IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 12, NO. 10, OCTOBER 2015 2041
Micro-Doppler Features Analysis and Extraction of
Vibrating Target in FMCW SAR Based on
Slow Time Envelope Signatures
Ying Liang, Qun Zhang, Senior Member, IEEE, Ying Luo, Member, IEEE, You-qing Bai, and Yong-an Chen
Abstract—The combination of frequency-modulated continuous
wave (FMCW) technology and synthetic aperture radar (SAR)
leads to lightweight, cost-effective, and low-power dissipation
imaging sensors of high resolution. For extracting the features
of vibrating targets on ground in FMCW SAR, the displaced
phase center antenna technique is introduced into the FMCW
SAR system to suppress the ground clutter, and then, the signal
characteristic is analyzed. It indicates that the energy of the joint
time–frequency distribution is presented as an uneven distribution
that is induced by a slow time envelope (STE). In this letter,
a novel extracting method of vibrating features based on STE
signatures is proposed. With this method, the vibrating frequency
and amplitude can be calculated from some extracted positions in
the STE. Some simulations are given for validating the feasibility
and effectiveness of the method.
Index Terms—Displaced phase center antenna (DPCA),
frequency-modulated continuous wave (FMCW) synthetic
aperture radar (SAR), slow time envelope (STE), vibrating target.
I. INTRODUCTION
M
ICROMOTIONS dynamics, such as the mechanical vi-
brations or rotations of an object or the structures on an
object, imposes a periodic time-varying frequency modulation
on radar signals. This is known as the micro-Doppler (m-D)
effect [1]. The m-D features reflect the unique dynamic and
structural characteristics of a target, which can provide addi-
tional information for the classification, recognition, and iden-
tification of the target [2]. In 2009, the synthetic aperture radar
(SAR)/micromotion target indication (MMTI) concept, which
incorporates micromotion techniques into SAR, was introduced
Manuscript received January 25, 2015; revised April 27, 2015; accepted
June 7, 2015. This work was supported by the National Natural Science
Foundation of China under Grant 61471386 and Grant 61172169.
Y. Liang and Y. Chen are with the School of Information and Navigation, Air
Force Engineering University, Xi’an 710077, China.
Q. Zhang is with the School of Information and Navigation, Air Force En-
gineering University, Xi’an 710077, China, with the Collaborative Innov ation
Center of Information Sensing and Understanding, Xidian University, Xi’an
710077, China, and also with the Key Laboratory for Information Science of
Electromagnetic Waves (Ministry of Education), Fudan Univ ersity, Shanghai
200433, China (e-mail: zhangqunnus@gmail.com).
Y. Luo is with the School of Information and Navigation, Air Force Engineer-
ing University, Xi’an 710077, China, with the Collaborative Innovation Center
of Information Sensing and Understanding, Xidian University, Xi’an 710077,
China, and also with the National Laboratory of Radar Signal Processing,
Xidian University, Xi’an 710071, China.
Y. Bai is with the School of Science, Air Force Engineering University, Xi’an
710051, China.
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LGRS.2015.2444658
to generalize the SAR/ground moving target indicator (GMTI)
techniques [3]. In addition, the influence of typical micromo-
tions on SAR images and the GMTI was analyzed in [4].
Vibration is a general kind of micromotions and characterizes a
specific target. The extraction of vibrating features such as the vi-
brating frequency and amplitude allows us todistinguish a diesel
engine of a bus from a gas turbine engine of a tank [5]. Some stud-
ies that focus on the analysis and extraction of vibrating features
using SAR systems h ave appeared in the past f ew years [5]–[9].
Generally speaking, the signals of vibrating targets in SAR sys-
tems are presented as sinusoidal in a joint time–frequency dis-
tribution (JTFD); thus, the vibrating features can be extracted by
mapping the JTFD onto the parameter space by pattern recog-
nition tools [10], such as the Hough transform (HT) [11], [12].
The r eceived signals of vibrating targets on ground are usu-
ally submerged in heavy ground clutter, which makes detection
and extraction difficult. Although the displaced phase center
antenna (DPCA) technique was introduced into bistatic SAR
to suppress the ground clutter and the vibrating features were
extracted from the JTFD in [9], it did not pay enough attention
to the influence of a time-varying envelope in the received
signal after the DPCA processing. In some cases, the m-D curve
will be fragmentary affected by the time-varying envelope. That
will make the performance of some extracting methods based
on the JTFD degenerate.
As a compact-size remote sensing instrument with light-
weight, cost-effective, and low-power dissipation, frequency-
modulated continuous wave (FMCW) SAR has brought
growing interest in recent years, particularly in small unmanned
aerial vehicle platforms, etc. [13]–[16]. The existing studies
about the FMCW SAR mostly focused on imaging and the
GMTI but seldom on the MMTI. The detection and characteri-
zation of vibrating targets in the FMCW SAR system should be
also paid enough attention.
In this letter, we introduce the DPCA technique into the
FMCW SAR with a transmitter and two receivers to suppress
the ground clutter. The time-varying envelope induced by the
DPCA processing is named as a slow time envelope (STE), and
the influence of the STE on the JTFD is analyzed. Finally, a
novel extracting method based on STE signatures is proposed.
II. S
IGNAL MODEL OF VIBRATING TARGET
Considering that the signal of the FMCW SAR is transmit-
ted and received continuously, it usually demands separately
dedicated transmitter and receiver. The FMCW SAR ge ometry
with a transmitter and two receivers in a 3-D space is shown
in Fig. 1. The FMCW SAR sensor travels along an ideal
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