GPS-tracked buoy for hydrographic survey applications

In producing nautical charts, acquisition of accurate three-dimensional water surface data is important to reduce hydrographic survey soundings to chart data. The NOAA National Ocean Service (NOS) is responsible for producing nautical charts along U.S. coastal waters for safe and efficient maritime commerce. These waters include about 500,000 square nautical miles of the U.S. Exclusive Economic Zone (EEZ). Traditionally, water levels are measured at structures on, or close to, shore and their use often requires analytical tide zoning techniques to extend survey coverage. In order to further improve the accuracies of sounding to chart datum reductions and to increase the efficiency and coverage for hydrographic surveys, a new GPS-tracked water level buoy system has been developed through the NOAA Small Business Innovation Research (SBIR) Program. This water level measurement system consists of a portable spherical buoy and a shore base station. Utilizing real-time kinematic (RTK) differential GPS techniques, accurate water level information are collected in real-time at six minute intervals complying with NOS water level measurement standards. The buoy's motion sensors provide automated corrections for wave-induced buoy motions. Several field tests conducted near NOS Next Generation Water Level Measurement System (NGWLMS) stations show that the root-mean-square (RMS) difference between the two systems is approximately 2 cm. The buoy system also can monitor surface waves. The system's portability and accuracy offer possibilities for other applications in coastal waters. This paper describes the GPS-tracked buoy system including its design and features, provides example field test results, and notes potential future modifications to transition the technology to operational use.