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Crustal Shortenning and tectonics of the NW Himalaya from GPS measurements
GPS Data Acquisition and Processing
We established nearly 50 GPS stations in the region covering the Indian states of Himachal Pradesh, Uttaranchal, and adjoining border areas of Jammu & Kashmir, Punjab and Uttar Pradesh (Fig.2 ). Out of these 50 stations, velocity vectors are computed for 26 stations, as many stations could not be re-measured, and some bad measurements were rejected after computing the vectors. Most of the GPS stations are monumented by inserting Bevis Steel Pin into hard rock. Small punched circular holes on top face of the steel pin are used as the measurement points. In few cases where the monuments were to be established in soil covered area, 2ftx2ftx6ft concrete structures were constructed underground, with a Bevis steel pin protruding from the top face of it, which is flush with the ground. All the campaign mode stations were occupied for 4-5 days during each campaign. Many of the stations that were established during 1998 and 1999, were reoccupied in 2000. Two permanent GPS stations were established at Dehra Dun, (Uttaranchal), and Naddi (Himachal Pradesh), and had been operating since the end of 1998. Both the permanent stations are equipped with Trimble 4000SSi receivers, and Trimble Choke ring antennas. All the campaign data were collected using Trimble 4000 SSE/SSi receivers. GPS data collections were initiated during 1995. Gradually more and more stations were added to the network.
Fig.2 More than 50 GPS stations were established in
the entire NW Himalaya (red stars). Red diomonds are sites to be installed in
future. Black stars are permanent stations at DehraDun and Naddi (Dharamsala).
Total 26 sites were reoccupied, and results are discussed in this paper.
The GPS data were processed using the GAMIT/GLOBK suite of software (Ver.9.95 ) . GAMIT and GLOBK are a comprehensive suite of programs developed by MIT, Scripps Institution of Oceanography, and Harvard University for analyzing GPS measurements primarily to study crustal deformation. GAMIT uses the GPS broadcast carrier phase and pseudorange observable to estimate three-dimensional relative positions of ground stations and satellite orbits, atmospheric zenith delays, and earth orientation parameters. GLOBK is a Kalman filter whose primary purpose is to combine various geodetic solutions such as GPS, VLBI, and SLR experiments. It accepts as data, or "quasi-observations" the estimates and covariance matrices for station coordinates, earth-orientation parameters, orbital parameters, and source positions generated from the analysis of the primary observations. The input solutions are generally performed with loose a priori uncertainties assigned to all global parameters, so that constraints can be uniformly applied in the combined solution.
In addition to the GPS data collected from our study area, we used data from IISC, KIT3, POL2 and LHAS IGS stations, as well as NAGA permanent station data from Nepal (Fig.1). These IGS sites were constrained to 5mm for horizontal, and 10mm for vertical coordinates. The a-pripori coordinates of the IGS sites were taken from global IGS solutions in ITRF97 reference frame. Two regional permanent stations, WIH2 and NADI, were similarly constrained tightly, after their accurate coordinates were obtained in ITRF97, from separate processing of a subset of the continuous station data. Fig. 3 shows time series for the entire span of the WIH2 continuously monitoring station. The tight constraining of the fiducial stations are useful only for resolving ambiguities, as loose solutions are used for finally combining daily solutions through GLOBK. In the primary processing using GAMIT, apart from the site coordinates, we also estimated parameters representing the effects of the atmosphere, Earth's rotation, and motions of the satellites. We included in our analysis, daily combined solutions of global IGS stations processed and archived as 'h-file's by SOPAC. Reference frame were defined by minimizing the departure from the a-priori ITRF97 values of the coordinates of a subset of IGS core stations. We used 24 IGS stations for position and velocity system stabilizations. This resulted in 2.4mm of RMS for position, and 1.6mm/yr of RMS for velocity stabilization. As UT1 and pole position are defined as single global parameters in GLOBK, we allowed Markov process for earth orientation parameters to allow day-to-day variations. We also allowed Markov process with tight constraints for satellite orbital parameters as well as radiation pressure parameters to absorb a given degree of mis-modelling of the satellite orbital motions. At the end, we have allowed large tolerance for uncertainties in station height component, effectively making the Kalman filtering process insensitive to height. After combining IGS site solutions with our regional solutions on a daily basis, total 735 daily solution files were combined to produce the final velocities of all the 63 sites, including both regional and global sites. We extracted the sites lying within our study area. In Fig. 4, we show few important regional site vectors relevant to India-Asia convergence process
Fig.3 Time series of the residual of WIH2 Permanent
station positions
(N, E and Height components) in ITRF97.
Data were processed
using GAMIT 9.95 software
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