Abstract
Persistent scatterer interferometry (PSI) is commonly applied to monitor surface displacements with millimetric precision. However, this technique still has trouble estimating non-linear displacements because the algorithm is designed for the slow and linear displacements. Additionally, there is a variety of non-linear displacement types, and finding an appropriate displacement model for PSI is still assumed to be a fairly large task. In this paper, the conventional PSI technique is extended using a non-parametric non-linear approach (NN-PSI), and the performance of the extended method is investigated by simulations and actual observation data processing with TerraSAR-X. In the simulation, non-linear displacements are modeled by the magnitudes and periods of the displacement, and the evaluation of NN-PSI is conducted. According to the simulation results, the maximum magnitude of the displacement that can be estimated by NN-PSI is two and a half times the magnitude of the SAR sensor’s wavelength (2.5λ that is roughly equivalent to 8 cm for X-band, 14 cm for C-band, and 60 cm for L-band), and the period of the displacement is about three months. However, this displacement cannot be reconstructed by the conventional PSI due to the limitation, known as the 2π displacement ambiguity. The result of the observation data processing shows that a large displacement with the 2π ambiguity can be estimated by NN-PSI as the simulation results show, but the conventional PSI cannot reconstruct it. In addition, a different approach, Small BAseline Subset (SBAS), is applied to the same data to ensure the accuracy of results, and the correlation between NN-PSI and SBAS is 0.95, while that between the conventional PSI and SBAS is –0.66. It is concluded that NN-PSI enables the reconstruction of non-linear displacements by the non-parametric approach and the expansion of applications to measure surface displacements that could not be measured due to the limitations of the traditional PSI methods.
Highlights
Spaceborne synthetic-aperture radar (SAR) sensors are informative resources for measuring surface deformations induced by natural disasters or human activities
These results prove that Conventional PSI (ConvPSI) works only with a small displacement, with a Dmax value of less than 0.25λ
We have extended persistent scatterer interferometry (PSI) using the NN-PSI and investigated NN-PSI in order to overcome the large non-linear displacement issues associated with PSI
Summary
Spaceborne synthetic-aperture radar (SAR) sensors are informative resources for measuring surface deformations induced by natural disasters or human activities. Based on the core algorithms of SBAS and PSI, it is implicitly indicated that there is no technology enabling reconstruction of non-linear displacements for the point targets that are needed to recognize among some complicated geometry, unless some specific models are formulated. In NN-PSI, no displacement model is defined or invented when reconstructing the displacements This proposed method is expected to help to simplify the estimation of non-linear displacements with full spatial resolution, and this simplicity will expand the applications of monitoring surface displacements that could not be measured due to the PSI limitations. MICC is the indicator based on the temporal coherence that shows the goodness of the fit between the observed displacement phase and the modeled one in time series This indicator provides the scattering distribution that highlights the difference of the velocity [27], and it is estimated by a non-parametric method [22]. The temporal coherence ranging 0 to 1, is used to estimate the stability of the displacement phase at each scattering point in the SDM
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