Application Of High Frequency Natural Resonances Extracted From Electromagnetic Scattering Response For Discrimination Of Radar Targets With Minor Variations

Abstract

Radars, as the name suggests, were traditionally used for Radio Detection and Ranging. Nevertheless, advances in high resolution electromagnetic simulations, Ultra Wide-Band sources, signal processing and computer technologies have resulted in a possible perception of radars as sensors for target discrimination. In this thesis, the feasibility of discrimination between targets even with minor variations in structure and material composition on the basis of radar echoes is effectively demonstrated.

It is well-known that the echoes from any target are affected by its natural frequencies which are dependent only on the shape and material composition of the target, and independent of the aspect angle or the incident waveform. The E-pulse technique is based on the fact that incident waveforms can be designed that uniquely annihilate the echoes from chosen regions of a target, and forms the basis of the method of discrimination proposed in this thesis.

Earlier methods reported in the literature, effectively discriminated only between different classes of targets with substantial variations in the overall dimensions of the body. Discrimination of targets of the same class with a minor structural modification or with a material coating on specific areas was rather difficult. This thesis attempts and successfully validates a method which comprehensively addresses this problem. The key idea of this method is to use the higher frequency resonances (which characterise the finer details of a target) in the E-pulse technique.

An obviously important aspect of target discrimination is therefore that of precisely estimating the natural frequencies for each target and understanding the changes in these frequencies, and their associations with the changes in structure and material composition. Current approaches to determine these frequencies are either based In the time or frequency domains. While the latter approach comprises the computation of the roots of a related determinantal equation, in the time domain, the natural frequencies are extracted from the response of a target to an impulse. Such a response can either be generated from actual experiments or by simulating the scattering response using Computational Electromagnetic (CEM) techniques. In this work, the impulse response is obtained from the frequency response of the scatterers in the frequency range of interest. Since no single CEM technique can effectively cover the entire range of frequencies needed for the E-Pulse synthesis. The Method of Moments and Physical Optics have been used for low and high frequency scattering respectively. The results obtained using the latter technique are validated by comparing with those obtained using Method of Moments at the transition frequencies and Geometrical Theory of Diffraction (GTD).

The natural frequencies (i.e., poles of a corresponding transfer function) are extracted from the impulse response using Prony's algorithm. One of the parameters in this method is the number of such poles (i.e.. the order of the transfer function) present in the response, and the accuracy of the computed pole values depends on this assumed order. Here, the Hankel singular values of a transfer function is used to estimate the number of poles. This in turn implies that a specific norm of the error between a transfer function corresponding to the frequency response generated earlier, and a transfer function with an assumed order obtained using Prony's method is minimised.

In the thesis, a wide range of target shapes are considered for purposes of illustration: wires, cylinders, spheres, plates and complex bodies such as aircraft, and the discrimination capability is demonstrated by introducing minor perturbations in their shape and/or material composition. .The following cases are considered here: (a) Wires: Conducting wires with a protrusion in one segment; conducting wire from another coated with a dielectric in a segment, (b) Cylinders: Conducting cylinders with one perturbed; conducting cylinders with a portion scrapped off in the middle, (c) Plates: Conducting plates with a elongation on one comer; conducting plate with another one with a hole in the centre, (d) Spheres: Conducting spheres with different radii; conducting spheres with Radar Absorbing Material coated spheres with different coating thickness; conducting spheres with chiral coated spheres with varying coating thickness, (e) Aircraft: Canonical model of MiG-29 aircraft from a similar one with stores placed under the wing.