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IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 44, NO. 2, APRIL 2019
Launch and Recovery of an Autonomous Underwater
Vehicle From a Station-Keeping Unmanned
Surface Vehicle
Edoardo I. Sarda
and Manhar R. Dhanak
Abstract—In-water tests of automated launch and recovery
(L&R) of a Hydroid REMUS 100 autonomous underwater vehicle
(AUV) from a station-keeping 16-ft wave adaptive modular vehicle
unmanned surface vehicle (USV) have been conducted to determine the feasibility of developed concepts of operation. The USV
is a catamaran with a custom-configured propulsion system that
enables it to maintain position and heading on the surface. AUV
launch is accomplished through lowering the AUV from the USV
top tray to the water surface and releasing it. Recovery is initiated
through requiring the AUV to navigate toward an acoustic homing
beacon on a taut line suspended from the USV. The taut line serves
as docking target and as a connecting link between the two vehicles during L&R. During these operations, the USV approximately
keeps station on the water surface. Once the docking is complete,
the USV moves forward, towing the AUV as it is extracted onboard
the USV via a customized winch mechanism. It was found that for
appropriate environmental conditions, L&R of the AUV from a
station-keeping USV is an effective alternative to the mobile L&R
method using the same vehicles, with the same launch and recovery
system.
Index Terms—Autonomous underwater vehicle (AUV), launch
and recovery (L&R), unmanned surface vehicle (USV).
I. INTRODUCTION
UTONOMOUS underwater vehicles (AUVs) and unmanned surface vehicles (USVs) are currently being utilized for a variety of applications where manned operations
can endanger lives. The different characteristics of an AUV
and an USV make each more suitable for certain applications
than the other. The two vehicles, collaboratively working together in a complementary fashion, enable a greater capability
of the combined system. Examples of applications that benefit
from such collaborations include surveying and surveillance in
remote unknown waters, mine-countermeasures, and port security. In these cases, an AUV alone may not be sufficient since
it typically lacks the ability to travel long distances at high
speeds, and it is unable to communicate with a surface ship or
A
Manuscript received April 10, 2018; revised August 10, 2018; accepted August 22, 2018. Date of publication September 20, 2018; date of current version
April 12, 2019. This work was supported by the Office of Naval Research under Grant N000141512724 [Program Manager: Kelly Cooper]. (Corresponding
author: Edoardo I. Sarda.)
Guest Editor: W. Kirkwood.
The authors are with the SeaTech—The Institute for Oceans and Systems
Engineering, Department of Ocean and Mechanical Engineering, Florida Atlantic University, Dania Beach, FL 33004 USA (e-mail:, esarda@fau.edu;
dhanak@fau.edu).
Digital Object Identifier 10.1109/JOE.2018.2867988
TABLE I
PRINCIPAL CHARACTERISTICS OF REMUS 100
shore without surfacing. A USV is a good mobile platform for
launch and recovery (L&R) since it can perform autonomous
navigation in an unknown marine environment.
A. Vehicles
Hydroid REMUS 100 AUV and the 16-ft wave adaptive modular vehicle (WAM-V 16, Advance Marine Research, Inc., Richmond, CA, USA) USV were selected as vehicles of choice for
this study.
REMUS 100 is a compact, lightweight platform suitable for
operations in coastal environments. Its compact size and low
weight make this AUV suitable for autonomous L&R operations
from mobile unmanned platforms. The principal characteristics
of REMUS 100 are provided in Table I.
A 3-bladed propeller, a rudder, and two vertical fins provide
for the trust and maneuverability of the AUV. The vehicle is
depth rated for 100 m and can achieve a maximum speed of
2.5 m/s. The vehicle configuration does not permit pure vehicle
motion in all six degrees of freedom used in its dynamic model,
therefore it is considered as an under-actuated system. The AUV
uses GPS to determine its position when it is on the free surface. It navigates underwater by dead reckoning, utilizing the
inertial navigation system to estimate its geoposition. REMUS
100 is equipped with an HG1700AG58 inertial measurement
unit (IMU) from Honywell for measurement of accelerations
and the rate of change of the orientation angles [1]. As acoustic
Doppler current profilers, REMUS 100 mounts a TD Explorer
R100 [1] to aid the dead reckoning navigation and to measure
the water currents. The acoustic modem on REMUS 100 allows
the vehicle to transmit and receive basic messages underwater.
In addition to the package described, REMUS 100 is equipped
with digital ultrashort baseline–acoustic positioning system
(DUSBL-APS). This is composed of an external transponder
array, and a receiver array with onboard electronics processing.
