Short- and Long-Range Passive Imaging in Millimeter-Wave-Band

A. Vertiy1, S. Ozbek1, A. Pavlyuchenko1, M. Tekbas1, A. Kizilhan1, H. Cetinkaya1, A. Unal1, and S.B. Panin2

1International Laboratory for High Technology, TUBITAK-MRC, MI, 1, Dr. Zeki Acar Street, 41470, Gebze-Kocaeli, Turkey, email: Alex.Vertii@mam.gov.tr; Sunullah.Ozbek@mam.gov.tr; Andy_Pavluchenko@ukr.net

2Department of Diffraction Theory and Electronics, Institute for Radiophysics and Electronics of NASU, 12, Acad.

Proskura Str., 61085, Kharkov, Ukraine, email: SerPanin@gmail.com

Abstract

The millimeter-wave passive radiometric imaging systems based on the single channel design are developed. The systems are intended for disclosure of dangerous objects concealed under persons' clothing in short distances and for passive visualization of remote targets. The low-noise compact receiver (noise factor 4.5dB for 90-100GHz) involving four stages was designed. Antenna systems based on aspheric lens and on parabolic reflector were adopted. The radiometric imaging peculiarities of the building materials were investigated. The systems successfully recognize the metallic weapon hidden under outer clothing at a distance about 10m and visualize remote targets at a distance of 170m.

1. Introduction

The passive sub-terahertz imaging systems are very promising for disclosure of dangerous objects (weapons, explosives, drugs, etc.) concealed under persons' clothing, without revealing itself. The high-resolution radiometric systems for the fast imaging in short distances are essential for personal weapon control in public places. Owing to ability to penetrate poor weather for surveillance in the long-range regime [1], the passive radiometric imaging holds great promise as a means to aid aviation in low visibility conditions.

However, millimeter-wave imaging systems have shortages of low speed of operation and the small spatial

resolution in comparison with optical systems [2, 3]. The first problem can be overcome by using multi channel scheme of the imaging sensor module. Such array of sensors is capable of 2D scenes in real time. Unfortunately this high-cost design has disadvantages: the channels should be identical. The spatial resolution constrained by the Rayleigh criterion is determined first of all by the receiver sensitivity and the image reproduction quality of antenna. Low-noise amplifiers and low aberration antenna systems are intended to improve the spatial resolution.

In this work the passive millimeter-wave imaging systems have been developed on the base of the single channel scheme, which includes our specially designed low-noise total power receiver. The quasi-optical antennas were developed for passive imaging in the different distance range. For scanning process acceleration the flapping reflector was incorporated in the antenna system that allowed us to perform the scanning in one dimension.

2. Imaging Systems

The 90-100GHz range passive imaging system proposed in this paper is composed of quasi-optical antenna system, an imaging sensor and a scanning platform with measurement software. It was developed and investigated two types of antennas based on lens or reflector. The originally designed receiver with microstrip line and the receiver with the discrete customary waveguide units were used. The input of both receivers was equipped by the pyramidal or conical horn. The proposed imaging systems are shown in Fig. 1.

Wideband low-noise receiver was designed and manufacture for 3-mm band passive radiometric measurements, see Fig. 2(a). Receiver is based on detector scheme and involves the active AsGa elements. The receiver has a waveguide input that allows using the different types of horn or dielectric-rod antennas and makes possible the measurements of receiver parameters by standard equipments.

Another low-noise wideband receiver used in radiometric experiments was constructed in accordance with standard scheme of total power; it involves three customary nodes in the waveguide performance by HXI [4], see Fig. 2(b).

978-1-4244-6051-9/11/$26.00 ©2011 IEEE

(a)

(c )

(b)

(d)

Figure 1. Photos of the passive imaging systems (a), (b) and (d); ray scheme of the periscope antenna (c).

The main specifications of the both receivers are presented in Tab.1. Noteworthy the first receiver (see Fig.2(a)) exceeds the second receiver (see Fig. 2(b)) based on HXI’s nodes in sensitivity, weight and size indexes, and in cost.

(a) (b)

Figure 2. Photos of the low-noise radiometric receivers.

The long-focus HDPE (high density poly ethylene) lens with the aperture size mA 5.0= and thickness md 05.0= was used for the reproducing of the radiometric image, corresponding to the brightness temperature of the

target, in the scanning plane. The lens has a profile shape ( )bz f x= defined by equation:

( ) ( )2 201 2 1 0,z F z xε μ ε μ− + − − =

( )2

20

12 4 1

AF dd

ε με μ

⎛ ⎞+= −⎜ ⎟⎜ ⎟−⎝ ⎠

, (1)

where 0F is a focal length of the dielectric plane-convex lens. For HDPE, permittivity ,3.2≈ε permeability 0.1≈μ and thus 0 1.147F m≈ . Imaging by single lens (1) and by “periscope scheme” antenna (i.e. the lens incorporated with the flapping plane aluminium reflector ( )mm 0.187.0 × ) was investigated, see Fig. 1(a), (b) and (c). For scanning in the image plane of the single lens the 2-axes precision Cartesian robot based on a raster mechanical scanning architecture is utilized. The ±3.5˚ swing of the flapping reflector around vertical axis is used for horizontal scan that allowed us to essentially accelerate the scanning process. Reflector swing and vertical movement of the receiver in the scanning plane are synchronized.

