The Wave Glider: A New Concept for Deploying

Ocean InstrumentationJustin Manley and Scott Willcox

The Wave Glider is a new class of persistent ocean vehicle. Development of the Wave Glider vehicle began in 2005 with a vision of enabling new types of ocean

observations independent of costly deep-water moorings or ship operations. Encouraged by immediate success with early prototype designs, Liquid Robotics, Inc. was founded in 2007 to further develop the platform for scientific, commercial, and military applications. The key innovation of the Wave Glider is its ability to harvest energy from ocean waves to provide essentially limitless propulsion. This provides an entirely new approach to deploying ocean instruments and thus enables new concepts of operations for ocean applications.

Wave Glider is a combination sea-surface and underwater vehicle comprised of a submerged “glider” that is attached to a surface float via a flexible tether. Fig. 1 shows the Wave Glider. It is propelled by the conversion of ocean wave energy into forward thrust, independent of wave direction. The wave energy propulsion system is purely mechanical; no electrical power is generated by the propulsion mechanism. Just as an airplane’s forward motion through the air allows its wings to create an upward lifting force, the submerged glider’s vertical motion through the comparatively still waters at the glider’s depth allows its wings to convert a portion of this upward motion into a forward propulsion force. As waves pass by on the surface, the submerged glider acts as a tug pulling the surface float along a predetermined course and is controlled by a single rudder on the glider. Separation of the glider by 23 ft (7m) depth from the float is a crucial aspect of the vehicle design. Fig. 2 shows the operating principles.

There is substantial power available in ocean waves, and the Wave Glider harnesses this power to maintain an average forward speed of 1.5 knots (2.8 km/h) in typical seas with one to three foot (0.3 to 0.9 m) waves. The Wave Glider’s forward speed is dependent upon the amplitude of the surface waves, the overall buoyancy force provided by the float, and the glider’s weight. Sea State 0 (0 m waves) has been observed to yield speeds of 0.25 to 0.5 knots (0.47 to 0.94 km/h) while Sea State 3 (1 m waves) and higher can result in speeds exceeding 1.5 knots (2.8 km/h). The Wave Glider’s mass and buoyancy

and the length of its tether have been tuned to provide excel-lent wave-energy propulsion performance in both energetic and calm seas.

While wave energy provides propulsion, Wave Glider uses two photovoltaic solar panels, each rated to deliver up to 43 W of peak power, to generate electricity for navigation, control, communications, and payload systems. These are harnessed to 665 W hours of rechargeable lithium-ion batteries to ensure continuously available power. This battery subsystem consists of seven smart battery packs that are electrically isolated from

Fig. 1. This Wave Glider photo shows the surface float (red) and glider (white) beneath–note the wake caused by motion through the water despite the low wave height.

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The Wave Glider: A New Concept for Deploying

Ocean InstrumentationJustin Manley and Scott Willcox

each other. Only two batteries are in use at any given time, and each battery has separate discharging and monitoring circuitry. The Wave Glider’s navigation, control, and com-munications systems require only 0.7 W of (average) con-tinuous power. The longest Wave Glider mission duration without a battery recharge (i.e., without the benefit of the solar panels) is about 23 days. This duration would decrease further with more payload sensors aboard.

In practice, the average continuous power delivered by the solar panels is substantially less than the combined 86 W of peak output power. Latitude and the season of the year have a significant effect on the power generated by the vehicle’s solar panels. There are also several other factors that influence the overall average continuous power produced by the solar panels including light level, temperature, shading, fouling, and conversion and storage efficiencies. Together, these factors reduce the (averaged) continuous power available to payloads to approximately 10 W. While just a few Watts is sufficient for many payloads – such as cameras and oceanographic sensors – additional development effort will increase available payload power to realize the full potential of the Wave Glider platform, particularly when operating in higher latitudes. Liquid Robotics is exploring concepts to harvest wave energy on a small scale for conversion to electric power to meet these needs. Also, a primary cell payload has been devised that provides over five kilowatt hours beyond the base battery capacity. With this option in place, missions of several months without benefit of solar energy are envisioned. Meanwhile the base power system configuration is proving beneficial in various real-world settings.

The Wave Glider employs a 12-channel GPS receiver as its primary navigation sensor and carries a tilt-compensated magnetic compass with three-axis accelerometers. Some vehicles also carry an optional water speed sensor, allowing for short-term dead reckoning. The Wave Glider’s typical navigation accuracy is better than three meters. The Wave Glider navigates autonomously to reach waypoints and to keep station. A Wave Glider’s course can include a series of

closely spaced waypoints (tracklines) where precise transit lines are required. Station keeping around a single waypoint of interest is also possible.

Wave Gliders are controlled via a simple web-based command and control interface. Each Wave Glider vehicle communicates with the shore-based web server by initiating an Iridium modem messaging session, which is received at an Iridium network ground station where the data is redirected onto the Internet. These sessions occur at configurable intervals, typically every five and fifteen minutes. Using the web-based interface, any number of operators (with the appropriate authorizations) can control any Wave Glider ve-hicle from any Internet-enabled computer, PDA, or cell-phone. Similarly, subscribers can monitor vehicle status and data on an as needed basis. Wave Gliders can also carry short-range, high-bandwidth radio modems for local area communications and acoustic modems for subsea telemetry.

Performance in Sea TrialsOver the past four years, Liquid Robotics has undertaken a vigorous series of sea trials, driving prototype and product-level vehicle development programs. The current production generation of the Wave Glider is the beneficiary of over seven years of combined sea time.

Since 2006, Wave Gliders have collectively traversed over 128,000 km. The longest single deployment was over 500 days. The longest point-to-point journey exceeded 5000 km.

Liquid Robotics, with U.S. Coast Guard permission, main-tains a test range in a one-mile by one-mile area offshore from Puako on the Big Island of Hawaii. Initial engineering trials and operational/acceptance testing of customer deliveries are conducted inside the Puako test range. Once trials have been satisfactorily completed, additional testing to expose the Wave Glider to a wider range of sea conditions is conducted farther offshore.

The Wave Glider vehicle “Red Flash” circumnavigated the Big Island of Hawaii, January 9-18, 2009, a 343 nautical mile (635 km) mission that the vehicle completed in 9 days, 2 hours with an average speed over ground of 1.57 knots (2.9 km/h). Sea conditions during this mission were estimated to be 10 ft

Fig. 2. The operational principles of the Wave Glider

The key innovation of the Wave Glider is its ability to harvest energy from ocean waves to provide essentially limitless

propulsion.

December2010 IEEE Instrumentation & Measurement Magazine 9

(3 m) seas and 15 knot (27.8 km/h) winds. A similar engineer-ing trial and demonstration was conducted between August 13 and September 23, 2009. The same vehicle, Red Flash, transited from Monterey Bay along the west coast of North America to Alaska. This mission lasted 41 days and covered just over 1300 nm (2407 km) for an average speed over ground

of 1.5 kn (2.8 km/h). During this mission severe weather exposed Red Flash to winds above 40 knots (74 km/h), swells exceeding 18 ft (5.5 m) and peak wave heights of over 21 ft (6.4 m). Red Flash endured these conditions and maintained core functionality. The vehicle was recovered for inspection and the antenna mast was found to have broken. As core ve-hicle antennas (GPS and Iridium) are directly deck mounted, this did not endanger the Wave Glider itself. Improved masts for payload sensors requiring elevation are currently being designed based upon these recent results. Fig. 3 shows “Red Flash” off the Pacific Northwest. Fig. 4 shows the long-range Wave Glider missions in 2009.

Additional long-term missions, including multi-month, basin-scale ocean transits, are currently underway. Finally, as an engineering endurance trial, one Wave Glider, “Stripes,” is conducting a station keeping mission in the Puako test range, augmented with frequent transits out into the open ocean. This Wave Glider demonstrated more than a full year of continuous operation on December 16, 2009. After a brief recovery to the deck of a support vessel and a brief cleaning of the hull, the vehicle was returned to the ocean. On May 8, 2010 the vehicle was returned to shore for maintenance after 508 days at sea.

Fig. 3. The Wave Glider “Red Flash” off the Pacific Northwest

Fig. 4. Major Wave Glider missions of 2009. Map credit: © 2009 Google; © 2009 Europa Technologies Data S10, NOAA, U.S. Navy, NGA, GEBCO; © 2009 Tele Atlas, US Dept. of State Geographer. The distinctive overlay of routes and labels on the Google map of the Pacific Ocean and bordering land masses is by Liquid Robotics, Inc.

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In operations to date in the Pacific Ocean (off Hawaii and the west coast of North America), the Gulf of Mexico and Atlantic Ocean, the Wave Glider has proven resistant to bio-fouling and entanglement. Subsea fouling has been observed in long-term tests off Hawaii, but performance impacts have been modest. Biofouling of the solar panels is mitigated by a constant wash due to the low freeboard. The panels never dry enough to become encrusted or become submerged enough to accumulate marine growth. Kelp has been encountered off California with mixed results. Many times Wave Gliders have safely passed through regions of kelp. In one instance, a Wave Glider entered a significant kelp bed and became entangled. By the time operators on a vessel reached it, the vehicle had worked itself free. The ocean environment is vast and diverse. Some regions will pose more significant hazards than others, but in most cases the Wave Glider has functioned quite well.

The Wave Glider vehicle has been designed to withstand extreme seas. Ultimately, the Wave Glider’s endurance is limited only by its robustness, as its propulsion power is effectively unlimited. In Hurricane Flossie in 2007 and during the 2009 Red Flash deployment, the Wave Glider demonstrated its ability to weather high seas and severe winds. The opposite extreme of very calm seas represents a greater challenge to successful Wave Glider operation. Without wave energy to harvest, the Wave Glider would not be able to maintain course and may not be able to keep station. Fortunately, the ocean is rarely so calm, and when it is, it rarely remains calm for long. Wave Gliders have been designed to make significant headway even in very mild seas (i.e., with wave heights of a few inches, ~ 5 cm, or less). Even in these extremely calm conditions, the Wave Glider is able to maintain a forward speed of 0.25 to 0.50 knots (0.13 to 0.26 m/s). This speed is usually sufficient to allow the vehicle to keep station against typical surface currents.

Demonstrated Oceanographic ObservationsThe Wave Glider has modular mechanical, electrical, and software interfaces to accept a wide variety of payloads. All command and control, communications, and navigations electronics are contained in a core electronics module, which also houses the batteries and their charging electronics. Dedicated forward and aft payload compartments house most payload sensor systems and support electronics. These compartments can be fitted with watertight dry boxes or left open to splash and wash. There is ready access to surface waters for instrument probes. While the topside float provides a large

volume and simple integration options, the subsea glider can also support payloads. The tether between the float and glider carries power and communications. A subsea payload compartment is an available option, and an industry standard connector is available for ease of integration. This configuration makes the system well suited to a variety of ocean instruments.

To date, several payload systems have been demonstrated on the Wave Glider, including passive hydrophones and towed hydrophone arrays, marine weather stations, still and video cameras, and acoustic Doppler current profilers (ADCPs) which measure the motion of water particles relative to the sensor. Fig. 5 shows an ADCP attached to a Wave Glider. The current generation of the Wave Glider has also demonstrated towing an instrumented buoy that was itself towing an acoustic modem at the end of a long cable. More recently, an acoustic modem and its support electronics have been integrated onto the Wave Glider float, eliminating the need to tow the hydrophone payload.

This payload flexibility has been applied to a variety of demonstration programs. Among other applications Wave Gliders have recently completed:

◗ a demonstration of a seafloor to surface acoustic link for the National Oceanic and Atmospheric Administration (NOAA) tsunami warning network,

◗ deployment of the Scripps Institution of Oceanography (SIO) high frequency acoustic recorder (HARP) for marine mammal monitoring,

◗ deployment of a 600 kHz RDI Sentinel ADCP for surface current monitoring, and

◗ deployment of a conductivity and temperature sensor for physical oceanography in Monterey Bay.

Tsunami warning stations consist of a seafloor pack-age, known as a bottom pressure recorder (BPR), which communicates with a surface mooring via acoustic signals. Unfortunately, the surface moorings experience occasional failures. When these occur in remote locales, the costs to repair or replace the buoy rise dramatically due to the long voyage of

Fig. 5. The Wave Glider has deployed common oceanographic instruments such as the acoustic Doppler current profiler (ADCP) shown here.

The ocean environment is vast and diverse. Some regions will

pose more significant hazards than others, but in most cases the Wave

Glider has functioned quite well.

December2010 IEEE Instrumentation & Measurement Magazine 11

a support vessel. In a demonstration program, Liquid Robot-ics has placed the telemetry systems commonly used on a tsunami buoy (undersea acoustic modems and satellite links) on a Wave Glider. This configuration has been demonstrated to provide connectivity to the BPR and thus offers a cost effective alternative to conventional moorings.

Passive acoustic systems are often used to monitor ocean regions for the presence of marine mammals. As with tsunami warning systems, conventional approaches make use of fixed moorings to deploy these undersea microphones and their associated recorders. Wave Gliders have been configured to carry such instruments, and field results are promising. A series of data sets collected off Hawaii in 2009 and 2010 are currently being reviewed. The results of this analysis were presented at the IEEE/MTS Conference, OCEANS 2010 in Seattle, Washington.

