Shanmugam Ganeshkumar1, Lal Samarakoon2 and Kiyoshi Honda 2
1GPS Centre, EEE Div. 5, Block S2 B4
Nanyang Technological University
Nanyang Avenue, Singapore 639798
Email: [email protected]
2Asian Centre for Research on Remote Sensing,
Asian Institute of Technology Pathumthani 12120, Thailand
Surveying, the way we measure the earth constants, has changed in the recent years with the advent of Global Positioning System (GPS). Proclaimed as ‘the next utility’ by Trimble, a major GPS vendor, GPS has more potential than what it is being used for today. With all the potential it is certainly the best invention after compass. One of the wide spectrums of GPS applications is the geodetic measurement. Geodetic measurement provides very accurate determinations of position of points on the earth’s surface. Because earth is a deformable, rotating body, the measured points on earth change relatively to tectonic plate movement, which can be as high as 100 mm/year, when measured over a period of few years. The required accuracy of the measurement 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. But general application for GIS professionals does not demand such accuracy level and is comfortable with the range of less than a metre to few centimetres. Though it is possible to have such very high accuracy of the order of mm, information available to the civil signal users are underprivileged of getting that accuracy by the fact of required necessary other information.
Developed and maintained by the US Department of Defence, NAVSTAR GPS is one of the two satellite-based positioning system that is currently in use. Other is the Russian system of GLONASS, which also provides similar service, although the constellation is not yet complete. Therefore synonymously GPS has been used 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 dependent. 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, on the globe. But the civil users around the world are supported with degraded information compared to that is available for military use (IRN-200C-002, DoD).
The positional information from the satellites is transmitted through two L band carrier waves called L1 and L2 with the frequencies of 1575.42 MHz and 1227.6 MHz respectively. The carrier of the message is also 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 19 and 24 cm.
With the accuracy level far exceeding the expected, only use of C/A code signal will not be of any substantial use since it is of the order of tens of metres. It is for the same reason that the Differential Positioning Method has been adopted, through which, it is possible to attain very high accuracy. 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 centre for horizontal and vertical components with various modes of measurements.
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 be taken in making the necessary transformation to convert the obtained information to the local coordinates. 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 right-handed, Earth-fixed orthogonal coordinate system”.
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) = 6378137.0 metres
Flattening (1/f) = 298.257223563 metres
Some of the reference ellipsoids have more than one semi-major axis associated with them. These different values of axis 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 was originally defined in yards in 1830. Here, changes in the yard to metre conversion ratio over the years have resulted in five different values for the constant (Table 1).
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 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 South-East 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 localised. As a result, the datum cannot be extended too far from the origin or else, very large geoid separations will occur. For this reason and the fact that the ties between local triangulation in South-East Asia are typically weak, the Indian Datum is probably the least satisfactory of all the major datums (DMA-TR 80-003). Different datums and their parameters, used in south and southeast Asia are listed in Table-2.
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 datum for which degraded parameter is currently being used widely is the Indian Datum.
The parameters associated with the Indian Datum are Indian State Secret and are available only with the military organisations and Survey of India. 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 countries 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.
Although it is possible to obtain few centimetres accuracy when surveyed with survey grade GPS receivers, 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 dual frequency GPS, it may not be possible to prove exactly. With the new Federal Radio Navigation Plan released in February 2000 indicating the introduction of two additional signals and withdrawal of selective availability from 1st May 2000, the approach towards surveying with GPS in the sub-continent should be given more thought. It will be possible even with normal hand-held receivers to obtain pseudorange accuracy of 5metres and with differential corrections applied, accuracy obtained can be 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.
Note: The article was written before the President Bill Clinton’s order on May 1st, 2000 to discontinue Selective Availability on Navstar GPS satellites.