Table 1. Receiver specifications.

Parameters Receiver 1 Receiver 2 Operation Band (GHz) 90-100 98-105 Gain (dB) 50-52 40-52 Integration time (ms) 1 1 Temperature Sensitivity (mV/K) 0.67 0.026

The antenna design in quasi-optics can be based on the geometrical optics approximation [5]. In our numerical

algorithms the cubic spline data interpolation was used to present the arbitrarily shaped lenses and reflectors in the general form. The ray antenna model obtained allows the antenna synthesis and optimization for arbitrarily shaped profiles of refractors and reflectors. The image formed by HDPE lens and its geometrical features are shown in Fig. 3.

(a) (b)

Figure 3. Lens image for different distances 0Z to the straight object of height mH 5.1= (a); the location of the curvature centre and the radius of image line for distance mZ 100 = to the object of height mH 5.1= (b).

The swing angle of the flapping reflector (see Fig. 1(b)) for imaging the target point ( )00 , ZX can be approximated by

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ+⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ−⎟⎟⎠

⎞⎜⎜⎝

⎛ Δπ

+≈ϕ5

0

3

00 51

31

218045

XXXzzz , (2)

where 0ZZmz −=Δ and mZ is z -coordinate of the point of intersection of the target and the antenna observation

axis for 45=ϕ , see Fig. 1(c). From (2) follows that the 5.2± turn of the flapping reflector around its vertical axis can be used for 1D horizontal scan of the target of size 1.5m placed at a 10m distance.

The aluminium parabolic reflector with the focal length mFm 32.1= and aperture size mAm 2.1= mounted on a scanning rotating platform was used for the long-range imaging, see Fig.1(d). The imaging sensor was fixed in the vicinity of the reflector focus.

3. Experimental Results

Using the radiometric system with the lens antenna described above, the radiometric measurements were conducted for disclosure of dangerous objects concealed under persons' clothing in short distances. The distance between the target and the receiver was 8m; the time of 2D scanning was about 900sec. Scanning in the domain 31×31cm we got the image consisting of 80×80 pixels. The measurement results are presented in Fig. 4 (a).

The imaging system based on the periscope scheme (see Fig. 1(b), (c)) allowed us to get a high-grade radiometric image of 80×80 pixels at a distances about 10m for 40sec. Note that the periscope scheme allowed us to accelerate the process more than in 20 times.

Parabolic dish reflector with the high sensitivity receiver were used for imaging in long distances, see Fig. 4(b). For the 170m distance the duration of 2D scanning was 360sec. The horizontal scanning region was from 5m to 50m that allowed us to get the object image with high resolution. Figure 4(b) demonstrates that the quality of radiometric image (of a tractor) is almost independent of engine exhaust fumes surrounding the target.

The passive radiometric investigations against the brightness temperature of sky are supposed to be conducted outdoors; therefore these measurements essentially depend on weather conditions. Withindoors, the induced radiation should be used to get sufficient contrast range for the radiometric measurements. It is particularly interesting the potentiality of the noise radiation sources placed behind the wall to provide the required level of the target illumination indoors. For that the incoherent signal transmittance of different building materials were investigated. As a noise source we use the noise oscillator based on silicon IMPATT diode and a solid-state amplifier. The noise source has output level of ENR about 35dB/kT0 in frequency range 98-105GHz. The results obtained shows that for the standard type of wall the noise source possessing ENR level of 40-70dB/kT0 is sufficient for radiometric imaging indoors when the fluctuation sensitivity of the receiver lies in 0.1-1.0mV/K.

(a)

⇓

(b)

⇓

Figure 4. Radiometric images obtained by lens (a) and reflector (b) imaging systems.

4. Conclusion

In this paper the sub-terahertz passive radiometric imaging systems based on a single channel design were developed. The systems can be employed for disclosure of dangerous objects concealed under persons' clothing in short distances and for passive visualization of remote targets. The several designs of antenna system and the radiometric receiver were developed and experimentally studied. The field measurements show that passive radiometric systems in 3-mm band are capable to successfully operate in short- and long-range regimes. The study of incoherent signal transmittance of the building materials shows the possibility of through wall radiometric imaging in the presence of the noise source with ENR level of 40-70dB/kT0..

5. Acknowledgments

This work was supported by the Turkish Republic Prime Ministry State Planning Organization Program (5075519) and by the TUBITAK 2221-Visiting Scientist Fellowship Program.

6. References

1. N. Skou, Microwave radiometer systems: design and analysis, Norwood, MA, Artech House, 1989.

2. A. Pergande, “New steps for Passive Millimeter Imaging,” Proceedings of SPIE in Passive Millimeter-Wave Imaging Technology, Orlando, FL, USA, vol. 6548, pp. 654802-1-654802-4, April 2007.

3. A. Vertiy, H. Cetinkaya, and M. Tekbas, "Active Microwave and Millimeter-Wave ISAR Imagine and Millimeter-Wave Passive Radar Receiver Design", Proceedings of the fourth world congress on Aviation in XXI-st century, Safety in Aviation and Space Technologies, Kyiv, Ukraine, vol.2, pp. 22.97-22.100, September 21-23, 2010.

4. Online available: http://www.hxi.com

5. M. Born and E. Wolf, Principles of optics, Cambridge: Cambridge University Press, 1999.