Kangwook Kim

Multistatic Ground-Penetrating Radar Experiments

A multistatic ground-penetrating radar (GPR) system has been developed and used to measure the response of a number of targets to produce data for the investigation of multistatic inversion algorithms. The system consists of a linear array of resistive-vee antennas, microwave switches, a vector network analyzer, and a 3-D positioner, all under computer control. The array has two transmitters and four receivers which provide eight bistatic spacings from 12 to 96 cm in 12-cm increments. Buried targets are scanned with and without surface clutter, which is a layer of rocks whose spacing is empirically chosen to maximize the clutter effect. The measured responses are calibrated so that the direct coupling in the system is removed, and the signal reference point is located at the antenna drive point. Images are formed using a frequency-domain beamforming algorithm that compensates for the phase response of the antennas. Images of targets in air validate the system calibration and the imaging algorithm. Bistatic and multistatic images for the buried targets are very good, and they show the effectiveness of the system and processing.

Design of a resistively loaded vee dipole for ultrawide-band ground-penetrating Radar applications

A new resistively loaded vee dipole (RVD) is designed and implemented for ultrawide-band short-pulse ground-penetrating radar (GPR) applications. The new RVD is improved in terms of voltage standing wave ratio, gain, and front-to-back ratio while maintaining many advantages of the typical RVD, such as the ability to radiate a short-pulse into a small spot on the ground, a low radar cross section, applicability in an array, etc. The improvements are achieved by curving the arms and modifying the Wu-King loading profile. The curve and the loading profile are designed to decrease the reflection at the drive point of the antenna while increasing the forward gain. The new RVD is manufactured by printing the curved arms on a thin Kapton film and loading them with chip resistors, which approximate the continuous loading profile. The number of resistors is chosen such that the resonant frequency due to the resistor spacing occurs at a frequency higher than the operation bandwidth. The antenna and balun are made in a module by sandwiching them between two blocks of polystyrene foam, attaching a plastic support, and encasing the foam blocks in heat-sealable plastic. The antenna module is mechanically reliable without significant performance degradation. The use of the new RVD module in a GPR system is also demonstrated with an experiment.

A COMPACT BEAM RECONFIGURABLE ANTENNA FOR SYMMETRIC BEAM SWITCHING

In this paper, two radiation pattern-reconflgurable antennas are designed to operate near the frequency of 1.8GHz. The geometry of the proposed antennas is symmetric with respect to the vertical center line. The electrical shapes of the antennas are composed of a monopole-loaded loop and an open wire. The open wire functions as either a director or re∞ector for the two antennas. Depending on the switching state, the antennas can select between two beam directions with no input impedance difierence. The sizes of each antenna are then optimized to achieve beam switching capability using PIN diodes and FETs. The re∞ection coe-cients and gain patterns for two bias conditions using both switches are measured and compared with the simulated results. The measured results show that the proposed antennas can clearly alternate their beam directions using the switching components.

Multi-feature based detection of landmines using ground penetrating radar

In this paper, we present a method for detecting anti- tank or anti-personnel landmines buried in the ground. A set of data generated by a ground penetrating radar is processed to remove the surface re∞ection and clutter, yielding signals for possible landmines. In order to detect landmines in the signals, features are computed and compared against a database, which contains those of various landmines. Three features are proposed to use; principal components from principal component analysis, Fourier coe-cients and singular values from singular value decomposition method, each of which is chosen to represent each landmine uniquely. Detection is performed using Mahalanobis distance-based method. Examples show that the proposed method can efiectively detect landmines in various burial condition.

Design and Realization of a Discretely Loaded Resistive Vee Dipole on a Printed Circuit Board

A discretely loaded resistive vee dipole is designed and realized for use in pulse radiation applications, such as ground-penetrating radars. The resistive vee dipole is capable of radiating a broadband pulse whose shape is simply related to the input signal. In addition, it mostly eliminates the multiple reflections between the surface of the ground and the antenna because of its low radar cross section. Other investigators have studied the resistive vee dipoles using continuous loading. The antenna presented in this paper is printed on a circuit board and discretely loaded with off-the-shelf surface-mount chip resistors, making it easy, inexpensive to build, and mechanically stable. The characteristics of the antenna on a circuit board are measured and compared with the characteristics of the antenna in free space, which is numerically modeled using the method of moments code. The effects of the balun on the performance of the antenna are also presented.

