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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
satellites cover the last 12 years. In this paper, the most
relevant aspects of a flexible DInSAR procedure for
deformation measurement have been discussed. The
procedure works with multiple interferograms over the same
scene, i.e. with stacks of SAR images. This represents the
key factor to achieve quantitative DInSAR deformation
monitoring capabilities. Three main aspects of the procedure
have been discussed. Firstly, the interferometric procedures
to process SAR image stacks, which include a phase
unwrapping algorithm that works on irregular networks of
sparse pixels. With this algorithm, only the pixels that
remain coherent over the observation period (say, few years)
arc used. This limits the deformation monitoring to the areas
that remain coherent over long periods, like the urban,
suburban and industrial areas. Secondly, the least squares
adjustment employed to estimate the deformations has been
illustrated. The estimation strategy has been described,
detailing few important aspects of the modelling of the phase
components, like the residual topographic component and
the atmospheric contribution. Thirdly, the DInSAR
geometric aspects have been addressed, emphasizing the
importance of the geometric model, which connects the SAR
image space to the object space. This model plays a key role
in the geocoding of the DInSAR products. A rigorous SAR
model has been briefly described. In our procedure an
accurate geometric model is achieved by refining the model
parameters by LS adjustment using GCPs.
Two applications based on the proposed DInSAR procedure
have been described. The first one is a screening analysis,
whose main goal is the detection of unknown subsidence
phenomena using a limited set of SAR images. The second
one is a quantitative analysis of a urban subsidence of small
spatial extent. Without any a priori information on the
analysed area, which has an extension of 340 kn, using 10
ascending interferograms, different deformation areas have
been detected. This result shows the great potential of the
technique to perform a fast and low-cost deformation
analysis over large areas. The analysis of the subsidence of
small spatial extent has been based on two independent SAR
datasets. Despite the relatively reduced number of available
observations (13 and 14 interferograms for the ascending
and descending dataset, respectively), the two derived
velocity fields are very consistent, both in terms of shape
and magnitude of the estimated deformations. This confirms
the capability of DInSAR to quantitatively assess
deformation phenomena, and opens the possibility to exploit
this technique in different applications and operational
contexts. This is also confirmed by the great number of
projects that are based on the DInSAR technique, see e.g.
(Strozzi et al., 2001; and Colesanti et al., 2003).
The main limitation of the DInSAR technique 1s that it only
provides information on the urban and industrial areas, which
however represent a very important type of land cover, where
most of the economical and social activities are concentrated.
The capabilities of the procedure described in this paper will
be improved in the future. A first step will be the joint
estimation of the deformations by fusion of the ascending and
descending datasets. A further step will include advanced 3D
modelling tools to separate the deformation phase component
from the atmospheric contribution.
ACKNOWLEDGMENTS
This work has been partially supported by the Spanish
Ministry of Science and Technology, through the research
project REN2003-00742. The author is greatly indebted to
169
Prof. Bruno Crippa. from the University of Milan, for his
continuous support and fundamental help during this work.
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