The Journal of Engineering
IET International Radar Conference (IRC 2018)
Baseline designing and implementation
approach for a Geo-SAR tomography system
eISSN 2051-3305
Received on 19th February 2019
Accepted on 2nd May 2019
doi: 10.1049/joe.2019.0171
www.ietdl.org
Zhao Bingji
1
, Zhang Qingun
1
, Dai Chao
1
, Shu Weiping
1
, Xu MingMing
1
1
Beijing Institute of Spacecraft System Engineering CAST, Beijing 100094, People's Republic of China
E-mail: zachary_zbj@163.com
Abstract: This study proposes a new method to design and implement the baseline of a geosynchronous Earth orbit synthetic
aperture radar (Geo-SAR) tomography system. Due to the satellite's orbital property, its ideal revisit cycle is 1 day, and it could
be changed through raising or lowering the orbital height. As a result, the local ascending node longitude would move because
of the orbital shifting, and all the passes' trajectories are nearly parallel. A baseline designing and implementation approach is
advanced according to this characteristic. The cell array configuration and baseline length are illustrated by considering the
demand of cross-range resolution and ambiguity. This study analyses and simulates the Geo-SAR tomography system baseline
properties. Furthermore, a series of parallel trajectories are implemented by controlling orbital altitude as demanding during
each cycle. A simulation experiment is performed to verify the efficiency and superiority of this approach, and the results show
that it has a good effect on an L-band mode Geo-SAR tomography system with three-dimensional resolution ∼10 m.
1 Introduction
Due to various application demand, an effective remote-sensing
approach, a synthetic aperture radar (SAR), independent of the
weather condition and light illumination is approved to achieve the
microwave images of the Earth's surface. The application of low-
Earth-orbital SAR (Leo-SAR) for the global remote sensing has
been validated by the current spaceborne SAR systems, such as
TerraSAR-X/TanDEM-X, ERS-1/ERS2, and GF-3. Leo-SAR's
capability is limited by its orbital altitude; however, there are
several weaknesses associated with it. The geosynchronous Earth
orbit SAR (Geo-SAR) can faultlessly deal with these problems. As
its orbital height is ∼35,790 km, Geo-SAR, firstly proposed by
Tomiyasu in 1978, is different from any other kind of spaceborne
SAR. It has a short revisit period, a wide swath bandwidth, and an
excellent maneuver tracking observing capability [1–3].
Tomography imaging SAR (Tomo-SAR) theory, proposed by
Knaell in 1995, combines the two-dimensional SAR images
obtained from different track heights to achieve focusing in the
elevation direction and to generate three-dimensional (3-D) SAR
images ． Different from the traditional interferometric SAR
technology, Tomo-SAR can effectively deal with the
misinterpretations caused by layover and ambiguity. In a general
way, it is considered as a new and important technique measure to
get a true 3D image, and it does not require extra complex
trajectory controls. As a result, it has a great research value in the
field of radar imaging. It is mostly studied in the airborne SAR and
Leo-SAR conditions; however, the Geo-SAR Tomography system
is hardly discussed now. In fact, there are several advantages of a
Geo-Tomo-SAR system [4–6]. Firstly, it could obtain a 3D image
of a larger scale of area than Leo-SAR. As a result, the echo data
processed could be employed to analyse the target area property in
a great range by some application departments showing interest in
it, such as woodland growing state, mountain physiognomy, and
the architecture property of a large city (New York, for instance).
Secondly, a 3-D image would be obtained quickly by a Geo-SAR
system. Due to its short orbital period and the nearly constant
overlapping fight trajectory, the baseline required may be formed
in <1 month; however, Leo-SAR needs more than 1 year possibly.
Finally, the scatter characteristic variety of physiognomy is
comparatively unobvious because of the short imaging time. Thus,
Geo-Tomo-SAR has an advantage over Leo-SAR.
This paper illuminates a new approach to design and implement
the baseline of the Geo-Tomo-SAR system. Its 3-D resolution and
ambiguity are analysed based on a geometry model. The interval of
every pass is confirmed by cell configuration demand, and the
circle numbers are decided by the baseline length. As a result, the
satellite's height, raised or lowered from the ideal geosynchronous
orbital altitude, could be decided by the interval calculated above.
This paper is structured as follows. In Section 2, the geometry
model and imaging process are presented. Furthermore, its
resolution and ambiguity are analysed. The orbital altitude
adjusting project, according to the baseline demand, is illuminated
in Section 3. The simulation results are shown in Section 4 to
verify our theory. Then the conclusions are given in Section 5.
Finally, acknowledgements are expressed in Section 6.
2 System working process and geometry model
Referring to the conventional synthetic aperture in the flighting
direction, high azimuth resolution could be obtained through a
synthetic aperture. A synthetic aperture could be similarly formed
in the normal direction of the range and azimuth plane, and the
tomography 3D resolution in the elevation direction can be
obtained. Being similar to the airborne SAR or Leo-SAR condition,
a Geo-SAR tomography system geometry model is illustrated as
follows [7].
As depicted in Fig. 1, the geometry model is discussed in the
Earth's rotation coordinate, where θ is the look angle, β (equal to θ)
is defined as the baseline obliquitous angle, α is the Earth's core
angle, R
e
is the Earth radius, R
0
is the slant range, H is the satellite
height, and H
d
is the elevation of the target. The N cell arrays are
presented by red dots. They are projections of satellite positions of
each circle on the cross-range line. As illustrated in Fig. 1, the Geo-
Tomo-SAR system baseline is composed of N passes over the same
area, and all the passes are parallel to each other. In each pass for
the conventional two-dimensional imaging SAR systems, the linear
frequency modulated signal is transmitted during the flight, and the
two-dimensional imaging processing is operated on the received
signal, which is reflected by the illuminated targets [8]. So the two-
dimensional SAR image obtained by the nth pass could be
expressed as
S
r
τ, η = A
0
ω
r
τ −
2R η
c
ω
a
η − η
c
⋅ exp −
j4πR η
λ
exp jπK
r
τ −
2R η
c
2
(1)
J. Eng.
This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License
(http://creativecommons.org/licenses/by-nc/3.0/)
1
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