The DART is a customized deep water version of the MARES AUV [Cruz & Matos, 2008], with a new 5000m rated housing. The MARES AUV was developed with a modular approach, with a single pressure housing holding all batteries and electronics. In the deep-water version, this housing is replaced by a glass cylinder to withstand the extreme pressures of the operational scenario. All external subsystems are installed in flooded sections, and therefore can be reused from the original vehicle. The thruster configuration, with two independent thrusters for longitudinal motion and two independent thrusters for vertical motion, allows the AUV to hover in the water column, which is very useful for close-range video inspection of seafloor features. To facilitate the descent in the water column, while saving energy, the vehicle has a drop weight in the nose, that is released once it approaches the desired depth. The energy is provided by rechargeable Li-Ion batteries, with a current total of 600Wh of energy. There is still some room for increasing battery size, if the first operational tests demonstrate its need.
The navigation package includes a pressure sensor, hydrophones for acoustic navigation, an inertial unit with compass and inclinometers, a Doppler sensor to measure velocity with respect to the surrounding water and the bottom, and a digital echo sounder to measure distance to the bottom. An onboard CTD will also be used to determine several characteristics of the water column: the water density will allow an accurate conversion from pressure to depth, while the sound velocity profile will improve the accuracy of range measurements based on time of flight of acoustic signals.
In terms of payload, the PISCES configuration of the vehicle has:
- A down-looking underwater digital camera (and lights) to capture digital pictures and videos of the sea bottom and features. This camera is a COTS IP camera from Mako, installed inside an aluminum pressure proof housing. It has been tested independently for shallow waters, and was tested in the hyperbaric chamber before being installed in the vehicle;
- A multibeam echo sounder, providing a direct reading of multiple distances to the sea floor. The multibeam echo sounder currently available is a Delta T from Imagenex. Although the range (altitude) is quite limited (100 meters), the main purpose of the echo sounder is to provide the calibration for the surface-based long range bathymetry sensor. If necessary to increase swath, this multibeam echo sounder may be replaced by another with a larger range;
- An array of acoustic recorders located along the outside of the hull, as part of the distributed long range bathymetry system. These acoustic recorders will receive and register all the echoes of the sound transmitted from the surface and reflected in the sea bottom;
- An onboard CTD to determine several characteristics of the water column: the water density will allow an accurate conversion from pressure to depth, while the sound velocity profile will improve the accuracy of range measurements based on time of flight of acoustic signals.