Accurate Positioning of Marine Vessels Using GPS

Abstract

Offshore geophysical studies usually require that accurate position of the various observations that are collected on board a moving platform be documented and referenced to absolute coordinates. Shore-based systems such as Mini-Ranger, Raydist, Trisponder, and Microfix have traditionally been used for this purpose. However, the advent of the Global Positioning System (GPS) and its potential for operating with longer distances from shore will certainly change the methods for offshore positioning. The geometry of GPS orbits and the highly structured nature of transmitted GPS signals imply that the system may be used in a precision differential mode to achieve high accuracy at considerable distances from shore using a C/A code receiver that is small, light weight, and relatively inexpensive.

This paper presents results of recent marine vehicle tracking tests using the Motorola Eagle GPS receiver. This receiver, which has only recently been introduced to the market, is a relatively modest cost, digital, 4-channel, C/A code GPS receiver with several features that produce very high performance. The receiver incorporates fully integrated carrier-aided tracking of the independent C/A codes from four satellites to acheive a high level of precision in a dynamic environment. Simultaneous integrated carrier phase measurements and C/A code phase measurements are obtained for four satellites in this receiver and are used in computing a precise estimate of position via an 8-state Kalman filter. Test results indicate that a geodetic level of measurement precision can be obtained which is unprecedented in a receiver of such modest cost, operating in a real-time dynamic environment. The availability of such equipment, especially at this early stage of GPS development, indicates that the technology of marine positioning is very dynamic indeed. In this, and many other realms of positioning technology, GPS will clearly have a very dramatic impact.