Detection of Explosives in a Flow of Vehicles

The threat potential by simple access to professional explosives via black channels, and the simple production of amateur explosives from publicly available substances, presently is high and will still increase in future. That high availability of explosives makes the construction of improvised explosive devices (IEDs) rather simple, at high damaging and life-threatening impact in case of terroristic attacks. Hence, the detection of such devices right on time is a primary task of security forces at all such events where many people meet together or where the damage to critical infrastructures can be high. Of course this is a quite challenging duty due to the manifold articulations of the problem. Consequently many tools have to be used and applied in order to guarantee a maximum of security. One of the most challenging situations is the case where large crowds of people or large accumulations of vehicles are in permanent motion, as it is the case for people on public areas or events and vehicles in dense traffic. Here the problem is the big number of objects to be investigated rather in parallel, and the fact that the objects may mask each other, preventing proper and complete scanning of them. Consequently sensor types and architectures have to be explored in order to overcome such problems. A useful technology for such applications is given by microwave remote sensing where a range of frequency bands between 1 and 300 GHz typically is used. Profiling or two- and even three-dimensional imaging sensors can be built. Sensing can be done actively as by radar using its own illumination source and a receiver, or passively as by a radiometer using only a receiver to measure naturally generated radiation. Radar allows the measurement of range and object reflectivity and radiometers provide a mixture of object temperature, emissivity and reflectivity. Depending on frequency and material many obstacles as well can be penetrated to image objects behind them. Now for the above mentioned problems various microwave sensor constructions are of basic interest, either by directly indicating a potential threat, or by indirectly giving indications which together with auxiliary information define a threat. The presentation will introduce both active and passive microwave sensing technologies, and show theoretical and experimental results for scenarios, being useful to investigate the problem of detecting threats in a flow of vehicles. Opportunities and limitations will be discussed.