Dennis P. Nyquist

The singularity expansion method and its application to target identification

The singularity expansion method (SEM) for quantifying the transient electromagnetic (EM) scattering from targets illuminated by pulsed EM radiation is reviewed. SEM representations for both induced currents and scattered fields are presented. Natural-resonance-based target identification schemes, based upon the SEM, are described. Various techniques for the extraction of natural-resonance modes from measured transient response waveforms are reviewed. Particular attention is given to the aspect-independent (extinction) E-pulse and (single-mode) S-pulse discriminant waveforms which, when convolved with the late-time pulse response of a matched target, produce null or mono-mode responses, respectively, through natural-mode annihilation. Extensive experiment results for practical target models are included to validate the E-pulse target discrimination technique. Finally, anticipated future extensions and areas requiring additional research are identified. >

Identification of propagation regimes on integrated microstrip transmission lines

There has been a resurgence of interest in the propagation characteristics of open integrated microstrip transmission lines. This is due in part to the discovery of diverse propagation regimes for higher-order modes on open lines. In contrast to the dominant EH/sub 0/ mode, three distinct propagation regimes exist for higher-order modes on microstrip transmission lines. In this paper, a rigorous spectral-domain integral equation formulation is used to analyze propagation in all three regimes. This formulation provides a clear physical picture of the different propagation regimes based on the mathematical location of poles and branch points in the complex spectral-variable plane. As an illustration, the formulation is applied to the case of an isolated uniform microstrip transmission line. The integral equation is discretized via the method of moments, and entire-domain basis functions incorporating suitable edge behavior are utilized to provide convergence with relatively few terms. The results obtained are compared to the results of other workers, and good agreement is observed. >

Radar target discrimination by convolution of radar return with extinction-pulses and single-mode extraction signals

A new method of radar target discrimination and identification is presented. This new method is based on the natural frequencies of the target. It consists of synthesizing aspect-independent discriminant signals, called extinction-pulses (E-pulses) and single-mode extraction signals which, when convolved numerically with the late-time transient response of an expected target, lead to zero or single-mode responses. When the synthesized, discriminant signals for an expected target are convolved with the radar return from a different target, the resulting signal will be significantly different from the expected zero or single-mode responses, thus, the differing targets can be discriminated. Theoretical synthesis of discriminant signals from known target natural frequencies and experimental synthesis of them for a complex target from its measured pulse response are presented. The scheme has been tested with measured responses of various targets in the laboratory.

Frequency domain E-pulse synthesis and target discrimination

A frequency domain approach to the E -pulse radar target discrimination scheme is introduced. This approach is shown to allow easier interpretation of E -pulse convolutions via the E -pulse spectrum, and leads to a simplified calculation of pulse basis function amplitudes in the E -pulse expansion. Experimental evidence obtained using aircraft models verifies the single-mode discrimination scheme, as well as the aspect-independent nature of the E -pulse technique. This leads to an integrated technique for target discrimination combining the E -pulse with single mode extraction waveforms.

Extraction of the natural frequencies of a radar target from a measured response using E-pulse techniques

A new scheme is introduced for extracting the natural resonance frequencies of a radar target from a measured response. The method is based on the E -pulse technique and is shown to be relatively insensitive to random noise and to estimates of modal content. Verification of the technique is accomplished by comparing the natural frequencies extracted from the measured responses of a thin cylinder and a circular loop with those obtained from theory. The applicability of the technique to low- Q targets is also demonstrated, using the measured response of a scale model aircraft.

An excitation theory for bound modes, leaky modes, and residual‐wave currents on stripline structures

The nature of the current on a general multilayered printed-circuit stripline structure excited by a delta-gap source is investigated. The current is obtained through the construction of a semianalytical three-dimensional (3-D) Greens function, which accounts for the presence of the infinite conducting strip and the layered background structure. The 3-D Greens function is obtained by Fourier transforming the delta-gap source in the longitudinal (z) direction, which effectively resolves the 3-D problem of a delta-gap source into a superposition of 2-D problems, each of which is infinite in the z direction. The analysis allows for a convenient decomposition of the strip current into a sum of constituent parts. In particular, the strip current is first resolved into a set of bound-mode current waves and a continuous-spectrum current. The continuous-spectrum current is then represented as a set of physical leaky-mode currents in addition to a set of “residual-wave” currents, which arise from the steepest-descent integration paths. An asymptotic analysis reveals that the residual-wave currents decay algebraically as z−3/2. Far away from the source, the residual-wave currents dominate the continuous-spectrum strip current. Results are shown for a specific type of stripline structure, but the analysis and conclusions are valid for arbitrary multilayer stripline structures.

Performance of an automated radar target discrimination scheme using E pulses and S pulses

Previous studies have demonstrated the viability of natural resonance based target discrimination using extinction pulses (E pulses) and single-mode pulses (S pulses). These studies qualitatively demonstrated the principles of resonance annihilation by forcing the interrogating pulse to have zeros at the complex natural resonance frequencies of the target. Here a quantitative scheme for evaluating discrimination using the E pulse and the S pulse is given. The performance of an automated E-pulse and S-pulse discrimination scheme is evaluated using numerically derived scattering data with varying amounts of noise. >

Integral Formulation for Analysis of Integrated Dielectric Waveguides

A polarization integral equation is advanced for use in the conceptual and numerical analysis of a broad class of integrated dielectric waveguiding systems. The equation is applied to axially uniform waveguides, in which case the axial integral becomes convolutional in nature, prompting a Fourier transform on that variable. Inversion of the transformed guiding region field, aided by complex analysis, allows identification of two components of that field the surface-wave modes and the radiation field. These are found in terms of the sources exciting the system, leading to a new formulation for the excitation of such waveguides. Analysis of the behavior of the kernel of the transformed integral equation in the complex plane leads to a general criterion for surface-wave leakage from the guiding region. Numerical results for the propagation characteristics of step- and the graded-index rectangular strip and rib waveguides are obtained from the integral equation by application of the method of moments and by a quasi-closed-form solution technique. These results are compared to those of other formulations. Further application of the integral equation is discussed, and several promising areas for further study are identified.

Scattering center analysis of radar targets using fitting scheme and genetic algorithm

Development of successful radar target discrimination schemes using ultrawideband signatures hinges on an accurate understanding of the scattering behavior of complex radar targets. Since it is very difficult to calculate the scattered field of complex targets theoretically, a mathematical model (Altes (1976) model) representing scattering center impulse response has been developed to describe the scattered field. The extraction of temporal positions, pulse responses, and transfer functions of target scattering centers is demonstrated using artificially created and measured responses. Two different scale aircraft models (B-58 and B-52) are utilized. The fitting scheme based on the least squares method is quite satisfactory but its accuracy deteriorates when the overlapping of scattering-center pulse responses is severe. To overcome this problem a genetic algorithm is used to improve the results. While the genetic algorithm gives much better accuracy, it consumes much more computer time due to its global nature and lack of derivative information. The purpose of this analysis is to provide a method to reduce data storage for ultrawideband signatures in target discrimination.

Radar target identification using a combined early-time/late-time E-pulse technique

The E-pulse technique has been applied in the past to both the early- and the late-time components of a transient radar response. While the late-time E-pulse technique uses aspect-independent waveforms, the early-time E-pulse technique requires a separate waveform for each target aspect angle and thus significantly more storage and processing time. This paper discusses a combination of the two techniques that employs the early-time technique to remove ambiguities generated from application of the late-time method. By narrowing the possible range of aspect angles of the potential targets, the early-time technique can be employed more efficiently.