Novel Radar Techniques for Humanitarian Demining
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DETERMINE: Novel Radar Techniques for Humanitarian Demining Federico Lombardi A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of University College London. Department of Electronic and Electrical Engineering University College London July 17, 2019 To my Wife, who always supported me, whatever path I took. To my Son, who will always support my wife, whatever path I may take. 3 I, Federico Lombardi, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the work. Federico Lombardi Abstract Today the plague of landmines represent one of the greatest curses of modern time, killing and maiming innocent people every day. It is not easy to provide a global esti- mate of the problem dimension, however, reported casualties describe that the majority of the victims are civilians, with almost a half represented by children. Among all the technologies that are currently employed for landmine clearance, Ground Penetrating Radar (GPR) is one of those expected to increase the efficiency of operation, even if its high-resolution imaging capability and the possibility of detecting also non-metallic landmines are unfortunately balanced by the high sensor false alarm rate. Most landmines may be considered as multiple layered dielectric cylinders that interact with each other to produce multiple reflections, which will be not the case for other common clutter objects. Considering that each scattering component has its own angular radiation pattern, the research has evaluated the improvements that multistatic configurations could bring to the collected information content. Employing representative landmine models, a number of experimental campaigns have confirmed that GPR is capable of detecting the internal reflections and that the presence of such scattering components could be highlighted changing the antennas offset. In particular, results show that the information that can be extracted relevantly changes with the antenna separation, demonstrating that this approach can provide bet- ter confidence in the discrimination and recognition process. The proposed bistatic approach aims at exploiting possible presence of internal structure beneath the target, which for landmines means the activation or detonation assemblies and possible internal material diversity, maintaining a limited acquisition effort. Such bistatic configurations are then included in a conceptual design of a highly flexible GPR system capable of searching for landmines across a large variety of ter- rains, at reasonably low cost and targeting operators safety. Impact Statement The research programme, funded by the charity Find A Better Way (http:// www:findabetterway:org:uk/), has been fundamentally directed towards the ex- perimental mapping and characterisation of landmine electromagnetic signatures, as well as developing new configurations for low-cost, accurate and efficient GPR equip- ment. Special focus has been placed on accurately characterising landmine signatures over a wide range of bistatic angle. The foremost contribution of the work described in this thesis consists in the per- spective of discriminating landmines based on the presence of scattering contributions generated by the internal assemblies of the target, which has demonstrated to be an effective and robust feature for target characterisation. Hence, the presented material, in addition to advancing the knowledge of what can be extracted from the GPR signa- ture of a landmine, opens the possibility to identify targets with internal structure and discriminate the buried anomalies accordingly. The work has resulted in some key achievements, whose impact is two-fold: firstly, results have highlighted the importance of using neutralised landmines for effectively exploiting the potential of GPR of detecting the internal structure of a target and prop- erly characterising its signature. Secondly, the investigated features are characteristic of the target itself, and are not only source but also scenario independent. This might lead to the development of a common landmine signature database, so that the out- comes can be embedded in a range of GPR systems and made available to a wide range of demining teams. In addition, yielding the same level of information at a lower computational cost, the suggested bistatic/multistatic approach allows the conceptualisation of a 2D GPR system with the same performance as a 3D one. This consisted of an autonomous ground based platform capable of remotely acquiring dense and regular bistatic data. Impact Statement 6 Considering that mine clearance operation is a time consuming activity, such concep- tual product represents a relevant improvement, outperforming the currently employed hand-held system and possibly relieving the manufacturing