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Overview | Earthquake | Drought | Fire | Flood & Cyclones | Landslide & Soil Erosion | Volcano
Application of GPS in crustal deformation studies: Some case studies
C.D. Reddy
Indian Institute of Geomagnetism, Colaba (P.O)
Mumbai-400005
cdreddy@iig.iigm.res.in
Introduction
The Global Positioning System (GPS) is a space based navigation system, consisting of a constellation of 24 satellites, in six orbital planes with 55° inclination to the equator. The satellites are placed at a height of about 20,200 km with 12 hours orbital period and operated by the United States Department of Defense (DOD) for accurate determination of position, velocity and time. All the GPS satellites are controlled by system tracking stations, ground antennae and the master control station.
In each satellite two rubidium and two cesium atomic clocks with stability 1013 to 1014 are used to derive the fundamental frequency
fo = 10.23 MHz. The GPS signals are transmitted at two frequencies, designated
L1 (154 fo = 1575.42 MHz) and L2 (120
fo =1227.6 MHz) which are derived from the fundamental frequency
(fo). Two codes are used, one of which is called C/A (coarse acquisition code,
fo /10) and the other is called P (precise, fo) code. As the rate of P code is 10 times the rate of C/A code, its precision is 10 times better than C/A code. The
L1 and L2 are modulated by Pseudo Random Noise (PRN) code, (each satellite is identified by this code) and transmitted after biphase modulation with the carrier.
The distance to GPS satellite is estimated by measuring the time a radio signal takes to reach us from the satellite. This is accomplished by cross -correlation of pseudo-random code generated by the satellite and the receiver. The distances from receiver to satellite measured in this way are called code pseudo ranges. Minimum four satellites are required for estimating the coordinates of a point on the Earth’s surface. The position accuracy that can be estimated this way depends on our ability to account for various error sources (Reddy, 2001). The textbooks, such as Seeber (1993, p. 209-348), Hofmann et al. (1994), Leick (1995), Parkinson and Spilker (1996), Kaplan (1996) provide very good reference on this subject.
While the use of the GPS is extensive in defense, navigation and surveying applications, it is being used in geo-science, ionospheric & atmospheric studies, global climate changes, observing polar motion & earth rotation rate, mapping the gravity field, detecting seismo ionospheirc effects, transport and communications, environment management, for accurate time and frequency etc.
At present we are able to achieve much better accuracies due to processing techniques which circumvent the purposeful degradation of the GPS signals. This motivated earth scientists to use GPS for monitoring the slow and relentless crustal deformation by employing a technique called carrier tracking which allows to determine baseline length within a few millimeters. The methodology is that the changes in position coordinates and baseline lengths in three orthogonal directions computed with GPS data during successive visits will enable us to assess the crustal deformation. Changes in deformational rates have intrinsic value in understanding the physics of the earthquake processes.
Crustal deformation studies have received new impetus all over the world with the full complement of satellites for adequate coverage, availability of comparatively low-cost receivers, sophisticated post processing softwares and international cooperation through International GPS Services for Geodynamics (IGS). In many countries the receivers are used permanently in a network mode with data telemetered and processed continuously to have upgraded baseline vectors regularly. For the past few years, regional GPS networks designed mainly to monitor strain for earthquake research and forecasting have been operated in many countries all over the world and have proved useful in detecting the crustal displacements.
GPS data collection and analysis
Exposed bed rocks or well settled concrete pillars were chosen with unobstructed view of the sky and with non reflective environment as GPS sites. The monuments which were made on vast expanses of bed rocks are considered to have least site instabilities. Dual frequency GPS receivers were used in re-occupation mode in collection GPS data. Generally, every year the data has been collected during winter when the humidity is very low to minimize the effects of troposphere. The sampling interval and elevation were fixed at 30 sec and
15o respectively throughout the survey.
The GPS data were organized into 24 hours segments covering a UTC day to facilitate the combination of data from some of the surrounding IGS sites; IISC, DGAR, BAHR, KIT3, LHAS, YAR1, KERG to constrain the site co-ordinates. Then the data were processed using the GAMIT software developed at MIT and SIO (King and Bock, 1991) to produce estimates and an associated covariance matrix of station positions for each session with loose constrains on the parameters. To get a combined solution (site positions and velocities), all such covariance matrices are input to GLOBK which is a Kalman filter. The basic algorithms and a description of this technique are given in Herring et al. (1990) and its application to GPS data is in Feigl et al. (1993). By introducing global h-files, we have obtained coordinates and velocity vectors at each site in the ITRF96 reference frame (Boucher et al., 1998. The horizontal components of these velocity vectors are further used to estimate the horizontal strain field by Least-Squares Prediction (LSP) method (Reddy et al., 2000).
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