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IEEE GEOSCIENCE AND REMOTE SENSING LETTERS 1
Holographic SAR Tomography Image
Reconstruction by Combination of
Adaptive Imaging and Sparse
Bayesian Inference
Qian Bao, Yun Lin, Wen Hong, Member, IEEE, Wenjie Shen, Yue Zhao, and Xueming Peng
Abstract—In this letter, we propose an imaging algorithm
for the holographic synthetic aperture radar tomography in the
circumstance of sparse and nonuniform elevation circular passes.
Considering the anisotropic behavior of scatterers and the off-
grid effect of sparse signal recovery, the algorithm combines the
2-D adaptive imaging method for circular SAR and the sparse
Bayesian inference-based method for elevation reconstruction.
For each circular pass, the azimuth-range 2-D image can be
formed by the adaptive imaging method, which depends on the
preretrieved maximum azimuth response angle and the azimuth
persistence width. To deal with the off-grid effect in elevation
reconstruction, which is caused by the deviation between the true
scatterers and the discretized imaging grids, the off-grid sparse
Bayesian inference method jointly estimates the scatterers and
elevation off-grid error by applying their hierarchical priors.
Compared with the conventional compressive sensing method
that does not concern the off-grid effect, the proposed algorithm
can provide more accurate 3-D reconstruction for pointlike
targets, which is verified by the real-data experiments.
Index Terms—Adaptive imaging, holographic synthetic
aperture radar (HoloSAR) tomography, off-grid effect, sparse
Bayesian inference.
I. INTRODUCTION
T
HE 3-D synthetic aperture radar (3-D SAR) has attracted
an increasing interest, since it can obtain true 3-D
scene scatterers’ distributions and scattering properties [1]–[5].
Among various kinds of 3-D SAR, circular SAR (CSAR) [6]
has the potential of 3-D reconstruction over full 360°, allowing
to provide more information and better reconstruction of
targets. With a single circular pass, CSAR imaging needs
the focused height to be that of the target; otherwise, cone-
shaped sidelobes caused by the poor resolution in the direction
perpendicular to the line of sight will arise.
Holographic SAR (HoloSAR) tomography [5] using multi-
ple circular passes at different elevation angles is an extension
of the CSAR and interferometric SAR [7], which uses two
passes to detect the average height of targets within one
Manuscript received September 8, 2016; revised December 20, 2016 and
April 9, 2017; accepted May 8, 2017. This work was supported by the National
Natural Science Foundation of China under Grant 61431018, Grant 61571421,
Grant 61601003, and Grant 61501210. (Corresponding author: Qian Bao.)
The authors are with the Science and Technology on Microwave Imag-
ing Laboratory, Institute of Electronics, Chinese Academy of Sciences,
Beijing 100190, China, and also with the University of Chinese Academy
of Sciences, Beijing 100190, China (e-mail: baoqiancherry@163.com).
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.2017.2704601
azimuth-range resolution pixel. By the combination of circular
aperture and the elevation diversity, the HoloSAR combines
the approaches of holographic and tomograghy and can reduce
the cone-shaped sidelobes and provide 3-D reconstruction with
more detailed elevation information. Compared with the multi-
pass straight-line SAR, also known as the side-looking tomog-
raphy SAR (TomoSAR), the HoloSAR can detect targets that
have orientation components in different azimuth directions.
For targets with aspect-dependent reflectivity characteristics,
the assumption about isotropic scattering for traditional radar
imaging methods is violated. Thus, the anisotropic behavior
of scatterers should be concerned for HoloSAR imaging,
especially for man-made objects [2], [8]. Another factor that
should be considered for HoloSAR imaging is the sparse
and nonuniform elevation flight paths. Since the expense of
real flight is high and the actual flight paths are hard to be
controlled uniformly and constantly, the sparse and nonuni-
form elevation sampling is unavoidable and finally limits
the image quality in terms of sidelobe power and geometric
resolution.
For targets, such as buildings and man-made objects, which
usually behave pixelwise spatial sparsity [1]–[3], compres-
sive sensing (CS)-based methods [9] have been widely used
for the elevation reconstruction of the HoloSAR and side-
looking TomoSAR with superresolution [1]–[4]. However,
conventional CS assumes that the scatterers are located on the
discretized grids; otherwise, the off-grid effect will arise [10].
In this letter, we use the “2-D + 1-D” approach [1] for
HoloSAR imaging, which first forms a set of 2-D SAR
images from each CSAR observation and then 1-D parametric
estimation is applied to estimate the elevation dimension.
We propose an imaging algorithm concerning the HoloSAR
imaging problems of aspect-dependent scatterering properties
and the off-grid effect in elevation sparse reconstruction.
By preretrieving the reflectivity characteristics of each pixel in
the azimuth-range 2-D imaging region [8], the valid azimuth
angles of each pixel can be used for focusing the ground plane
2-D images from each pass. By stacking those 2-D images,
we introduce the off-grid sparse Bayesian inference (OGSBI)
method [11] for the elevation reconstruction, which jointly
estimates the scatterers and their off-grid error in the elevation.
By applying hierarchical priors for sparse signal and off-
grid error, the OGSBI possesses the properties of off-grid
effect reduction and user parameters easy to choose. The
proposed algorithm can provide HoloSAR 3-D reconstruction
of pointlike targets with high resolution and accuracy, which
is verified by the real-data experiments.
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