IEEE - Aerospace and Electronic Systems - October 2021 - 9

Chan et al.
Table 1.
System Parameter of GBSAR
Design parameter
Polarization
Transmit power
Antenna gain
3 dB antenna
beamwidth
Synthetic length
Range resolution
Azimuth resolution
Maximum sensing
distance
PHASENOISEFILTERING
Due to the noise generated by the system itself, and noise
introduced in the data processing process, resulting in a
reduction of the SNR of the interferogram subsequently
cause the reduction in phase coherence. These noises will
greatly affect the information extraction via phase measurement
and the accuracy of the GBSAR system. Adopt
periodic mean filtering and other phase estimation methods
to reduce the phase noise and eventually improve the
deformation measurement accuracy.
PHASEUNWRAPPING
Since the phase value of each pixel in the interferogram is
modulo 2p and the range is between (-p,p), the real phase
value of the target area needs to be added to an integer
multiple of 2p. Phase unwrapping is a particularly important
step in the data processing of the GBSAR system to
obtain the true phase value. Common phase unwrapping
algorithms include branch cut method, quality guided
method, and least squares model estimation method [21].
DEFORMATIONVALUECALCULATION
After unwrapping the phase of the interferogram, the
deformation value of the target area can be obtained via
the true phase calculated. According to Equation (8), the
true deformation value of the target at the viewing direction
can be calculated. However, the monitoring range of
the GBSAR system is wide, and the working mode can be
changed according to the monitoring target. In order to
obtain high-precision measurement results, the conversion
relationship between the visual deformation and the true
OCTOBER 2021
value of the deformation according to different geometric
observation models need to be determined.
Design value
Operating frequency 17.2 GHZ
Bandwidth
1 W
16 dBi
50 (azimuth), 25
(elevation)
1.5 m
0.15-0.5 m
5.81 mrad@ 1 km
2000 m
where dftopo is the terrain phase, dfflat is the phase due to
flat earth effect, dfmov is the phase change caused by the
target line of sight deformation, dfatm is the phase delay
caused by the atmosphere, and dfnoise is the phase change
caused by noise. Unlike spaceborne InSAR, the initial
interference phase of the GBSAR system does not include
the flat earth phase and terrain phase. Therefore, the interference
phase of the ground-based InSAR system can be
expressed as
;int ¼ d;disp þ d;atm þ d;noise
(10)
where dfdisp is the phase difference caused by the displacement
ofthe target. For the scattering phase difference
of the target, because the observation interval of the
GBSAR system is very short, it is generally considered
that the difference of two measurements before and after
is very similar.
SYSTEM IMPLEMENTATION
The detailed specifications of the GBSAR system are
listed in Table 1. This radar system is operated at Ku-band
with a center frequency of 17.2 GHz. The bandwidth is
selectable from 300 MHz to 1 GHz and the correspond
achieved range resolution will be from 0.15 to 0.5 m. The
synthetic length of this radar system is 1.5 m and therefore
it produces an azimuth resolution of 5.81 mrad at a distance
of 1 km. The implementation of the ground-based
radar system based on the specifications is shown in
Figure 5.
This GBSAR consists of RF module, antenna system,
SAR controller, embedded SAR processor and data
recorder, and a linear scanning platform (linear guide)
with linear scanner controller. The liner guide will act as
the moving platform of the SAR antenna system. The
antenna system will move along the linear scanning platform
in order to complete the synthetic length, and the
received radar echo will be digitized and recorded via
embedded SAR data recorder.
EXPERIMENT AND RESULTS
A series of indoor experiments and outdoor field measurements
have been performed to verify the performance of
IEEE A&E SYSTEMS MAGAZINE
9
300 MHz-1 GHz
Single, VV
GROUND-BASED INSAR PHASE COMPOSITION
The phase of the spaceborne InSAR system can be
expressed as
;int ¼ d;topo þ d;flat þ d;mov þ d;atm þ d;noise
(9)
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