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

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 where the components of the base line are expressed in terms of Cartesian coordinate differences (Remondi 1984). 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 (El-Mowafy and Schwarz 1994, Feng and Kubik 1996, Huff and Gallagher 1996, Morse et al. 1996), but still not yet widely used. NGS and CS, which are offices of NOS, National Oceanic and Atmospheric Administration, propose to transfer this technology to the Port Authority of Oakland, California. NOS would provide the technical personnel to set up and conduct a demonstration of this application on large container ships 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 project for real-time GPS heights on marine vessels consists 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 in the GPS coordinate system; and (4) relate the real-time positioning of the ship to the bottom of the channel.

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 visibility-related harbor delays. This project is a major component in realizing electronic charts for navigation.

Phase 1

The purpose of this phase of the test is to determine the most cost-effective procedure for measuring accurate GPS heights in real-time on marine vessels. This application of GPS technology has been successfully tested on many types of vessels and vehicles (El-Mowafy and Schwarz 1994, Feng and Kubik 1996, Huff and Gallagher 1996, Morse et al. 1996). This test will obtain GPS data and use post-processing techniques to determine an accurate relationship of vessel dynamic draft as a function of ship's speed for underkeel clearance and an independent measurement of vessel roll and pitch, with the ultimate goal of performing this process in real-time.

The first phase will incorporate a set of GPS systems. Each set consists of GPS receivers, antennas, associated cables, and DC power sources. It is expected that the equipment will be placed on test vessels prior to departure from the Port of Long Beach bound for the Port of Oakland. The equipment will collect data from the GPS satellite constellation beginning with its departure from Long Beach and ending when the vessel completes docking at the Port of Oakland. In San Francisco Bay, two GPS receivers, located within 2 kilometers of each other, will collect data while the ship approaches San Francisco until it completes docking at the Port of Oakland. These two base stations will be used to validate the GPS carrier-phase ambiguities that were fixed to integers using the on-the-fly (OTF) technique (Remondi and Hilla 1993).

GPS equipment will be placed on the vessels so several equipment configurations can be tested. This will allow for the ultimate selection of specific equipment configurations that can best fit a variety of requirements for individual vessel operations.

The GPS equipment will be deployed on the vessels in some combination of the following:

• a set of four GPS systems; two near the bow and two near the stern of the vessel and
• two Attitude Determination Units (ADU); one each at the bow and stern of the vessel.

An ADU is simply a single GPS receiver connected to four antennas set in a "+" configuration. The primary receiver/antenna configuration measures vessel speed and positions, while the additional antennas allow for determination of pitch, roll, and heading.

GPS signals are affected by obstructions which either block the signal from the satellite to the antenna or which can cause the signal to be reflected enroute to the antenna. This latter concern is called multipath. (Multipath is similar to "ghost" images on a television screen.) Such obstructions will have to be minimized or, if possible, eliminated. Placement of the antennas is thus a critical part of the test. The GPS receiver should be placed less than 30 meters from the antenna or else additional preparations will have to be incorporated. The GPS signal is relatively weak and amplifiers would have to be added to the antenna cable if the cable is too long.

GPS data will be post-processed ("after the fact") by NOS personnel in its Silver Spring, Maryland, headquarters. Some preliminary data processing will be accomplished within about

1 day of the data acquisition to demonstrate some of the GPS capabilities. Additional vessel-log data such as independently obtained speed, heading, pitch, and roll data should also be made available to NOS. Since the GPS data are accurately time-tagged, the data will provide an independent verification of vessel-logged data. At Silver Spring, NOS will process the data using a variety of equipment configurations. The benefits of these different configurations will be described so vessel operators can determine the configuration that best suits their needs.

NOS anticipates close cooperation and "hands on" involvement with its partners in the planning and execution of the test. NOS can also provide training in GPS theory and operation to facilitate transfer of the technology to users of real-time GPS operations.

NOS personnel have discussed the project with several manufacturers of GPS equipment: Ashtech Inc. , Leica Inc., NovAtel Communication Ltd., and Trimble Navigation Ltd. It appears that each has equipment currently available which could be used for this phase of project.

Phases 2-4

Phase 2 will be similar to phase 1, except it will be performed in real-time and the GPS configuration will be based on the results of phase 1. Phase 2 will involve obtaining GPS data simultaneously with receivers at ground points with accurately determined positions and will test the radio transmission and incorporation of all the data sets aboard the test vessels. We expect that this testing will be done on container ships and other vessels as well, perhaps U.S. Coast Guard (USCG) vessels.

Phase 3 will acquire information necessary to describe the location and shape of the navigational channel, supporting Physical Oceanographic Real-Time System (PORTS) water levels and current information concurrent to acquiring GPS phase data on hydrographic vessels equipped with GPS. GPS data obtained in the test must be related to a consistent, uniform ground network of latitude, longitude, and ellipsoid height. Results from dredging operations and bathymetry mapping surveys that were positioned using GPS, where the ellipsoid height of the bottom of the Bay can be accurately obtained, will be used in Phase 3. Ultimately the entire shipping channel in the Bay will have to be accurately mapped using survey vessels that are positioned using GPS.

Phase 4 will relate the real-time positioning of the ship to the bottom of the channel.