Concealed Weapon Detection with Active andPassive Millimeterwave Sensors, Two Approaches

Helmut Essen1, Hans-Hellmuth Fuchs1, Manfred Hagelen1, Stephan Stanko1, Denis Notel1,Sreenivas Erukulla1, Johann Huck1, Michael Schlechtweg2, Axel Tessmann2

1Research Institute for High Frequency Physics and Radar Techniques (FGAN-FHR),Neuenahrer Str. 20, D-53343 Wachtberg, Email: Essen@fgan.de

2Fraunhofer Institut fur Angewandte Festkorperphysik (IAF)Tullastr. 72, D-79108 Freiburg, Email: Michael.Schlechtweg@iaf.fraunhofer.de

Abstract— The increasing interest in the security of publicspaces leads to a demand for sensor technology beyond metaldetectors. Two different approaches of concealed weapon detec-tion using millimeter wave systems are presented in this paper.The design of a passive radiometric sensor in the W-band ispresented. On the active side, an FMCW radar system at 94 GHzis introduced for the scanning of persons. The resulting imagesare shown which enable a first comparison of both approaches.

I. I NTRODUCTION

Sensors used for security purposes have to cover the non-invasive control of humans, baggage and letters with theaim to detect weapons, explosives and chemical or biologicalthreat material. Those sensors have to cope with differentenvironmental conditions. Preferably, the control of people hasto be done over a longer distance. In times of increasing threatby terrorist attacks the control of passengers at airports andstations is one of the major items. People carrying concealedweapons or explosives or those, who have other terroristicattacks in mind, have to be detected under all circumstances.Very similar requirements have to be met for all aspects ofhomeland security. Currently, emphasis is placed on systemconcepts and technology for this type of applications employ-ing millimeterwave, submillimeterwave and terahertz sensors.This is based on the capability of these frequency bands tolook through textile material and the possibility to achievea geometric resolution, which is sufficient to resolve criticalitems within the necessary range.

II. M ILLIMETERWAVE TECHNOLOGY FORSECURITY

APPLICATIONS

Due to other applications, civilian and military, the fre-quency region around 94 GHz is best developed. Both, devicesand components, have been designed and manufactured byFraunhofer IAF, who have been involved with the demon-strators discussed below. Key components are low noise andmedium power HEMT amplifiers [1] and a miniaturized singlechip FMCW radar at 94 GHz [2].

Demonstrators have been set up using active and passivesensors. A single channel Dicke type radiometer was designedusing three stacked LNAs and a PIN SPDT to switch between

Fig. 1. Radiometric channel of the Dicke type (top), layout of an LNA (right)and LNA performance (left) [1].

Fig. 2. Functional diagram of single chip FMCW radar (left) and layout at94 GHz (right)[2].

receiver and the matched second port, which serves as a ref-erence. While the bandwidth of the amplifier chain is 20 GHz,the total bandwidth is limited by the PIN switch to 4 GHz.The overall gain, in this case, is 60 dB. The general set-upand a photo of the LNA layout as well as typical performancecurves are shown in Figure 1. To show the rapid improvementsin LNA development, the system was then modified to workas a total power sensor. By definition, a total power setup istwice as sensitive as a Dicke type system and offers a lowersystem noise temperature as no PIN switch is necessary at itsinput. The second demonstrator, which was involved in thecomparative test, is based on the single chip FMCW radar at94 GHz sketched in Figure 2. It was combined with a linearscanning antenna [3] for the azimuth scan and mounted on a

Fig. 3. Scanning beam antenna for the FMCW radar at 94 GHz

single axis positioner for the elevation scan. Figure 3 shows aphoto of the scanning beam antenna.

III. SHORT RANGE IMAGING FOR CONCEALED WEAPON

DETECTION AT 94 GHZ

A. Passive Sensing

Passive mm-wave imaging with or without artificial illumi-nation by an incoherent noise source is optimal for detectingconcealed weapons, which are worn under any kind of clothes.Because of the incoherent illumination, no polarizing effectson the surface or in the clothing material itself occur.

Fig. 4. Photo of sample array of holes and its radiometric raw image

The measurements were done with natural illuminationthrough a window and to be more representative for indoorapplications with an incoherent noise source illuminating thescene. First results were achieved at relatively slow scanningspeed using a dummy instead of a human being. During theseexperiments the scanning scheme was optimized. Figure 4shows the measurement result for a sample consisting of alinear array of holes in a metal plate without any imageprocessing applied. The radiometric image demonstrates thedistortions due to scanning and also the limits of resolutionwith respect to reproduction of the true geometry and due tonoise.

To test the possibility of detecting concealed objects, eithermetallic or not, like a gun, in a first step measurements wereconducted with a dummy wearing a coat with a hidden gunand two other metallic objects. To enhance the contrast of themetallic items the radiometric image was processed using athreshold algorithm. Using different sample objects made upof metal, ceramic and plastic material, it was shown that it

(a) (b) (c)

Fig. 5. Indoor images at 94 GHz (Dicke radiometer) for a human with aconcealed gun without illumination (a), a dummy with illumination (b) andan illuminated human carrying chocolate and a gun (c).

is possible not only to detect suspicious materials but also toget a significant image showing the outline of the object. Dueto limitations in the scanning algorithm the scanning time forthese first experiments was unacceptably low at approximately20 minutes.

With the advent of LNAs with an improved noise figure,as described in Section 1, effort was put into an optimisationof the scanning process. The scanning time was reduced to2.5 minutes using only a single channel receiver. This enabledmeasurements of sample objects under the clothing of realhumans even in indoor environments [4]. Fig. 5a shows aradiometric image of a man with a gun hidden under hisjacket. Fig. 5b shows a dummy in a similar arrangementin a closed room but with artificial illumination. As thedetection of plastics and explosives is an important issue,measurements using chocolate as a replacement for explosiveswere conducted. Fig. 5c shows the result of a human carryinga gun and a block of chocolate under the clothes, illuminatedby a noise source.

LNALNA LNA4GHz Filter

Quadratur

VideoLowpass

Detector

Fig. 6. Radiometric sensor in total power setup avoiding the Dicke typeswitch. The overall gain of the LNAs is around 60 dB. This system is twiceas sensitive as a Dicke type sensor.

Using radiometers with LNAs so stable that a Dicke typesystem can be replaced by a total power sensor (see also Fig. 6)leads to reduced noise temperature and therefore increaseddynamic range of the system.

Figure 7 shows an example of a radiometric outdoor mea-surement using a total power radiometer at 94 GHz. Weaponsand other devices can be detected although hidden inside acoat. It must be emphasized that the ceramic knife is detected,which is not possible with a conventional metal detector. Thismeasurement shows well the high spacial resolution of thescanning system, especially when looking at the cell phone andthe PDA, where you can see many details of the objects. The

Fig. 7. Photo (left) and radiometric image (right) of a person scanned witha total power sensor under outdoor conditions. In the left picture the weapons(gun and ceramic knife) were put in front of the coat for clarification reasons.While producing the radiometric image (left) they were hidden inside the coat.The high spacial resolution of the system using 94 GHz, is noticed from theimage, which is why so many details of the scene are clearly visible. Forexample, the darker display and the brighter housing of the PDA are clearlydistinguishable. A cell phone is visible on the other hanging upright. Detailslike the display as well as the keyboard and the small external antenna aredetected.

Fig. 8. Radiometric outdoor images of persons concealing weapons underleather jackets using a Dicke radiometer [5].

pictures in Figure 8 show measurements of humans wearingconcealed weapons under leather jackets.

B. Active Imaging

For a direct comparison between an active and a passivescanning system a demonstrator based on a FMCW radarmodule at 94 GHz was developed und assembled (Fig. 9).Due to the scanning beam antenna a 2-dimensional scan ofa person can be performed much faster than by using a twoaxis positioner. The duration of a complete measurement istypically in order of a few minutes. Because the antenna’s fieldof view is limited to±10◦ the optimum distance to the targetis about 3 to 4 meters. This range combined with the widthof the antenna beam provides a spacial resolution, which ismuch worse than the resolution of the passive system describedabove. The resonant structure of the antenna (Fig. 3) limitsthe useful bandwidth to 400 MHz, which makes this type ofantenna inapplicaple for passive radiometric systems, wherehigh bandwidth is a crucial factor. In a radar based system

Fig. 9. Image of the FMCW radar scanner mounted on a positioner.

this bandwidth allows to discriminate the scene in range witha resolution of approximately 37 cm. In combination with ascanning system a 3-dimensional image of the target can becreated.

