The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
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interferograms, on which the fringes caused by baseline errors
is first estimated with FFT in range and azimuth direction and
then eliminated. The nonlinear algorithm is performed in phase-
unwrapped maps, where a quadratic surface function is used to
fit the trend of unwrapped phase and then the trend is subtracted.
The quadratic function can be described by:
f\x,y) = a, + a 2 ■ x + a 3 • y + a 4 ■ xy + a 5 ■ x 2 + a 6 ■ y 2 (3)
where a n (hi = 1 • • • 6) = polynomial coefficients
x,y = coordinates in range and azimuth directions.
3. RESULTS
To verify the effectiveness of linear and nonlinear methods for
mitigating the baseline errors, ESA Envisat ASAR data at Bam
area are selected. The selected image pairs and baselines are as
follows.
Master
Slave
Temporal
Perpendicular
Baseline
Baseline
9192
6687
-175 days
475 m
9192
9693
475 days
530 m
Table 1. SAR images pair and baselines
The data was processed by DORIS software with two-pass
diffemential method, in which SRTM3 DEM was used to
remove the topographic effects. The results are as follows.
Figure 5. Figure 6.
Figure 1. Interferogram (9192-6687,coarse orbit used)
Figure 2. Residuals of Fig. 1 after subtracting bilinear fringes
Figure 3. Interferogram (9192-6687,precise orbit used)
Figure 4. Rresiduals of Fig.3 after subtracting bilinear fringes
Figure 5. Interferogram (9192-9693,precise orbit used)
Figure 6. Residuals of Fig.5 after subtracting bilinear fringes
From the comparison of interferogram between Figure 1. and
Figure 3., we can see that the pattern of fringes mainly takes on
the characteristics of linear trend. The fringes on Figure 1 and
Figure 3. are mainly ascribed to baseline errors, for no obvious
deformation takes place during the 175 days of SAR images
acquirement. The pattern difference between them are only due
to the difference of the orbit accuracy: Figure 1. with precise
orbit and Figure 3. with coarse one. Figure 1. is generated
during BAM earthquakes, on which the batterfly-look fringes
are very clear. However the three other linear fringes cannot be
due to earthquakes, and they should be due to the baseline
errors. If the linear fringes are not eliminated, they will affect
the interpretation of the earthquakes. Maps Figure 2, Figure 4,
and Figure 6 are respectively the results of Figure 1, Figure 3,
Figure 5 after subtracting the linear fringes estimated from FFT
spectrum.
The fringes caused by baseline errors can be removed in two
methods. One is bilinear method, which is performed on
interferograms, the other is nolinear method performed on
phase-unwrapped maps. The results of both methods are as
follows.
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