Region-enhanced passive radar imaging

The authors adapt and apply a recently-developed region-enhanced synthetic aperture radar (SAR) image reconstruction technique to the problem of passive radar imaging. One goal in passive radar imaging is to form images of aircraft using signals transmitted by commercial radio and television stations that are reflected from the objects of interest. This involves reconstructing an image from sparse samples of its Fourier transform. Owing to the sparse nature of the aperture, a conventional image formation approach based on direct Fourier transformation results in quite dramatic artefacts in the image, as compared with the case of active SAR imaging. The regionenhanced image formation method considered is based on an explicit mathematical model of the observation process; hence, information about the nature of the aperture is explicitly taken into account in image formation. Furthermore, this framework allows the incorporation of prior information or constraints about the scene being imaged, which makes it possible to compensate for the limitations of the sparse apertures involved in passive radar imaging. As a result, conventional imaging artefacts, such as sidelobes, can be alleviated. Experimental results using data based on electromagnetic simulations demonstrate that this is a promising strategy for passive radar imaging, exhibiting significant suppression of artefacts, preservation of imaged object features, and robustness to measurement noise

Forward scatter radar for air surveillance: Characterizing the target-receiver transition from far-field to near-field regions

A generalized electromagnetic model is presented in order to predict the response of forward scatter radar (FSR) systems for air-target surveillance applications in both far-field and near-field conditions. The relevant scattering problem is tackled by developing the Helmholtz-Kirchhoff formula and Babinet's principle to express the scattered and the total fields in typical FSR configurations. To fix the distinctive features of this class of problems, our approach is applied here to metallic targets with canonical rectangular shapes illuminated by a plane wave, but the model can straightforwardly be used to account for more general scenarios. By exploiting suitable approximations, a simple analytical formulation is derived allowing us to efficiently describe the characteristics of the FSR response for a target transitioning with respect to the receiver from far-field to near-field regions. The effects of different target electrical sizes and detection distances on the received signal, as well as the impact of the trajectory of the moving object, are evaluated and discussed. All of the results are shown in terms of quantities normalized to the wavelength and can be generalized to different configurations once the carrier frequency of the FSR system is set. The range of validity of the proposed closed-form approach has been checked by means of numerical analyses, involving comparisons also with a customized implementation of a full-wave commercial CAD tool. The outcomes of this study can pave the way for significant extensions on the applicability of the FSR technique

Geometric polarimetry - part II: the Antenna Height Spinor and the Bistatic Scattering Matrix

This paper completes the fundamental development of the basic coherent entities in Radar Polarimetry for coherent reciprocal scattering involving polarized wave states, antenna states and scattering matrices. The concept of antenna polarization states as contravariant spinors is validated from fundamental principles in terms of Schelkunoff's reaction theorem and the Lorentz reciprocity theorem. In the general bistatic case polarization states of different wavevectors must be related by the linear scattering matrix. It is shown that the relationship can be expressed geometrically, and that each scattering matrix has a unique complex scalar invariant characterising a homographic mapping relating pairs of transmit/receive states for which the scattering amplitude vanishes. We show how the scalar invariant is related to the properties of the bistatic Huynen fork in both its conventional form and according to a new definition. Results are presented illustrating the invariant k for a range of spheroidal Rayleigh scatterers

Bistatic 3D Electromagnetic Scattering from a Right-Angle Dihedral at Arbitrary Orientation and Position

A method is created to extend a bistatic 3D electromagnetic scattering solution for a dihedral at a given orientation and position to the case of arbitrary orientation and position. Results produced using this method are compared to shooting and bouncing rays (SBR) and method of moments (MoM) predictions, as well as measured data for applicable cases. The model in this thesis shows excellent agreement in magnitude and phase with SBR predictions. It also shows good agreement in magnitude with MoM predictions. Small phase differences between model and MoM data occur due to differences in the underlying scattering solution and the more exact MoM prediction. The model accurately predicts bistatic scattering from a dihedral at arbitrary orientation and position and is computationally more efficient than SBR and MoM methods