The long-term goal of this project is to develop a cost-effective means to observe the ocean in three dimensions and time. Much of what is known about ocean circulation comes from ship-based transects and time series from fixed locations. Both methods suffer from limited resolution in either time or space. Our goal is to develop a means to measure profile time series inexpensively enough that many locations can be observed simultaneously, as if dozens or hundreds of moorings reported data through a large range of depths.
The technical objective of this project is to develop a small network of comparatively inexpensive autonomous vehicles that can collect temperature and salinity profiles continuously from the upper km or so of the ocean simultaneously while maintaining their geographic positions. this network is to report measurements in near real-time and be controllable from shore to alter sampling strategy. To be scientifically useful, the network must be able to operate unattended for at least several months.
Our approach is to design, construct and test a prototype battery-powered ocean glider carrying a CTD package as payload. This vehicle, which we call the Virtual Mooring glider, will be able to stem ambient currents of up to about 25 cm/s relatively efficiently. To achieve the desired mission lifetimes, we have chosen to use a low-drag laminar flow hull shape to which wings are appended to provide lift, a nearly neutral compressible pressure vessel, and low power electronics. The overall power consumption of the vehicle needs to be a fraction of a watt in order to meet the endurance requirement.
The Virtual Mooring will dive from the surface along a glide path to maintain a horizontal position within a modest tolerance. The station-keeping ability o f the vehicle will be superior to that of the upper portion of a conventional surface mooring. That is, the profiles will be as close to vertical as possible. At depth, the vehicle will pump a small amount of oil into an external bladder to make itself buoyant and return to the surface. Upon reaching the surface, the vehicle will extend an antenna above the surface to determine its position from GPS and dive along a newly-determined trajectory. Every several profiles, the vehicle will spend an extra minute or two at the surface to make a cellular telephone call to transmit data and receive any updated instructions on sampling strategy.
Virtual Mooring gliders will be reusable and relatively inexpensive. The unit cost will be about $45K for "one-off" production or as little as half this in quantity. The vehicles are designed to be maintainable by technical staff of average skill.
A prototype vehicle ;is being constructed for test in local waters, first Lake Washington, then Puget Sound, then Monterey Bay: all areas in range of land-based cellular telephone networks. When worldwide cellular telephone service is available (projected as soon as 1998), the vehicle can be used in the open ocean.
The first open-ocean test selected for us is a severe one: the Labrador Sea in winter. Communication there will require higher power because it is to precede the commercial debut of worldwide cellular telephone service. The environment there will severely test the capabilities of our glider.
The experience gained in testing of the prototype will allow us to incorporate changes in the design and construction of several more gliders. The small fleet will then be applied to a limited demonstration experiment.
The hydrodynamic performance of the vehicle shape chosen has been verified with wind tunnel tests on a model. Glide slopes of as little as 0.2 to 0.125 appear attainable while maintaining laminar flow over most of the vehicle. Attack angles of up to 8 degrees can be used. That is, the vehicle will be able to stem currents several times its ascent or descent rate, thus conserving power. A nearly neutrally compressible pressure hull and low power electronics are incorporated into the final design drawing package now being assembled. Fabrication of the prototype is underway. A GPS antenna that can be protected from pressure has been developed.
The development tasks in this project are on track after just over one year of support.
Virtual Mooring gliders should have an enormous impact on how the ocean is sampled. Everything from routine CTD casts from ships to basin-scale monitoring of climate change will be made more cost effective by this technology.
This technology is expected to be transferred to the commercial sector when development is complete.
This project is part of the MURI/AOSN program