Autonomous underwater vehicles (AUVs) are small, unmanned submarines. Most commonly these vehicles follow a preprogramed track consisting of several waypoints. The waypoints can consist of position information in combination with the water depth or a certain distance from the seafloor. On their way through the ocean they carry different instruments to measure several parameters from the temperature of the water to the amount of light penetrating the ocean. At the end of a mission the AUV and the supply vessel meet at a preprogramed position and the AUV is recovered.
AUV enable us to reach areas that are hard to access with conventional tools. They are able to sample horizontally close to the underside of the ice or just above the seafloor. The scientific payload they can carry depends on their size and the design of the vehicle. At AWI we aim at developing customized payload sections for the respective scientific questions. These sections are interchangeable for different missions.
Technical details about the AWI vehicle PAUL
Diving depth: 3000 m
Length: 4.5 m
Diameter: 53 cm
Weight in air: ca. 500 kg, depending on payload)
Weight in water: + 3 kg buoyancy
Average speed: 3-4 kn or 5-7 km/h
Mission duration: depending on payload 4-8 h
Manufacturer: Bluefin Robotics
Vehicle type: BF-21
Several Lithium-Ion batteries supply two different electrical circuits. One circuit consists of three pressure compensated batteries (30 VDC and an energy of 4500 Wh) providing power to the main computer and to the thruster. This circuit can be extended to up to six batteries (9000 Wh). Power for the scientific payload is supplied by the second circuit with a voltage of 14.8 VDC and an energy of 460 Wh.
Navigation and tracking
As long as the AUV is at the surface, a GPS antenna is used to determine the position of the AUV. Below the surface an Inertial Navigation System (INS) is used. The INS measures the travelled distance, the three-dimensional acceleration as well as the angular acceleration and determines its position via dead reckoning. Additionally a Doppler Velocity Log (DVL) measures the velocity over ground as long as the vehicle stays within 130 m distance to the seafloor.
At the surface, the AUV can be located with a radio beacon. It can also send its position via radio to the supply vessel. Outside the range of radio communication systems a GPS/Iridium transceiver determines its position via GPS, which is then transmitted via Iridium SBD.
For underwater tracking the AUV is equipped with a transponder that can be located using hydrophones underneath the ship. The exact travel time of the signals is used for determining the position of the AUV in relation to the supply vessel. For safety reasons the power supply of all tracking systems is independent of the vehicle.
Sensors for navigation of the AUV
GPS – Ashtech DG-14, Thales Navigation
Inertial Navigation System – KN-5053, Kearfott
Pressure sensor – Paroscientific Digiquartz
Doppler velocity Log – Workhorse Navigator DVL 300 kHz, Teledyne RDI
Tracking system – GAPS, iXBlue
CTD – conductivity, temperature and pressure sensor
ADCP – Acoustic Doppler Current Profiler
MSP – Microstructure probe for turbulence
Water sample collector consisting of 22 tubes with 220 ml volume each
Fluorometer – Chlorophyll a and Colored Dissolved Organic Matter (CDOM)
Light sensor – Photosynthetically Active Radiation (PAR): 400 – 700 nm
Wulff, T., Lehmenhecker, S., Bauerfeind, E., Hoge, U., Shurn, K., Klages, M. (2013). Biogeochemical research with an Autonomous Underwater Vehicle: Payload structure and arctic operations. OCEANS - Bergen, 2013 MTS/IEEE, doi:10.1109/OCEANS-Bergen.2013.6608043.