Gliders

The main application of gliders at NOC is observing ocean convection. The traditional oceanographic measurement techniques are based on moorings and profile casts. Moorings provide high time resolution data of the vertical structure of the ocean, but as they are expensive to deploy, the spatial resolution is extremely poor. Profiling from research ships is costly too, but provides a higher spatial resolution at the expense of the time resolution. Moreover, profiling is labour intensive and can be virtually impossible in hostile sea conditions. The obvious solution is the use of Autonomous Underwater Vehicles (AUV) which have the same time resolution as moorings, but as they move around in the water, a larger area can be observed, and thus increasing the spatial resolution. Except from the launching and recovery, weather conditions and sea state do not seriously affect the use of AUV's.

Different types of AUV's have been used in oceanography. In this project we use a relatively new class of AUV's, gliders. In contrast to other AUV's, gliders use their capability to change their buoyancy to sink or rise. Forward propulsion is achieved by wings, so that the gliders glide through the ocean.

At NOC we have a fleet of three gliders (still). The image below proudly presents them: from left to right: Ammonite, Bellamite and Coprolite. How do we distinguish between them? In principle, they are all the same, but effectively, they are not... Internally (software), each glider has its own name, which it uses when communicating with the base on shore. Externally, they are all yellow, but each of them is labelled with a piece of the most versatile tool of all: tape. Ammonite, Bellamite and Coprolite are colour coded as blue, red and green.

The NOC fleet: Ammonite, Bellamite and Coprolite

Principle of operation

For a submersed object like the glider, the principle forces are due to gravity, buoyancy, and, when it moves, friction from the hull and lift from the wings. Inside its nose, the glider has a cylinder and piston, so that it can take in water (reducing its volume and therefore decreasing the buoyancy) or push out water (increasing its volume and buoyancy). Besides buoyancy control, this mechanism also provides a rough pitch control. Fine tuning of the pitch is achieved by a movable battery pack. Heading control is achieved by a rudder in the tail fin.

Each glider is equipped with GPS and an Iridium phone/modem. With these devices, the glider knows its position from GPS when floating at the surface and also it can communicate to a shore-based computer through a (satellite) Iridium connection. This two-way communication allows us to download measurement data from the glider, including its current position and upload new instructions, such as a new set of waypoints.

Next: Example data