The Surveying Application of a New Loran-C Type. System

Abstract

ABSTRACT New pulse-compression techniques have made feasible the design and construction of solid state transmitters emitting Loran-C signals. When coupled with new receivers, there are tangible benefits for every user and operator of position-fixing services:ranges that are at least double what is presently obtainable with shore-based CW phase-comparison survey networks;portable transmitters can be erected and operated where needed;unambiguous, yet continuous, position-fixing for multiple users; andacceptable accuracies. Initial operation of two networks were in the Gulf of Mexico where offshore operations are now extending to the continental slope; and in the Java Sea where a survey chain was erected specifically for geophysical exploration. INTRODUCTION As resource exploration and production has spread to the edge of the continental shelves, the need for accurate electronic positioning to cover these areas of activity has arisen. Land-based positioning networks for survey work are presently utilized out to the limit of their range. However, new technical advances have made possible the design and fabrication of solid-state transmitters for Loran-C signals. In addition, new receiver designs are available. Combining these two components, it is now possible and practical to set up Loran-C chains in any part of the world where accurate, medium range survey fixing is demanded. (By medium range is meant 100 to 300 miles from the network baselines, and up to 500 miles from any one transmitter station.) The positioning operator is not required to track and maintain lane count, thus the system is non-ambiguous. There is no signal deterioration due to sky wave effect within the medium range area. Initialization is not required when entering the system. EQUIPMENT Since this paper deals with the survey application of this new Loran-C equipment, the theory of operation will not be discussed. Similarly I rather than offer an explanation of Loran-C transmissions, the authors refer to the many excellent summaries available. One such summary is Coast Guard Publication CG-462, "Loran-C User Handbook." Of course, new solid-state transmitters do not emit the same peak power as do the Government transmitters, and hence do not have the same ranges. However, the Coast Guard Loran-C chains are designed for long-range navigation by ships and aircraft. The new Loran-C system is intended to provide the same potential accuracy within the survey area, easier mobilization of chains I and economy of operation. Each chain comprises a master transmitter, and two or more secondary transmitters, all emitting properly timed signals on their ' assigned Group Repitition Interval (GRI.) GRI's are now selectable in ten microsecond increments and are chosen so as to minimize adjacent chain interference. The transmitters are normally controlled by a cesium standard to provide the most stable transmissions possible. However, each station has a timing receiver to provide back-up for the cesium standard as an alternate transmitter control. The transmitting stations, in addition to the cesium standard, consists of the transmitting equipment, the antenna system, and a prime power source. The transmitter is made up of eight compact units with interconnecting cables that will fit on a standard office desk.