High fidelity hydrographic surveys using and autonomous surface craft

Abstract

There is a vital need to update the hydrographic database of the United States. NOAA statistics show that with current survey technologies it will take nearly 40 years to update U.S. nautical charts. Hydrographic surveys require a careful record of depth, position, tide, and the motions of the survey platform. The U.S. Army Corps of Engineers is one highly regarded organization which performs hydrographic surveys. They impose a strict standard of accuracy for certain surveys. For these Class 1 surveys, position must be within 6 meters and depth must be measured within 0.5 feet. This thesis documents the development of a new technology to meet these needs and provide hydrographic surveys in more cost effective ways than existing techniques. Since 1993 Autonomous Surface Craft (ASC) have been under development at the MIT Sea Grant College Program. Hydrographic surveying was the first practical mission approached by an ASC. The ASC ARTEMIS used simple navigation and control systems and a basic recreational depth sounder to demonstrate the possiblity of performing surveys with ASC. This background led to the developments presented here. This project had two goals, the first was to develop an ASC which was better suited for hydrographic surveys than ARTEMIS. This required designing and constructing a new ASC with improved endurance, speed, payload, and stability. This goal was met with the development of the ASC ACES (Autonomous Coastal Exploration System). The development of ACES and its preliminary field tests, which provided a hydrographic survey which was 78% Class 1, are documented in this work. The second goal of this effort was to configure the new ASC for high fidelity hydrographic surveys. This required selection of new sensors to measure position, depth, tide, and the motions of the ASC. Conventional systems were evaluated and a final design was selected which incorporated the latest developments in the application of the Global Positioning System (GPS). By using GPS sensors to account for all variables except depth, ACES is able to meet the high standards of a Class 1 survey. Using an Acoutisc Doppler Current Profiler to measure depth provides ACES with a high quality and versatile sensor to employ in such surveys. This project has demonstrated the potential for ASC to be used in the field of hydrographic surveys. ACES, A system capable of providing high fidelity hydrographic surveys to meet the needs of the U.S. survey community has been designed and built. This system has matched the USACE surveys with 78% accuracy in a prototype configuration. The final high fidelity survey configuration of ACES will provide Class 1 or better surveys more cost effectively than manned survey vessels.