The DUSBL-APS is essential for docking the AUV, as it enables REMUS 100 to determine the position of the USV on the
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SARDA AND DHANAK: LAUNCH AND RECOVERY OF AN AUV FROM A STATION-KEEPING UNMANNED SURFACE VEHICLE
surface. The receiver array is mounted on the nose of AUV and it
consists of a single transceiver configuration composed of four
transducers or acoustic microphones. To complete the DUSBLAPS, a transponder needs to be mounted on the target dock. The
DUSBL-APS on REMUS 100 is based on estimating the distance between the target, where a transponder is mounted, and
the transceiver on the AUV, and the time it takes for a specific
acoustic signal from the source to reach the target. In particular, an acoustic pulse is transmitted by the AUV and when the
transponder detects the signal, it responds with its own acoustic
pulse. When this return pulse is detected by the transceiver on
the AUV, the DUSBL-APS is able to convert the time of transmission into a range, representative of the distance between the
AUV and the USV line on which the transponder is mounted.
In addition, the four transducers on the transceiver are able to
measure the vertical and horizontal bearings, based on the direction from which the acoustic signal reached them. The system
on the AUV can interrogate the transponder every three seconds. The time delay necessary for the signal to loop is range
dependent. The major advantage of the DUSBL-APS is its accuracy in the estimates, which enables REMUS 100 to reach the
desired target within 1 m. Numerous sea trials have been carried out to evaluate the performance of the DUSBL-APS. These
results are described in details in [2] and summarized in [3].
Although the ultrashort baseline (USBL) performance matched
the system specifications, sea trials highlighted some major limitations of the system. The main limitation of the DUSBL-APS
is that REMUS 100 only utilizes its USBL system as a secondary instrument to correct a predetermined cross target path.
The DUSBL-APS on REMUS 100 therefore does not serve as
the main sensing device to determine the vehicle’s path. For this
reason, if the AUV is programmed to reach a target location that
is not within 25 m of the acoustic estimate, the information of
the DUSBL-APS is assumed to be erroneous and is ignored by
the main guidance navigation and control (GNC) system on the
AUV. In such a case, the AUV simply dead reckons to the predetermined target location without any aid from the DUSBL-APS,
leading to large docking errors [2]. This limitation implies that
in order for the DUSBL-APS to be utilized for L&R from a USV,
the docking target needs to be stationary as the AUV performs
its docking maneuver. Another limitation of the DUSBL-APS
on REMUS 100 is its inability to consistently detect the acoustic
response, when the vehicle is located further than 50 m from the
transponder. This can become a major problem if the vehicle is
operating in an environment heavily disturbed by currents and
waves, which can compromise the dead reckoning navigation
of the AUV.
The WAM-V 16 (see Fig. 1) is a twin hull, pontoon style
USV. The vessel structure consists of two inflatable pontoons, a
payload tray connected to the pontoons by two supporting arches
and a suspension system. This USV is designed to mitigate the
heave, pitch, and roll response of the payload tray when the
vehicle operates in waves. The vehicle’s physical characteristics
are given in Table II.
A modular GNC system was developed to enable USV’s autonomy [4]. The GNC system is housed in a plastic, waterproof
box, which contains a single board computer, an IMU with GPS
Fig. 1.
291
WAM-V 16 USV during on-water tests in the Intracoastal Waterway.
TABLE II
PRINCIPAL CHARACTERISTICS OF THE WAM-V 16
capability, a tilt-compensated digital compass, a RF transceiver,
a pulse with modulation signal generator, and a custom-built
printed circuit board for power distribution and communications between the computer and instrumentation. A detailed
description of this GNC system can be found in [5]. For the
purpose of this project, the key components of the GNC system
are the sensor suite (IMU/GPS and digital compass), computer,
and RF transceiver. The IMU/GPS is an Xsens MTi-G sensor,
which is used to estimate the position and orientation of the
USV during operations. The GPS is wide area augmentation
system (WAAS) enabled and can provide up to 1 m accuracy
in both latitude and longitude, depending on cloud cover and
satellite availability. The digital compass is used to monitor the
vehicle heading and has a resolution of 0.1◦ . The lightweight
communication and marshalling (LCM) system [6] is utilized as
the underlying architecture for the GNC software. Sensor data
was transmitted using drivers incorporating the LCM system to
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292
Fig. 2.
IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 44, NO. 2, APRIL 2019
LARS design.
the control architecture, and logged at 4 Hz. A handheld remote
control is used for operating the thrusters and the actuators in
manual mode and for initiating autonomous operations.
An AIRMAR Weather Station 100WX was installed on the
WAM-V 16. This ultrasonic anemometer is used to measure the
apparent wind speed and direction at a sample rate of 1 Hz. The
dynamic range of the anemometer is 0–40 m/s with a resolution
of 0.1 m/s. The anemometer is located in an elevated position
at the aft end of the payload tray to avoid the effects of wind
blockage and interference from other structures. Owing to the
relatively small size of the USV, it is assumed that the wind speed
and direction measured by a single point sensor is representative
of the wind flowing past the entire vessel.
