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A comparative study of results from GPS data processing software
Supriya Likhar, Madhav N. Kulkarni*, V.S. Tomar, Praveen Pillai
Department of Civil Engineering, I.I.T. Bombay, Mumbai, India - 400076
*kulkarni@civil.iitb.ac.in
Abstract
The Global Positioning System (GPS) has become an important tool for various applications, including scientific studies to monitor crustal deformations due to earthquakes. Dual-frequency geodetic GPS receivers are used to carry out the GPS observations. It becomes very important to post-process this data using software that gives the desired accuracy for estimating the crustal deformations. A variety of softwares are available in the market today for processing the GPS observations. Vendor-supplied GPS data processing softwares like the GPSurvey, Trimble Geomatics Office (TGO), SKI, etc., and scientific softwares like Bernese, GAMIT are available, that provide different levels of accuracy. This paper deals with the performance of different softwares used for processing short and long baselines. The softwares under analysis are Bernese, GPSurvey and TGO. The main objective of this study is to investigate the effectiveness of these software packages in different configurations. The work is based on the data collected by the GPS team of Indian Institute of Technology, Bombay (IITB), during the GPS observations carried out in Bhuj region of Gujarat, India, in February 2002.
Introduction
GPS related 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 software, etc. Many applications of GPS demand precise positions, for which extensive data processing is required. There are many GPS data processing software available: both scientific and commercial. In this study, a comparison of the output from the different software is carried out, in order to understand the effectiveness, precision and utility of different kinds of software (Kulkarni, 2000).
The effective monitoring and management of natural calamities requires the most recent terrain information and quick updating of the digital maps. The GPS technique can be used for this purpose, to yield very accurate results in an efficient and economic way. In February 2001, immediately after the 26th January earthquake in the Bhuj region of Gujarat, the GPS team from IIT Bombay carried out GPS observations, to understand the post-earthquake crustal deformation pattern, and to monitor any future crustal dynamics in this region. This GPS field campaign, with a total of 14 stations, including 5 old GT stations, has been repeated in February 2002. This data has been used in the present study.
GPS data
The GPS data, which basically consists of either pseudo-range or phase differences or both, can yield the three-dimensional position of the receiver antenna at the instance of observation (either in Cartesian Coordinates: x, y, z, or latitude, longitude and height), and the precise time. In addition to this important information, other parameters of interest for specific applications, such as atmospheric (tropospheric and ionospheric) delays, clock errors, satellite orbital elements, Earth rotation parameters and model parameters etc. can also be estimated. The choice of the software and technique to be used for such processing would depend primarily on the level of accuracy desired, and other factors such as the hardware available, surveying technique used to collect the data, etc. The GPS data processing software can be broadly classified into two categories - the receiver-specific vendor-supplied software and the scientific processing software. Even though the GPS data collected using different makes of the receivers is in different format, the facility of converting all these to a common internationally accepted ASCII format, called RINEX (Receiver IN dependent data Exchange format) makes it possible to process any kind of data using the scientific software, for achieving higher accuracy (Kulkarni, 1999).
GPS observation should be post-processed in relative mode using suitable software, to get accurate results. For most surveying applications, use of the vendor-supplied GPS data processing software is sufficient to yield the required accuracy. However, for high precision scientific applications, special scientific data processing softwares have been designed. Some of the scientific GPS data processing softwares being used world wide are: the BERNESE, developed by the University of Bern, GAMIT developed by MIT, GEODYN by NASA, GIPSY by JPL, TOPAS, Germany, GPSOBS by ESA, TEXGAP by Univ. Texas, GEPHARD by GFZ, Germany, etc. Of these, BERNESE software is being used by few research institutes/organizations in India, including Survey of India, IIG, IIT Bombay, CMMACS, WIHG, NGRI, etc., and GAMIT is also being used in more recent times.
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