Page 56: of Marine Technology Magazine (March 2015)
Oceanographic Instrumentation: Measurement, Process & Analysis
Seaf oor Mapping de? nition cameras and powerful lighting to illuminate the ice from the base ship above; the second trails behind Nereid con- surface where the all-important algae cling. These are the very nected by a hose—an umbilical, as the engineers call it. base of the food web. Nereid will eventually boast a robotic Each canister acts like a ? shing reel with ? sh on the line: arm capable of plucking samples directly from the sea bot- Nereid moves, the tension increases and cable spools out. The tom, or taking samples from the ice. It will even be able to spool is so sensitive that surface tension between the water chemically test those samples while at depth using instru- and the cable is enough to tug the ? ber out of its cocoon. Un- ments mounted aboard the vehicle. like ? shing reels, however, once the ? ber is spooled out, it
Nereid also has many instruments designed to measure the cannot be reeled back in. The cable pays out over a wheel that environment around the vehicle, including Doppler-based keeps track of the distance meted out. sensors that are able to evaluate currents in the water. Oth- “It’s pretty exciting from an engineering standpoint. You end er sensors will monitor temperatures, as well as oxygen and up with a vehicle that is free to move great distances both ver- chlorophyll levels in the water column. Likewise, there are tically and horizontally,” Whitcomb says.
instruments able to measure the amount of light that makes This is important because the dangers are many under the it through the ice fueling the hungry blooms of algae with ice. Jagged ? oes are always nearby and at the bottom rock energy. outcroppings could present snagging hazards. Whitcomb
Of all the cutting-edge engineering that went into Nereid, notes, as well, that he cannot rule out the threat of “biological however, perhaps the most notable is the tethering system that aggression”—an animal masticating on or running into this uses a ? ber optic cable just a few times thicker than a human lightweight tether. In the face of these many risks, Nereid is hair. elaborately programmed with fail-safe mechanisms.
The tether carries high-de? nition video and navigational “If the tether gets severed during a dive, Nereid will auto- data back and forth to the mother ship at the speed of light. matically descend to a pre-programmed depth below the ice
This cable provides a gigabit Ethernet data stream to the sur- and use acoustic communications to signal its position. Then face, as well as a high-de? nition video feed that will allow the team sends acoustic commands to guide it back to its scientists to observe the under-ice world as never before and mother ship for recovery,” he says.
as close to ? rsthand as is presently possible.
Nereid’s lightweight expendable tether enables it to have Dead Reckoning horizontal and vertical mobility that is not possible with con- In an era of ubiquitous GPS positioning, it is hard to imag- ventional steel armored tethers, mostly used until now. These ine that navigation is an issue for Nereid, but the thick shroud far heavier steel cables severely limit horizontal mobility and of ice and water render GPS signals useless below the sur- restrict a vehicle’s depth range to about 7,000 meters. Beyond face. Nereid, therefore, must go old school and rely upon dead that length, McFarland says, the steel breaks under its own reckoning combined with other methods to ? x its position. Of weight. course, it does so with some sophisticated technology. Nereid “The ? ber optic system is based on the one we developed for is equipped with a north-seeking gyrocompass that helps track
Nereus, but the world below the ice presents some very dif- its heading. It also features upward- and downward-facing ferent and dif? cult challenges. The ice is often jagged, pitted, acoustic sensors that gauge speed. Whitcomb has been a key and deeply contoured—all of which present the possibility of leader in creating the navigation systems for Nereid and, be- snagging,” Whitcomb says. fore that, Nereus. The acoustic sensors bounce sound waves
Nereid’s tether is expendable. It is so thin and lightweight off nearby surfaces (such as the sea ? oor or the underside of that 12 miles of it can be wound into a single canister the size sea ice) and use the Doppler shift to calculate speed by listen- of a lunchbox, greatly improving both Nereid’s economics and ing closely to the returning signal. The returning sound waves its range. The ? ber costs about a dollar per meter, McFarland get altered—higher in frequency— says. While such a ? gure is not exactly inconsequential—a as the vehicle approaches an object, and then farther apart— single spool holds about 60,000 feet of ? ber—neither is it pro- lower in frequency—as the vehicle moves away.
hibitive, should Nereid need to cut and run. “It’s like when an ambulance passes and your perception of “We published ? ve or six papers on the spooling system the pitch of the siren changes as it goes by. This is the Doppler alone. The tether pays out from the spool, like a ball of twine,” shift in action. If you know the frequency of the outgoing sig-
Whitcomb says. “It’s a subtle design. The biggest challenge is nal, you can use this shift in frequency of the returning signal to not double back on yourself, which is pretty easy to avoid.” to calculate the vehicle’s speed,” McFarland says.
The ? ber itself is neutrally buoyant and simply hangs in the If Nereid is within 200 meters of the sea? oor, it is able to get water, free to move wherever Nereid, or the water currents, an acoustic ? x. Or, it can use the ice above as a guide, though takes it. The system employs two canisters, one suspended the ice is less accurate because it is in constant in motion. In a
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