Use of the TRANSIT Navigation Satellite System for offshore oil exploration and for land geodetic survey began in 1968. (To realize that we are well into the second decade of using this space-age technology for commercial applications was a slight shockto this author.) Many of the advances made in the state-of-the-art have been reported at previous Offshore Technology Conferences (see references 1, 2, 3, and 10). The purpose of this paper is to review the progress made since the previous reports and to examine trends affecting future developments.

This report covers three major areas. The first is the field of land geodetic survey in which three developments are reported. First is a multi-station network adjustment program called MAGNET. The second is a field translocation capability which has been implemented within the MX 1502 Satellite Surveyor instrument. The third development is a real-time translocation capability based on a communications link between two or more MX 1502 instruments.

The second area of this report focuses on the evolution of integrated navigation systems for offshore oil exploration. The objective is to show how these systems have matured since they were first described in references 1, 2, and 3. Also, trends affecting the direction of future developments are discussed.

Finally, this report evaluates the coming impact of NAVSTAR, the Global Positioning System. The future potential when the full satellite constellation is provided, the present potential with the interim six-satellite constellation, and concerns of the civil user community are discussed.

Geodetic Positioning

One very important application of the TRANSIT Navigation Satellite System is for fixed point survey. In contract with satellite position fixes taken on a moving vessel, in which each position fix is used independently to update the dead-reckoned position, fixed point survey requires data from multiple satellite passes at a single location. With appropriate processing techniques such data can be used to obtain excellent survey results, thus avoiding the land traverse required with conventional survey techniques in which distance and angle measurements must be made from one point to the next, regardless of the intervening terrain.

Multiple pass satellite data can be processed in either of two ways: point positioning or translocation. In point positioning, data from a single receiver is used to obtain the .latitude, longitude, and altitude of its antenna. The only reference datum for this process is the satellite systemitself, and from 10 to 50 satellite passes normally are employed when operating in the point pos1tioning mode. A horizontal positioning repeatability of five meters rms can be expected with 25 satellite passes.

A variation on the point positioning technique is to employ a "precise ephemeris" rather than the orbit parameters obtained from the satellites themselves.

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