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Shouhuan Zhou - One of the best experts on this subject based on the ideXlab platform.
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spatial carrier phase shifting algorithm based on principal component analysis method
Optics Express, 2012Co-Authors: Guoying Feng, Javier Vargas, Shouhuan ZhouAbstract:A non-iterative spatial phase-shifting algorithm based on principal component analysis (PCA) is proposed to directly extract the phase from only a single spatial carrier Interferogram. Firstly, we compose a set of phase-shifted fringe patterns from the original spatial carrier Interferogram shifting by one pixel their starting position. Secondly, two uncorrelated quadrature signals that correspond to the first and second principal components are extracted from the phase-shifted Interferograms by the PCA algorithm. Then, the modulating phase is calculated from the arctangent function of the two quadrature signals. Meanwhile, the main factors that may influence the performance of the proposed method are analyzed and discussed, such as the level of random noise, the carrier-frequency values and the angle of carrier-frequency of fringe pattern. Numerical simulations and experiments are given to demonstrate the performance of the proposed method and the results show that the proposed method is fast, effectively and accurate. The proposed method can be used to on-line detection fields of dynamic or moving objects.
H A Zebker - One of the best experts on this subject based on the ideXlab platform.
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persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis with application to volcan alcedo galapagos
Journal of Geophysical Research, 2007Co-Authors: Andrew Hooper, Paul Segall, H A ZebkerAbstract:[1] While conventional interferometric synthetic aperture radar (InSAR) is a very effective technique for measuring crustal deformation, almost any Interferogram includes large areas where the signals decorrelate and no measurement is possible. Persistent scatterer (PS) InSAR overcomes the decorrelation problem by identifying resolution elements whose echo is dominated by a single scatterer in a series of Interferograms. Existing PS methods have been very successful in analysis of urban areas, where stable angular structures produce efficient reflectors that dominate background scattering. However, man-made structures are absent from most of the Earth's surface. Furthermore, existing methods identify PS pixels based on the similarity of their phase history to an assumed model for how deformation varies with time, whereas characterizing the temporal pattern of deformation is commonly one of the aims of any deformation study. We describe here a method for PS analysis, StaMPS, that uses spatial correlation of Interferogram phase to find pixels with low-phase variance in all terrains, with or without buildings. Prior knowledge of temporal variations in the deformation rate is not required for their identification. We apply StaMPS to Volcan Alcedo, where conventional InSAR fails because of dense vegetation on the upper volcano flanks that causes most pixels to decorrelate with time. We detect two sources of deformation. The first we model as a contracting pipe-like body, which we interpret to be a crystallizing magma chamber. The second is downward and lateral motion on the inner slopes of the caldera, which we interpret as landsliding.
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correction for interferometric synthetic aperture radar atmospheric phase artifacts using time series of zenith wet delay observations from a gps network
Journal of Geophysical Research, 2006Co-Authors: F Onn, H A ZebkerAbstract:[1] Radar Interferograms provide precise (millimeter level) measurements of crustal deformation at fine resolution over wide areas. The dominant error source in many Interferograms results from spatial heterogeneity of the wet component of atmospheric refractivity, resulting in excess path length of the radar signal propagating through the neutral atmosphere. In this report, we introduce a method to compensate for atmospheric phase artifacts in a radar Interferogram using spatially interpolated zenith wet delay (ZWD) data obtained from a network of Global Positioning System (GPS) receivers in the region imaged by the radar. Our method, based on Taylor's “frozen-flow” hypothesis, uses GPS data recorded prior to and after the individual radar acquisition times to infer denser spatial networks of control points for interpolation than is suggested by the sparse and irregular spatial distribution of GPS receivers. We test this approach by correcting phase fluctuations observed in a radar Interferogram over an area covered by the Southern California Integrated GPS Network stations, using maps of atmospheric delay interpolated from the denser network of GPS ZWD measurements generated by our method. We find that the largest improvement results from removing an altitude-dependent model derived from the GPS data: the uncorrected Interferogram in our example has an RMS fluctuation of 16.1 mm, which falls to 8.6 mm after the altitude correction. A map derived from ZWD measurements recorded only at the instances of radar acquisition reduced overall fluctuations in the data from 8.6 to 8.1 mm RMS, while using the inferred denser network reduces the error further to 7.5 mm RMS. Most of this residual variation is due to decorrelation of the signal and does not result from atmospheric propagation, however. After smoothing to remove decorrelation noise, the residual drops from 4.7 to 3.9 mm (GPS stations only) and then to 2.7 mm RMS when the denser network algorithm is used.
