Instrumentation

Autonomous ocean gliders can be built relatively cheaply, are controlled remotely and reusable allowing them to make repeated subsurface ocean observations at a fraction of the cost of conventional methods. Gliders descend and ascend through the water column by changing their volume and hence their buoyancy; this momentum is converted to forward motion by wings resulting in an average horizontal velocity of 0.25 - 0.40ms-1. Dive pitch is controlled by moving an internal mass (battery pack) and steering is achieved through the use of an active rudder (Slocum) or by the rotation of an off-centre internal mass (Seaglider). Using GPS, internal dead reckoning and altimeter measurements the glider autonomously navigates its way to a series of waypoints. Near real-time data can be uploaded to a base computer via two way Iridium satellite communication, which also allows for remote updating of glider control parameters and waypoints. IMOS glider fleet consist of two types of gliders, Slocum (shallow water ~ 200m) and Seagliders (deeper water > 1000m). 

Slocum gliders

The Slocum glider is 1.8 m long, 21.3 cm in diameter and weighs 52 kg.  Manufactured by Webb Research Corporation, the glider is designed to operate in coastal waters of up to 200 m deep where high maneuverability is needed.  Moving at an average forward velocity of 25 - 40 cm/s the glider travels in a sawtooth pattern through the water navigating its way to a series of pre-programmed waypoints using GPS and altimeter measurements.  Operating on C cell alkaline batteries, typical deployments can range up to 500 km with a duration of approximately 30 days.  Communication with the glider is by iridium satellite and each glider is fitted with an ARGOS transmitter to aid in recovery.

All communication and GPS antennas are located within the glider’s tail fin which is positioned out of the water during surfacing.  Currently, Slocum gliders operated by the Ocean Glider facility are instrumented with a Seabird-CTD, WETLabs FLBBCD 3 parameter optical sensor (measuring Chlorophyll-a, CDOM & 700nm Backscatter), an Aanderaa Oxygen optode and SAtlantic 4-channel OCR (Ocean Colour Radiometer) downwelling irradiance sensor.

Schematic of a Slocum glider traveling in a sawtooth pattern through the water column.
Diagram showing the design and components of a Slocum glider.

Seagliders

A Seaglider at the surface. Note the projecting antenna allowing data transfer and new waypoint instructions to be relayed.

Seagliders were originally built and developed by the University of Washington and are now buit by Kongsberg. They are designed to operate in the open ocean in depth up to 1000m.  Moving at an average forward velocity of 25 cm/s, Seagliders travel more slowly, alternately diving and climbing along slanting glide paths.  The gliders obtain GPS navigation fixes when they surface, which they use to glide through a sequence of programmed targets.  As they are deployed in the open ocean, Seagliders have a very large range sufficient to transit entire ocean basins and can be deployed for periods of up to 6 months.  During deployment, the gliders regularly dive to depths of 1000 m to collect high resolution profiles of physical, chemical, and bio-optical variables.  Currently, Seagliders operated by the Ocean Gliders facility are instrumented with a Seabird-CTD, WETLabs BBFL2VMT 3 parameter optical sensor (measuring Chlorophyll-a, CDOM & Backscatter) and Seabird Oxygen sensor.

Inside a Seaglider.
Seaglider dive path schematic. Image from http://aquarius.umaine.edu/images/ed_seaglider_path.jpg