Through-Wall Imaging With UWB Radar System - KEMT FEI TUKE

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4.1 Through-Wall TOA Estimation 42 The conventional method for computing T OA with constant velocity model, which does not consider different velocity in the wall and air, introduces an error in estimation of target shape and position [141]. Therefore, more or less accurate methods based on a ray theory and Snell’s law are used in algorithms of T OA computation to compensate the presence of the wall [61, 5, 86, 6]. The T OA for multilayer model cannot be computed directly, some numerical minimization method [106], has to be chosen. In general, the optimization techniques use T OA as an error function, and a vector of independent variables over which the error is minimized [76]. The number of variables is equal to the number of layers. This is a very time consuming process, but for small number of layers (especially for airwall-air structure) a few improvements that significantly reduce the computation complexity will be presented. 4.1.1 Properties of the Waves Penetrating Through the Wall Through-wall imaging requires waves to penetrate through the specific building materials such as concrete blocks, clay bricks, mortar, plaster, drywall, asphalt shingles, fiberglass insulation, etc. The transmitted signal is attenuated several times due to the free space loss, scattering from air-wall interface, loss in the wall, and the scattering from objects. The propagation loss inside the wall is a function of the frequency [147]. Electromagnetic waves are able to penetrate through the concrete walls without massive attenuation of up to approximately 3 - 4 GHz [101]. With regard to this fact, M-sequence UWB radar technology is very attractive for through-wall object imaging. TOA is commonly referred to the flight time of the wave between transmit antenna, target, and receive antenna. To simplify the notation we denote T OAAT as the flight time of wave between transmit antenna and target, or receive antenna and target. T OAAT corresponds to the summation of distances between antenna and the target divided by appropriate velocities in air and the wall (4.1.1). When the wave arrives to the wall under a certain angle, the wave in the wall will continue its propagation but with changed direction. This holds in the case when the permittivity of the wall differs from the permittivity of the layer in front of the wall. Even more, when the wave is leaving the wall, it changes its direction once more. In the case that the permittivity of the wall is greater than the permittivity of the air εw > εa, the wave velocity in the wall would be lower than in the air. When the relative permittivity of the air εra is equal to 1, the wave velocity in the air is equal to the velocity of the light c. The velocity in the wall in this case is given by (2.5.5). In the case when εa1 of the air in front of the wall and εa2 of the air behind the wall are the same, εa = εa1 = εa2, the incidence angle of the incoming wave
4.1 Through-Wall TOA Estimation 43 is the same as the angle of the outgoing wave which is leaving the wall, so the wave behind the wall has the same direction as that in front of the wall. This condition is fulfilled only for a homogeneous wall or if the wall consists of the arbitrary numbers of homogeneous layers in parallel to the wall. This section is focused on computation of the true T OAAT between antenna and target, with the wall in between them. The losses in the wall are not considered here. 4.1.2 True TOA Between Antenna and Target To estimate the correct position of the objects behind the wall, the exact T OAAT between the antenna and the object has to be known. To compute this time, the model that is shown in Fig. 4.1.1 is used. Z Antenna � a Wall �w �a Ax [ A,zA] d a1 �� dw �� W [ x ,z ] d 1 �� W1 W1 W [ x ,z ] H X 2 �� da2 W2 W2 Target Tx,z [ T T] H Z W di Fig. 4.1.1: True flight distance model in through-wall scenario. There are several assumptions: the wall is homogeneous with constant permittivity and constant thickness; the relative permittivity of the air in front of the wall and behind it is the same and equals to one; the coordinations of the antenna A[xA, zA] and the target T [xT , zT ], the permittivity of the wall εw, the thickness of the wall Dw, and distance of the wall from antenna in Z direction Wdi have to be known. Because the material inside the wall is homogeneous, the angles φ1 = φ4 and φ2 = φ3. The true flight distance between antenna and target is not equal to straight distance |AT | between them. It has to be computed as summation of distances D w X
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TECHNICAL UNIVERSITY OF KOˇSICE Fa
Page 3 and 4:
Acknowledgement I give a sincere gr
Page 5 and 6: CONTENTS v 3 Goals of Dissertation
Page 7 and 8: LIST OF FIGURES vii List of Figures
Page 9 and 10: LIST OF FIGURES ix 4.4.3 Aquarium f
Page 11 and 12: LIST OF FIGURES xi List of Symbols
Page 13 and 14: LIST OF FIGURES xiii kx - Horizonta
Page 15 and 16: Chapter 1 Introduction 1.1 Motivati
Page 17 and 18: 1.3 Thesis Organization 3 In additi
Page 19 and 20: Chapter 2 State Of The Art 2.1 UWB
Page 21 and 22: 2.2 Through-Wall Radar Basic Model
Page 23 and 24: 2.3 SAR Scanning 9 and shall remain
Page 25 and 26: 2.4 Calibration and Preprocessing 1
Page 27 and 28: 2.4 Calibration and Preprocessing 1
Page 29 and 30: 2.5 Radar Imaging Methods Overview
Page 55: Chapter 4 Selected Research Methods
Page 59 and 60: 4.1 Through-Wall TOA Estimation 45
Page 71 and 72: 4.2 Measurements of the Wall Parame
Page 81 and 82: 4.3 Highlighting of a Building Cont
Page 91 and 92: 4.4 Measurements of the Practical S
Page 99 and 100: 5.1 Conclusion and Future Work 85 5
Page 101 and 102: 50 100 150 200 250 300 350 400 Z 45
Page 103 and 104: 50 100 150 200 250 300 350 400 Z 45
Page 105 and 106: 50 100 150 200 250 300 350 400 Z 45
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50 100 150 200 250 300 350 400 Z 45
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Z [cm] Z [cm] 50 100 150 200 250 30
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Z [cm] 50 100 150 200 250 300 b) Em
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BIBLIOGRAPHY 99 [13] T. W. Barrett,
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BIBLIOGRAPHY 101 [46] G. Derveaux,
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BIBLIOGRAPHY 103 [81] S. Kidera, T.
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BIBLIOGRAPHY 105 [115] J. Sachs, M.
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BIBLIOGRAPHY 107 [150] O. Yilmaz, S
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Through-Wall Imaging With UWB Radar System - KEMT FEI TUKE

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