An Overview of Real-Time GPS Heights On Marine Vessels in San Francisco Bay

Since early 1983, the National Geodetic Survey (NGS) has performed control survey projects in the United States using satellites of the Global Positioning System (GPS). Analysis of GPS survey data has shown that GPS can be used to establish precise relative positions in a three-dimensional Earth-centered coordinate system. GPS carrier phase measurements are used to determine vector base lines in space, with the components of the base line expressed in terms of Cartesian coordinate differences. These vector base lines can be converted to distance, azimuth, and ellipsoidal height differences (dh) relative to a defined reference ellipsoid.

With the availability of high-accuracy, differential GPS results in real-time, there is a new opportunity to use GPS to accurately measure a vessel's settlement, squat, trim, roll, pitch, and heading. This application of GPS is very promising, but still not yet widely used. NGS and the Coast Survey (CS), offices of the National Ocean Service (NOS), National Oceanic and Atmospheric Administration, are transferring this technology to the Port Authority of Oakland, California. NOS provides the technical personnel to set up and conduct demonstrations of this application on large container ships and oil tankers operating in the Port of Oakland. The overall goal of this project is to provide the position of a vessel's keel in real time to within 10 centimeters (about 4 inches) relative to the bottom of the shipping channel.

This real-time GPS heights on marine vessels project consisted of four activities (phases): (1) to demonstrate the feasibility of determining accurate GPS heights in real-time on large marine vessels by obtaining GPS data and using post-processing ("after the fact") techniques to determine an accurate relationship of vessel squat (dynamic draft) for underkeel clearance, as well as an independent measurement of vessel trim, roll, pitch, and heading; (2) repeat phase 1 in real-time, using the GPS configuration based on the results of phase 1; (3) acquire information necessary to describe the location and shape of the navigational channel using GPS phase data collected on designated vessels as they transit into or out of the Port of Oakland; and (4) relate the real-time positioning of the ship to the bottom of the channel. NOS has accomplished all four activities and is now integrating the effort as simulated in the attached figures.

This technology transfer effort will enable the Port of Oakland to better conduct business with their client shippers, to better interface with NOS future electronic chart products, and to more accurately plan for future increases in vessel size. The integration of real-time GPS positioning on marine vessels with electronic charts will facilitate the development of accurate docking charts. These charts will enable large cargo ships to dock in major harbors under extremely poor visibility conditions and will drastically reduce weather-related harbor delays. This project is a major component in realizing electronic charts for navigation.