Detection of an artificial target with low radar crosssection 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
reflectarray.
I.
cross-section,
target
detection,
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 nonspecular direction in order to facilitate improved operational
performance.
II.
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.
THEORY
A. Rotational phase shift
In 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
not found.. 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
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
cos 𝜃𝑟 − sin 𝜃𝑟 1 0
cos 𝜃𝑟 sin 𝜃𝑟 1
1
𝑬𝑅 = (
)(
)(
) ⁄2 ( )
±𝑗
sin 𝜃𝑟 cos 𝜃𝑟
0 −1 −sin 𝜃𝑟 cos 𝜃𝑟
where θr is rotation angle, and ± is to distinguish RHCP
and LHCP waves.
1 1
(1)
𝑬𝑅 = 𝑒 ±𝑗2𝜃𝑟 ( )
2 ∓𝑗
It follows from (1) 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 ±2𝜃𝑟 . 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 steering
In 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 𝑛𝜃𝑠ℎ𝑖𝑓𝑡 + 𝜃𝑟 .
The steering angle of array is determined by the
progressive phase shift as follows [5]:
2 𝜃𝑠ℎ𝑖𝑓𝑡
(2)
𝑘𝑑
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 𝜃𝑠𝑡𝑒𝑒𝑟 have been calculated
for both reflected CP polarization types. The sign in (2)
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.
0 𝑡𝑜 180∘ and the same hand polarization phase response
measured.
0
III.
Phase ()
𝜃𝑠𝑡𝑒𝑒𝑟 = ± asin
Fig. 2. Measurement setup.
-180
-270
-360
0
30
60
90
rot ()
120
150
180
Fig. 4. Comparison of measured and simulated LHCP phase vs 𝜃𝑟 for
position of
the beam
maximum defined by 𝜃𝑠ℎ𝑖𝑓𝑡 = 30∘ .
LHCP/RHCP(dB)
30
MEASUREMENT
The structure has been measured with the bi-static setup in
an anechoic chamber shown in Error! Reference source not
found.. 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 90degree 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..
measured
simulated
-90
20
measured
simulated
10
0
0
50
100
rot ()
150
Fig. 5. Comparison of measured and simulated LHCP to RHCP isolation vs
𝜃𝑟 for position of the beam maximum defined by 𝜃𝑠ℎ𝑖𝑓𝑡 = 30∘ .
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) 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]
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 𝜃𝑠ℎ𝑖𝑓𝑡
was set at 30∘ . All disks were then rotated through 𝜃𝑟 =
[2]
[3]
[4]
[5]
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