In this paper, we describe the recent development of new algorithms applied to short-range radar imaging. Facing the limitations of classical backpropagation algorithms, the use of techniques based on Fast Fourier Transforms has led to substantial image computation accelerations, especially for Multiple-Input Multiple-Output systems. The necessary spatial interpolation and zero-padding steps are still particularly limiting in this context, so it is proposed to replace it by a more efficient matrix technique, showing improvements in memory consumption, image computation speed and reconstruction quality.
KEYWORDS: Mining, Radar, Antennas, Data acquisition, Land mines, Video, General packet radio service, Ground penetrating radar, Polarimetry, Scattering
The results of the measurement campaign, which has been held recently at the test facilities for landmine detection systems located at TNO-FEL (The Hague, the Netherlands), are presented. The test facilities give an opportunity to evaluate system performance in different environment (such as grass, sand, clay, etc.) under controlled conditions. The test lanes contain various types of antipersonnel and antitank mines. In this campaign we used the Video Impulse Ground Penetrating Radar that has recently been developed in the IRCTR. The design of the radar allows us to perform simultaneous full-polarimetric measurements in two ultra-wide frequency bands. Furthermore, the scattered from subsurface electromagnetic field is measured in quasi-monostatic and essentially bistatic antenna configurations. The acquired during the measurement campaign data are of high quality in terms of time stability, radar positioning and signal-to-noise ratio. This has allowed to extract full-polarimetric target responses and to analyze them. The obtained results are of importance for target classification.
KEYWORDS: Antennas, General packet radio service, Fourier transforms, Resistance, Algorithm development, Reflection, Calcium, Transmission electron microscopy, Chemical elements, Evolutionary algorithms
In this paper we present the electric field integral equation (EFIE) method for the analysis of transient metallic antennas above a lossy half-space. This formulation is suitable for the analysis of a wide class of metallic GPR antennas that does not contain any dielectric material. Ground influence is properly taken into account by the Green's function for layered media. The EFIE is solved numerically by the method of moments using the wellknown Rao-Wilton-Glisson basis function. For GPR antennas, the free-space Green's function is replaced by the dyadic Green's function for layered media. The time-domain solution for transient antennas is obtained using Fourier transform method and a time-window technique. By using time window to remove reflections from antenna open ends, the solution for transient antennas is obtained. To speed up computations, several approaches are used, which include the exploitation of antenna symmetry, a special treatment for integration over singularity, and an interpolation method to expedite the evaluation of Sommerfeld integrals. We have found that those approaches allow rapid and accurate computations for obtaining the time-domain solution. Good agreement between the computation and measurement of the input impedance of a wire dipole and a wire bow-tie antenna is obtained.
KEYWORDS: Correlation function, Interfaces, Dielectrics, Erbium, General packet radio service, Scattering, Monte Carlo methods, Numerical analysis, Reflection, Surface roughness
The clutter caused by scattering from a rough air-ground interface is analyzed numerically. The simulations have been done using Monte-Carlo approach. An ensemble of surface profiles with the desired probability distribution and autocorrelation spectrum is simulated numerically. For each realization of the interface profile the scattered field has been calculated by a deterministic approach. Averaging of the scattered filed over ensemble of the surface realizations has been done numerically. Statistical properties of the scattered field have been analyzed. It has been found that the magnitude of the surface clutter caused by reasonably smooth surfaces can exceed easily the mean value of the ground reflection. Furthermore it is demonstrated that if the magnitude of the surface clutter is considerably less than the mean value of the ground reflection then the correlation function of the reflected field coincides with the correlation function of the rough surface.
In this paper a comprehensive characterization of transient bow-tie antennas for GPR applications is presented. In particular the characteristic impedance of a transient bow-tie antenna above different types of ground, as a function of its flare angle is given. The time-domain reflection coefficient of the transient bow-tie antenna above the ground is derived and also plotted as a function of flare angle. This result shows that there exists an optimal flare angle which gives minimal reflection at the antenna terminal for various ground types. Additionally, here we also investigate the characteristic impedance as a function of antenna elevation above the ground. Moreover, in this paper the subsurface footprints of bow-tie antennas as a function of flare angle for different ground types are shown. This result shows that depending on the target's size, the size of footprint can be adjusted by varying the flare angle in order to reduce clutter from ground surface. The analysis is carried out in the frequency-domain using the mixed-potential integral equation (MPIE) method for problems in multi-layer media, which is solved numerically by the method of moments. The solution in the time domain for transient antennas is obtained using the Fourier transform method and a time window technique to remove open-end reflections.
A full-polarimetric GPR front-end has been developed. The front-end comprises a generator section, a multi-static antenna system and a receiver based on a multi-channel sampling converter. The front-end allows to perform subsurface imaging with resolution and accuracy sufficient for antipersonnel mine recognition. In comparison with commercially available video impulse GPR systems the key advantages of the front-end are considerably larger bandwidth, very high precision of measurements of scattered field and ability to measure polarimetric structure of the scattered field.
