Progress in predicting the performance of ocean gliders from at-sea measurements

With over 100 commercially-available ocean gliders being used by researchers around the world, there is strong evidence that these platforms have become the tool of choice for those who require continuous sampling of ocean properties over a range of user-controllable depths. Researchers continue to add new sensors to these vehicles usually on the external surfaces where a sensor can work in an essentially unobstructed flow condition. These added sensors change the behaviour of the glider. For the purpose of improving our predictions of the behaviour of a glider during steady-state glides and course-changing manoeuvres, it is useful to have a simple analytical hydrodynamic model which can be validated quickly using at-sea measurements during several descending and ascending glides. The purpose of this paper is twofold: (i) to show how the hydrodynamic properties which govern steady-state gliding can be extracted from measurements made with on-board sensors, and, (ii) to show how these hydrodynamic properties can be used to predict the performance of ocean gliders (e.g. glide angle, glide speed, duration of voyage etc.). We describe a three-parameter model which has proved useful in representing the behaviour of an ocean glider during straight-line descents and ascents. This parametric model has been validated with at-sea measurements during multiple glides. Estimates for these parameters can be obtained from the measurements of four quantities on-board a Slocum ElectricTM glider, namely (i) the fore-and-aft position of the pitch-control battery, (ii) the volume of seawater which is ingested or expelled by the buoyancy engine, (iii) the glider pitch angle, and, (iv) the glider depth. We describe briefly a method for obtaining estimates for three of these parameters and show some results in terms of the glider drag and lift coefficients over a wide range of operating conditions. Additional work is outlined to obtain estimates for the parameters which determine the pitching moment behaviour of this ocean glider.