must be made to get enough information to calibrate the system. The calibration method adopted here involves measurements of a trihedral corner reflector and a dihedral reflector with two inclinations. The reflector scattering properties are assumed to be known. The ISAR images produced for the TTW scenes show that the human can be discerned from the background. The contrast is greatest with vertical polarization at transmit and receive, with less contrast using crosspolarization or horizontal polarization. The asymmetry between vertical and horizontal polarizations is probably due to the horizontal grain structure in the wooden walls. These results are partly different from those of published simulations, which assume homogeneous wall. Polarization Difference Imaging (PDI), formed in different ways of ISAR images, gives some enhanced contrast between the human and the background but the improvement is not great. A classification scheme that has come to be used with SAR has been tested in this work with a view to discriminating between different target objects in the cabin. The method shows some promise, but a reliable classification has not yet been attained since the polarimetric features chosen in the scheme show some overlap.