The U. S. Navy made its navigation satellite system available for private and commercial use in 1968. From its beginnings as a general worldwide aid to navigation, the TRANSIT satellites have become a primary reference network for precision navigation and geodetic surveying. Among commercial users, the offshore geophysical industry has provided impetus for continued refinement of positioning systems, techniques and performance.
This paper reviews specific improvements which have been made in the operation of the TRANSIT satellite system, and in user equipment and procedures. In particular, the improvements have increased the accuracy of positions derived from the doppler satellite measurements:
Portable surveying systems regularly achieve single fix accuracies (one sigma) of the order of 15–25 meters.
Simultaneous reduction of data from multiple satellite passes gives a three dimensional position accuracy within 5 meters, generally in 24 hours or less on site.
The use of translocation offers single-fix accuracy of 1–5 meters relative position between two satellite receivers.
The TRANSIT navigation satellite system was developed by the U. S. Navy as a unified system for worldwide navigation. When the system was made publicly available for commercial use in 1968, it was quickly adopted as a tool both for navigation and for precision geodetic surveying.
During the intervening period of time, several authors have discussed the application of satellite navigation to precision geophysical navigation, particularly with respect to integrated navigation systems for seismic survey vessels (References 1 and 3).
During the past two years, several improvements have been made in the operation of the TRANSIT system and in user equipment and techniques. It is the purpose of this paper to analyze the benefit of these improvements, and to evaluate the accuracy of satellite position determination currently available to the offshore operator.
Position determination plays an essential role in geophysical exploration; seismic surveys are tied together and interpreted on the basis of the positioning system of the survey vessel (or vehicle), lease concessions are sold and administered on the basis of boundary coordinates, and drilling operations are keyed to the coordinate location which is interpreted to have potential for greatest yield. If the positioning system is inaccurate, surveys fail to tie, boundary disputes arise, or the drilling fails to correspond to the structures expected from interpretation. Conversely, high accuracy in positioning can save time in moving and position verification of expensive equipment, the greatest saving being in avoiding the necessity for redrilling "misses".
An analysis of accuracy must be based upon examining the opposite of accuracy error. Most of the improvements in system performance have come through proper identification of error sources, and through subsequent elimination or reduction of the error contribution.