Figure 10 shows the radar images of two men and a dummycreated using the radar based security scanner. The comparisonof these results with images generated using a passive scannerdemonstrates, that both systems point out different propertiesof the measured target. It is planned to combine the passiveand active approaches in order to detect more details aboutthe test object in order to increase the reliability of securityapplications.

However, investigations of the detectability of metallicobjects by means of the active radar scanner revealed that thissystem in stand-alone mode does not match the requirementsof concealed weapon detection. The first reason for this is thatthe surface of most weapons, like knifes and guns, consistof a small set of flat metal facets. The dominant type ofscattering on flat surfaces is specular reflexion, therefore theseobjects can only be detected, if one of the illuminated surfacesis perpendicular to the radar line of sight. Certainly, underconvenient conditions the radar will see a flash of the metalobject, but this is not enough for a reliable detection. Thisshortcoming can be fixed by application of several radarsensors in a bi- or multistatic configuration, which wouldsignificantly increase the probability of detection. Figure 11shows images of an unarmed man and a man with a gun onhis chest. The position of the gun is marked with a red circle.Although the weapon was not covered by clothing, the radarimage gives no clue about it, while the passive scanner usuallyshows even the shape of the gun.

Moreover, the detection of concealed weapons by meansof a radar scanner is complicated because of the differentscattering and absorption properties of fabric. While woolenclothes are nearly transparent in the millimeterwave range,heavy cotton can cause strong specular scattering itself, which

(a) (b)

Fig. 10. Images of two men (a) and a dummy (b) generated using a 94 GHzFMCW radar scanner.

(a) (b)

Fig. 11. Images of an unarmed man (a) and a man with gun (b). The positionof the gun is marked with a red circle.

disturbes the image and leads to a masking of reflections frommetallic objects. Under certain conditions even the wrinklesof the clothing can act as small scattering centers. In order tounderstand all these phenomena further investigations on thisfield are necessary.

IV. RESULTS

Two approaches for concealed weapon detection weretested, one using a radiometer, the other employing a miniatureFMCW radar. The passive system delivers encouraging resultswith good image quality, at rather long scanning times. Itwas shown that indoor use is feasible passively and can beenhanced by artificial illumination sources. The active sensoroffers higher scanning speed and works indepentend of theambient temperature.

Due to the monostatic geometry, which is in favour ofspecular reflections even from fabric, depending on its type, itis much more difficult to detect and image concealed metallicobjects using an active scanning system. The probability ofdetection can be improved using several radar sensors in a

multistatic configuration. For the future it is also plannedto introduce fully polarimetric methods, which are likely toallow a discrimination between different types of scatteringand acting as a polarimetric filter for reflections from thesurface of the fabric and from underneath.

REFERENCES

[1] M. Schlechtweg, A. Tessmann, A. Leuther, C. Schworer, H. Massler,M. Mikulla, M. Walther, R. Losch, Advanced Millimeter-Wave ICsUsing Metamorphic HEMT Technology, 32nd International Symposiumon Compound Semiconductors (ISCS05), September 2005.

[2] A. Tessmann, S. Kudszus, T. Feltgen, M. Riessle, C. Sklarczyk,W.H. Haydl, Compact Single-Chip W-Band FMCW Radar Modules forCommercial High-Resolution Sensor Applications, IEEE Transactions onMicrowave Theory and Techniques 50 (2002), 2995.

[3] Waveband Corporation, Van Ness Ave, Suite 1105, Torrance, CA 90501,info@waveband.com

[4] J. Huck, Personenscanner, Diploma Thesis, Fachhochschule Koblenz,Koblenz, June 2005.

[5] S. Erukulla, Design and Optimisation of Millimetrewave Sensors forSecurity Imaging, Master’s Thesis, Chalmers University of Technology,Goteborg, February 2006.