by Penny Holliday
After many, many days of CTD stations, data processing and running samples, the days begin to merge together into one tiring monotony. So when something new is expected, it can bring great excitement and anticipation. This morning’s treat was the prospect of finding and recovering a glider called Bellatrix. I was up an hour and a half early so I could watch and film the recovery operation, and I wasn’t the only one!
Gliders are relatively new technology and they are a brilliant additional tool for our work to understand the way the ocean works. They use the same basic principle as floats for collecting measurements of the temperature and salinity of the upper part of the ocean. They change their buoyancy relative to the water they are in with the use of an external oil bladder which is repeatedly filled and emptied. As they sink and re-surface they collect data through the water. Gliders are more sophisticated than floats though because they have fins which means they fly through the water, travelling in the direction we want them to. Just like gliders you see circling overhead on currents in the air, these sea glider don’t have propulsion motors but use their fins to move forward. They are controlled by pilots back at base who can alter their flight through communication via satellites. They can be carried sideways by the strongest currents, but our skillful pilots keep them on the right track.
This glider has been making measurements along the Extended Ellett Line, which we traditionally survey once a year with ships. The gliders give us a lot of extra data from the top part of the ocean in between those ship-based surveys. This extra data will help us understand the variability of the water in between those CTD surveys.
The glider pilot at SAMS (Scottish Association for Marine Science www.sams.ac.uk) had been up in the night to send instructions to Bellatrix to get her into position close to our CTD station. The glider was reporting its position very frequently so we could assess its movement and decide how to approach it for recovery. The glider is small; about the size and weight of me actually, so you can imagine that it takes great skill to position the ship close enough for us to capture it, but not to run it over.
Fortunately for us, the conditions were ideal; only a low swell and a light breeze. Nevertheless it was hard to spot the glider with the naked eye. They sit vertically in the water with only a thin orange antenna showing, so keen eyes are required to find it. The ship was maneuvered into position so that the glider would pass slowly along the starboard side where we waited with poles and ropes.
The plan was to use a long, light, carbon-fibre pole to lasso the glider at a strong point, around the stern fins. This is harder than you might think – the long pole can be hard to control when the ship and the glider are moving independently. Not only that, but the glider was sitting low in the water with the fins below the surface and ropes tend to float. Stefan had been practising handling the pole (including lassoing my feet in the lab) but it was still quite tricky. It took several attempts, but in the end Stefan managed to get the rope around the fins and pulled the lasso tight. After that it was a question of lifting the glider up with the winch, trying not to let it bash against the side of the ship, and lowering it gently into the cradle.
Once it was on board, a very pleased Stefan strapped it into the cradle and we carried it into the rough workshop where he used a PC to connect to it, check it over, and turn it off. It will now be packed into a crate, and taken back to the lab to be serviced, the sensors calibrated and new batteries installed for the next mission. In all it was a very successful recovery, and a pretty good spectator sport too.
Images: the first view of Bellatrix is her tiny orange tail, as the ship maneuvers alongside the float we get a better view, Stefan wields a carbon fibre pole holding a lasso, he successfully lassos her fins, attached to the winch the glider is lifted out of the water, Stefan happy to have her back, Bellatrix in her cradle. (all by Penny Holliday)