Loran-C Expansion: Impact on Offshore Technology

ABSTRACT Expansion of the Loran-C Navigation System will have a substantial impact on offshore navigation in the U. S. coastal Confluence Zone. Loran-C provides a precise, continuously-available method of vessel location, permitting more effective harvest of natural resources through considerable savings in time, money and energy. This paper summarizes the expansion plans and discusses the benefits of Loran-C for the offshore user. INTRODUCTION As you have heard previously [1] Loran-C has been selected as the Government-provided navigation system for the U. S. Coastal Confluence Zone (CCZ). Loran-C is not, however a new system. It has been in use since the late 1950's by the Department of Defense and more recently by the civil community [2,3, A, 5]. Loran-C already covers 16 million square miles of the northern hemisphere. Figure 1 shows the existing Loran-C coverage. Note that this diagram shows both sky wave and ground wave coverage. The ground wave limits indicate an area inside of which the user can expect a Loran-C derived position to be within 1/4 nautical mile (NM) of his true position with a probability of 0.95 throughout the year. In general, as the user progresses toward the transmitting stations the accuracy improves, with much of the area having geodetic accuracies on the order of 500 feet or better. As the predecessor to Loran-C, Loran-A has been serving the maritime and aviation communities since World War II. This system is still in operation and will remain so for a few more years. While Loran-C can be used to identify a position that is accurate to 1/4 NM, Loran-A's accuracy is from 1 to 5 NM. A brief review of fix accuracy terminology is appropriate at this point. The three most common terms for expressing fix accuracy are geodetic (or absolute) accuracy, repeatable accuracy and relational accuracy. Briefly, these terms refer to accuracy with respect to some recognized grid system, accuracy with respect to a previous indication of the same position, and accuracy with respect to the position of another user of the system. The definitions are contained in Figure 2. The Loran-C system provides excellent repeatability, on the order of 100 feet, without the need for calibrations or the use of calibrated charts required to obtain the stated geodetic accuracy. The availability of this accurate navigation/positioning system offers tremendous potential for the entire offshore community. In testimony before the House Subcommittee on Coast Guard & Navigation concerning the Coast Guard's fiscal year 1975 budget, Mr. Carl Savit, speaking on behalf of the International Association of Geophysical Contractors, stated that expansion of Loran-C services throughout the CCZ would result in a cost savings of $20 million to $30 million per year to the oil and gas exploration industry alone. EXPANSION PLANS FOR U. S. COASTAL CONFLUENCE ZONE (CCZ) The on-air dates for the new CCZ Loran-C chains were published in 1974 [6] and have been cited in Mr. Haislip's paper [1]. Figure 3 reiterates this schedule.