Ocean currents are of major interest to scientists and ocean operators including those related to the offshore oil and defense industries. A key tool in monitoring these currents is the ADCP, which uses four acoustic beams to measure particle motion in the water column. Wave Gliders have been configured to carry ADCPs and have demonstrated successful current monitoring in the open ocean.

One of the most common oceanographic data require-ments is the conductivity temperature depth (CTD) profile. This provides a vertical record of the salinity and temperature of the water column and is one key to understanding ocean physics. While the Wave Glider does not currently provide for a vertical measurement, it has deployed industry-leading sensors to collect horizontal profiles of conductivity and temperature distribution. By providing precise position-ing and real-time connectivity while collecting such data, the Wave Glider provides a significant new capability to oceanographers.

A New Paradigm for Ocean ObservationAs a persistent ocean platform, the Wave Glider complements fixed buoys, undersea vehicles, drifting floats and vessels. Fig. 6 shows a Wave Glider alongside a fixed buoy. The ocean is vast, and observing requirements are not met by current platform availability. Regions beyond traditional shipping routes are especially difficult to access. Wave Glider ’s long-range capability and persistent station-keeping make it well suited to observing in these regions. In sufficient numbers, Wave Gliders can provide oceanographic data from transects across ocean basins. In 2009, this capability was demonstrated when two Wave Gliders crossed the Pacific Ocean from Hawaii to California. Fig. 4 shows their paths. Additionally, areas of significance (e.g. upwelling sites, seamounts, marine protected areas) can be observed by station keeping Wave Gliders serving as virtual moorings. Mobile and stationary Wave Gliders, operating for up to one year without service, will greatly increase the number of platforms observing the ocean as well as their temporal and spatial distribution.

Wave Gliders are affordable, with a favorable capital cost compared to moorings. While their capital costs are modest, the potential savings in operations and maintenance are noteworthy. Wave Gliders do not require large vessel support. Liquid Robotics runs most operations from a 30 ft (9.1 m) rigid hull inflatable boat. This is more than adequate to move vehicles beyond the immediate coastal region into waters deep enough for effective Wave Glider operation, where the vehicles take over their own “deployment.” Launches directly from shore have been demonstrated. Recovery is equally straightforward. During the Red Flash mission described above, the vehicle was recovered by a commercial fishing vessel. The captain and crew safely and effectively recovered the vehicle without any direct on-site support from Liquid Robotics staff. The use of vessels of opportunity to support Wave Glider operations is the anticipated norm. The smaller vessels required for these operations and the rarity of launch and recovery requirements present a significant cost savings

Fig. 6. Wave Gliders complement traditional ocean observing platforms such as fixed moorings.

Liquid Robotics has placed the telemetry systems commonly

used on a tsunami buoy (undersea acoustic modems and satellite links) on a Wave Glider. This configuration has been demonstrated to provide

connectivity to the BPR and thus offers a cost effective alternative

to conventional moorings.

12 IEEE Instrumentation & Measurement Magazine December2010

over conventional ocean platforms dependent on full-size, and costly, research vessels.

The Wave Glider is a cost effective platform and also quite versatile. With significant experience derived from engineer-ing trials and scientific demonstration programs, Liquid Robotics is ready to apply the persistent ocean presence of the Wave Glider to ocean observation.

AcknowledgmentsThe Wave Glider has benefited from the support of customers and collaborators. The authors express their appreciation to their colleagues at Liquid Robotics and to financial and in-kind sponsors of projects discussed here. These include, but are not limited to, The Jupiter Foundation, Teledyne RDI, SAIC, The National Oceanic and Atmospheric Administration and Scripps Institution of Oceanography.

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Justin Manley (justin.manley@liquidr.com) holds MS and BS degrees in Ocean Engineering. He has taken unmanned technologies to sea in expeditions across the globe. Mr. Manley is Director of Scientific and Commercial Business at Liquid Robotics. He is the Vice President of Government and Public Affairs for the Marine Technology Society and a Senior Member of IEEE.

Scott Willcox holds MS degrees in Electrical Engineering and Ocean Engineering and a BS degree in Electrical Engineering. He was a founder and Chief Technology Officer at Bluefin Robotics Corp. Mr. Willcox joined Liquid Robotics in May 2009 and serves as the Director of Defense Business and Principal Technologist.

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