An Advanced Precision Navigation System
Authors: Knox W.A. , Moody Alton B. 
Published: October 2008

An integrated navigation system for use at sea has been developed, tested, and placed in commercial operation; Novel features include an inertial system with doppler-sonar velocity damping, automatic position-error feedback from satellite fixes, and continuous digital magnetic-tape recording of all navigational parameters. An onboard computer makes all the navigation calculations and controls a steering display and a real-time graphical plot of ship's track. Computer post-processing of the recorded navigation data allows mapping of a vessel's travels to an accuracy of 100 meters or better.

INTRODUCTION

In the fall of 1542, Juan Cabrillo explored the coast of California (1). He measured his latitude using a cross staff (fig. 1) but his longitude was indeterminate because he had no chronometer. One historian comments that Cabrillo's latitude determinations were 1.0 to 1.5 degrees too high and that it was necessary to recompute his surveys.

Three hundred years later, John Fremont explored the region between Missouri and California in the summer of 1842 (2). Having a well-made sextant and a chronometer "which of course, I carried upon my person", Fremont was able to locate each night's encampment with fair accuracy, despite the fact that he spent half the night making and reducing his observations, following a strenuous day in the saddle.

Today, automatic equipment gives us our latitude and longitude to an accuracy of a hundred meters or so (fig. 2), without reference to celestial bodies or shore-based navigation aids. This paper will describe such a system and summarize the results actually achieved.

SHIPBOARD NAVIGATION SYSTEMS

The first self-contained, shipboard navigation systems used for geophysical exploration consisted of a doppler-sonar, gyrocompass combination for dead reckoning and a satellite receiver to provide periodic bench marks upon which to base the dead reckoning. Navigation parameters were manually recorded on log sheets and/or hand-plotted on a map.

System accuracy was limited by gyrocompass acceleration errors, doppler-sonar errors caused by the cross-coupled effects of ship's pitch-roll-heave, doppler-sonar tracking errors, and 108s of sonar bottom-lock in deep water. A further disadvantage was restriction of ship's maneuvers during satellite passes, constant course and speed during this period of eight to 16 minutes being required. Manual recording of data was inefficient and subject to error. Post-plotting of manually-recorded data was slow, tedious, and inaccurate.

To eliminate the problems we have described and to provide navigational capabilities suitable for detailed seismic exploration as well as reconnaissance work, a magnetic tape recorder and an inertial platform were integrated with the doppler sonar and satellite receiver through a central computer.

Addition of the inertial system makes possible corrections to the doppler sonar for pitch-roll-heave. Unaffected by accelerations, the stable platform provides a heading reference with an error of less than 0.1 degree. During periods when the doppler sonar suffers temporary losses of bottom-lock or when the sonar indicates a track error, the inertial system bridges the navigation gap by continuing to provide ship's velocities, and to compute and display navigator's coordinates with great accuracy.