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Atmospheric correction of DInSAR phase for land subsidence measurements using an integrated approach

Kamarajugedda, Shankar Acharya (2013) Atmospheric correction of DInSAR phase for land subsidence measurements using an integrated approach.

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Abstract:Differential Interferometric Synthetic Aperture Radar (DInSAR) is an effective tool to measure the surface deformations. The measurements derived from this technique are affected due to the errors caused by the earth’s atmosphere on the propagating RADAR signal. Troposphere and Ionosphere are the main layers of the atmosphere which induce the atmospheric error in DInSAR measurements. Many studies were performed to mitigate the effects of atmosphere on the RADAR signal traversing through it. The RADAR signals when propagating through the atmosphere undergo refraction. Research studies have shown that the temperature, pressure and water vapour pressure are the causative factors of RADAR signal refractivity in troposphere. The ionospheric refractivity is dependent on the Total Electron Content (TEC) present in the ionosphere. This study explores the integrated approach of using ground meteorological observations and satellite based meteorological data to model the atmospheric effects on the interferograms generated from RADARSAT-2 single look complex (SLC) data. Jharia coal field in Jharkhand was taken as the study area. The coal field is prone to land subsidence cause by mining activities. The DInSAR was generated from three RADARSAT-2 SLC’s (acquisition dates: 12-10-2012, 29-11-2012 and 23-12- 2012). The topography removal from the interferogram was performed using a 10m resolution DEM (Digital Elevation Model). Automated weather station (AWS) was used to obtain the ground meteorological observations and MODIS (Moderate Resolution Imaging Spectrometer) water vapour product (MOD07) was used for satellite based meteorological data. Since the time of pass of RADARSAT-2 is not in synchronization with MODIS, a regression analysis was performed to define a relationship between the meteorological data from AWS and MODIS. Hence the temperature, pressure and water vapour pressure were modelled for the time of acquisition of RADARSAT-2. The modelled bands were used to calculate the amount of tropospheric path delay. The tropospheric delay for the three acquisition dates ranged between 5.0 m and 5.07 m. The TEC data from IRI-2007 model was used to model the ionospheric path delay. The ionospheric path delays for the three acquisition dates were -0.189 m, -0.137 m and -0.118 m respectively. The negative sign indicates the phase advance caused due to presence of ions in the layer. The total delays observed for the three dates were subtracted from the phases of the differential interferograms generated. Three differential interferograms were generated for this study. Atmospheric correction was performed on the phase of all the three interferograms and the residual phase was analysed. Two differential interferograms exhibited excessive phase noise because of temporal decorrelation. These pairs were discarded for subsidence measurements. The better differential interferogram was used to extract the subsidence fringes. The subsidence rates for the fringes were validated with respect to ground levelling data. Two test sites were used in this study where ground levelling was performed. Subsidence fringes were observed from the differential interferogram for one of the test site (Bastacolla). The subsidence rates from DInSAR showed good correlation with the ground levelling data. No subsidence fringes were observed in the second test side (Mahespur). The low coherence caused due to temporal decorrelation might be one of the factors. The low coherence exhibited by the interferometric pairs lead to phase unwrapping problems. This approach provided a platform to explore the tropospheric and ionospheric path delays effects on the interferograms generated from RADARSAT-2 SLC’s. Accurate subsidence measurements can be achieved when atmospheric correction is performed on interferometric pairs exhibiting higher coherence. Keywords: DInSAR, troposphere, ionosphere, path delay, integrated approach, coherence, temporal decorrelation, total electron content, phase, subsidence, fringes.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Subject:38 earth sciences
Programme:Geoinformation Science and Earth Observation MSc (75014)
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