cost of such a platform No clear IP has been identified so far, but might likely emerge in the areas of detec- tion and classification algorithms, signal analysis and processing, bistatic measurement methodologies and antenna development for radar imaging. Acknowledgements I would like to thank a number of people who helped me throughout the course of my PhD and contributed to make the work presented in this thesis possible. First thanks to my supervisor Professor Hugh Griffiths for his constant support and excellent supervision during these years, ensuring successful delivery of this research. This achievement is a result of his constant encouragement and motivation. My gratitude goes to Dr. Alessio Balleri who has genuinely collaborated for the success of the project and has given me insightful suggestions and counsels throughout the research, as well assistance for the experimental trials. A special mention for Dr. Maurizio Lualdi for his critical insight and constructive collaboration. The work would not have been possible without the funding support of Find A Better Way, who has sponsored this project, and without the support of the Defence Academy of the UK, who has made available to us the devices employed in the project. Finally, to my wife Carla who gave me all her unconditional support every day anytime during my PhD. Last but not least I would like to thank my family and my older and newer friends for their precious support throughout the time of my PhD. Time flies when you’re having fun. Contents 1 Introduction 27 1.1 Motivations of the work . 28 1.2 Research scope and methods . 31 1.3 Scientific innovation and perspective . 33 1.4 Personal publications . 34 1.5 Thesis outline . 35 2 The landmine problem and the response of technology 37 2.1 Evolution of mine warfare . 42 2.2 The path to the global ban . 48 2.3 Landmine classification . 53 2.4 Standards and Definitions . 58 2.5 Background on Sensor Technology . 62 2.6 Ground Penetrating Radar . 70 2.6.1 Operational principles and survey techniques . 72 2.6.2 Performance factors . 78 2.6.3 Radar cross section and clutter . 84 2.6.4 GPR Data presentation . 86 2.7 Summary . 88 3 Research Context 90 3.1 Scattering from composite targets - landmines perspective . 91 3.2 System design . 94 3.2.1 Multistatic GPR . 94 3.2.2 Polarimetric GPR . 99 Contents 9 3.3 Summary . 105 4 GPR Design and Modelling 107 4.1 Electromagnetic principles of GPR . 108 4.1.1 Physics of propagation . 108 4.1.2 Propagation in a dielectric . 119 4.2 System design . 126 4.2.1 GPR range equation . 126 4.2.2 Bistatic GPR corrections . 130 4.3 GPR design for landmine internal structure detection . 132 4.3.1 Detection of the landmine internal structure . 135 4.4 System modelling . 135 4.4.1 Computational methods comparison . 136 4.4.2 Finite Difference Time Domain scheme . 139 4.4.3 Forward modelling . 143 4.5 Summary . 159 5 Methodology and Results 160 5.1 Target description . 161 5.2 Evidence of the internal structure: radar signature . 165 5.2.1 Off the ground measurements . 165 5.2.2 Buried targets measurements . 174 5.2.3 Comments . 182 5.3 Evidence of the internal structure: radar profiles . 184 5.3.1 Trials description . 184 5.3.2 Validation of results . 187 5.3.3 Comments . 188 5.4 Evidence of the internal structure: radar images . 189 5.4.1 Trials descriptions . 189 5.4.2 Validation of results . 197 5.4.3 Constraints on GPR imaging . 200 5.4.4 Comments . 209 5.5 Bistatic characterisation of landmine signature . 211 Contents 10 5.5.1 Trials descriptions . 211 5.5.2 Validation of results . 219 5.5.3 Comments . 221 5.6 Summary . 222 6 Concept for a bistatic system 224 6.1 Motivations and platforms . 225 6.2 Concerns regarding technology . 231 6.3 Sensor fusion . 233 6.4 Conceptual design . 235 6.4.1 Considerations on dual sensor equipment . 244 6.5 Summary . 245 7 Conclusions and future work 247 7.1 Summary of findings . 248 7.2 Future work . 252 Bibliography 255 List of Figures 2.1 Contamination status as of December 2017. 39 2.2 Casualties from remnants of war as of December 2017. 40 2.3 Recorded casualties as of December 2017. 40 2.4 Support for mine action as of December 2017. 42 2.5 The evolution of concealed traps. 43 2.6 Landmine warfare precursors. 44 2.7 Early German landmines. 45 2.8 Remotely delivering mine system. 46 2.9 Mine contamination of the Sarajevo area in 1997. 47 2.10 The 1949 Geneva Convention. 48 2.11 The Ban campaign. 50 2.12 Mine Ban accomplishments as of December 2017. 51 2.13 Pyramid of Shoes against landmines. 52 2.14 Examples of blast mines. 54 2.15 Examples of stake-mounted fragmentation mines. 55 2.16 Examples of directional fragmentation mines. 55 2.17 Examples of bounding fragmentation mines. 56 2.18 Examples of anti-vehicle landmines. 56 2.19 Examples of explosive remnants. 57 2.20 Examples of improvised explosive devices.