“Among the sensors the scientists placed on the ice in March were a set of eight acoustic navigation beacons. These have base-stations at the surface, which fix their locations using GPS. They then rebroadcast that information from loudspeakers hanging 100 metres down below the ice, in the transmission layer. If a Seaglider can detect two or more beacons while it is travelling through this layer, it can swiftly compute its own position.
This may not always work, because the Seagliders might stray too far from the beacons. In that case, the researchers have a pair of robotic guide dogs to assist. These are called Wave Gliders (pictured at the top of the story). One part of each Wave Glider stays on the surface, generating electricity from solar panels during the Arctic’s 24-hour summer daylight. The other part is an array of hydrofoils suspended four metres underwater. The difference in motion between the waves above and the calm below causes water to move over the hydrofoils and propel the Wave Glider forward up to twice as fast as a Seaglider. Although Wave Gliders broadcast far above the sound layer, and thus have shorter ranges than fixed beacons, they can be programmed to shadow the Seagliders, and keep them within earshot.”