Detection of an artificial target with low radar cross-section in presence of backscatter

Dmitry Zelenchuk1, Vincent Fusco1

1 The Institute of Electronics, Communications and Information Technology (ECIT), Queen's University of Belfast,

Belfast, United Kingdom, d.zelenchuk@ecit.qub.ac.uk, v.fusco@ecit.qub.ac.uk

Abstract—This paper presents an approach to improve the detection of an artificial target with low radar cross-section in presence of clutter. The target proposed in the paper modulates the phase response of the circularly polarized incident signal by means of rotation. The same physical phenomenon can be used to steer the modulated response in a non-specular direction. The bi-static measurements of the response of the target have demonstrated good agreement with theoretical prediction as well as with full-wave simulation.

Index Terms—radar cross-section, target detection, reflectarray.

I. INTRODUCTION The detection of a target in the presence of high levels of

background reflected backscatter has numerous applications for RF sensor identification, and communication in multipath environments. For instance, as an RFID tag modulates its RCS, a signal scattered from surrounding environment degrades the signal-to-noise ratio and extra processing is necessary to extract the tag information [1], [2]. In the particular case of a tag mounted on a large backing metal plate this unwanted signal amplitude can easily exceed the one generated by the tag itself. Hence, a method of RCS modulation that allows physical isolation of the clutter response would be extremely helpful.

One way to approach this problem is to ensure polarization isolation between the signal reflected by the environment and that reflected by the target. We propose the use of a rotational phase shift phenomenon which ensures polarization isolation of a specifically designed target and background clutter [3]. The same phenomenon has been extended to allow beam steering enabling the target to direct its response in a non-specular direction in order to facilitate improved operational performance.

II. THEORY

A. Rotational phase shiftIn order to demonstrate the principle of operation we

employ a reflectarray with mechanically rotatable patches [4]. The proposed structure is shown in Error: Reference source notfound. Five 2 mm thick brass circular patches are suspended 5 mm above an aluminum ground plane. Each patch is designed to resonate at 3.1 GHz. There are three 2 mm wide shorting pins: one at the centre and two at opposite edges. Thus the

patch is seen as a "short circuit" for a wave polarized along the pins and "open circuit" for the orthogonal polarization.

Fig. 1. The reflectarray with rotatable patches.(rd=23.5 mm, ax=380 mm, ay=180 mm, d=70 mm).

Let us analyze the scattering response of a single patch upon CP excitation. On applying the Jones calculus [3] one can derive the reflected field as

ER=(cosθ r −sinθ r

sin θr cosθr)(1 0

0 −1)( cosθr sin θr

−sin θr cosθr) 1

2 ( 1± j)

where θr is rotation angle, and ± is to distinguish RHCP and LHCP waves.

ER=e± j 2θr 12 ( 1∓ j)

1\*MERGEFORMAT()

It follows from 1\* MERGEFORMAT () that upon reflecting from the target an incident CP wave will preserve the hand of the incident will polarization and gain a phase shift of ± 2θ r. The sign of the phase shift depends on the hand of the incident wave.

The back plate behind the elements behaves as a ground plane. This reverses the hand of the incident CP signal upon reflection from it. In addition the ground plane does not encode a rotational phase shift onto the reflected signal. Consequently the signal reflected from the reflectarray can have phase information encoded upon it and also be readily indentified from the much larger levels of background backscatter coming from the ground plane. Furthermore we can add a progressive offset phase shift between each of the reflecting elements in the reflectarray for the purpose of steering, [5], the reflectarray backscatter beam into a desired non-specular direction.

B. Beam steeringIn order to induce beam steering capacity the five elements

of the array were rotationally offset, as shown in Error:Reference source not found, to provide a progressive phase shift, i.e. nth patch is rotated by angle n θ shift+θr.

The steering angle of array is determined by the progressive phase shift as follows [5]:

θsteer=± asin2θshift

kd2\*

where k is the wavenumber and d is separation between the adjacent elements.

