With the search for the earth's resources pushing farther offshore, even past the edges of the continental shelves, there is an increasing need for accurate long range radio positioning systems including self-contained systems no longer slaved to land based stations.
This paper review the systems used during the past decade and explains the newer systems which are now available or expected to be available in the near future (1970). Special emphasis is placed on principle of operation, limitations and ranges. For purposes of discussion, the systems are divided into two categories - land based and self contained.
In a recent issue of Surveying and Mapping, Paul D. Thomas of the United States Oceanografic office has reasonably expressed the gravity measurements in geophysical surveys. Now while these accuracies are greater than those generally needed in support of other geophysical instruments used in petroleum exploration, they represent a definitive goal to achieve. (Figure 1 - Tabulation) Mr. Thomas states "No long range positioning system is likely now, or in the near future, to meet the stringent requirements. But the requirements do exist."
In order to determine how close the state of the art is to meeting these requirements, it may do well to review a little of this history of radio positioning in offshore exploration and the systems which have been used in the past.
In 1946, when offshore exploration commenced in earnest, various evolutions of techniques used on land came about wire line stretched thousands of feet to measure distance weather or barrage balloons covered with mirrors and flown over the geophysical vessel so as to mark its position for triangulating by optical instruments ashore the more sophisticated World War II Radio Detecting and Ranging Systems. and others. Surprisingly enough, one Radar system survived even until now, and will be discussed more in this paper. Still other systems made their way to the forefront as the petroleum industry pushed ever seaward in the tidal areas of the world. LORAC, RAYDIST, DECCA longer range systems of which most of you probably have an intimate knowledge. And nearly always, a radio positioning system to meet the need. But now it's August 1968 the Glomar Challenger brings up a coring from the Sigsbee Knoll hydrocarbons are present and the nearest land 200 to 350 miles away! I There's no way to give radio positioning coverage with the systems of yesterday. So new systems must come into being to serve the needs of a vital industry in a dynamic age. Is the answer the satellite navigation system? Inertial systems? Sonar doppler? Combinations of these? Is the land-based system relegated to a diminishing role?
Let me review the general classes of medium range systems which are in use. First, There is the pulse time-distance method. In this scheme, the mobile unit transmits a pulse of energy which is reflected or target on shore. The mobile station measures the time required for the round trip transmission, and since the velocity of constant, the transmission time is easily convertible into distance between mobile unit and shore base target.