[Cruise Journal: 18-19 December]
For those of us on the science team who have never seen the AUV Sentry deployed from the deck of a research vessel before, today was a bounty: two launches for the price of one.
Waypoint 1 is the point in the Pacific Ocean closest to our port of departure in Hawaii that reaches a depth comparable to the bulk of our area of interest in the Jurassic Quiet Zone. That’s why it’s the best place to perform the first Sentry dive of the cruise.
Here’s what happened, according Sentry Expedition Leader Dana Yoerger:
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“This was an Engineering test day for Sentry. That’s a day specifically dedicated to testing improvements on the vehicle. Of course it’s also a chance to test our readiness for the upcoming science dives. These days give us to chance to try big changes to software and hardware that would not be prudent in the midst of intense science operations.
We had two dives, both revealed important aspects of some changes we had made in the vehicle’s hardware and software. We test all our changes as carefully as we can before we launch the vehicle. We do simulations, which test the higher level aspects of our software. And we do tests on the vehicle on deck to look at the interfaces between our software and the vehicle’s hardware. Nevertheless, until we actually run the vehicle in deep water we can’t be sure our changes are good.
Our first dive ended very early, when the vehicle reached about 100m. We had revised our code that handles the vehicle’s depth sensor, but that code had a bug in it. So Sentry ended its descent early (very early). As soon as we got the vehicle on deck and the data offloaded, the problem was obvious and the code repaired and tested in a few hours. The science team took advantage of the time to test the deep tow magnetometer; we try to never let the vessel be idle.
On our second dive, Sentry got quite deep (about 5600m) but never started surveying. After launch, Sentry’s descent weight forces it to go down without using much energy. Normally, Sentry descends until it senses it’s close to seafloor (within 80 meters) using its Doppler Velocity Log (DVL). Then Sentry drops its descent weight and starts its programmed mission. In this case, Sentry “thought” it saw the seafloor within 80 meters but in fact it was sensing some sort of “false bottom”. Sentry’s software applies a series of tests to judge if the readings are genuine. Sentry was very deep (over 5500 meters). The bottom appeared to be close enough (within 80 meters) and the distance to the seafloor was decreasing. When the software sees a series of readings in a row meeting those criteria, it drops the descent weight and starts the survey. All this worked as expected. Except the sensor was giving us erroneous readings, it was sensing something other than the seafloor. The readings were consistent, sensible, but wrong! Some sort of deep layer? We don’t know.
![Sentry has been here before. The pink track is depth data from a previous cruise; the multicolored track is our current survey of the test dive site](http://web.whoi.edu/jurassicmagnetism/wp-content/uploads/sites/33/2014/12/test_dive_site-300x160.jpg)
Sentry has been here before. The pink track is depth data from a previous cruise; the multicolored track is our current survey of the test dive site
We were able to command the vehicle to continue its mission using acoustic commands to force it to drive deeper. But in the end, we had another problem: the vehicle was too light and it simply couldn’t fight its positive buoyancy. So we decided to end the dive and bring the vehicle home.
We’re always disappointed when a dive does not go as planned. But despite the problems, we were able to check most of the changes to Sentry’s software and hardware. We’ll be working on solutions to the “false bottom” problem. We have a 9-day transit, so we’ll have time to solve the problem.”
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With the engineering test dives completed, we are now under way toward the first area of the JQZ on our survey agenda. While we’re transiting, we’ll have some time to tell you more about the ship and the scientific instruments on board.