Tsz-King Chan

An imaging technique using confocal circular synthetic aperture radar

This paper presents a theory and its experimental demonstration of an imaging technique based on three-dimensional (3D) space-time confocal imaging and circular synthetic aperture radar (SAR). The theory is an extension of the conventional straight-path SAR-to-SAR on an arbitrary curved path. Next, a general formulation for the curved SAR is applied to circular SAR geometry, which has two important features. First, it allows the maximum attainable resolution to be an the order of a wavelength. Second, it makes 3D confocal imaging possible, X-band (7-13 GHz) imaging experiments are conducted to demonstrate this technique.

Experimental studies on circular SAR imaging in clutter using angular correlation function technique

This paper describes a novel approach for three-dimensional (3-D) imaging in a heavy-clutter environment. The proposed approach consists of two techniques. The first calls for the use of a synthetic aperture radar (SAR) moving in a circular orbit, whereas the second involves using angular correlation function (ACF) measurement for achieving clutter suppression. To demonstrate the imaging and clutter suppression capabilities of this combined approach (circular-correlation SAR), microwave imaging experiments are conducted at X-band frequencies (7-13 GHz). It is found that in heavy-clutter environments, this combined SAR approach outperforms conventional SAR, resulting in clearer images with enhanced target-to-clutter ratio.

Subsurface detection of a buried object using angular correlation function measurement

Abstract A new technique is proposed for subsurface detection of buried objects using the angular correlation function (ACF) measurement of scattered waves. Compared with the traditional detection technique which relies on radar cross section (intensity) measurement, this new ACF-based technique results in better signal-to-clutter ratio and thus higher target visibility. Laboratory experiments were conducted at millimetre-wave (80–105 GHz) and X-band (7–13 GHz) frequencies to illustrate the potential effectiveness of this new correlation approach over the traditional cross section approach.

Experimental studies of bistatic scattering from two-dimensional conducting random rough surfaces

Despite the recent development of analytical and numerical techniques for problems of scattering from two-dimensional rough surfaces, very few experimental studies were available for verification. The authors present the results of millimeter-wave experiments on scattering from two-dimensional conducting random rough surfaces with Gaussian surface roughness statistics. Machine-fabricated rough surfaces with controlled roughness statistics were examined. Special attention was paid to surfaces with large rms slopes (ranging from 0.35 to 1.00) for which enhanced backscattering is expected to take place. Experimentally, such enhancement was indeed observed in both the copolarized and cross-polarized returns. In addition, it was noticed that at moderate angles of incidence, the scattering profile as a function of observation angle is fairly independent of the incident polarization and operating frequency. This independence justifies the use of the geometric optics approximation embodied in the Kirchhoff formulation for surfaces with large surface radius of curvature. When compared with the experimental data, this analytical technique demonstrates good agreement with the experimental data.

Angular memory effect of millimeter‐wave scattering from two‐dimensional conducting random rough surfaces

An experimental study was conducted to investigate the angular memory effect of millimeter-wave scattering from two-dimensional conducting random rough surfaces. The surfaces under investigation were machine-fabricated with known Gaussian roughness statistics, and the copolarized and cross-polarized angular correlation functions (ACFs) of scattering amplitudes were measured. It was found that for the case of reference antenna positions located bistatically in a backward direction, the measured ACF exhibits broad response when single scattering dominates but two peaks when multiple scatteringdominates. These observations are in good agreement with the second-order Kirchhoff approximation (KA2). Specifically, the observed broad and peak responses are analytically identified to be due to the first-order and second-order (ladder and cyclical) scattering components, respectively, in KA2.

Detection of a target in a homogeneous medium using angular correlation function

A novel technique for detecting a target buried under a homogeneous medium is presented. This technique is based on a phenomenon known as the angular memory effect, and involves measurement of the angular correlation function (ACF) for scattered waves observed at antenna positions where the masking effect of the medium on the desirable target ACF return is minimized. Millimeter-wave experiments were conducted to demonstrate the effectiveness of this technique.

Numerical study of the time-reversal effects on super-resolution in random scattering media and comparison with an analytical model

The time-reversal effects on super-resolution in random scattering media are analysed using numerical finite-difference time-domain (FDTD) simulations. The analytical solutions and results have been presented previously in the literature, which provide confirmation of spot-size reduction and also explanations of the shower curtain effects and backscattering enhancement. However, the analytical solutions are based on several approximations. Thus, validation of the analytical results against realistic scattering events is necessary. Two-dimensional FDTD Monte Carlo simulations have been employed for this investigation to simulate wave propagation and scattering in a random medium. The scattering environments are created by randomly locating cylindrical rods in the background medium. The simulation process involves a point source emitting a Gaussian pulse wave that propagates through the scattering medium, gets time-reversed, and then back-propagated into the same scattering medium. The focusing behaviours including the location of the focal point and its spot-size as a function of its transverse position are analysed. The shower curtain, particle size, and time domain effects are also investigated. In comparison, the behaviours of focusing derived by numerical results are consistent with those of previously reported analytical results. However, there are some differences, which we speculate to be mainly because of the different phase functions.

Circular SAR imaging using frequency and angular correlation techniques in a clutter environment

This paper outlines a brief study on imaging by circular SAR using two different correlation techniques in a clutter environment. The first technique calls for correlation between images over frequency using frequency correlation function (FCF) measurement; whereas, the second involves correlation images over aperture using angular correlation (ACF) measurement. Microwave SAR imaging experiments are conducted at a frequency range of 7-13 GHz to study the relative effectiveness of these SAR processing algorithms in a clutter environment. It was found that for imaging of point scatterers heavily embedded in clutter, the former (ACF technique) surpasses not only the latter (FCF technique), but also the conventional SAR processing approach, resulting in clearer images and hence enhanced target-to-clutter ratio.

Numerical and experimental studies of the angular memory effect of discrete random media in the azimuthal plane

In this paper the angular memory effect of discrete random media in the azimuthal plane was investigated using numerical and experimental techniques. Good agreement was achieved between the experimental and numerical results. Both results showed rapid angular decorrelation in the backscattering direction, which is in direct agreement with previous studies on the angular memory effect in the two-dimensional case.

Feasibility study on localized subsurface imaging using circular synthetic aperture radar and angular correlation function measurement

The traditional linear synthetic aperture radar (LSAR) flying along a linear flight path is modified to circular synthetic aperture radar (CSAR) flying along a circular flight path. When operating in down-looking spotlight mode for imaging purposes, CSAR, in comparison with LSAR, can provide higher pixel resolutions on any confocally focused plane within the volume illuminated by the antenna beams. By combining this 3D imaging technique with the circular angular memory effect, it is possible to reconstruct volume radar images in localized subsurface imaging applications where poor signal-to-noise ratio (SNR) is a critical issue. Laboratory experiments at X-band (7-13 GHz) and W-band (75-100 GHz) frequencies were conducted to illustrate the capability of CSAR to perform 3D imaging, and the analytical basis of circular angular memory effect due to random surface scattering based on the first-order Kirchhoff approximation.