ACTIVE PEOPLE SCREENING WITH TAARDIS
Future approaches for active body scanners were explored within the framework of the EDA project TIPPSI (THz Imaging Phenomenology Platforms for Stand-off IED Detection). Fraunhofer IAF and Fraunhofer FHR contributed the passive millimeter wave system TAARDIS (Terahertz Accelerated Aperture Remote Detection Imaging Scanner).
Passive millimeter wave scanner TAARDIS
Highly sensitive, broadband receivers at 94 GHz, 220 GHz and 300 GHz were researched for the passive scanner in a cooperation between Fraunhofer IAF and Fraunhofer FHR. The receive channels were supplemented with imaging optics. These focus the received millimeter waves and direct them to the receive channels thus facilitating the scanning of the image section. The scanning process was set up in such a way that the functional principle of the system could be demonstrated with just a few channels and a mechanical scan. An increase in the number of channels reduced the necessity for mechanical scanning and accelerated the imaging speed. This is achieved using a confocal Gregorian offset reflector system with five reflectors, as was realized with the TAARDIS scanner.
One of the important requirements of the TIPPSI project was the performance of body scans at larger distances, as is currently standard in the area of civilian security screening: with TAARDIS, a predefined volume is scanned within a range of 5 m to 50 m as opposed to scanning at a distance of just 1 m. To facilitate the detection of persons and, at the very least, larger foreign bodies at such distances, the TAARDIS system was equipped with an aperture with a diameter of 90 cm which can optionally be enlarged.
The aim is to achieve an image repetition rate of 1 Hz, which can be realized with an optimized scanning mechanism in a configuration with 6 or more receive channels. As the required short-term stability could not, however, be realized in all of the channels, the results for TIPPSI were measured at a reduced repetition rate. The missing channels were gradually stabilized and added to the system. The scanning mechanism then underwent further optimization.
Results of the TIPPSI project
Previous research revealed the extent to which the detection result is influenced by the relative orientation of the person who is to be measured and the objects that are to be detected as well as the orientation to the scanner. As two scanner systems and a simulation environment were to be compared within the framework of the TIPPSI project, a realistic model of a human body was created at Wojskowa Akademia Techniczna (WAT) in Warsaw. The body has a distributed heating system which allows the realization of a temperature distribution very similar to that of human skin. This model was dispatched to Fraunhofer FHR and FOI (Totalförsvarets forskningsinstitut) in Sweden for measurement purposes. It was also included in the FOI simulation chain in the form of a 3D model. This simulation environment combines 3D-animated persons, elaborate clothing and a mixture of objects concealed on the body, e.g. mobile phones and improvised explosive devices (IEDs), with a three-dimensional simulation for active and passive millimeter wave scanners. The specific imaging properties of the scanners and the material database from WAT were both taken into account. Although the moving person scenario could not be investigated with this heatable model, the broad consistency between the simulations and the measurement results for static scenes counts as one of the greatest successes of the TIPPSI project. The active scanner was created in a close cooperation between the THz Sensing Group of the Technical University of Delft, which was responsible for the imager optics, and the Terahertz and Millimetre Wave Laboratory at the Chalmers University of Technology, which supplied the 220 GHz radar modules. System integration took place at FOI. TNO in the Netherlands contributed a detailed analysis of the means necessary to supplement the existing active scanner with a MIMO arrangement, which would make a large share of the mechanical scanning movements unnecessary.
End-of-project demonstration in Warsaw
In the closing phase of the TIPPSI project, joint measurements of the heatable model were carried out with both scanners and the results were compared with the simulations. The measurements took place in September 2016. All obtained results were presented in Warsaw in October 2016. The presentation and, not least, the concluding live demonstration, clearly highlighted the excellent results that were achieved within the framework of the TIPPSI project.