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FEATURE ARTICLE

The Robot Ocean Network

Automated underwater vehicles go where people cannot, filling in crucial details about weather, ecosystems, and Earth’s changing climate.

Oscar Schofield, Scott Glenn, Mark Moline

Ocean Workhorses

The most vital component of the rapidly growing ocean sensor network is the AUV. These devices come in various types, carry a wide variety of sensors, and can operate for months at a time with little human guidance, even under harsh conditions.

2013-11SchofieldF3.jpgClick to Enlarge Image Underwater robotics has made major advances over the past decade. Key technological gains include an affordable global telecommunication network that provides sufficient bandwidth to download data and remotely control AUVs from anywhere on the planet, the miniaturization of electronics and development of compact sensors, improved batteries, and the maturation of platforms capable of conducting a wide range of missions.

The AUVs being used in the ocean today generally come in three flavors: profiling floats, buoyancy-driven gliders, and propeller vehicles. In the first category, the international Argo program has deployed more than 3,500 relatively inexpensive profiling floats (costing about $15,000 each) throughout the ocean, creating the world’s most extensive autonomous ocean network. These 1.3-meter-long platforms decrease their buoyancy by pumping in water, sinking themselves to a specified depth (often more than 1,000 meters), where they remain for about 10 days, drifting with the currents. The floats then increase their buoyancy by pumping out water, and rise to the surface. During the descent and ascent, onboard sensors collect vertical profiles of ocean properties (such as temperature, salinity, and a handful of ocean color and fluorescence measurements). New chemical sensors to measure pH and nutrients are also available. Data are transferred back to shore via a global satellite phone call. After transferring the data, the floats repeat the cycle.

Profiling floats are incapable of independent horizontal travel, leaving their movements at the mercy of the currents, but they are extremely efficient. A single battery pack can keep a float operating for four to six years. The combined data from large numbers of floats provides great scientific value, offering a comprehensive picture of conditions in the upper 1,000 meters of the ocean around the globe. When these data are combined with global satellite measurements of sea-surface height and temperature, they allow scientists to observe for the first time climate-related ocean variability in temperature, salinity, and circulation over global scales.

2013-11SchofieldF4.jpgClick to Enlarge Image Cousin to the profiling floats are the buoyancy-driven gliders, which were highlighted in Stommel’s original vision of a networked ocean. Several different types exist, but generally they are 1 to 2 meters long and maneuver up and down through the water column at a forward speed of 20 to 30 centimeters per second in a sawtooth-shaped gliding trajectory. They operate by means of a buoyancy change similar to that for floats, but wings redirect the vertical sinking motion due to gravity into forward movement. A tailfin rudder provides steering as the glider descends and ascends its way through the ocean, which makes these devices more controllable than the floats. They are more expensive, however, costing around $125,000. Therefore, they are often deployed for specific scientific missions.

A glider’s navigation system includes an onboard GPS receiver coupled with an attitude sensor, a depth sensor, and an altimeter. The vehicle uses this equipment to perform dead reckoning navigation, where current position is calculated using a previously determined position, and that position is then updated based on known or estimated speeds over elapsed time and course. Scientists can also use a buoyancy-driven glider’s altimeter and depth sensor to program the location of sampling in the water column. At predetermined intervals, the vehicle sits on the surface and raises an antenna out of the water to retrieve its position via GPS, transmit data to shore, and check for any changes to the mission.

Because their motion is driven by buoyancy, the gliders’ power consumption is low. They can coast for up to year on battery power. These robots are also modular: Researchers can attach sensors customized to one particular science mission, and then remotely reprogram what the sensors are searching for in near real time, based on collected data.

2013-11SchofieldF5.jpgClick to Enlarge Image The most advanced, but also the most expensive, underwater robots are the propeller-driven AUVs. Costs can range from $50,000 to $5 million, depending on the size and depth rating of the AUV. They are powered by batteries or fuel cells, and can operate in water as deep as 6,000 meters. Like gliders on the surface, propeller AUVs receive a GPS fix and relay data and mission information to shore via satellite. While they are underwater, propeller AUVs navigate by various means. They can operate inside a network of acoustic beacons, by their position relative to a surface reference ship, or by an inertial navigation system, which measures the vehicle’s acceleration with an accelerometer and orientation with a gyroscope. Travel speed is determined using Doppler velocity technology, which measures an acoustic shift in the sound waves that the vehicle bounces off the seafloor or other fixed objects. A pressure sensor measures vertical position.

Propeller-driven AUVs, unlike gliders, can move against most currents at 5 to 10 kilometers per hour, so they can systematically measure a particular line, area, or volume. This ability is particularly important for surveys of the ocean bottom and for operations near the coastline in areas with heavy traffic of ships and small crafts.

Most AUVs in use today are powered by rechargeable batteries (such as lithium ion ones similar to those in laptop computers). Their endurance depends on the size of the vehicle as well as its power consumption, but typically ranges from 6 to 75 hours of operation under a single charge, with travel distances of 70 to 400 kilometers over that period. The sensor cargo they carry also depends on the size of the vehicle and its battery capacity.

Because of the additional power of propeller AUVs compared to gliders, they can run numerous sensor suites, and they remain the primary autonomous platform for sensor development. Hundreds of different propeller AUVs have been designed over the past 20 or so years, ranging in size from 0.5 to 7 meters in length and 0.15 to 1 meter in diameter. Most of these vehicles have been developed for military applications, with a few operated within the academic community. By the end of this decade, it is likely that propeller AUVs will be a standard tool used by most oceanographic laboratories and government agencies responsible for mapping and monitoring marine systems.




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