Below the array has been simulated under the influence of incident LHCP excitation applied normal to the array and both phase shift and magnitude at angle θ steer have been calculated for both reflected CP polarization types. The sign in 2\*MERGEFORMAT () depends on the incident wave polarization: "+" for RHCP and "-" for LHCP.

The beam steering capacity can be used in a reverberant or strong multipath environment to steer the response to a direction of the null of the clutter radiation pattern. This can improve clutter response separation in addition to the polarization isolation discussed earlier.

III. MEASUREMENT

The structure has been measured with the bi-static setup in an anechoic chamber shown in Error: Reference source notfound. The reflector is illuminated with a dual-polarized horn antenna fed through a 90 degree hybrid coupler in order to send a CP wave. The signal then scattered by the reflector and received by a dual-polarized square patch fed through a 90-degree hybrid coupler to receive a CP wave. The reflector and the patch are placed onto a polystyrene foam fixture attached to a rotating table as shown in Error: Reference source not found.

Fig. 2. Measurement setup.

Fig. 3. Measurement fixture with reflectarray and the Rx dual-polarized patch.

The square patch and array are oriented so as to measure the maximum of backscattered field whose polarization is the same as that of the incident field, i.e. θb=θsteer , whilst the angle of incidence θ0=0. Below we present measured phase response for θsteer=13∘. In order to maintain the main beam at this particular angle the progressive rotation angle θ shift was set at 30∘. All disks were then rotated through θr=0¿180∘ and the same hand polarization phase response measured.

0 30 60 90 120 150 180-360

-270

-180

-90

0

rot (

)

Pha

se ( )

measuredsimulated

Fig. 4. Comparison of measured and simulated LHCP phase vs θr for

position of the beam maximum defined by θ shift=30∘.

0 50 100 1500

10

20

30

rot (

)

LHC

P/R

HC

P(d

B)

measuredsimulated

Fig. 5. Comparison of measured and simulated LHCP to RHCP isolation vs

θr for position of the beam maximum defined by θ shift=3 0∘.

Comparison between the measured response and the one simulated in CST Microwave Studio is presented in Fig. 4. One can readily observe a 2:1 linear phase dependence in full agreement with theoretical prediction of 1\*MERGEFORMAT () and numerical EM simulation. The

measured phase does follow the simulated response with discrepancies at θr=90 …120∘.

A shown in Fig. 5, steering the beam in non-specular direction results in prevalence of signal scattered by the array (LHCP) over the signal reflected by the ground plane and the fixture (RHCP). The target positioning uncertainty and scattering from the fixture are to explain the difference between the measured and simulated results.

In further work the mechanical rotation of the reflect array can be substituted for electrical switching of diagonal pairs of element peripherally located pins between open and short circuit states as has previously been demonstrated in spiraphase reflectarrays [6]. This would then permit the reflectarray to encode its same hand polarized signal with BPSK or QPSK.

REFERENCES

[1] P. Nikitin and K. V. . Rao, “Theory and measurement of backscattering from RFID tags,” IEEE Antennas Propag. Mag., vol. 48, no. 6, pp. 212–218, Dec. 2006.

[2] S. J. Thomas, E. Wheeler, J. Teizer, and M. S. Reynolds, “Quadrature Amplitude Modulated Backscatter in Passive and Semipassive UHF RFID Systems,” IEEE Trans. Microw. Theory Tech., vol. 60, no. 4, pp. 1175–1182, Apr. 2012.

[3] R. C. Jones, “A New Calculus for the Treatment of Optical Systems. I,” J. Opt. Soc. Am., vol. 31, no. 7, pp. 488–493, Jul. 1941.

[4] V. Fusco, “Mechanical beam scanning reflectarray,” IEEE Trans. Antennas Propag., vol. 53, no. 11, pp. 3842–3844, Nov. 2005.

[5] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd Edition, 2nd ed. Wiley, 1996.

[6] A. E. Martynyuk, J. R. Zamudio, and N. A. Martynyuk, “Reflectarray based on three-bit spatial phase shifters: Mathematical model and technology of fabrication,” in European conference on Antennas and Propagation, 2009, pp. 2774–2778.