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
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.
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.
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.
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;
- Semi-major Axis (a)
a = 6378137.0 meters
- Flattening (f)
1/f = 298.257223563 meters
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.
A typical example of such an ellipsoid is Everest whose semi-major axis (a) was originally defined in yards in 1830. Here, changes in the yard to meter conversion ratio over the years have resulted in five different values for the constant (a) as shown in figure 2 and table 1.
Figure 2: Ellipsoids that are used in South and Southeast Asia
Datum Datum is the most important function that one need to know before carrying out GPS surveys as they provide the vital information about the mapping system of a country. i.e which defines the local system of survey. Though typically there are two datums associated, one with horizontal and other vertical, mostly it is the horizontal that is used. As defined by NIMA, the horizontal geodetic datum may consist of the longitude and latitude of an initial point (origin); an azimuth of a line (direction) to some other triangulation station; the parameters (radius and flattening) of the ellipsoid selected for the computations; and the geoid separation at the origin. A change in any of these quantities affects every point on the datum. For this reason, while positions within a system are directly and accurately relatable, data such as distance and azimuth derived from computations involving geodetic positions on different datums will be in error in proportion to the difference in the initial quantities.
The Indian Datum has been used for India and several adjacent countries in Southeast Asia. It is computed on the Everest Ellipsoid with its origin at Kalianpur in Central India. Derived in 1830, the Everest Ellipsoid is the oldest of the ellipsoids in use and is much too small. As a result, the datum cannot be extended too far from the origin or very large geoid separations will occur. For this reason and the fact that the ties between local triangulation in Southeast Asia are typically weak, the Indian Datum is probably the least satisfactory of the major datums. (DMA-TR 80-003)
The different datums, which are, currently used in the sub-continent, are shown in figure 3.
Figure 3: Datum that are used in South and
Southeast Asia. Improper use of datum and parameters that are associated with the datum is still the major issue in the GPS surveying circle. Though some parameters are available through various organizations like NIMA, it is believed that the published figures are not very precise and it may not be possible to obtain geodetic quality accuracy (few cm to mm). One such degraded parameter that is currently being used widely is of Indian. (Clifford J. Mugnier, 2000).
The parameters associated with the Indian Datum are Indian State Secret and are available only with the military organizations of UK and US. This has crippled the GIS and GPS user community and the academic researcher to test the full potential of the GPS. With the recent Federal Radio Navigation Plan unveiling future plans with increased accuracy for civilians, the Indian and other states which have held their parameters should come out to release them to public. Also, maintaining them as a state secret at the information age where one could get satellite information with a resolution of 1m (IKONOS, EarthImaging) the purpose gets nullified.
Though there are many other factors that one need consider while using like, Error sources and geometry, they remain outside the scope of this paper.
Two research studies have been carried out in the center with the full capability of real time kinamatic and wide area differential. The research were conducted on the Horizontal Component for road surveying (Ganesh, 1999) and Vertical component for level surveying (Dinesh, 1998).
The study on the horizontal components with different positioning techniques such as the Real Time Kinematic, Differential with Wide Area System (OmniSTAR), Pseudorange Differential has resulted in very comprehensive results. The tests carried out on two different sets of GPS receivers concluded that even with differential the accuracy depends on the quality of the receiver to large extent.
Figure 4: Accuracy levels at 0cm at
different vehicle speeds.
The results below on figure 3 shows the accuracy obtained at 0cm from the centerline of the road at different vehicle speed and different measurement options.
With the same setting at a different offset from the road reveals some undesired results but at the same time the accuracy by the small receivers multiply.
The study on the vertical components by Dinesh in 1998 indicates that it is possible to obtain high level of accuracy of the order of 4cm by applying proper correction algorithm thought out the results provided we do with a calibration point.
The study was conduected only with Real Time Kinematic (carrier phase) and tied to level survery carried over that area.
The results of the studies have indicated that it is possible to obtain few centimeter accuracy when surveyed with survey grade GPS receivers. But the problem of accuracy remains a question with the parameters that are used for conversion being not accurate enough to prove the results. Though it is possible to achieve even high accuracy provided we use the both the frequency spectrums it may not be possible to prove exactly.
Figure 5: Accuracy levels at 100cm at
different vehicle speeds.
Figure 6: Vertical accuracy with RTK With the new Federal Radio Navigation Plan released in February 2000 indicating the introduction of two additional signals and withdrawal of selective availability by the year 2006, the approach towards surveying with GPS in the sub-continent should be given more though. It will be possible even with normal hand held receivers to obtain pseudorange accuracy of 5meters and with differential accuracy that are good enough for any of the applications that one can think of.
With the opensky information policy being adopted by GPS developers, the governments that are holding out conversion information should come out to release to make the best of navigation service that it available today for the mankind – the global utility.
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