TeraSCREEN - fast security checks at the airport
A promising approach for the fast screening of moving persons at the airport, achieved through the combination of active and passive high frequency technology in different spectral bands.
European Joint Project
The acronym TeraSCREEN comes from the research project "Multi-Frequency Multi-Mode TeraHertz screening for border checks" and describes a European joint project that focuses on passenger screening in the high-security areas of the airport. The joint project involves a total of 12 partner institutes from 6 European countries (Fig. 1). It is planned to construct a passive scanner at 94, 220 and 360 GHz as well as an active scanner within the framework of the project. The active scanner with a medium frequency of 360 GHz, which will be used to measure moving persons, will be designed, developed and constructed at FHR.
Concept of the active scanner
When measuring moving persons, the high resolution screening and detection of body-worn objects concealed under clothing is a huge challenge. This applies in particular when such measurements are carried out in the planned frequency range of 360 GHz. On the one hand, very few high frequency components are currently available for this spectral range and, on the other hand, the waveguide-based transport of electromagnetic waves has practically reached its physical limits in production technology. Furthermore, a high resolution image of the entire body must be acquired within a very short time – and this with a system that itself only has a low number of high frequency pixels. A full pixel matrix in this frequency range would undermine each economic approach.
In spite of the difficult technological conditions, a high resolution image of a moving person can be created through the utilization of special imaging techniques. In the present case, a mechanically swiveled panned MIMI approach with 16 transmitters and 16 receivers was selected. These are arranged in such a way that an array with 256 virtual antennas is created with the help of a suitable switch matrix to control the transmitters and receivers. This array is installed transversely to the person so that the width of the person can be presented with pixel widths of just a few millimeters. The image line created in this manner is subsequently guided over the person with the help of a swivel mechanism so that a complete image of the person can be created within a fraction of a second. Thanks to the underlying mode of operation of the frequency modulated continuous wave radar (Fig. 2) with a very high bandwidth of 30 GHz, it is still possible to achieve an enormous degree of detail in the depth resolution.
Design of the high frequency technique
The design of the high frequency technique is a very challenging aspect of the overall development process. Based on the MIMO concept, an antenna cassette was developed which contains the 32 antenna structures and facilitates the transport of the electromagnetic wave in the cassette from the antenna to the standardized flange by way of complex waveguide structures. The challenge here lies in the minimal dimensions of the waveguide – sometimes in the submillimeter range – in this frequency range. In concrete terms, the mechanical production possibilities for the design of the split block antenna cassette (see Fig. 3) will have to be considered.
In addition to the complex antenna concept, the high frequency technique comprises many additional elements, such as the basis FMCW module. The other required elements, e.g. frequency multiplier, power amplifier and subharmonic mixers will be developed and made available by the project partners. The integration and characterization of the system will take place at FHR.
The international project has a number of important goals. The new approach aims to enhance security at airports with faster processing times while at the same time safeguarding personal privacy. Furthermore, the technological development of new frequency ranges should lead to the generation of new foundations and processes which will be useful to European society in general.