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a new method for measuring deformation on volcanoes and other natural terrains using insar persistent scatterers
Geophysical Research Letters, 2004Co-Authors: Andrew Hooper, H A Zebker, Paul Segall, Bert KampesAbstract:[1] We present here a new InSAR persistent scatterer (PS) method for analyzing episodic crustal deformation in non-urban environments, with application to volcanic settings. Our method for identifying PS pixels in a series of Interferograms is based primarily on phase characteristics and finds low-amplitude pixels with phase stability that are not identified by the existing amplitude-based algorithm. Our method also uses the spatial correlation of the phases rather than a well-defined phase history so that we can observe temporally-variable processes, e.g., volcanic deformation. The algorithm involves removing the residual topographic component of flattened Interferogram phase for each PS, then unwrapping the PS phases both spatially and temporally. Our method finds scatterers with stable phase characteristics independent of amplitudes associated with man-made objects, and is applicable to areas where conventional InSAR fails due to complete decorrelation of the majority of scatterers, yet a few stable scatterers are present.
David Huang - One of the best experts on this subject based on the ideXlab platform.
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postprocessing algorithms to minimize fixed pattern artifact and reduce trigger jitter in swept source optical coherence tomography
Optics Express, 2015Co-Authors: Gangjun Liu, Ou Tan, Simon S Gao, Alex D Pechauer, Byungkun Lee, James G Fujimoto, David HuangAbstract:We propose methods to align Interferograms affected by trigger jitter to a reference Interferogram based on the information (amplitude/phase) at a fixed-pattern noise location to reduce residual fixed-pattern noise and improve the phase stability of swept source optical coherence tomography (SS-OCT) systems. One proposed method achieved this by introducing a wavenumber shift (k-shift) in the Interferograms of interest and searching for the k-shift that minimized the fixed-pattern noise amplitude. The other method calculated the relative k-shift using the phase information at the residual fixed-pattern noise location. Repeating this wavenumber alignment procedure for all A-lines of interest produced fixed-pattern noise free and phase stable OCT images. A system incorporating these correction routines was used for human retina OCT and Doppler OCT imaging. The results from the two methods were compared, and it was found that the intensity-based method provided better results.
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measurement of absolute flow velocity vector using dual angle delay encoded doppler optical coherence tomography
Optics Letters, 2007Co-Authors: Cameron J Pedersen, David Huang, M A Shure, Andrew M RollinsAbstract:Single-beam laser Doppler measurements of flow velocity are only sensitive to the velocity component parallel to the optical axis. We describe a simple modification to a standard Doppler optical coherence tomography (OCT) system using a single sample beam that provides velocity information from multiple angles within the beam. By introducing a glass plate midway into the OCT beam path, the sample beam is divided into several components, each with a different group delay and each providing a separate Interferogram with its own effective Doppler angle. By combining the Doppler shift measured in each of these component Interferograms, the flow velocity vector is fully determined.
Pierre Briole - One of the best experts on this subject based on the ideXlab platform.
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time series analysis of mexico city subsidence constrained by radar interferometry
Journal of Applied Geophysics, 2009Co-Authors: Penelope Lopezquiroz, Mariepierre Doin, Florence Tupin, Pierre Briole, Jeanmarie NicolasAbstract:Abstract In Mexico City, subsidence rates reach up to 40 cm/yr mainly due to soil compaction led by the over exploitation of the Mexico Basin aquifer. In this paper, we map the spatial and temporal patterns of the Mexico City subsidence by differential radar interferometry, using 38 ENVISAT images acquired between end of 2002 and beginning of 2007. We present the severe Interferogram unwrapping problems partly due to the coherence loss but mostly due to the high fringe rates. These difficulties are overcome by designing a new methodology that helps the unwrapping step. Our approach is based on the fact that the deformation shape is stable for similar time intervals during the studied period. As a result, a stack of the five best Interferograms can be used to compute an average deformation rate for a fixed time interval. Before unwrapping, the number of fringes is then decreased in wrapped Interferograms using a scaled version of the stack together with the estimation of the atmospheric phase contribution related with the troposphere vertical stratification. The residual phase, containing less fringes, is more easily unwrapped than the original Interferogram. The unwrapping procedure is applied in three iterative steps. The 71 small baseline unwrapped Interferograms are inverted to obtain increments of radar propagation delays between the 38 acquisition dates. Based on the redundancy of the interferometric data base, we quantify the unwrapping errors and show that they are strongly decreased by iterations in the unwrapping process. A map of the RMS interferometric system misclosure allows to define the unwrapping reliability for each pixel. Finally, we present a new algorithm for time series analysis that differs from classical SVD decomposition and is best suited to the present data base. Accurate deformation time series are then derived over the metropolitan area of the city with a spatial resolution of 30 × 30 m.
