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

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2.4 Calibration and Preprocessing 12 where Spline is a cubic spline function and k is an oversampled discrete time. This step does not improve hardware resolution of the radar system, but could help to find crosstalk and time zero more precisely and slightly improve the image after migration (Section 2.5). 2.4.2 Time Zero Estimation Time zero τ(k) is the time instant (or corresponding digitized sample) in which the transmitted signal leaves the transmit antenna. Bs(X, k) has to be shifted, so that the τ(k) is at the beginning of the dataset (Fig. 2.4.1 c)). Bsτ(X, k) = Bs(X, k − τ(k)). (2.4.2) In case of M-sequence radar the M-sequence is transmitted periodically around. The exact time at which the TX antenna starts emitting the first chip from Msequence is time zero [79]. This time depends mostly on antennas cables lengths, antennas itself and radar hardware. Finding time zero in case of M-sequence radar means to rotate all the received impulse responses so that the first chip corresponds with spatial position of the TX antenna. There are several methods how to find the τ(k). Most often the crosstalk signal CA(k) is used [149]. Since the geometry of antenna system is known, and the transmission medium of the crosstalk is air, the time needed for the signal to arrive from TX to RX can be easily estimated as: tT X RX = dT X RX/c (2.4.3) where dT X RX is the known distance between the antennas. As can be seen in Fig. 2.4.1 a), the crosstalk has both a negative and a positive peak. The study [149] has shown that the best and most reliable part of the crosstalk response to estimate its position is the first peak. In principle, for determination of τ(k) whichever measured object can be used instead of crosstalk. Position in data that corresponded to the well known spatial position of object has to be found. 2.4.3 Antenna Crosstalk Removing After the crosstalk CA(k) has been used to determine time zero, it may be removed out from the data. The crosstalk is the part of the signal that travels directly between the emitting and receiving antennas. It is the first and often the largest peak in the A-scan signal,thus its removal is really important. The crosstalk can be obtained by measuring with radar in the free space, or with absorbers around (anechoic chamber room). This reference measurement can be then subtracted from the data. Bsτc(X, k) = Bsτ(X, k) − CA(k − τ(k)). (2.4.4)
2.4 Calibration and Preprocessing 13 Practically, it is very difficult to make a good quality crosstalk measurement. It can be done in anechoic chamber room like it is shown in Fig. 2.4.2 a). In this case the results are very close to ideal ones, but mostly they are useless. It is very difficult to transport the whole measurement setup including the antennas system and motion system that are necessary for SAR measurement into the anechoic chamber room without any mechanical changes in the setup. If even minor part of mechanical (mostly metal) setup changes within real measurement and anechoic chamber room measurement, the shape of the crosstalk changes too. The better results are mostly obtained when crosstalk measurement is done during real measurements with absolutely the same arrangement of the whole measurement system. However, in this case it is very difficult to find a ”free space”. The crosstalk measurement inside a building is shown in Fig. 2.4.2 b). It can be seen that lots of another artifacts like floor, roof, near walls etc. are present. Fig. 2.4.1 d) represents examples of data after the crosstalk has been removed. Amplitude 1 0.5 0 -1 Crosstalk measured in anechoic chamber room Crosstalk Amplitude 1 0.5 0 Crosstalk measured inside a building Crosstalk -1 10 20 30 40 50 10 20 30 40 50 tns [ ] tns [ ] a) b) Clutters Fig. 2.4.2: Example of crosstalk measured a) in an anechoic chamber room with horn antennas b) inside a building with double-ridge horn antennas. 2.4.4 Deconvolution An effort to remove influence of the antenna and the whole radar system impulse response to the measured data is the most complex calibration process. There exist lots of more or less complicated methods how to provide deconvolution described e.g. in [125,123,42,126,113,11,55]. Long impulse response of antenna causes that any object that reflects the wave energy and is received by RX is presented in data also in long area. Influence of the whole radar system, and mainly the antennas, to the received signal can be reduced by deconvolving the whole data with radar
Page 1 and 2: 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: 2.4 Calibration and Preprocessing 1
Page 29 and 30: 2.5 Radar Imaging Methods Overview
Page 55 and 56: Chapter 4 Selected Research Methods
Page 57 and 58: 4.1 Through-Wall TOA Estimation 43
Page 71 and 72: 4.2 Measurements of the Wall Parame
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4.2 Measurements of the Wall Parame
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4.2 Measurements of the Wall Parame
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4.3 Highlighting of a Building Cont
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4.4 Measurements of the Practical S
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5.1 Conclusion and Future Work 85 5
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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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