IEEE - Aerospace and Electronic Systems - October 2021 - 13

Chan et al.
Figure 15.
(a) Setup ofGBSAR in the cabin. (b) Three trihedral were used in the long-term measurement.
waveform represents the displacement of a point on the
ground and T2 waveform represents the displacement
measured at the metro station main structure. We can
clearly observe that waveform T2 fluctuated across the
time, whereas T1 is much more stable. The fluctuation of
T2 is due to the vibration of the metro station when the
train is traveled in or out from the building. It can be concluded
that the ground-based InSAR system is able to
detect the movement of the man-made structure and perform
the health monitoring for the building.
LONG-TERMFIELDMEASUREMENT
In this outdoor measurement, the GBSAR system was
placed at the cabin for long period to monitor the slope.
Figure 14 shows the setup of the experiment. Three trihedral
(two fixed and one tuneable) are placed at different
distance (30, 40, and 50 m) and different azimuth position
(0, 10, 0 m). The photograph of the measurement setup is
shown in Figure 15. The ground-based radar system is
placed in a cabin with solar power supply so that 24-h testing
can be performed.
The measurement has performed for five days. During
the five days test, about 950 usable data are collected. One
of the SAR images generated is shown in Figure 16. Four
strong reflections can be observed in the SAR image. The
locations ofeach point are as follows.
peak point 1: Imax ¼ 14.68 dB, [x ¼ 30.45 m, y ¼ 0.52 m]
peak point 2: Imax ¼ 7.57 dB, [x ¼ 40.45 m, y ¼ 10.72 m]
peak point 3: Imax ¼ 6.70 dB, [x ¼ 50.25 m, y ¼ 0.52 m]
peak point 4: Imax ¼ -7.51 dB, [x ¼ 38.00 m, y ¼
-8.78 m]
Point number 1-3 is trihedral, whereas point number 4
is the short metal pole on the ground.
The power reflected by the four point targets is plotted
in Figure 17(a). It can be observed that the power
received is consistent for all the four point targets.
Figure 17(b) shows the displacement plot by using the
target P1 as the reference. During the five days test, there
is no obvious displacement observed except the measurement
number 390 shown there is a movement for P2, P3,
and P4. This is due to the whole radar platform is moved
from its original position; therefore, this causes all the
return from P1-P4 experienced phase shift that translate
to surface movement, which have been detected by the
InSAR system.
NEARFIELDANDLONGDISTANCEMEASUREMENT
An outdoor field measurement has been performed at
Keelung Ocean Centre, Taiwan in February 2020. During
this field trip, both the near range and far range measurements
have been conducted. Figure 18(a) shows the
ground-based radar system in operation as well as the
near-range scene. The corresponding SAR image generated
is shown in Figure 18(b). It can be observed that the
similarity in terms of the features in SAR images compared
to optical image. Strong reflections can be observed
by the metal poles that form the fence along the walking
path.
For the same scene, long distance SAR images were
Figure 16.
SAR amplitude image generate via the scene setup at Figure 15
(b).
OCTOBER 2021
generated as well. Figure 19 shows the SAR image overlays
with the Google Earth optical images. The image
generated with a distance of 2200 m and viewing angle
of 80
can be observed. Thus, the coverage of this
ground-based system for a distance of 2.2 km is about
3072 m2.
IEEE A&E SYSTEMS MAGAZINE
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IEEE - Aerospace and Electronic Systems - October 2021

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