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Surveying in the sub-continent with global positioning system
Mr.Shanmugam Ganehkumar, Dr. Lal Samarakoon, Dr. Kiyoshi Honda Asian Center for Research on Remote Sensing, Asian Institute of Technology PO Box 4, Klong Luang, Pathumthani 12120 Thailand sganesh@ait.ac.th
Abstract:
Traditional surveying has now been replaced with GPS since it provides accuracy that are acceptable to geodetic applications. Though the accuracy level that is obtainable through the GPS is of millimeter level the question of reliability of the results still remain a question to most of the researchers in this sub-continent since the conversion parameters remain a state secret. We have analyzed the potential of the Global Position System with the architecture with the description of the various reference systems that are currently used in the region. The paper also analyses these issues in detail and with the experiments on horizontal and vertical components. Introduction Surveying, the way we measure the earth consonants, has changed in the recent year and will have a major impact with the advent of Global Positioning System (GPS). Proclaimed as "the next utility" by Trimble, a major GPS vendor, the GPS has more potential than what it is being used for today. With all the potential that it has to offer the global users, it is certainly the best invention after compass. One of the wide spectrums of applications of GPS is Geodetic Measurements. Geodetic measurement provides very accurate determinations of positions of points on the earth's surface. Because earth is a deformable, rotating body, the measured points on earth change relative to tectonic plate movement these can be as high as 100 mm/year when measured over a few years 1. The required accuracy of the measurements depends on the requirements of the study. Accuracy of 5-10 mm/year is often adequate to assess the magnitude of the rate of strain accumulation. These accuracy's being the general requirement of Geodetic measurements, it has become a pleasure to monitor and obtain such accuracy levels using GPS. But general application for GIS professionals do not demand such accuracy level and are comfortable with the range of less than a meter to few centimeters. Here we discuss the relevance of information required to conduct a GPS survey in this South and Southeast Asian region, in other words the sub-continent. Though it is possible to have such very high accuracy of the order of mm, the information available to the civil signal users are underprivileged of getting that accuracy by the fact of required necessary other information. An in-depth of the parameters that are required is also been discussed in this paper. We also have made a brief description of the whole system as to make the concept clear. In conclusion we see the disadvantage of using certain datum compared to others. GPS Architecture Developed and maintained by the SU Department of Defense, NAVSTAR GPS is one of the two satellite based positioning systems that is currently in use. Though Russian system of GLONASS also provides the similar service, there are no hand held receivers till we write this paper. Therefore we synonymously use GPS for the NAVSTAR system in this paper. With a constellation of 24 satellites in 6 orbital planes of 6 satellites each, the system provides positional information on the WGS-84 reference gird. The system also provides highly accurate time information through its on board rubidium and cesium atomic clocks. Though this information is required to calculate the positional information, the time information is used by many other sectors which are time depended. The GPS time has become a global standard because of its accuracy. Though primarily a military system, the positional information from the GPS is free and can be used by anyone in the globe2. But the civil users around the world are supported with degraded information compared to that is available for military use (IRN-200C-002, DoD) As per specifications laid in the document, the civil users are supported with 100m horizontal and 156m vertical accuracy. This is achieved through the mechanism called Anti-Spoofing and Selected Availability in the civilian spectrum of signal service called Standard Positioning Service (SPS). (GPS Standard Position Service, Signal Specification, July 1995) The positional information from the satellites are transmitted to the surface through two L band frequency called L1 and L2 with the frequencies of 1575.42 MHz and 1227.6 MHz. respectively. The carrier of the message is also an important since it can be used to acquire more accurate information than the simple use of Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code sequence. The L1 and L2 band have carrier wavelength of 19x and 20x cm. The importance of the wavelength has been discussed later in the paper. Positioning Methods With the accuracy level far exceeding the expected, only use of CA code signal will not be of any substantial use since its of the order of tens of meters. 
Figure 1: Accuracy attainable through various methods It is for the same reason that the Differential Positioning method has been adopted, through with it is possible to attain very high accuracy. Accuracy of the order of mm using both the frequencies. As shown in figure 1, accuracy of the order of mm can be obtained using the carrier frequency of the signals and processing later through resolving ambiguity and other errors. Adopting the right method to observe the position is very important as to achieve the desired accuracy. It is possible to achieve 2cm and 20cm on the fly through NovAtel's OEM Cards. With simple differential correction through RTCM protocol can give 1 to 3 m accuracy. Experiments have been carried out in the center for horizontal and vertical components with various modes of measurements. Some of the results of the experiments have been discussed later in the paper. GPS Reference System The most important aspect of the position information that is obtained from the GPS satellite is its reference system. Since most of the countries adopt their own projection system, care should taken in making the necessary transformation to convert the obtained information to the local. As defined by the NIMA, "The WGS 84 Coordinate System is a Conventional Terrestrial Reference System (CTRS). The definition of this coordinate system follows the criteria outlined in the International Earth Rotation Service (IERS) Technical Note 21. Also it is a is a right-handed, Earth-fixed orthogonal coordinate system" Ellipsoid Global geodetic applications require three different surfaces to be clearly defined. The first of these is the Earth's topographic surface. This surface includes the familiar landmass topography as well as the ocean bottom topography. In addition to this highly irregular topographic surface, a definition is needed for a geometric or mathematical reference surface, the ellipsoid, and an equipotential surface called the geoid. The ellipsoidal parameters used in the WGS-84 are given as;
Some of the reference ellipsoids have more than one semi-major axis (a) associated with them. These different values of axis (a) vary from one region or country to another or from one year to another within the same region or country.
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