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time series analysis of mexico city subsidence constrained by radar interferometry
Fringe 2007 Workshop, 2009Co-Authors: Penelope Lopezquiroz, Mariepierre Doin, Florence Tupin, Pierre Briole, Jeanmarie NicolasAbstract:article i nfo InMexicoCity,subsidenceratesreachupto40cm/yrmainlyduetosoilcompactionledbytheoverexploitationof theMexicoBasinaquifer.Inthispaper,wemapthespatialandtemporalpatternsoftheMexicoCitysubsidenceby differential radarinterferometry, using38ENVISAT imagesacquiredbetween end of 2002andbeginningof 2007. WepresentthesevereInterferogramunwrappingproblemspartlyduetothecoherencelossbutmostlyduetothe high fringe rates. These difficulties are overcome by designing a new methodology that helps the unwrapping step. Our approach is based on the fact that the deformation shape is stable for similar time intervals during the studied period.As a result,a stack of the five bestInterferograms can beusedto compute anaverage deformation rate for a fixed time interval. Before unwrapping, the number of fringes is then decreased in wrapped Interferograms using a scaled version of the stack together with the estimation of the atmospheric phase contribution related with the troposphere vertical stratification. The residual phase, containing less fringes, is more easily unwrapped than the original Interferogram. The unwrapping procedure is applied in three iterative steps. The 71 small baseline unwrapped Interferograms are inverted to obtain increments of radar propagation delays between the 38 acquisition dates. Based on the redundancy of the interferometric data base, we quantify theunwrappingerrorsandshowthattheyare stronglydecreasedby iterationsintheunwrappingprocess.Amap of the RMS interferometric system misclosure allows to define the unwrapping reliability for each pixel. Finally, we present a new algorithm for time series analysis that differs from classical SVD decomposition and is best suited to the present data base. Accurate deformation time series are then derived over the metropolitan area of the city with a spatial resolution of 30×30 m.
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ground motion measurement in the lake mead area nevada by differential synthetic aperture radar interferometry time series analysis probing the lithosphere rheological structure
Journal of Geophysical Research, 2007Co-Authors: Olivier Cavalie, Mariepierre Doin, Cecile Lasserre, Pierre BrioleAbstract:[1] We measure ground motion around the Lake Mead, Nevada, using synthetic aperture radar interferometry. The lake water level has fluctuated through time since impoundment in 1935. To quantify the deformation due to water level variations over the past decade, and to constrain the crust and mantle rheological parameters in the lake area, we analyze 241 Interferograms based on 43 ERS images acquired between 1992 and 2002. All Interferograms have a high coherence due to arid conditions. Most of them show strong atmospheric artefacts. Tropospheric phase delays are estimated and corrected for each Interferogram by analyzing the phase/elevation correlation. Corrections are validated using data from the ERA40 global atmospheric reanalysis. Corrected Interferograms are inverted pixel by pixel to solve for the time series of ground motion in the lake area. Temporal smoothing is added to reduce random atmospheric artefacts. The observed deformation is nonlinear in time and spreads over a 50 × 50 km2 area. We observe a 16 mm subsidence between 1995 and 1998 due to an 11 m water level increase, followed by an uplift due to the water level drop after 2000. We model the deformation, taking into account the loading history of the lake since 1935. A simple elastic model with parameters constrained by seismic wave velocities does not explain the amplitude of the observed motion. The two-layer viscoelastic model proposed by Kaufmann and Amelung (2000), with a mantle viscosity of 1018 Pa s, adjusts well the data amplitude and its spatiotemporal shape.
Javier Vargas - One of the best experts on this subject based on the ideXlab platform.
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spatial carrier phase shifting algorithm based on principal component analysis method
Optics Express, 2012Co-Authors: Guoying Feng, Javier Vargas, Shouhuan ZhouAbstract:A non-iterative spatial phase-shifting algorithm based on principal component analysis (PCA) is proposed to directly extract the phase from only a single spatial carrier Interferogram. Firstly, we compose a set of phase-shifted fringe patterns from the original spatial carrier Interferogram shifting by one pixel their starting position. Secondly, two uncorrelated quadrature signals that correspond to the first and second principal components are extracted from the phase-shifted Interferograms by the PCA algorithm. Then, the modulating phase is calculated from the arctangent function of the two quadrature signals. Meanwhile, the main factors that may influence the performance of the proposed method are analyzed and discussed, such as the level of random noise, the carrier-frequency values and the angle of carrier-frequency of fringe pattern. Numerical simulations and experiments are given to demonstrate the performance of the proposed method and the results show that the proposed method is fast, effectively and accurate. The proposed method can be used to on-line detection fields of dynamic or moving objects.
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two step demodulation based on the gram schmidt orthonormalization method
Optics Letters, 2012Co-Authors: Javier Vargas, Antonio J Quiroga, C O S Sorzano, J C Estrada, J M CarazoAbstract:This Letter presents an efficient, fast, and straightforward two-step demodulating method based on a Gram-Schmidt (GS) orthonormalization approach. The phase-shift value has not to be known and can take any value inside the range (0,2π), excluding the singular case, where it corresponds to π. The proposed method is based on determining an orthonormalized Interferogram basis from the two supplied Interferograms using the GS method. We have applied the proposed method to simulated and experimental Interferograms, obtaining satisfactory results. A complete MATLAB software package is provided at http://goo.gl/IZKF3.