IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 8, NO. 3, MAY 2011 411
Micro-Doppler Signature Extraction and ISAR
Imaging for Target With Micromotion Dynamics
Kai-ming Li, Xian-jiao Liang, Qun Zhang, Senior Member, IEEE, Ying Luo, and Hong-jing Li
Abstract—The micromotion of a target will generate a micro-
Doppler (m-D) effect in the frequency domain. The m-D effect is
regarded as a unique property of the target, which has special
significance in target detection, identification, and classification.
The classical range-Doppler algorithm cannot obtain a clear in-
verse synthetic aperture radar (ISAR) image due to the m-D effect
induced by micromotion dynamics. The m-D effect induced by
periodical micromotion is represented as a sinusoidal modulation
in a spectrogram, whereas the Doppler induced by a main body is
depicted as the form of a straight line. Therefore, the extraction of
an m-D signature is transformed into the separation of a sinusoid
and a straight line. The cancellation technique is a classical method
for removing ground clutter. Based on the same principle, the
cancellation technique is applied to the spectrogram in this letter,
which successfully achieves the separation of the m-D signature
and gets the clearer ISAR image of the main body. The effective-
ness and robustness of the algorithm are proved by simulation
results.
Index Terms—Automatic target recognition (ATR), micro-
Doppler (m-D) effect, micromotion, radar imaging, spectrogram
cancellation, time–frequency analysis.
I. INTRODUCTION
A
SIDE from the translation of a target centroid, there are
slight movements of the target body or components, such
as vibration, rotation, or acceleration, which are named as
“micromotions” [1]. Rotation and vibration are two of the most
common forms of micromotions. In inverse synthetic aperture
radar (ISAR) imaging, the target with rotating or vibrating
parts, such as a helicopter, will generate additional frequency
modulation aside from the Doppler frequency shift caused by
the target body, which is called the “micro-Doppler (m-D) ef-
fect” [1]–[3]. The m-D effect is referred to as a unique property
of the target, which has special significance in the detection,
classification, and recognition of the target with micromotions.
Manuscript received May 27, 2010; revised August 4, 2010; accepted
August 19, 2010. This work was supported in part by the National Nature
Science Foundation of China under Grant 60971100 and in part by the Nature
Science Foundation of Shaanxi Province under Grant 2010JQ8007.
K. Li, X. Liang, and Y. Luo are with the Institute of Telecommunica-
tion Engineering, Air Force Engineering University, Xi’an 710077, China
(e-mail: likaiming1982@163.com; drdd111586@163.com; luoying2002521@
163.com).
Q. Zhang is with the Institute of Telecommunication Engineering, Air Force
Engineering University, Xi’an 710077, China, and also with the Key Labo-
ratory of Wave Scattering and Remote Sensing Information (Ministry of Ed-
ucation), Fudan University, Shanghai 200433, China (e-mail: zhangqunnus@
gmail.com).
H. Li is with the College of Geographical Science, Southwest University,
Chongqing 400715, China (e-mail: lihongjing3@163.com).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LGRS.2010.2081660
Since the concept of the m-D effect was proposed by Chen
in 2000 [2], the extraction and separation techniques of the
m-D signature have developed rapidly. The m-D effect induced
by periodic rotation or vibration will be shown as a form
of sinusoidal modulation in the spectrogram [1]–[3]. Chen
systematically researches the m-D effect and extracts the m-D
feature by mainly using the time–frequency analysis techniques
[2], [3]; Thayaparan et al. detect and separate the m-D signature
by utilizing the adaptive joint time–frequency analysis and
wavelet transform theory [4]; Setlur et al. parameterize the m-D
signal and implement its separation [5]; Chen [6] decomposes
the m-D signal as the linear combination of independent basic
functions; Li and Ling [7] put forward an adaptive chirplet de-
composition algorithm, which has a heavy calculation burden;
Cai et al. [8] and Bai et al. [9] propound the experimental-mode
decomposition (EMD) method and the complex-valued EMD
method for the m-D signature extraction, respectively. Then,
the methods for image processing have also been introduced for
the separation of the m-D feature; Stankovic et al. [10] apply the
Radon transform to extract the m-D signature while Zhang et al.
[11] apply the Hough transform in a spectrogram to extract the
m-D signature of the targets with large rotors, but its calculation
cost is also high. Moreover, some literatures have researched
the application of the m-D effect in the signature extraction of
a target, such as the geometrical signature extraction of wheels
[12], the micromotion signature extraction of a helicopter [4],
the urban sensing and indoor sensing [13], the gait and activity
analysis of a pedestrian [1]–[4], [7], [12], [14], etc., which are
effective for automatic target recognition.
Due to the influence of the m-D effect, the image quality
of the target is deteriorated when using a traditional range-
Doppler algorithm, which brings difficulties for target recog-
nition. Therefore, it is necessary to separate and extract the
m-D information and then implement the ISAR imaging. A
cancellation technique is used for removing ground clutter
[15], which based on the amplitude of the echoes generated
by stationary objects on the ground is relatively stable. In this
letter, the cancellation technique is applied to a spectrogram
in terms of the similar situation under a chirp signal system,
which is effective for the extraction of the m-D signature and for
getting a clearer ISAR image. The performance of the method
is simulated and discussed, and the method is also perfected for
signature extraction and recognition for the micromotion target
with a small rotating or vibrating radius by combining with the
time–frequency analysis technique.
II. m-D E
FFECT
By taking the rotating target for example, the point scatterer
model of ISAR imaging is shown in Fig. 1. The radar is
located at the origin O of the radar coordinate system XOY .
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