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what's to eat?

Sink or swim
July 29, 2007 (posted July 30, 2007)
by Lonny Lippsett

The robotic underwater vehicle Jaguar was launched at 1 a.m. Sunday. Twenty-four hours later, it had not come home. It had aborted its mission more than 4,200 meters (13,779 feet) deep and was not responding to commands to resurface.

The autonomous underwater vehicle (AUV) team launched Jaguar on a mission to survey an area where evidence of hydrothermal venting had been found. It was programmed to go toward the seafloor, about 4,100 meters (13,451 feet) deep, and then level out and maintain an altitude of about 20 meters (65 feet) above the seafloor.

As a safety measure, the engineers programmed the vehicle to abort its mission if it exceeded the target depth of the seafloor—so if for some reason the vehicle began to go too deep, it would stop and not continue to drive even deeper.

In Sunday’s case, the AUV team factored in a margin of error of an additional 100 meters (328 feet). Jaguar was programmed to abort its mission if it flew deeper than 4,200 meters (13,779 feet).  During the mission, Jaguar’s depth sensor reported that it was 4,200 meters deep, so the vehicle aborted its mission, as it was programmed to do.

What happened? Measurements from the depth sensor can stray slightly from reality, and the error gets larger the deeper the vehicle goes down. In this case, the error was large enough that the sensor gave a false reading of 4,200 meters, triggering the abort command, said WHOI geophysicist Rob Reves-Sohn, the expedition’s chief scientist.

In retrospect, the AUV team might have given Jaguar more “wiggle room” before programming it to abort, Reves-Sohn said. But operating robotic vehicles to depths of 4,000 meters (2.5 miles) or more in the Arctic Ocean has never been attempted before this expedition, he said. Engineers and scientists are learning on the job.

By itself, aborting the mission should not have put Jaguar in a difficult spot, however. But the AUV team could not send acoustic commands to the vehicle, including a command to surface. 

Why? Jaguar was programmed to “lock in” to a certain depth above the seafloor to do its mission. Without the ability to send Jaguar new commands, the engineers could not override that original command with a new one to tell Jaguar to rise.

At a depth of 4,000 meters, the vehicle was near the limit at which it could receive acoustic signals. In addition, it was in a harsh, rocky terrain, said WHOI engineer Hanu Singh, head of the AUV team. Signals from the ship to Jaguar were reverberating off the rocky seafloor, which Jaguar was hovering just above, making it difficult for clear signals to reach the vehicle. Signals from the vehicle to ship, however, were transmitted up clearly through the water to the ship.

“We could hear Jaguar, but we could not talk to it,” Singh said.

What to do? Through the early morning hours Monday, engineers took out the acoustic modem—a device that sends and receives sound signals in seawater—from Jaguar’s sibling robotic vehicle, Puma. They attached it to the Camper vehicle, which will be sent to the seafloor Monday to look for hydrothermal vents.

Camper will have a dual mission. It will take Puma’s acoustic modem to the seafloor, where it can send signals from a much shorter distance to Jaguar.

 “It gives you a better shot at getting an acoustic command to the vehicle,” Reves-Sohn said, and getting the command to surface to Jaguar before 4:30 p.m. Monday (10:30 a.m. Eastern Time) is a high priority. At that time, roughly 36 hours after the start of Jaguar’s mission, it is programmed to “time-out,” shut off its thrusters, and float up to the surface. This is another feature to ensure that if something goes wrong, the vehicle will still return to the surface.

If Camper and Puma’s modem can successfully get a command to Jaguar before the “time-out,” Jaguar will still have use of its thrusters. It will not only come to the surface more quickly, but engineers will also be able to use the thrusters to direct Jaguar to an ice-free pool of water for recovery.

If 4:30 p.m. arrives, and the Camper/Puma strategy hasn’t worked, Jaguar will rise to the surface. The AUV team will not be able control over where it surfaces, so it may well come up under ice. But the team still should be able to locate Jaguar’s position.

In that scenario, the researchers and crew will have to try to go get it under less than ideal conditions. But that is exactly what happened after Jaguar’s first mission on July 21, and they proved they could do it.

Read the new interview

Susan HumphrisSusan Humphris:Thirty-one years ago, no one had ever found a hydrothermal vent on the seafloor. But a graduate student named Susan Humphris—analyzing rocks dredged from the ocean bottom—found telltale chemical clues that vents existed and were out there, waiting to be discovered. Read the interview »

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