IEEE Geoscience and Remote Sensing Magazine - June 2020 - 123

threshold should be the optimal result
after trial and error to ensure that the
overall network of the baseline connection graph is connected.

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FIGURE 9. A time-position plot.

INTERFERENCE WORKFLOW
This step is the core step in SBAS data processing. It mainly
includes generating an interferogram, removing the topographic phase, and PU. According to different combinations
of SB interference pairs, the corresponding pixel values of
the registered complex images are conjugate-multiplied to
obtain an interference phase, and an interferogram is generated. Next, the external DEM data are used to obtain the
elevation data in the radar coordinate system, and the DEM
calculation in the radar coordinate system is used to generate the simulated topographic phase according to parameters such as the baseline and the incident angle. Because
the orbital parameter information of the main image is not
accurate, the simulated SAR image generated by a DEM in
the radar coordinate system usually has a deviation from
the real SAR image. Therefore, it is necessary to accurately
register the real and simulated SAR images to estimate the
pixel offset. Then, the DEM corresponding to the simulated
SAR image is resampled to generate a DEM in the final radar coordinate system. Finally, the simulated topographic
phase is removed from the interferogram to obtain a differential interferogram.
The differential interferogram is generally influenced
by noise. A differential interferogram can be filtered by a
low-pass filter. By setting the coherence threshold, the differential interferogram with a coherence coefficient lower
than the threshold is discarded. After the differential interferogram is screened, the connection of the interference
image pair should be checked. If it is not connected, some
interferograms with baselines exceeding the set threshold
but having good coherence can be added to make the network connect.
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SAR IMAGE REGISTRATION
Sentinel-1A IW level 1 products are
SLC. Each SAR image is composed of
three subswaths, and each subswath
is composed of chronologically arranged bursts in the azimuth direction. To eliminate the phase jump
between each burst during the interference and to get rid of phase offset
between the burst and subswath,
the accuracy of the data during registration must be very high, and the
registration accuracy in the azimuth
direction must reach one thousandth
of a pixel. During the process, the registration parameters must be adjusted
continuously until the accuracy meets
the requirements.

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IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

EDITING OF THE INTERFERENCE PAIR
CONNECTION GRAPH
During each step of data processing, the connection graph
of the interference pair can be edited. The interference images with large unwrapping errors, low coherence, and evident atmospheric influence are eliminated so that they do
not participate in the subsequent processing. The connection situation of an interference pair after deletion is shown
in Figure 9.
With this method, the latter result can avoid being affected by the poor quality interference pattern. This process
requires the data processor to have extensive experience with
InSAR data processing, and the results of each image in each
step must be viewed and selected. It should be noted that, in
the interference-removal process, each image must be connected as much as possible to ensure full use of the data,
but a situation in which the image data quality is very poor
should be removed at this point.
REFINEMENT AND REFLATTENING
This step is mainly used to estimate and remove the constant
phase and phase transitions remaining in the unwrapped
phase map. In this step, one needs to select the ground control point (GCP). Since the reflattening is performed for the
unwrapping results of all of the interference pairs, the best
way to choose GCPs is to select the most representative one of
the unwrapped phase maps. Then, select the GCPs from this
phase map that are applicable to most interferograms. GCPs
should be selected in a stable, phase-free, coherent region because the deformation of the GCP is considered to be zero
during processing. The results obtained in this step will be
used as input data for the first step of the SBAS inversion.
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IEEE Geoscience and Remote Sensing Magazine - June 2020

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