A resistive linear antenna for ground-penetrating radars

The resistive vee dipole (RVD) loaded with the Wu-King profile has many advantages for use in ground-penetrating radar (GPR) applications. It can be designed to transmit a temporally-short pulse to a small spot on the ground. The shape of the transmitted pulse is simply related to the input signal, e.g., a derivative. The RVD also has a low radar cross section. In addition, it can be easily manufactured using a circuit board and discretely loading it with chip resistors. One drawback of the RVD is that the input impedance of the RVD increases significantly with decreasing frequency and, therefore, has a high voltage standing wave ratio (VSWR) at low frequencies, which limits the low-frequency response of the antenna. To improve the low-frequency response, a discretely-loaded resistive linear antenna (RLA) has been developed, whose basic principle of operation is the same as that of the RVD. The RLA has curved arms loaded with a modified Wu-King profile instead of straight arms loaded with the Wu-King profile. With an appropriate selection of the curve and the loading profile, the low-frequency response is significantly better for the RLA than for the RVD. The RLA has been developed using a method of moments code. The performance of the RLA is validated both numerically and experimentally.

GPR-Based Landmine Detection and Identification Using Multiple Features

This paper presents a method to identify landmines in various burial conditions. A ground penetration radar is used to generate data set, which is then processed to reduce the ground effect and noise to obtain landmine signals. Principal components and Fourier coefficients of the landmine signals are computed, which are used as features of each landmine for detection and identification. A database is constructed based on the features of various types of landmines and the ground conditions, including the different levels of moisture and types of ground and the burial depths of the landmines. Detection and identification is performed by searching for features in the database. For a robust decision, the counting method and the Mahalanobis distance-based likelihood ratio test method are employed. Four landmines, different in size and material, are considered as examples that demonstrate the efficiency of the proposed method for detecting and identifying landmines.

Investigation of a combined seismic, radar, and induction sensor for landmine detection

An experimental system to collect co‐located ground penetrating radar (GPR), electromagnetic induction (EMI), and seismic data was developed to investigate the benefits of the fusion of these sensors. In the experiments, a range of mines and clutter objects were buried at various depths in the sandbox at Georgia Tech. Multiple burial scenarios were investigated with a variety of antipersonnel and antitank mines and typical clutter objects. The seismic system used in these experiments is an extension of our existing seismic mine detection system. The system uses electrodynamic shakers to generate a seismic wave which propagates across the simulated minefield, and a specially designed radar is used to measure the displacement of the surface caused by the seismic wave. The GPR makes use of modified resistive‐vee antennas and operates over the frequency range of 500 MHz to 8 GHz. These antennas are very clean in that they have very little self‐clutter and very low radar cross section to lessen the reflections...

Improved resistively-loaded vee dipole for ground-penetrating radar applications

A new RVD has been designed. The new RVD has curved arms and is loaded with a modified Wu-King resistive profile. The RVD showed improved responses in a broad range of frequency, which make it more suitable to use in a GPR system than a typical RVD. One way of implementing the antenna was shown.

A Broad-Band Conductively-Loaded Slot Antenna for Pulse Radiation

A slot antenna with a conductive loading profile, which is complementary to the dipole antenna with the Wu-King resistive profile, is derived based on a transmission line model. The drive point impedance of the nonreflecting slot antenna is determined using Babinets principle. The voltage distribution along the slot arms is shown to be an outward traveling wave with no internal reflection. In addition, the radiated electric field pattern and the efficiency of the nonreflecting slot antenna are discussed. The complementary nature between the Wu-King dipole and nonreflecting slot is numerically verified. The pulse radiation and reception characteristics of the antenna are then demonstrated both numerically and experimentally.