Thinned array of reflectors for Suicide Bomber detection applications

Borja Gonzalez Valdes , Jose A. Martinez and Carey M. RappaportWork supported by Gordon - CENSSIS, BombDetec, ALERT, and Northeastern University, Boston, MA 02115(bgvaldes@com.uvigo.es)

AbstractStandoff identification (50 m) of suicide bombers wearing explosive devices under

clothing is an increasingly important problem. To achieve such identification, the detectionof abrupt changes in shape on the target’s body using millimeter wave radar is a viableoption. To detect irregular contours it is necessary to scan a small beam across the bodysurface. The spot of illumination must be of the same order of the irregularities of interest.The only feasible solution to create such an spot is to use an array of reflectors. In thiswork, the design of such an array and its optimization in order to minimize the number ofelements for a given low Side Lobe Level (SLL) is presented.

Introduction• A recent work [1, 2] presents a new concept to detect explosives worn under

clothing around human bodies. It consists in detecting abrupt changes at theshape of the subject under test using radar.

Figure 1: General sketch of our van-based, highresolution radar system for standoff detectionof potential suicide bombers.

• To detect irregular contours it is ne-cessary to scan a small spot beamwith extent of the same order as thetarget irregularities across the bodysurface.

• If a small portion of the human chestis illuminated, it is possible to pro-cess the electromagnetic signature ofthis area separately from other illu-minated areas.

• For this application, an antenna pro-ducing a 5 cm. wide spot at fiftymeters would require a 1000 wave-length aperture.

• The only feasible solution is to usean array of reflectors

Center strategic goals and legacyThis project is one of the new Gordon-CenSSIS activities, and incorporates all

the three levels: first, it is a basic research project (level L1), with elements offundamental antenna theory. A PO code has also been used for electromagneticcharacterization of arrays of reflectors in the nearfield. The performance evalua-tion provides a strong proof of enabling technology which directly falls insidethe level (L2). The system is being highly demanded by national and internatio-nal agencies and solves a Real-World problem, in the security systems field, andextends in the system level (L3).

Broader impact and technology transferThis projects supplements the BomDetec project, and establishes an enabling

environment for further work in suicide bomber detection. There are many com-ponents inside this project that could potentially be technology transfer items:intellectual property, developed software, testbed results, and system applica-tions. The recent contract signed by Gordon-CenSSIS and partners with theHSARPA is a perfect fusion between government and industry partners wherethe technology transfer could occur.

Antenna configuration• In order to obtain an aper-

ture large enough, the pro-posed antenna configura-tion is composed of an arrayof reflectors with total ex-tent of 1000 wavelengths.

• Each reflector antenna iscomposed of a parabolicsurface with diameter 75wavelength illuminated bya horn.

Figure 2: Profile of each reflector element of the array.

• The maximum number of reflectors that can compose the array is thirteen ifthey touch each other (Figure 3). In order to narrow the beam in the planeorthogonal to the linear array axis several rows of the linear array must becombined.

Figure 3: Positions of the reflectors in the two dimensional full array

• For the selected range of fifty meters, the antenna must be able to scan ± 5beamwidths (BW) in order to cover the forty centimeters of the human chest.The array of reflectors is suitable for steering the antenna beam by introducinga linear phase shift at each reflector feed [3].• The aim of this work is to obtain a thin array, minimizing the number of

elements and optimizing their positions in the array in order to keep low SLLfor all the scanning conditions.

Array optimization by Simulated Annealing• The optimization of the array pattern requires evaluating the scattering from

the reflectors. At the distance of fifty meters, it is necessary to evaluate thenearfield produced by each reflector, because the target is not in the farfield ofthe full array. This complicates the design and optimization of the array.• To calculate the currents induced by the feed in each reflector the PO (Phy-

sical Optics) approximation is used. Then, from the induced currents J, thenearfield scattered by a perfect electric conductor (PEC) with surface Ω in theobservation points r, Es(r), can be computed as [4]:

Es(r) =− jkZ0

∫Ω

[g1J(r′)−g2(J(r′) · R)R]GdΩ

where

g1 = 1−1/(kR)2− j/(kR) , g2 = 1−3/(kR)2− j3/(kR)

R = r− r′ , R = |R| , R = R/R

Z0 =√

µ0/ε0

• The total nearfield is then calculated by the summation of the field contribu-tions at the observation point r for each reflector.

