An Improved Procedure for ZDC Reduction in High
Resolution Imaging of Rotating Targets
Liya Liang, Xiaojian Xu
School of Electronics and Information Engineering, Beihang University
37 Xueyuan Road, Beijing, 100191, China
Email: xiaojianxu@buaa.edu.cn
Abstract—Strong background clutter exists in many cases
such as inverse synthetic aperture radar (ISAR) imagery of
turntable objects, which results in severely corrupted images.
This work mainly aims at the reduction of zero-Doppler clutter
(ZDC) for fixed background in high resolution radar imaging of
rotating targets. Existing signal processing techniques for ZDC
suppression are first addressed. An improved procedure is
developed to avoid the defects of existing techniques.
Experimental results using outdoor range measurements are
presented to demonstrate the usefulness of the proposed
technique.
Keywords—Radar imagery; High resolution range profile
(HRRP); Inverse synthetic aperture radar (ISAR); Zero-Doppler
clutter (ZDC); Radar cross section (RCS)
I.
I
NTRODUCTION
Inverse synthetic aperture radar (ISAR) imaging on a
turntable-tower test range permits convenient generation of
high resolution one- and two-dimensional images of radar
targets under controlled conditions, typically for
characterization of the electromagnetic (EM) scattering
signatures of targets or for tests of synthetic aperture radar
(SAR) image processing and automatic target recognition
(ATR) algorithms [1]. In many outdoor test ranges, consistent
with conventional ISAR processing, a target is positioned on a
rotating turntable and backscattered data are collected at
specific elevation angles across the whole 360 degrees of the
azimuth. The ISAR antenna mainbeam continuously
illuminates the rotating target and receives echoes having
Doppler frequency shift. However, mainbeam and sidelobe
energy also illuminates surrounding terrain and other objects,
called "background", which have virtually zero velocity due to
the fact that there is no obvious motion between the radar and
the background. The stationary background induces no
Doppler shift and produces the so-called zero Doppler clutter
(ZDC) responses. The coherent summation of these zero
Doppler returns degrades ISAR images by either masking
actual target responses or inducing responses which are not
directly attributable to target features [2].
Existing techniques for ZDC reduction include preparing
ground surface carefully to minimize reflections, using
antennas with narrower beamwidths and shorter radar pulses
to “see” less background, applying post-processing algorithm
to extract and subtract ZDC components from the
measurement data [2], and so on.
II. ZDC
E
XTRACTION AND
R
EDUCTION
For most radar cross section (RCS) ranges, the target under
test is mounted either on a pylon or put directly on a turntable.
During the RCS data acquisition, the target is rotated by the
positioner or turntable for azimuth sampling; while the
surrounding objects including target pylon itself keeps
stationary without any rotation. As a consequence, at a
specified frequency, the scattering field caused by surrounding
objects in the test field (called background clutter) will be a
complex constant for any azimuth angle, meaning that the
background is independent of azimuth angles. Therefore, the
radar received signal as a function of frequency and azimuth
angle can be expressed as [4]
(,) (,) ()
ff
θθ
=+VSB
(1)
where
(,)f
θ
S
denotes the target or calibrator’s scattering
signal,
()
B
represents the background scattering.
Assume that the two-dimensional scattering function of the
target is
(, )
xy
, or
(, )
r
in polar formation. The expression
of the radar received signal for a rotating object may be
written as
2
0
00
4
( , ) ( , ) exp{ ( )}
L
f
fr j R R rdrd
c
π
π
θ
=−−
S
(2)
where
is the maximum size of the target;
0
R
is the distance
between the radar and the rotating center;
R
denotes the
distance between the radar and a point
(, )r
on the target and
is written as
22
00
2sin( )RRr Rr
θ
=++ −
(3)
For far-field case, we have
0
sin( )RR r
θ
≈+ −
(4)
Combining (2) with (4) results in
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