Eric K. Walton

Time-frequency distribution analysis of scattering from waveguide cavities

The time-frequency distributions (TFD) analysis of the electromagnetic scattering from a circular waveguide cavity is investigated. Two time-frequency representations, the Wigner distribution and the running-window Fourier transform, are applied to the frequency-domain scattering data and the results are compared. For the cavity, propagating modes and cutoff frequencies can be determined from the TFD, whereas neither time nor frequency representations will provide such information directly. >

The ohio state university compact radar cross-section measurement range

The ElectroScience Laboratory at The Ohio State University has recently installed a new radar cross-section measurement system. The system uses a Scientific Atlanta compact range reflector (an offset parabolic reflector) to generate a far-field plane wave in the confines of a 40 \times 20 \times 60 ft anechoic chamber. The system uses a computer controlled microwave frequency synthesizer and a multichannel computer controlled receiver. The target support/positioning system and an optical target alignment system are also interfaced to the computer. The parameters of the system are 1) operation from 1 to 30 GHz (eventual operation to 96 GHz has been confirmed by field probing); 2) a plane wave volume (test area volume) of 1.3 m diameter; and 3) a sensitivity of -50 dBsm. Of particular importance is the ability of the system to measure phase as well as amplitude. This permits vector subtraction of the background and system calibration using a reference (sphere) target. The development of this system and the performance characteristics obtained so far will be discussed. Some results which demonstrate the system performance will be shown. Of particular interest is the broad-band measurement of both amplitude and phase. This permits conversion of the results to the time domain. Examples will be shown in which the various system components (antennas, reflector, walls, ceiling, etc.) are separated in the time domain by this technique. The development of the range is continuing and planned future improvements will also be discussed.

Estimating the time-delay and frequency decay parameter of scattering components using a modified MUSIC algorithm

A superresolution technique to determine the time-delay and frequency response of individual components of electromagnetic scattering is presented. A modification of the MUltiple SIgnal Classification (MUSIC) algorithm is proposed for the data model which includes scattering components with frequency dependent amplitudes. The modified technique is based on pre-multiplying the scattering data by the inverse of an assumed frequency dependence parameterized by a variable /spl alpha/. Estimates of /spl alpha/ are then obtained by considering the roots of the root-MUSIC polynomial for various /spl alpha/. The algorithm is tested for simulated and experimental compact range data. For the example of measured scattering from a diagonal flat plate, different decay parameters corresponding to the single and double diffraction components are determined using the new algorithm. >

Ultrawide-band noise radar in the VHF/UHF band

A radar that transmits continuous band-limited random noise is considered. The target impulse response is constructed from the cross correlation of the received signal with the transmitted noise signal. The system uses a fixed-length delay line and relies on the target movement through the range gate. Range profiles of different automobiles are measured with this system and used in a target recognition example.

Application of maximum likelihood estimation to radar imaging

An efficient maximum likelihood (ML) estimator to obtain the scattering center locations of a target and the relative scattering level of these scattering centers from the scattered field data is described. In the proposed method, ML estimation is carried out in the image domain rather than in the frequency-aspect domain. A two-dimensional (2-D) inverse Fourier transform is used to transfer the scattered field data from frequency-aspect domain to the image domain (down-range/cross-range). As expected, the scattered field data in the image domain has some regions with high energy. The samples in the high-energy regions are used to obtain the initial guess for the ML estimator as well as for ML estimation. The ML estimator in the image domain is applied to both simulated and experimental scattered fields of some targets.

Compact range radar cross-section measurements using a noise radar

This paper discusses the measurement of radar cross section (RCS) with a very low-cost system that transmits band-limited random noise over the band from 1.0 to 4.0 GHz. The received signal is correlated with a delayed version of the transmitted signal. A variable delay line is used to obtain the response at various delay times. This yields the radar target impulse response as a function of delay. This can be transformed to yield both the amplitude and phase of the scattering matrix in the frequency domain.

Moving vehicle range profiles measured using a noise radar

The Ohio State University ElectroScience Laboratory has built and is operating a set of noise correlation radar systems. We discuss the construction and operation of such a system designed for the foliage penetration and vehicle identification band. A noise radar operates by generating a wide band noise signal and transmitting this signal as it is. The receiving system receives signals (including those scattered by the target) and forms a cross-correlation with a delayed version of the transmitted signal. This process must be done at base band, which in this case is the band from 50 to 600 MHz (but is often much higher). Thus analogue components rather than digital cross correlation are used.

Annular slot windshield antenna

An automotive AM/FM hidden antenna using the slot between the body of the automobile and a metal solar heat-reduction film in the front windshield is described. Such solar heat-reduction metal films are becoming common and thus represent an opportunity to incorporate an antenna at only marginal cost. The radio frequency (RF) input impedance and gain patterns are described, and techniques for impedance matching are shown. Theoretical and experimental antenna patterns are compared. A test range measurement system and experimental results is described. A mobile on-road system to characterize the antenna in urban and suburban environments is shown. The application of this mobile system to the measurement of calibrated gain patterns is also demonstrated.

High resolution imaging of radar targets using narrow band data

The impulse responses of a target for a limited bandwidth (at several aspect angles) are used to generate the ISAR (inverse synthetic aperture radar) image of the target. The high-resolution images for a test target are then compared to the FT (Fourier transform)-based ISAR images. For a 2-GHz bandwidth, the FT-based image and the image obtained from high-resolution PSD are shown. For this bandwidth it can be seen that the image generated from the high-resolution PSD more clearly resolves the individual components.<<ETX>>

Far-field measurements and maximum entropy analysis of lossy material on a conducting plate

The relationships between Fourier techniques and maximum entropy techniques for the case of scattering measurements from metal-plate-backed lossy dielectric material are described. A set of experimental measurements were made in the frequency and aspect-angle domains on square panels of a multilayer radar absorbing material (RAM) backed by a conducting plate (approximately 30*30 cm). The minimum scattered signal was observed in the 9-12-GHz band. Experiments were performed over a band of frequencies from 2 to 18 GHz. Analysis techniques are described for transformations to the time and Doppler (cross-range) domains. Interpretations of the results with respect to the penetration of the radar signal into the RAM and scattering from the edge discontinuity of the plate are given and discussed. >