Unmanned Systems Maritime Search & Rescue (UNSY 605 Assignment 2.4)

The REMUS 6000 Autonomous Underwater Vehicle (AUV), employing a wide array of underwater sensors, was critical to finding the wreckage of Air France flight 447. AF447 was en-route from Brazil to France when it encountered icing conditions over the Atlantic Ocean which blocked its pitot tubes, which are used to measure the aircraft's speed. The pilots responded incorrectly, stalling the airplane and ultimately crashing it into the ocean, killing all of the occupants (Ferrante, Kutzleb & Purcell, 2011). 

The REMUS 6000 incorporates numerous sensors to determine its position and navigate. Underwater navigation requires aggregating the data from many sources to determine an accurate position and track. To this end, it uses an Inertial Measurement Unit (IMU) that records linear and angular accelerations to estimate its travel from a known location. Augmenting this estimation is an Acoustic Doppler Current Profiler (ADCP), which takes acoustic measurements of the vehicle's movement over the seabed. Two transducers receive acoustic position data from Deep Ocean Transmitters (DOTs) pre-lain in known locations. The vehicle is equipped with a Global Positioning System (GPS) receiver, but this sensor is only usable on the water surface. To avoid collisions, the vehicle uses a pencil-beam sonar collision avoidance system that informs the control system of obstacles in the AUV's immediate path necessitating evasive action. Depth is measured using a pressure sensor combined with the ADCP measurement. The vehicle is also equipped with a conductivity (salinity) sensor and ground fault detection (Kongsberg, 2012). 

To search the ocean floor for wreckage, the purpose-built REMUS 6000 uses several exteroperceptive sensors, many developed specifically for the underwater environment. It uses Edgetech dual frequency side-scan sonar sensors to map the topography of the ocean floor and search for sonar returns characteristic of man-made objects, 400 to 700 meters to the left and right of the vehicle (Ferrante, Kutzleb & Purcell, 2011). It uses these sensors in a raster pattern. These are similar to side scan radar used on aircraft, but radio waves dissipate too quickly underwater, thus acoustic waves are used instead. When an object of interest is found, the REMUS 6000 employs a multi-beam profiling sonar, which creates a 3-dimensional sonar image. To confirm the existence of wreckage, the vehicle uses an electro-optical imager synchronized to a strobe light. When searching a known debris field, the vehicle employs a sub-bottom profiling sonar to search for buried debris (Woods Hole Oceanographic Institute, 2012). 

Would the REMUS 6000 be more effective if paired with an Unmanned Aircraft System (UAS)? Without some idea of where to begin to search, AUVs are very ineffective tools. AUVs scan the ocean bottom slowly (1-4 knots). Aircraft, manned and unmanned, scan the ocean surface much faster than AUVs scan the ocean floor. Any floating debris or oil slicks indicative of the last known position (LKP) of the aircraft will narrow the underwater search considerably and make the REMUS 6000 more effective. AF447's wreckage was found 6.5 nautical miles from its LKP (Ferrante, Kutzleb & Purcell, 2011).

Are AUVs more or less effective at carrying underwater sensing equipment than a manned submersible? Both vehicles are capable of carrying and employing the same sensors. However, a manned submersible carries a great deal of superfluous equipment, not the least of which being its human occupants. AUVs are smaller and more hydrodynamic, require less power for operation and locomotion, and only require small pressure vessels for its equipment and flood the remainder of the vehicle (Christ & Wernli, 2013). In the event of a catastrophic failure of the power system of pressure vessels, AUVs ideally jettison ballast and return to the surface. A manned sub can do the same, but possibly endangering its occupants. If an AUV can perform the same sensory tasks, perhaps this implies a moral obligation not to put humans in harm's way. Finally, the REMUS 6000 can perform its sensing missions for up to 16 hours. Human factors would preclude a manned submarine from staying on station for that amount of time. 

Are there any conceivable ways to improve upon the REMUS 6000's role in underwater search and rescue? Without being part of the design process, it is difficult to understand all of the system level trade offs. One idea is to simply increase the vehicle's battery size, ideally resulting in a longer time on station. However, since the data must be downloaded directly from the vehicle, perhaps 16 hours is the longest interval that its designers wanted to wait for data. Another idea is to employ a small, go-between AUV that offloads the large datasets from the REMUS 6000 and returns them to the control vessel. 

References:

Ferrante, B., Kutzleb, B, Purcell, M. (2011). AF477 Underwater Search and Recovery Operations: A Shared Government-Industry Process. Sterling, VA: International Society of Aviation Safety Investigators.

Christ, R. & Wernli, R. (2013). The ROV Manual, Second Edition. Oxford: Elsevier/ Butterworth-Heinemann Press. 

REMUS 6000 Specifications. (2012). Woods Hole Oceanographic Institution, Oceanographic Systems Laboratory. Woods Hole, MA. Retrieved 22 January 2015, from http://www.whoi.edu/page.do?pid=105976 (Links to an external site.)

REMUS 6000 Datasheet. (2012). Kongsberg: Hydroid, Inc. Retrieved 22 January 2015, from http://www.km.kongsberg.com/ (Links to an external site.)