The development of transient antennas for Ground Penetrating Radar would be difficult without numerical simulations of their performance. To this end FDTD code should be considered as a useful and powerful tool for transient antenna analysis. As an example of such analysis we describe simulations of the dielectric wedge antenna. Detailed investigations on such key model elements as the staircase approximation of 3D structures, the antenna feed model, the cell size and the size of the domain of computation were carried out. Very good agreement between theoretical predictions and experimental results was observed. Some of antenna characteristics (like the radial component of the electric field in the near zone, the radiated waveform and the antenna footprint in different grounds) cannot be measured easily, so the correct theoretical (numerical) model is essential for their determination. The numerical model of the antenna can be also used for optimization of the antenna.
KEYWORDS: Antennas, Dielectrics, General packet radio service, Radar, Land mines, Electromagnetism, Finite-difference time-domain method, Ground penetrating radar, Near field, Metals
The paper describes the design of an antenna system for a video impulse radar dedicated to landmine detection. The developed antenna system consists of a dielectric wedge antenna as transmit antenna and loop antennas as receive ones. The receive antennas are situated below the transmit one. The dielectric wedge antenna has been designed on the basis of the transmission line model, which has been later verified by means of FDTD modeling. The loop antenna has been designed on the basis of a semi-analytical model. The transient behavior of the antenna system has been investigated both experimentally and theoretically. It is shown that the developed system illuminates a limited spot on the ground surface with a short electromagnetic pulse. The field scattered by objects is received by the antenna system in a local point and without integration over large antenna aperture. The output of each receive antenna reproduces the waveform of the scattered field in a local point. To avoid any mechanical contact with hazardous objects the antenna system should be elevated at least 10cm above the ground.
KEYWORDS: Antennas, General packet radio service, Land mines, Radar, Video, Target detection, Pulse generators, Data acquisition, Ground penetrating radar, Mining
The paper describes the design of a cheap ultra wideband GPR front-end suitable for subsurface imaging with resolution sufficient for antipersonnel mine recognition. The front-end comprises a generator section, a multi-static antenna system and a receiving unit based on a multi-channel sampling converter. In comparison with commercially available video impulse GPR systems the key advantages of the front-end are considerably larger bandwidth, high precision of measurements of scattered field, ability to measure polarimetric structure of the scattered field and high pulse repetition rate resulting in fast data acquisition.
The video impulse ground penetrating radar (GPR) system for detection of small and shallow buried objects has been developed. The hardware combines commercially available components with components (e.g. antennas) specially developed or modified for being used in the system. The GPR system has been designed to measure accurately electromagnetic field backscattered from subsurface targets in order to allow identification of detected targets through solution of the inverse scattering problem. The GPR has been tested in different environmental conditions and has proved its ability to detect small and shallow buried targets.
In this paper a concept in designing an adaptive antenna for ground penetrating radar is presented along with its preliminary simulation results. The antenna considered is an array of wire dipoles arranged to form a wire bow-tie antenna. The input impedance of bow-tie antennas is known to be dependent mostly on their flare angle. For antennas situated on the ground, maximal radiation into the ground can be obtained if their input impedance is optimized with respect to the ground impedance. Bow-tie antennas can therefore be optimized to radiate maximum power into a certain type of ground by adjusting their flare angle. In a first approach, without matching the antenna to feeding line, it is found numerically that there exists a certain angle at which this antenna radiates maximal power into the ground for all types of ground. In this case, adaptation to the ground by varying flare angle is still not fully realized. In our future research, another approach will be used, in which the antenna is matched to the feeding line within the whole spectrum of input pulse in order to obtain a more effective adaptation to the ground.
The requirements on GPR technology for the application of humanitarian landmine detection are severe; 99.6% probability of detection and low false alarm rate. Trying to meet these challenging requirements, an impulse radar system has been designed specifically for the application of landmine detection. The radar system contains a dielectric filled TEM horn transmitting antenna and a small loop receiver antenna below the transmitting antenna. With this radar system three- dimensional measurements have been carried out over a test site containing surface-laid and shallowly buried landmines. The test site contains antitank and antipersonnel mines of metal and plastic. In order to show the performance of the new radar system we have to produce images of the subsurface. The imaging algorithms must then be tuned to the specific acquisition parameters. More specific, the refraction of the waves at the surface and the acquisition geometry of the transmitting and receiving antenna influence the arrivaltime of backscattered energy related to subsurface objects. Since imaging algorithms are based on coherent stacking over this energy we must take into account these factors. We produce clear images of landmines and other subsurface objects using adapted imaging algorithms on the data obtained with the new radar system.
A numerical technique which may be used to calculate scattering from a homogeneous cylinder witharbitrary smooth cross-section shape inside a dielectric slab waveguide has been developed. By meansof surface potentials method the initial diffraction problem is reduced to the system of two integralequations. A new more efficient solution procedure for the latest is proposed. A number of examples for thescattering and mode conversion of guided modes are presented.
Two-dimensional scattering from an arbitrarily inhomoge- neous body buried in a homogeneous layer has been considered usingthe integral equation formulation over the cross-section or thebody and the moment method. The arising system of linear algebraicequations is solved with the aid or conjugate gradient method andone dimensional fast Fourier transform. The solution can beapplied to investigate problems connected with microwave and opticdesign, non-destructive control and remote sensing. Some numericalresults are presented.
The unique possibilities ofpowerfu.l gas lasers operating on carbon dioxide for treatment of materials,
for production and heating of plasma, for exploring the atmosphere and other applications have been
represented in numerous studies into the physics of interaction between the infrared laser radiation
and gases, as well as condensed substances.
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