International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
A NEW PERSISTENT SCATTER NETWORK CONSTRUCTION ALGORITHM FOR
PERSISTENT SCATTER INSAR AND ITS APPLICATION TO THE DETECTION OF
URBAN SUBSIDENCE
Xiaojun Luo, Dingfa Huang, Guoxiang Liu, Letao Zhou, Keren Dai
Dept. of Remote Sensing & Geospatial Information Engineering, Southwest Jiaotong University, Chengdu, China
Ixj@swjtu.cn
Keywords: persistent scatters, three-dimensional persistent scatter Delaunay network, de-correlations, atmospheric delays, urban
subsidence
Abstract:
To extremely eliminate atmospheric delays for improving the accuracy of persistent scatter INSAR, the algorithm for constructing
three-dimensional Delaunay network of global positioning system (GPS) stations is introduced to construct three-dimensional
persistent scatter Delaunay network. The comparison with two-dimensional persistent scatter network indicates that
three-dimensional Delaunay network is stable and avoids the affect of landscape conversion from geography space to image space.
The urban subsidence of Lujiazui in Shanghai during 1992-2002 was effectively detected with InSAR based on three-dimensional
persistent scatter Delaunay network. The result shows that persistent scatter INSAR based on three-dimensional persistent scatter
Delaunay network can be used to efficiently and accurately detect ground deformation. The comparison with Leveling and InSAR
based on persistent scatter planar network indicates that the accuracy and reliability of InSAR based on three-dimensional persistent
scatter Delaunay network are significantly improved.
1. INTRODUCTION
Differential synthetic aperture radar interferometry (DInSAR)
is a potential technique for monitoring minor ground
deformation because of its pantoscopic view and high spatial
resolution. However, de-correlations and atmospheric delays
mitigate the accuracy of DInSAR. persistent scatter InSAR
promoted by Ferretti is at present regarded as one of the most
efficient approach in overcoming both de-correlations and
atmospheric delays (Zebker, 1992, Ferretti et al. 2000a, 2001b).
In persistent scatter InSAR, the persistent scatters are first
detected from time serial SAR images. Then the network of
persistent scatters is constructed (Luo et al. 2008a, Liu et al.
2008). Based on the network, the neighbourhood of persistent
scatters is defined along each arc, ie., connection of the
network and the increments of differential phases, called
neighbouring differential phases (NDP) between two
neighbouring persistent scatters are calculated. Finally, ground
deformations and terrain errors are deduced from NDPs.
Because of the homogeneity of atmosphere distribution in
certain range, atmospheric delays are strongly correlative in
small space scale such as 2km (Luo, 2007b). The NDPs
accordingly eliminate most of the atmospheric delays, which is
consistent with the fundament of differential GPS. Furthermore,
the more closely the two persistent scatters are adjacent, the
more clearly the atmospheric delays are removed from NDPs.
To eliminate atmospheric effects to a maximum extent, the pair
of persistent scatter decided by the network should be as far as
possible to close. So it's crucial to persistent scatter InSAR to
establish an appropriate persistent scatter network.
The common simple persistent scatter network introduced first
by Mora et al. (2003) is triangular irregular network (TIN)
established with Delaunay algorithm. However, some isolated
islands and singular points are easily generated while the arcs
longer than lkm are cut from TIN (Ferretti et al. 2000a,
Colesanti et al. 2003). The presence of isolated islands makes
deformation detection difficult. To avoid the generation of
isolated island in the network, an enhanced persistent scatter
network called freely-connected network (FCN) was proposed
by Liu et al. (2009). According to FCN algorithm, every
persistent scatter is connected to all the others. Therefore more
arcs than TIN are generated in FCN and the isolated islands and
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singular points are seldom formed though the FCN is optimized
by cutting the arcs longer than lkm. Furthermore, more
observations are obtained from the FCN which correspondingly
is useful to the network adjustment and the improvement of
deformation measurement accuracy. However, FCN is
constructed at the cost of computation time.
Both TIN and FCN are two-dimensional networks constructed
based on the image planar coordinate system. As atmospheric
delays change along both horizontal and vertical direction, the
NDPs derived from the planar network mainly mitigate the
horizontal atmospheric delays. Most of vertical atmospheric
delays still remain in the NDPs. On the other hand, because of
the special image distortion such as foreshortening in SAR, the
distance on the earth surface, i.e., ground distance between two
neighbouring persistent scatters is usually longer than the range
decided by the image resolution. Which means that the ground
distance with nominal 1km (the range threshold used to select
persistent scatter pairs) in image calculated by image resolution
may be much longer than lkm. So the NDPs derived from
planar network can not completely eliminate the atmospheric
effects.
In order to improve the correction of atmospheric effects in
persistent scatter InSAR, the algorithm for constructing
three-dimensional Delaunay network of GPS stations (Zhou et
al. 2007) is introduced in this paper to establish
three-dimensional persistent scatter Delaunay network
(TDPDN). The validation of TDPDN is confirmed by detecting
ground subsidence over Lujiazui in Shanghai during
1992-2002.
2. FOUNDMENTAL OF PERSISTENT SCATTER
INSAR
Given N+1 SAR images acquired at the ordered times over the
same area, N interferograms will be generated if an image is
specified as common master image. Suppose that M persistent
scatters is identified from the N+1 time serial images, the M
persistent scatters are combined to form a network with some
algorithm. Then the pairs of persistent scatters are defined
through the network and N time serial NDPs for any pair of
adjacent persistent scatters are calculated. The NDP is function
of the difference of terrain errors and the difference of linear
