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Defense & Security

Radar target identification using a ‘banded’ E-pulse technique

A resonance-based radar target recognition scheme shows potential for differentiating between small physical differences in similar targets.
5 March 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0593

We have recently designed a new resonance-based technique to address radar target discrimination problems.1 Resonance-based radar target recognition uses the natural resonances extracted from the late time signature of an electromagnetic transient scattered by a target. It is based on the singularity expansion method (SEM), first introduced in the 1970s, and which describes the late time transient scattered field as a sum of damped exponentials with complex natural resonances (CNRs).2 It is well known that CNRs can be used to identify targets because their characteristics depend on the physical geometry and electrical properties of the target.

The SEM description has accordingly led to various target identification schemes that make use of target CNR signatures. For example, the K-pulse3, S-pulse,4 and E-pulse4,5 discrimination schemes all use CNRs in various algorithms to recognize targets. In particular, the E-pulse is a synthesized time domain linear filter which matches all significant CNRs of the target response when convolved with the late time target response. A null convolution result only occurs when the E-pulse and target response are from the same target. Target identification can therefore be achieved by thresholding the energy level of the convolution in late time. The convolution with the lowest energy level identifies the target.

Let us consider what is involved in identifying a single target among a library of similar targets using a resonance-based method. In terms of a resonance description, each target has its own set of specific resonant frequencies that result from its physical structure and corresponding dielectric properties. When comparing two or more targets with similar physical properties, it is most likely that they would have similar CNRs. It is also likely that they will have some different CNRs corresponding to the physical properties that they do not share.

To improve target discrimination, we recently developed the ‘banded’ E-pulse (BEP) technique1 illustrated in Figure 1. The entire frequency band of the target response in the frequency domain is divided into a number of frequency sub-bands. The E-pulse is constructed using the resonant poles within each band,5 termed the BEP, and convolved with the corresponding banded target response (BTR), again constructed from the CNRs and residues within the same band. Target identification is achieved by convolving the BEP with the BTR of the various targets. To quantify the energy level of the convolution, the E-pulse discrimination number (EDN)4 and the discrimination ratio (DR) are computed for each band. For the BEP, these are respectively referred to as the EDNB,p,q and DRB,p,q values.1

Figure 1. The concept of the ‘banded’ E-pulse for target identification. The target response in the frequency domain is divided into N frequency bands. Within each band, E-pulses and target responses are constructed and convolved for individual consideration.

Numerical examples using two stick aircraft models were investigated. Both models had the same physical structure except for the rear wing segment, measuring 0.4m for model 1 and and 0.6m for model 2 (see Figure 2). Such wire segments correspond to fundamental resonant frequencies of 0.375 and 0.25GHz, respectively. The monostatic backscattered signal in the frequency domain for the 0–2GHz plane wave incidence was obtained using the method of moments. The entire frequency band was divided into four bands (0–0.5, 0.5–1, 1–1.5 and 1.5–2GHz), and the BEP was applied. The resulting computed DRB,p,q values are plotted in Figure 3.

Figure 2. Wire model aircraft used in our studies. Models 1 and 2 have r=0.4 and 0.6m, respectively.

Figure 3. DRB,p,q values for aircraft models 1 and 2. These values were also computed for the original E-pulse formulation using the same algorithm and considering the whole frequency band from 0 to 2GHz. The results demonstrate that the BEP technique yields a higher DRB,p,q value in most bands when compared with the E-pulse technique.

Band 1 has the highest DRB,p,q value, showing that the difference is most significant in this band. Physically, the geometrical difference between the two models is based purely on the different lengths of the rear wing segment, and the resonant frequencies corresponding to this section fall into band 1. Thus the BEP technique is capable of differentiating physical differences between similar targets. We also followed the original E-pulse technique3 by considering the entire 0–2GHz band. Results showed that the BEP technique gave a higher DRB,p,q value for most cases, demonstrating improved discrimination performance.

In conclusion, we have designed a banded E-pulse technique and applied it to the radar target discrimination problem. Numerical examples using high-Q aircraft wire models were used to test the proposed method. The results clearly show that the banded approach provides another level of information to discriminate between targets and may be applied as a further level of confirmation in target discrimination schemes, with potential for improved performance.

Hoi-Shun Lui, Nicholas Shuley 
School of Information Technology and Electrical Engineering,
University of Queensland
St. Lucia, Queensland, Australia 

Hoi-Shun Lui was born in Hong Kong. Currently, he is a PhD student in the School of Information Technology and Electrical Engineering at the University of Queensland, Australia. His current research interests include resonance-based radar target identification and electromagnetic scattering.

Nicholas Shuley received his BE and ME in electrical engineering from the University of New South Wales, Australia, and his PhD also in electrical engineering from Chalmers University, Göteborg, Sweden. He is currently with the University of Queensland in Brisbane, where he conducts research in areas related to ultrawideband radar target identification.