114 Untethered UUV Dives: Lessons Learned
James M. Walton
Richard W. Uhrich
NCCOSC RDT&E DIV
53560 Hull Street
San Diego, CA. 92152 USA
Presented at MTS/IEEE Oceans95, San Diego, CA, October 10-12, 1995
Abstract
The Ocean Engineering Division at the Naval Command Control and Ocean
Surveillance Center RDT&E Division (NRaD) has developed and fielded two
successive untethered, supervisory controlled UUV systems: a prototype, and an
improved model. These robotic vehicle systems were part of the Advanced
Unmanned Search System (AUSS) program which had its genesis in the early
1970s. At that time the idea of performing useful work with untethered robots
in the deep ocean was met with some skepticism. Because of this program and
the verity of these two vehicles, especially the improved one, we now know
that supervisory controlled systems can be employed effectively.
A systems engineering approach was applied to the design and construction of
the prototype. Nevertheless, it was only after the prototype was fielded, many
lessons were learned, and the prototype experienced major evolutionary changes
that system feasibility was fully demonstrated. By that point the prototype
had become outdated.
As a consequence of the prototype experience, the improved model was developed
with confidence. The second system was a complete redesign, using
state-of-the-art subsystems and technologies. The resulting product was both
capable and reliable, yet flexible, creating a plethora of system evolutionary
possibilities. Sea tests, improved tactics, and systems engineering became
synergistic and interactive. Increases in vehicle autonomy enhanced the human
operator's capability to supervise by decreasing piloting and navigating
burdens. The resulting system significantly exceeded expectations, and was
delivered to the fleet.
AUSS involved pioneering research in underwater search and in UUV systems.
Important knowledge was also gained in systems analysis, system engineering,
and program evolution. Invaluable experience was gained from 114 successful
untethered dives. The purpose of this paper is to share many of the lessons
learned during the AUSS program.
INTRODUCTION
The Advanced Unmanned Search System (AUSS) program was born in 1973, after the
searches for the submarines USS Thresher and USS Scorpion, and an H-bomb
search mission near Spain provided evidence that a need existed to improve the
U.S. Navy's capability to conduct deep-ocean search. The first action on this
program was to collect the people who were involved in these missions together
with other search experts to discuss how to improve deep ocean search. This
study group identified and documented several deficiencies in the existing
state-of-the-art search approach. The study group, in total and in part,
continued to participate and contribute to the effort until 1976.
AUSS program evolutions encompassed a search data base, computer modeling of
search, subsystems evaluation, the test-bed prototype search system, and
finally the improved delivery system. Throughout this effort, from 1973 until
1993, engineers at NOSC (now NRaD) continued the AUSS program, acquiring
experience and applying their knowledge to improve both search technology and
vehicle technology.
THE UNTETHERED ADVANTAGE
The Cable May Not Be Your Friend
Many of the deficiencies flagged by the AUSS study group were related to the
tow cables and tether cables used by undersea vehicles in the search mission.
These deficiencies included long search vehicle turn time, vehicle navigation
error, and vehicle control error. Attaching a long tow or power cable to a
small vehicle cripples its potential maneuverability, slows its advance speed,
drastically increases its turn times, and relegates primary vehicle control to
the forces transmitted from the massive surface platform through the cable.
The focus of the AUSS program became finding a means to decouple search
vehicles from the effects of cables. After analysis, modeling, field testing,
and candidate systems tradeoffs, it was concluded that the most effective
approach to solving this problem was to develop and field the technology which
would eliminate the cable completely.
Cut The Cable To Decrease Risk
Cables and tethers have historically been the most troublesome component of
underwater systems. They twist, tangle, chaff, break, blow up, fatigue, and
can even entomb the attached system forever if snagged on the bottom.
Nevertheless, many are more comfortable with a tether attached to an expensive
underwater system, even though it means a loss in performance.
But consider the AUSS experience. Two systems have been built and fielded
which in combination have experienced 114 untethered launches (and 114
successful recoveries) to depths between 2500 and 12,000 feet. Like a faithful
hound which can be trusted to come when called, AUSS proved it didn't require
a long, clumsy, and potentially dangerous leash.
SUPERVISORY CONTROL ENHANCEMENT OF UNTETHERED SYSTEMS
AUSS program systems tradeoff studies and analysis showed that an untethered
search vehicle with supervisory control outperformed all other tethered,
towed, and untethered options. Fortunately, NRaD engineers were concurrently
inventing the underwater acoustic communications capability which would make
the cable unnecessary.
Untethered Systems Have Strengths and Weaknesses
Potential strengths of properly designed untethered systems are agility,
stability, hovering in three dimensions, high forward speeds, rapid turns,
combined with low risk of loss. Untethered systems, however, will not have
enough self intelligence in the foreseeable future to replace the human
decision making capability afforded by vehicle/operator communications.
Operators Have Strengths
The human operator, when allowed to supervise the operation of an untethered
system, fills in where the untethered system is deficient: in complex decision
making. The human plans the mission, and decides how to alter the mission
based upon information obtained from both the support ship systems and the
vehicle itself. The human analyzes vehicle sensor data, and decides which
anomalies in the data are of interest to the mission and therefore deserve
further investigation, and which are not. He can also alter the tactics
pursued by the vehicle based upon environmental changes indicated in sensor
data. Finally, the human operator is uniquely qualified to declare when the
mission is completed.
Autonomous Systems Benefit From Supervisory Control
Autonomous systems can profit by the inclusion of a real time (cable or fiber
optic) or near real time (acoustic) communications capability during
development testing. With this approach, the developers/operators have an
opportunity to interact with the system and monitor system performance in real
time while they work out system bugs. More autonomy is allowed as the system
earns it.
The evolution toward more autonomy with an untethered system can be carried to
completion if the mission permits. The AUSS mission was not such a mission.
Underwater search and inspection are too complex and too spontaneous for
unsupervised machines; these tasks require human judgment and curiosity.
Increases in AUSS vehicle autonomy have enhanced the human operator's
capability to supervise by relieving him of piloting and navigating burdens,
and even allowed the matured AUSS vehicle to be used for certain complex,
fully autonomous functions. These included performing sonar search patterns
covering several square nautical miles, and transiting long distances, without
operator commands being sent for hours.
SYSTEMS ENGINEERING, EVOLUTION, AND EXPERIENCE
Two Systems Link Systems Engineering, Evolution, and Experience
An early AUSS hardware objective was to build a prototype system as a testbed
for various types of sensors and methods of search, then smooth it out for
delivery to the fleet as an operational system. This became clearly
impractical. The prototype involved several emerging technologies applied
together for the first time, and it was the first source of performance data
for the approach taken. This level of RDT&E proved to require an evolution to
upgrade the mechanical, electronic, acoustic, and computer capabilities of the
prototype.
Thus the AUSS developers had to design and field a prototype before they could
realize the requirements for a deliverable system. But the results were far
better than what could have been achieved in a single generation. Many
essential enabling technology areas were matured with the prototype. With it,
an acceptable approach to tailoring search sensors to the UUV search task was
established, many system deficiencies were defined, and most were reduced to
solvable engineering problems. A search demonstration with the highly
compromised prototype showed that the AUSS concept was a feasible approach,
and that the potential existed to significantly improve search by continuing
the AUSS effort.
The Improved System Benefitted From Prototype Lessons Learned
The prototype was a product more of evolution than of its original system
engineering. Post- design breadboard-level implementations existed throughout.
Add-on vehicle wiring was a major contributor to electromagnetic interference
with the onboard analog and digital communications. The signal-to-noise ratio
in the acoustic link system was not acceptable. Transmissions of high-quality
images through the acoustic link required so much time that the rate at which
the system could search was below optimal. Some of the computer processing
capabilities were tapped out, the hardware being several years and a
generation old. The vehicle buoyancy system consisted of a pressure vessel
providing less than adequate displacement supplemented by ad hoc, oddly shaped
pieces of syntactic foam. The vehicle fiberglass fairings suffered from
extensive modifications including holing, sawing, and gluing.
Enter the improved system. Its ground up design based upon the prototype
lessons paid off handsomely. The electrical and acoustic signal- to-noise
ratios were excellent. The vehicle computer systems were expanded and upgraded
to the best available technology, with processing capability to spare.
Hard-to-change, contractor- supplied surface console software was rewritten
and ported to a network of off-the-shelf industrial computers. Original image
compression algorithms were developed so optical and sonar images were seen by
the operator within seconds of acquisition, and the advance speed of the
vehicle was optimized during sonar imaging for the travel time of the sonar
pings.
No Syntactic Foam Edict Allows Efficient Design
An important AUSS goal was to produce a small lightweight system that could be
transported easily and placed upon a large cross section of ships of
opportunity. As with any overall vehicle system, the size of AUSS depends
heavily upon the weight and size of the undersea vehicle. If the vehicle is
allowed to increase in size, the launch and recovery gear, the handling gear,
and the maintenance areas grow in kind. There is also a vicious cycle of
growth associated within the self-powered vehicle design. A larger vehicle
requires more propulsion power requiring more energy for the same speed and
endurance. More energy leads to more weight and volume in the energy source,
which leads to a larger vehicle.
Deep service syntactic foam is a much less efficient form of buoyancy than
properly designed pressure vessels. Syntactic foam was used extensively on the
AUSS prototype vehicle, as has been the case for many undersea vehicles. A
commitment was made to avoid its use on the improved vehicle. To meet this
objective, several measures were taken.
Extremely efficient graphite pressure hull technology was developed with the
prototype, and applied to the improved system. A 30 inch diameter graphite
cylinder was manufactured to provide all of the buoyancy required for the
improved vehicle. Other measures taken were the use of SpectraTM (which has a
specific gravity very close to that of sea water) for the free flooded
fairings, magnesium for the chassis inside the vehicle, titanium for the wet
connectors, and titanium and aluminum for redesigns of various sensor
housings. The commitment to relying solely on the graphite epoxy hull for
buoyancy was met. The only syntactic foam in the system was the deployable
nose float used for recovery.
POST COMPLETION REVELATIONS
Evolution Is Sea Test/Development Interactive
A major lesson learned was that a system such as AUSS must be developed
interactively with the use of the ocean as a laboratory. From the beginning of
the improved system sea testing until its final search demonstrations,
operations provided catalysts for improved strategy, refined tactics, better
software algorithms to relieve the operator of mundane tasks, and completely
new methods for performing supervisory controlled search.
Furthermore, only actual in situ experience can identify certain hardware
component weaknesses or system deficiencies. In particular, the complex
interactions between multiple acoustic devices and other potentially noisy
subsystems on board cannot be ultimately proofed elsewhere, nor can the
cumulative effects of sources of electromagnetic interference.
More Autonomy Begets Range Independence
The time required for signals to travel between the surface and vehicle is
dependent on speed of sound in water and the distance between the two. Range
of operation therefore affects the response time of the vehicle to supervisory
commands, and it also affects the delay time taken for sensor information to
reach the supervisor. These delays will increase with operational range,
amounting to a round-trip delay of ten seconds or more at 20,000 ft depth with
moderate standoff. The only way to prevent degradation of performance with
range in an acoustically supervised system is to develop strategies which
utilize vehicle autonomy.
An example of more autonomy yielding better range insensitivity by the system
is an approach developed during the AUSS interactive seatest/development
process for viewing objects on the bottom of the ocean. Neither the prototype
nor the improved vehicles had side thrusters, and hovering over an object in a
current proved impossible. With the prototype, pictures of the object were
taken while the vehicle glided over it at some forward velocity. The operator
had to guess when to command the vehicle to take a picture. The combined
acoustic link/supervisor reaction time increased with range to the vehicle.
This process was marginally possible for ranges of 2500 feet, and would have
been nearly impossible at the maximum range of 20,000 feet.
During the improved vehicle evolution, an autonomous "hover at a radius"
algorithm was implemented. This simple algorithm is analogous to a boat
standing off from a buoy; the vehicle points at a position and maintains a
given standoff from that position. The vehicle "weathervanes" into the
current, but remains aimed toward the target object. If the standoff distance
is selected to be equal to the distance between the imaging camera (at the
front of the vehicle) and the Doppler sonar (which is aft of the camera and is
used to determine the position of the vehicle), the camera stays over the
target. This is a completely autonomous routine which is range insensitive and
requires only one supervisory command to send the vehicle to a target.
If It Works, It Can Become Friendly
As the AUSS system became operational and more dependable, a number of other
innovative supervisory control system advances were invented to simplify the
supervision of the undersea vehicle operations. Among these was target
marking, wherein the location of a target object in the vehicle's onboard
navigation coordinate system is automatically calculated when a cursor is
placed over its image. Target marking was applied to Side Looking Sonar (SLS),
Forward Looking Sonar (FLS), and Cooled Charged Coupled Device (CCD) imaging
portions of the AUSS mission.
