An integrated Geophysical Navigation System Using Satellite-Derived Position Fixes

Abstract

ABSTRACT As geophysical offshore exploration has become more sophisticated and as theareas to be explored have become more remote, the need has developed for anavigation system not tied to shore-based facilities and not limited by time ofday or geographical location. Three developments have occurred which make sucha navigation system possible. The Navy Navigation Satellite System has beenreleased for commercial use, thus providing a world-wide source of preciseposition fixes. Pulsed Doppler sonar devcies have been developed which providea very precise measurement of ship's speed relative to the bottom in waterdepths up to 1000 ft. Finally, the age of the integrated circuit, generalpurpose digital computer is upon us. Availability of these small but powerfulcomputers permits joining speed, heading and position sensor subsystems into anoptimized, integrated navigation system. This paper describes the Geophysical Navigation System proposed by the MagnavoxCompany for advanced offshore exploration. As part of this description, extensive coverage is given to the accuracy now being obtained by commercialusers of the Navy Navigation Satellite System. INTRODUCTION For many years, offshore geophysical exploration has been accomplished with theaid of a number of radio navigation systems. Although differing in technique, these systems have common characteristics of very high accuracy but relativelyshort range, expecially at night. In general, such navigation devices areprovided on a leased basis. Because of the cost and because of logistics and technical limitations of theradio navigation devices, there exists a need for another form of offshorenavigation. In addition to providing accurate position, the new system shouldhave four other important characteristics:World-wide coverage not limited by range from shore or time of day.Independence from shore-based facilities.Automatic control and/or recording of shot points.Simple, continuous displays to aid in maintaining the desired course. The Magnavox Company came to understand these requirements as a direct resultof supplying equipment for navigation with TRANSIT, the Navy's NavigationSatellite System. This system, which is now available to commercial users, provides excellent accuracy and is available world-wide. Magnavox felt thatTRANSIT would be valuable for offshore exploration and began to pursue thismarket. Similarly, those engaged in offshore exploration were interested inlearning about this new navigation aid. The original navigation system offered by Magnavox is termed the MXl702/hp. Figure 1 shows that it consists of an antenna and preamplifier unit to recoverthe satellite signals, a receiver to process these signals, a Hewlett Packard2115A Computer programmed to calculate a position fix each time a TRANSITsatellite passes by, and an ASR-33 teleprinter for input/output functions. Anumber of these MX/702/hp units have been sold to the offshore explorationcommunity with excellent results. However, it soon became evident that sellinga black box just to obtain position fixes was not sufficient. Many users do nothave the background or facilities to create the sophisticated type ofintegrated system necessary to provide: