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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
In the test sites, a subsidence bowl with radius of 150m is 
expected given that the width of each longwall panel is about 
200-300m. Therefore, theoretically the expected maximum 
deformation that can be detected (without phase discontinuity) 
is approximately 8cm, 7cm, 48cm, 86cm, 39cm and 39cm for 
the wavelengths of the ERS, ENVISAT, JERS, ALOS, 
TerraSAR-X and COSMO SkyMed satellites respectively 
(assuming resolution of 25m, 30m 18m, 10m, 3m and 3m 
respectively), along the LOS direction. 
A simulation is carried out to investigate this effect using a 
subsidence model (Figure 2) derived from an ALOS PALSAR 
DInSAR result. The model has a peak subsidence of 50cm. The 
subsidence model is rescaled based on the ground resolution of 
each satellite and is converted into absolute phase using 
equations (2) & (3). Differential interferograms are simulated 
by wrapping the absolute phase (Figures 2). The simulated 
differential interferograms are then converted back into LOS 
displacement by unwrapping the phase in the simulated 
differential interferogram using the MCF method (Costantini, 
1998). The temporal and spatial decorrelation is not considered 
in this simulation. Phase saturation has been observed in both 
differential interferograms derived from ERS and ENVISAT 
data due to the high phase gradient in the subsidence model. In 
contrast, the phase fringes in the differential interferograms 
from ALOS, JERS, TerraSAR-X hnd COMOS SkyMed data are 
reasonably clear. 
Figure 2. Simulated differential interferograms from various 
SAR satellites based on the subsidence model under noise-free 
conditions. 
Figure 3. Detectable subsidence errors with different 
magnitudes of peak subsidence under noise-free conditions. 
The simulation is repeated using a subsidence model with 
different peak subsidence (from 5cm to 150cm). The detectable 
subsidence errors (RMSE) with different peak subsidence are 
shown in Figures 3, which shows that the ALOS, TerraSAR-X 
and COSMO-SkyMed data are able to be used to measure 
larger displacement with much lower errors. The L-band ALOS 
PALSAR is able to maintain a low subsidence error with 
relatively high maximum detectable subsidence. High RMSE is 
observed in the ENVISAT and ERS results, with peak 
subsidence greater than 10cm. 
4. RESULTS 
ALOS and ENVISAT images with similar temporal coverage 
were searched for the test sites. The two-pass DInSAR 
technique with a 25m resolution DEM was used to estimate the 
location and amplitude of ground deformation. The 
performance of earlier SAR satellites ERS-1/2 and JERS-1 have 
already been discussed in a previous study (Ge et al., 2007). 
4.1 ENVISAT ASAR 
More than 90 ENVISAT images have been acquired over the 
same site during the period 07 July 2006 and 10 March 2008. 
The images were acquired from 7 different tracks, in both 
descending and ascending passes, with four different imaging 
modes. Although the location of the subsidence bowls can be 
identified from many ENVISAT differential interferograms, 
strong phase discontinuities and decorrelation have been 
observed in almost all ENVISAT interferograms, and hence it is 
not possible to generate displacement maps. Figure 4 shows an 
example of a differential interferogram generated using 
ENVISAT pairs for both mine sites. The interferogram derived 
from ENVISAT pairs show phase saturation near the centre of 
the subsidence bowl in the case of the Westcliff Mine, while the 
fringes at the rims of that subsidence bowl are reasonably clear. 
The phase of the interferogram in Figure 4 is unwrapped and is 
converted into vertical displacement. The maximum subsidence 
detected by the ENVISAT pair detected from the 
interferograms is about 5cm, whereas the expected subsidence 
is greater than 40cm. This is because the phases in the 
ENVISAT differential interferograms fail to correctly 
unwrapped due to phase saturation. The ENVISAT differential 
interferogram again shows phase saturation in the centre of the 
subsidence bowl in the case of the Appin Mine. Unlike the 
subsidence bowl in the Westcliff Mine, the fringes at the rim of 
the subsidence bowl in Appin are only clear in the upper parts 
of the image (low vegetation area) and are very noisy for the 
lower parts (heavily vegetated area). This suggested that 
ENVISAT images can be affected strongly by vegetation. 
4.2 ALOS PALSAR 
There are 10 ALOS PALSAR acquisitions available for the 
period from December 2006 to March 2008, from both 
ascending and descending passes, with two different imaging 
modes (FBS and FBD). Seven differential interferograms were 
generated based on the ALOS PALSAR images (Table 1). The 
ALOS PALSAR FBD data are oversampled by a factor of 2 in 
the range direction so that they can be co-registered with ALOS 
PALSAR FBS data for DInSAR processing. Figure 5 shows the 
differential interferograms generated by ALOS PALSAR pairs 
for a similar time period to the ENVISAT pairs (Figure 4). The 
fringes in the differential interferogram derived from the ALOS 
pair are very clear even at the centre of both subsidence bowls.