The synergy of hover at a radius and target marking made a significant
contribution to the efficiency with which the system could view objects
(targets) on the bottom. Each step in the target marking/hover at a radius
sequence brings the AUSS closer to the objective target using successively
shorter range, higher resolution sensing. A SLS target mark is used to
determine a position for the vehicle to go to, hover at, and obtain an updated
target mark with the FLS. FLS target mark is used to determine a position for
the vehicle to go to, hover at, and obtain the first CCD image. Finally, the
cursor is moved about on the CCD screen to mark positions for the vehicle to
go to and obtain CD image coverage of the target area.
A CAPABILITY DEMONSTRATED
Word of the impending end to AUSS development funding was received on April
Fools Day, 1992. This bad joke provided a good opportunity to obtain a
snapshot of the system capability and for AUSS to "showcase" for a period of
time. And showcase it did! 65 hours of bottom time were logged during 8 dives
between 5 April and 24 June. These 8 dives produced some compelling results.
The Glory Days Vindicate AUSS
During the showcase, SLS search rates were as high as 1.5 sq-nmi/hr, and contact
evaluations (the process by which targets are found and imaged with CCD)
typically took between 10 and 15 minutes (time between the operator's
identification of a potential target on SLS, and the time when the vehicle was
once again searching with SLS). Fully operational dives between 2500 and
12,000 feet, depth-independent supervisory controlled search tactics, and
excellent compression-enhanced acoustic link performance to 12,000 feet, were
all demonstrated. This is where it all came together.
During a single dive at 4000 ft, consistent SLS search was conducted at speeds
between 4.5 and 5 knots with a swath of 2000 ft. The area searched during the
dive was 7.5 sq-nmi, and the time to conduct SLS search and contact evaluations
was 8.5 hours. This demonstrated a SLS search rate better than 1.5 sq-nmi/hr and
an overall search rate (including contact evaluations) of 0.9 sq-nmi/hr.
In another 4000 ft dive, over 2.5 sq-nmi were searched, including several
lengthy contact evaluations and three photomosaics (series of overlapped CCD
images taken while the vehicle performs a small search pattern over a target
area). The contact evaluations included a 55-ft yacht (
figure 1) and a Korean War vintage Skyraider night fighter aircraft
( figure 2) which were both discovered and position
pinpointed during the dive. An autonomous 5 nmi transit was also performed
during the 14 hours the AUSS vehicle was submerged.
12,000 Foot Dive Proved Depth Independence
The passenger compartment of a 1940 Oldsmobile was searched for, found, and
inspected (figure 3) during a dive at 12,000 ft. The
vehicle operated at 12,000 ft for 11 hours. The images were compressed and
transmitted through the acoustic link at 2400 bps. Communications during the
12,000-ft dive were excellent, and search and contact evaluation tactics were
proven to be depth insensitive.
SUMMARY
Although years of study, research, and computer modeling preceded the
fabrication of the prototype AUSS, capabilities and tactics are far better
than originally conceived. The evolution of two generations of supervisory
controlled search systems was guided by the lessons learned at sea. AUSS has
been an able teacher.
Figure 1. 55-ft Yacht
Figure 2. Skyraider Aircraft
Figure 3. 12,000-ft Target
BIBLIOGRAPHY
Mackelburg, G. R., S. J. Watson, and W. D. Bryan. 1992. "Advanced Unmanned
Search System (AUSS) Acoustic Communication Link Development." NRaD TR 1531
(Nov). Naval Command, Control and Ocean Surveillance Center, RDT&E Division,
San Diego, CA.
Uhrich, R. W., and S. J. Watson. 1992. "Deep-Ocean Search and Inspection:
Advanced Unmanned Search System (AUSS) Concept of Operation." NRaD TR 1530
(Nov). Naval Command, Control and Ocean Surveillance Center, RDT&E Division,
San Diego, CA.
Walton, J. 1992. "Evolution of a Search System: Lessons Learned with the
Advanced Unmanned Search System." NRaD TR 1529 (Nov). Naval Command, Control
and Ocean Surveillance Center, RDT&E Division, San Diego, CA.
This technology is covered by U.S. Patents #5,018,114, 4,905,211, 4,432,079
and 4,418,404 and others assigned to the U.S. Government. Parties interested
in licensing this technology may direct inquiries to SSC SD Legal Counsel.
Address all questions/comments to:
uuv-web@spawar.navy.mil
Last update: 2 December 1998
Figure 1. 55-ft Yacht
Figure 2. Skyraider Aircraft
Figure 3. 12,000-ft Target