In April 2016,
the National Transportation Safety Board announced that the cargo ship El Faro’s voyage data recorder was
located at the site where the vessel sunk after the ill-fated voyage during a hurricane
last year. The research vessel Atlantis, operated by Woods Hole Institute
found the recorder using Sentry, an autonomous underwater vehicle that has an
extensive sensor suite depending on the application. Along with the other remote vehicles onboard Atlantis,
they were able to find the object about the size of a basketball in almost
three miles underwater. NSTB Chairman
Christopher Hart stated that it would not have been possible to find it without
the help of the institutions and agencies involved in the search and the
equipment provided (NSTB locates Sunken Cargo Ship, April 2016).
The autonomous
vehicle Sentry was developed with funding from the National Science Foundation,
and designed and built by Woods Hole Institute.
The vehicle is operated the National Deep Submergence Facility (NDSF),
funded by several US agencies. The NDSF operates,
maintains and coordinates the use of these deep ocean research vehicles (NSTB
locates Sunken Cargo Ship, April 2016).
Based on the
finding provided by Sentry, the NSTB announced on June 6th, that
USNS APACHE with the remote vehicle CURV-21 will be displaced in July to
commence recover efforts of the recorder (NSTB to Launch Mission, June 2016).
APACHE and CURV-21 were initially used last November to locate the sunken ship. CURV-21 is a tethered remote operated vehicle
with two 7-function manipulators and tool packages, which will be used to
retrieve the recorder (CURV 21, n.d.).
Sensor Systems
Sentry is a
unique remote underwater vehicle capable of operating down to 6,000 meters,
fully autonomously. It is designed for quick maneuverability, and
shaped to allow for faster ascents and descents. It carries an array of exteroceptive
sensors to produce bathometric, side-scan, sub-bottom, and magnetic maps. It has an acoustic navigational system that
performs as a proprioceptive sensor. The
navigational system uses a Doppler velocity log and an inertial navigation,
assisted by an acoustic navigation system. This
acoustic navigational system also has proprioceptive attitude sensors to
determine the pitch, roll and heading via a vertical reference unit (VRU) and
gyrocompass. All these sensors were designed for the maritime environment (Sentry,
n.d.; USBL Overview, n.d.).
Modification to the existing system
Sentry is currently used as a search platform. Woods Hole Institute operates other manned
and unmanned systems to complement Sentry when a full search and rescue
operation is performed. It also collaborates
with the US Navy, NOAA and other agencies to use their unmanned platforms as
required by the mission. Sentry
currently does not have any manipulators.
This is the reason the CURV 21 is going to be used to retrieve the recorder. If Sentry had the capability to use
manipulators, it could have retrieved the recorder when it initially found it.
Now another effort must be done to retrieve the recorder.
UAS use to complement the existing system
In the
cases where Sentry is used for deep dive searches, the use of UAS would have
minimal value. However, the use of UAS
for the initial search and rescue operations would be beneficial as an
additional search platform or as a communications relay for other search
platforms. UAS with maritime or
synthetic aperture radars and electro-optic/infrared camera system would be
invaluable to initially search a maritime area of interest. It could cover magnitudes more amount of area
than ships. Operating sensors at higher
heights and higher velocities, the aerial system will provide greater coverage and
more frequent revisit rates.
Advantages of unmanned maritime systems
Sentry
has significant advantages over manned systems since it can operate
autonomously in a very harsh environment. The hull shape allows Sentry to get
on station quickly, and can operate at great depths for long periods of time. The ability to be autonomous provides the
search and rescue team greater flexibility than a tethered manned or unmanned
system. They can do missions with Sentry
in more hazardous areas and at a faster pace than with other systems in their
inventory.
In the
case of Sentry, it can cover an ocean floor effectively to find objects with
the array of sonars and cameras. At the
same time, it is a small footprint. Therefore, it can operate effectively at a
site, such as a shipwreck without disturbing the environment. A manned tethered system that can carry a
similar sensor array would not be able to explore the site as effectively
because it would be difficult to maneuver the vehicle in the tight
environment. With visibility being so
poor at great depths, a manned system may not be able to take the photographs
or other sensor measurements as can the autonomous Sentry.
References
NTSB
Locates Sunken Cargo Ship's Voyage Data Recorder. (2016, April 26). Retrieved
June 06, 2016, from http://www.ntsb.gov/news/press-releases/Pages/PR20160426b.aspx
NTSB to
Launch Mission to Recover Voyage Data Recorder. (2016, June 6). Retrieved June
06, 2016, from http://www.ntsb.gov/news/press-releases/Pages/PR20160606.aspx
CURV 21.
(n.d.). Retrieved June 06, 2016, from http://www.supsalv.org/00c2_curv21Rov.asp?destPage=00c2
USBL
Overview. (n.d.). Retrieved June 06, 2016, from
http://www.sonardyne.com/products/positioning/usbl-all-systems.html
Sentry.
(n.d.). Retrieved June 06, 2016, from http://www.whoi.edu/main/sentry
