IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING, VOL. 8, NO. 4, APRIL 2015 1721
MOCO for High-Resolution ScanSAR via
Full-Aperture Processing
Ning Li, Robert Wang, Senior Member, IEEE, Yunkai Deng, Member, IEEE, Jiaqi Chen, Yabo Liu,
Zhimin Zhang, and Fengjun Zhao
Abstract—In this paper, a novel Doppler rate (DR) estimation-
based two-dimensional (2-D) subaperture motion compensation
(MOCO) approach is proposed for high-resolution scanning syn-
thetic aperture radar (ScanSAR) mode. It can estimate space-
variant phase errors in different subswaths simultaneously by
exploiting data of multiple subswaths. High-resolution ScanSAR
images, produced by full-aperture processing algorithm, are often
defocused by uncompensated phase errors. As a result of the
periodic gapped data in each subswath in ScanSAR mode, the per-
formance of the commonly used stripmap MOCO approaches will
be degraded or even not work when using full-aperture processing
algorithm. The effectiveness of the proposed approach is demon-
strated by an airborne ScanSAR real data set containing different
types of terrain, with a high geometric resolution of about 3.5 m
(mid or far range) or 5 m (near range) in azimuth working at C
band.
Index Terms—Doppler rate (DR), full-aperture, motion com-
pensation (MOCO), multiple subswaths, scanning synthetic aper-
ture radar (ScanSAR).
I. INTRODUCTION
S
CANNING synthetic aperture radar (ScanSAR) is
designed to image wide swaths by sweeping the antenna
beam periodically from near range to far range in a step-wise
manner [1]. Each subswath of the ScanSAR data consists of
a set of bursts. Different algorithms have been developed for
ScanSAR data processing [1]–[5]. Compared with other algo-
rithms, full-aperture processing is not a very efficient approach
[3]. However, it has the advantage that all the existing standard
high-precision stripmap synthetic aperture radar (SAR) imag-
ing algorithms can be used for ScanSAR data with very little
modifications. In addition, this kind of processing preserves
the phase information, and yields a product that shares the same
geometric properties with the corresponding stripmap images
[1], [3]. Because of these, full-aperture processing algorithm is
still used in many applications [6].
Manuscript received June 18, 2014; revised August 04, 2014; accepted
September 18, 2014. Date of publication October 12, 2014; date of current
version May 26, 2015. This work was supported in part by the “Hundred Talents
Program” of the Chinese Academy of Sciences, in part by the National Natural
Science Foundation of China under Grant 61301025, and in part by China
Postdoctoral Science Foundation under Grant 2013M541035.
The authors are with the Department of Space Microwave Remote Sensing
System, Institute of Electronics, Chinese Academy of Sciences, Beijing
100190, China (e-mail: lining_nuaa@163.com; yuwang@mail.ie.ac.cn;
ykdeng@mail.ie.ac.cn; ybliu@mail.ie.ac.cn; cjq19840130@163.com).
N. Li is also with the University of the Chinese Academy of Sciences, Beijing
100039, China.
J. Chen is also with the College of Computer and Information Engineering,
Hohai University, Nanjing 210098, China.
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/JSTARS.2014.2359953
In full-aperture processing algorithm, the gaps are filled with
zeros for a given subswath, and the ScanSAR data are processed
coherently as stripmap data. Although the azimuth resolution of
the resulting ScanSAR images is only determined by the burst
duration, the synthetic aperture is usually shared between sev-
eral bursts [1]. Phase errors induced by uncompensated motion
of the airborne platform or orbit error of the spaceborne plat-
form in any burst within the synthetic aperture may degrade the
image quality. Therefore, motion compensation (MOCO) tech-
niques, which have been proved to be a useful tool for phase
errors compensation, are necessary to obtain the sharpest image
focus [7]–[16], especially in high-resolution case processed by
full-aperture approach.
In past years, several signal-based MOCO approaches have
been developed [7]–[16]. The well-known phase gradient auto-
focus (PGA) technique was proposed for spotlight mode [9],
and its improvements have also been successfully used in
stripmap mode [10], [11]. Doppler rate (DR) estimation-based
techniques were proposed to estimate the quadratic and high-
order phase errors, which is effective for both stripmap and
spotlight mode and robust to different types of scenes [12]–[16].
However, all the above-mentioned MOCO techniques are inval-
idated for ScanSAR mode when using full-aperture algorithm,
due to periodic gapped data in each subswatch.
In [16], an innovative DR-based MOCO (DRM) approach
was proposed for the high-resolution SAR, which compensated
the excessive range cell migration (RCM) and space-variant
phase error in two steps. The phase error function of the
approach is brought by the double integral of DRs to avoid
the “stitching problem” and without introducing any models
for the complete phase error function. Inspired by their work,
in this paper, an improved DR-based MOCO (IDRM) approach
is proposed for ScanSAR mode, which is modified by the pre-
vious work intended for stripmap mode [16]. In the proposed
approach, assuming that the excessive RCM has been corrected
by the inertial navigation system (INS), we only focus on the
step of space-variant phase error compensation. After range
compression operation, each burst is first divided into multiple
blocks in both range and azimuth direction. Second, the DRs
in both dimensions that belonged to different subswaths are
estimated. Third, range-dependent DR errors (DREs) of each
azimuth subaperture for total ScanSAR swath are obtained by
least-square error (LSE) method and then integrated to obtain
space-variant phase errors for MOCO. Airborne ScanSAR real
data set is used to test the proposed approach.
The manuscript is organized as follows. In Section II, some
related works are briefly reviewed. In Section III, the proposed
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