• Then, in order to optimize the antenna pattern with lowest maximum SLL aSimulated Annealing (SA) [5] algorithm is used. The object of the optimiza-tion is the position of each reflector element in the linear array, contained invector ~p.

Figure 4: Diagram of the SA algorithm used inthe optimization.

• The nearfield scattered by the arrayelements in positions ~p has an as-sociated cost function, cost(~p), de-pending on the combined maximumSLL of the antenna pattern for all thescanning situations.

• Each step of the SA algorithm repla-ces the current positions by a ran-dom solution, T~p+ ~∆p, where ~∆p isa random variable and the parame-ter T (called the temperature), is agradually decreased value during theprocess through a variable dt. Theparameter N controls the number ofiterations.

• Figure 4 resumes the SA algorithmused in the optimization.

ResultsResults of the optimization show that in order to obtain a reasonable SLL the

minimum number of reflectors to use is seven. Figure 5. presents three rows ofthe designed linear array that comprise a new two dimensional array of 1000 x250 wavelengths.

Figure 5: Positions of the reflectors in the two dimensional thinned array

Figure 6 presents the nearfield antenna pattern in the plane containing the arrayaxis for several scanning conditions. Figure 7 presents the nearfield beam ofthe two dimensional array on the surface of a human chest model placed at fiftymeters from the array and when the array scans from 0 to 5 BW. With the desig-ned array, the approximated BW at 3 dB is 3 cm. in Y and 12 cm. in X. at thefrequency of 77 GHz.

ConclusionsThe design and optimization of a radar array antenna aimed to detect explosives

worn under clothing has been presented. Results show that the number of ele-ments in the array and their positions can be optimized in order to minimize theSLL. The designed array accomplishes the requirements of BW and SLL neces-sary to illuminate the human chest with tight spots at fifty meters and allowingprocessing of their electromagnetic signatures separately from other illuminatedareas.

(a) (b)

(c) (d)

(e) (f)

Figure 6: Nearfield pattern [dB] when the seven elements non-uniform array is scanning a)0 BW, b)1 BW , c)2 BW , d)3BW, e)4 BW , f)5 BW

(a) (b)

(c) (d)

(e) (f)

Figure 7: Body model illumination [dB] when the array is scanning a)0 BW , b)1 BW , c)2 BW, d) 3BW , e)4 BW , f)5 BW

References[1] Martinez-Lorenzo, J. A., Rappaport, C. M., et al. , “Standoff Concealed Explosives Detection Using Millimeter-Wave Radar to Sense Surface Shape Anoma-

lies”. , AP-S 2008, IEEE AP-S International Symposium, San Diego, CA, USA, Jun. 2008.

[2] Martinez-Lorenzo, J. A., Rappaport, C. M., Sullivan, R. and Pino, A. G., ”A Bi-static Gregorian Confocal Dual Reflector Antenna for a Bomb Detection RadarSystem”. , AP-S 2007, IEEE AP-S International Symposium, Honolulu, Hawai’i, USA, Jun. 2007.

[3] Gonzalez-Valdes, B., Martnez-Lorenzo, J. A., Pino, A. G. and Rappaport, C. M ”Zooming and scanning a beam by hybrid mechanical-electronic pointingsystems”. ESA Antenna Workshop on Space Antenna Systems and Technologies, Noordwijk, Netherlands, Jun. 2007.

[4] C. A. Balanis, Antenna Theory Analysis and Design, 3rd ed. Hoboken, NJ: Wiley, ch. 12.

[5] S. Kirkpatrick, J. Gelatt, and M. Vecci. Optimization by simulated annealing. Science, vol. 220, pp. 671-680, 1983.