Feature ArticleSeaExplorer Glider Breaks Two World Records
By Hervé Claustre • Laurent Beguery • Patrice PLA
Ocean gliders are revolutionizing underwater surveys and missions for ocean data collection. Navigating autonomously in water for weeks or months, depending on the mission, and covering thousands of kilometers, gliders persistently collect a variety of data (physical, chemical, biological, acoustic, etc.) along the water column. Those data are then sent by satellite telemetry to a ground station, avoiding the need for a support ship.
The main difference between a glider and an AUV is a glider's propulsion principle, which is based on buoyancy variation, instead of a propeller. This allows efficient navigation at low speed, resulting in lots of energy savings. Materials with higher density than water sink, and ones with lower density are buoyant. Materials with the same density remain stable in water. Most gliders change their density through an external bladder. When the bladder deflates, the glider dives and glides using wings. When the bladder inflates, the glider climbs to the surface and glides the same way. This results in a particular sawtooth path. The actuator allowing bladder inflation and deflation for propulsion is named the ballast. This propulsion device allows months of endurance, which makes for another big difference between gliders and AUVs, as well as making gliders a very convenient platform to collect data from the water column at a very cost-effective price.
In terms of telemetry, gliders regularly surface to establish a communication link for supervision, data transfer and piloting. Once its antenna is in the air, its GPS position is computed and data collected are transmitted by Iridium communication link to the control station based onshore. The supervision and piloting station (SPS), which is accessible from a simple Web interface allows, when required, new navigation instructions to be delivered back to the glider for any modification of the previously planned mission file.
In some situations, close communication can be established with a glider by radio frequency giving convenient wireless access for test procedures before a mission or making recovery at sea easier.
The first gliders were traditionally designed with wings. At-sea experiences demonstrated that this design was a drawback, leading to critical situations, such as wing break during launch/recovery or entanglement in seaweed, plastic debris or fishing nets. It explains why the latest generation of gliders now uses a typical hull shape and short fins instead of wings.
Another identified weakness of gliders designed with external actuators is that users have reported leaks at the mechanical interfaces over time. To tackle this problem, smart devices using internal mass displacement as internal actuators were designed and introduced on gliders. But for scientists, the strongest recommendations have always concentrated on a more open scientific payload architecture to finally achieve the capability to enable the integration of custom sensors by themselves.
With this idea came the need for sensor payload interchangeability and modularity. In terms of piloting, low speed (a particularity of gliders due to propulsion generated by the ballast) makes glider navigation quite challenging in current.
The new SeaExplorer glider is a cooperation between the ACSA company (Meyreuil, France), several French oceanographic institutes such as Ifremer and the CNRS (Centre National pour la Recherche Scientifique), and ACRI-ST and ACRI-IN as industrial partners. Taking into account end-users' feedback, the SeaExplorer glider integrates lots of innovations.
First, the project resulted in a wingless platform design (no wing, no break). Benefits of such small wingspan fins include a reduced drag and less surface area subject to biofouling, which modifies buoyancy and compromises piloting.
The SeaExplorer is steered by two internal actuators for attitude. The first one displaces the moving mass forward and backward to pitch up/down, and the second one rotates the moving mass port and starboard for roll and heading control. Regarding attitude actuators, the SeaExplorer is fitted with long-range displacement, providing ease in piloting and more flexibility for glider calibration. To continue this article please click here.
Hervé Claustre is a senior scientist at the Laboratoire d'Océanographie de Villefranche. He is also the team leader of the Oceanographic Autonomous Observations group.
Laurent Beguery is a marine instrumentation engineer responsible for marine seismic instrumentation and a fleet of 16 underwater gliders at the French National Center for Scientific Research.
Patrice PLA is the sales and marketing director at ACSA, Alcen Group.