Determining DGPS Coordinates Directly on the Grid

Yosef Melzer

Abstract:	In 1993, GPS is scheduled to achieve Initial Operation Capability. It will bring more users to the growing GPS community. During the last two years, DGPS (Differential GPS) technology has progressed remarkably, probably as an answer to SA (Selective Availability). Let us remember that satellite orbits, as well as user position, refer to the WGS-84 datum. This World Geodetic Datum maybe conventional to most users in the future, but it is not today. User requirements call for transforming GPS WGS-84 position to their local datum. Transformation to the local datum could be obtained by using known transformation parameters. Where 2D grid coordinates are needed, two major steps are needed to transform the GPS WGS-84 coordinates. The first, transformation from WGS-84 datum to the local datum, the second, presenting the position on the grid by using the local mapping equations. Some users would like to transform the heights to the geoid. This procedure requires information about the separation between the ellipsoid and the geoid (undulation). It is well known that in precise datum transformation seven parameters should be solved. For this procedure some common points are needed (3 is the minimum) at both source and target datums. But these technical requirements cannot be met in all regions where datums different from WGS-84 are in use, while access to the local datum is not available. Sometimes, information about the local grid is more readily available. Positioning with DGPS, while occupying one receiver on a known local grid point, could solve the access to the local grid, without processing datum transformation. The alternative procedures require two things in principle: One known point from the local grid, and, DGPS vector5 b tween the known point to the unknown points. If only the above requirements are available, the solution may suffer from unknown accuracy. But, the addition of another local grid point, observed by DGPS, could help to transform the 3D GPS coordinates to the local grid with good results. The article will describe some of the alternative procedures, and will present examples of some experiments which were made to verify the models.
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	89 - 95
Cite this article:	Melzer, Yosef, "Determining DGPS Coordinates Directly on the Grid," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 89-95.
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