Long Term Autonomous Ocean Remote Sensing Utilizing the Wave Glider

Rising costs of ship time and increasing budgetary restrictions make installation and maintenance of fixed, mid-ocean buoys a logistical and financial challenge. The cost associated with launch, recovery, and maintenance has resulted in a limited number of deployed buoys, restricting data on mid-oceanic conditions. To address these challenges, Liquid Robotics (LRI) has developed the Wave Glider, an autonomous, mobile remote sensing solution. This system utilizes wave energy for propulsion allowing for long duration deployments of up to one year while providing real-time data on meteorological and oceanographic conditions. In November 2011, LRI deployed four Wave Gliders on a mission to cross the Pacific Ocean (the PacX) from San Francisco to Australia (two vehicles) or Japan (two vehicles) while transmitting data on weather conditions, wave profiles, sea surface temperatures, and biological conditions (fluorometry) in real-time. This report evaluates the vehicle's ability to operate as an ocean going data platform by comparing data from the onboard weather sensors with two moored buoys, NDBC 46092 in Monterey Bay and NDBC 51000 200 nmi northeast of Maui. The report also analyzes data transmitted from all four vehicles as they passed directly through a tropical storm 580 nmi northeast of Hawaii.

The route traveled from San Francisco to Australia directed all four vehicles past the two aforementioned NOAA buoys. Upon arriving at a buoy, gliders continuously circled for a period of two days at a distance of three to eight nautical miles to build a comparative dataset. Data from both platforms were streamed in near real time enabling mid-mission evaluation of the performance of sensors. Overall, results varied from a <0.5% difference in barometric pressure between buoy NDBC 46092 and the gliders to high disagreement in wind speed and direction. While comparisons to moored buoy data can provide valuable insight into the relative accuracy of each platform, differences in agreement on variables such as wind speed and direction were attributed to micro-spatial variability in oceanic conditions. Also, disagreements in sea surface temperature were attributed to the buoys reporting the hull temperature of the mooring and not a true in-situ water temperature.

In addition to data collected for comparison between existing moored buoys, all four PacX vehicles collected data from directly within a tropical storm off the coast of Hawaii. Starting on February 5th, 2012, the vehicles measured sustained winds of 40 knots for 4 days with gusts up to 80 knots at the height of the storm. The vehicles also measured sustained wave heights of 7m along with a barometric pressure drop to a low of 985 mbar. A pressure between 965 and 979 mbar is comparable to that of a category two hurricane while the measured wind speeds fall within the range of a tropical storm on the SSHS. The wind data from the vehicles compares favorably to satellite imagery from the ASCAT satellite data of the same storm but with much higher spatial resolution.

In conclusion, the Pacific crossing has provided solid evidence that the Wave Glider as deployed would provide a suitable and highly efficient platform for the observation of sea surface and lower atmospheric conditions over extended sampling periods. The system could be used to quickly and efficiently increase the operational density of ocean observations without the need for expensive deployment and recovery vessels. In future studies, data from the PacX will be compared with additional satellite and oceanic data sources to provide ground truthing of collected oceanographic data. In addition, two Wave Gliders will be deployed from Puerto Rico to monitor storm conditions from directly within a hurricane.