KEYWORDS: Detection and tracking algorithms, Deconvolution, Dielectrics, Metals, General packet radio service, Radar, Finite-difference time-domain method, Ground penetrating radar, Land mines, Algorithm development
This paper presents a novel deconvolution algorithm designed to estimate the impulse response of buried objects based on ground penetrating radar (GPR) signals. The impulse response is a rich source of information about the buried object and therefore very useful for intelligent signal processing of GPR data. For example, it can be used in a target classification scheme to reduce the false alarm rate in demining operations. Estimating the target impulse response from the incident and scattered radar signals is a basic deconvolution problem. However, noise sensitivity and ground dispersion prevent the use of simple deconvolution methods like linear least squares deconvolution. Instead, a new deconvolution algorithm has been developed that computes estimates adhering to a physical impulse response model and that can be characterized by a limited number of parameters. It is shown that the new algorithm is robust with respect to noise and that it can deal with ground dispersion. The general performance of the algorithm has been tested on data generated by finite-difference time-domain (FDTD) simulations. The results demonstrate that the algorithm can distinguish between different dielectric and metal targets, making it very suitable for use in a classification scheme. Moreover, since the estimated impulse responses have physical meaning they can be related to target characteristics such as size and material properties. A direct application of this is the estimation of the permittivity of a dielectric target from its impulse response and that of a calibration target.
This paper presents a study on using polarimetric ground penetrating radar (GPR) for the identification of plastic antipersonnel mines. In general, the polarimetric radar response of a surface-laid or buried object depends on the orientation of the object with respect to the transmitting and receiving antennas. Hence, in order to make identification possible, it is crucial to measure the full scattering matrix and transform the data into the target frame, in which the response is orientation independent. In this paper, we present an impulse ultrawideband ground penetrating radar with a polarimetric antenna system. Using this radar, the scattering matrices for a set of surface-laid targets with different shape and internal structure have been measured. The measurements were done for different target orientations. Transformation of the measured response into the target frame was achieved by matrix diagonalization in the frequency domain. The eigenvalues obtained by matrix diagonalization constitute a set of orientation invariant features and have been studied as possible target discriminators. In particular, we addressed the problem of classifying targets with respect to shape (rotationally symmetric versus elongated). The results suggest the possibility to distinguish between targets by looking at how the eigenvalues change as a function of frequency. Moreover, matrix diagonalization yielded an angle of orientation and the significance of this angle for small minelike targets and elongated targets is discussed. The analysis was repeated for scattering matrices acquired over buried targets and the results are compared against those obtained for the surface-laid objects.
KEYWORDS: Land mines, Finite-difference time-domain method, Radar, Scattering, Signal attenuation, Backscatter, 3D modeling, Ground penetrating radar, 3D acquisition, Radio propagation
This paper analyzes the early-time radar response of buried penetrable targets such as plastic landmines. The Born approximation is used to derive simple analytical expressions relating target and soil properties to the earlytime response. Understanding these dependencies is crucial for target identification under varying soil conditions. The derived expressions include the transfer function and the impulse response of a penetrable target embedded in an unbounded homogeneous lossy medium and illuminated by a uniform plane wave. Using a truncated circular cylinder having the dimensions of a PMA-3 mine as an example, the early time responses predicted by the Born approximation are compared against responses obtained by threedimensional finite-difference time-domain (FDTD) simulations. The results demonstrate that with the Born approximation it is possible to predict the general shape of the target response, i.e. the number of amplitude peaks, as well as the amplitudes of those peaks that relate to backscatter from the top of the example target. To improve the fit between the predicted and simulated responses, two phenomenologically motivated modifications to the earlytime response expressions are proposed. The modified expressions are able to accurately predict not just the general shape of the early-time response, but also the influence of the host medium conductivity on the target impulse response.
This paper presents a parametric study on the influence of target and soil properties, including depth of burial, on features extracted from ground penetrating radar (GPR) data. Understanding this influence is crucial for designing a classifier that uses these features for mine detection and identification. Two types of features have been studied. These are the Wigner-Ville distribution and geometric moments. Using a fast forward modeling program, synthetic GPR data were created for six buried objects, including two plastic minelike objects, for a wide range of soil properties and depths of burial. Both non-lossy and lossy soils were considered. From the computed data the above features were extracted and correlated with each other. The results show that the Wigner-Ville distribution performs much better in discriminating between objects than geometric moments. Furthermore, the features were found to be practically invariant to changes in mine-soil permittivity contrast and depth of burial provided that the soil is non-lossy. In the presence of losses, the GPR pulse is reshaped at the air-ground interface and as it propagates through the soil. As a result of the reshaping, the target response and hence the features can differ substantially from the non-lossy case.
This paper discusses the possible use of strapdown inertial navigation for real-time ground penetrating radar (GPR) antenna position and orientation estimation along arbitrary three-dimensional acquisition lines. Strapdown inertial navigation theory has been studied extensively in the literature for aircraft, missile and space navigation. Here, we give an overview of the theory as it applies to the antenna position and orientation problem. This includes the definition of the relevant coordinate frames and attitude parameters, a discussion of the measured acceleration and angular velocity, and a description of the four primary computational tasks pertinent to strapdown inertial navigation. These are the initial alignment of the system, the integration of angular velocity into attitude (attitude updating), the acceleration transformation and integration into velocity (velocity updating) and the integration of velocity into position (position updating). The key elements of using a low-grade versus a high-grade inertial measurement unit (IMU) are pointed out. The actual performance of a commercially available low-grade IMU is evaluated based on a series of navigation experiments. The experimental results show that the tested IMU is far from being accurate enough for completely self-contained antenna positioning and that the precise calibration for scale factors, biases and axis misalignments is vital. The observed orientation accuracy (error of less than 1 degree after 60 seconds) suggests the integration of the tested IMU with odometry, extending the applicability of the latter to environments with topography or where changing of the profile direction due to obstacles is necessary. Another possible use of low-grade IMUs might be for more sophisticated 'rubber sheeting' techniques.
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