B. Launch and Recovery System (LARS)
An automated LARS has been designed and implemented
to accommodate transporting a REMUS100 AUV onboard a
WAM-V 16 USV. A flexible line was mounted on the USV to
provide a connection tether to the AUV during L&R. This line
can be extended into the water during recovery or stay retracted
on board the USV otherwise. A weighted hydrofoil at the end
of the line is used to keep the line taut underwater. The latch
on the AUV is necessary to allow the connection to take place.
A winch is necessary to allow the line assembly to be moved
in and out of the water. A second winch is used to move the
carriage structure in and out of the water surface, to deploy the
AUV during launch, and to extract it onboard the USV during
recovery. The set up for this type of system is shown in Fig. 2.
The possibility of automated L&R of an AUV from a USV
platform has been explored previously through simulations [7],
[8] and subsystem testing [9]. This capability has been used to
enable the USV to deploy or intercept the AUV, while both vehicles are in motion. An alternative method, which is described
here, is letting the USV station-keep, while the AUV is launched
and subsequently recovered. In either case, launching is accomplished through lowering and subsequently releasing the AUV
from the top tray of the USV onto the water surface. The USV
may be station-keeping or mobile during launch, depending on
the operating conditions. In calm conditions, the USV keeping
station until the AUV is launched may be desirable, while in
the presence of waves, wind, and currents, a mobile USV, controlled by the adaptive heading and speed controller presented
in [10] better ensures that the AUV is safely clear downstream
of the USV once launched. Furthermore, launching the AUV
from a station-keeping USV may be a necessity, if the operating
environment does not guarantee adequate space for the surge
motion of the USV, which is required for a mobile launch. Recovery is initiated through requiring the AUV to navigate toward
an acoustic homing beacon on a taut line suspended from the
USV. The taut line serves as docking target and as a connecting link between the two vehicles during L&R. Throughout the
entire operation, the USV approximately keeps station on the
water surface. Once the docking is complete, the USV moves
forward towing the AUV as it is extracted onboard the USV via
a customized winch mechanism. During recovery, requiring the
USV to station-keep may be desirable under certain operating
conditions to maintain more stable communication between the
two vehicles. Station-keeping of the USV also facilitates implementation of a simplified control scheme for identifying a
desired state for recovery and for driving both systems toward
this desired state through acoustic positioning.
Since this LARS enables two alternatives, using either a mobile or station-keeping USV to launch and recover an AUV,
an algorithm that can formulate an intelligent decision on an
optimal method for L&R based on sensor data and simulation estimates was also developed and implemented on the
systems. More precisely, data collected during in-water field
testing of the station-keeping controllers described in [4] and
[5] is utilized to train a predictive model for position and heading error of the USV. Based on these predictions, the USV
can then dictate the methodology for L&R that maximizes
the probability of success. While the effort of this algorithm
was essential to test these two alternatives for L&R, the focus
of this paper is on the sea trials of L&R of an AUV from a
station-keeping USV.
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SARDA AND DHANAK: LAUNCH AND RECOVERY OF AN AUV FROM A STATION-KEEPING UNMANNED SURFACE VEHICLE
II. BACKGROUND
In the past decade, much research has been devoted toward AUV L&R [11], [12]. Most previous studies suggested
that the most effective way to overcome the challenge of recovering an AUV autonomously is by letting it home to a
docking station, which may be anchored to the seafloor [13]–
[16] or towed by the mother vessel [17], [18]. In addition, several previous works provide manned options to overcome the
challenge [19]–[21].
In [22], the main aspects that need to be considered in designing a device for launching an AUV from a USV are briefly
explained. This launching system consists of a USV mounted
cradle that carries REMUS 100. Upon command, a servomechanism lowers the aft end of the cradle, and the AUV slides
off into the water. Once in the water, REMUS 100 waits for
a command to begin its mission. Overall, this method represents a very robust and secure way to deploy an AUV, but the
proposed design does not allow for AUV recovery from the
same USV.
Another approach to launch an AUV is described in [20].
The intent of this project was to allow one man to easily and
safely launch and recover a Gavia class AUV from a small
inflatable boat. The LARS developed for this project was not
autonomous, but it required minimum human interaction. The
final design selected for this purpose consisted of a crane that
allowed launching the AUV from either side of the boat. A
combination of pulley wheels and a winch were used to lower
the AUV into the water.
Hydroid, a Kongsberg company, has developed a mobile dock
for REMUS 100 [18], as evolution of their previous designs [23],
[24]. The idea was to allow REMUS 100 to intercept and mate
with a submerged towed docking cone, using USBL navigation
to refine the position of the moving dock underwater. This mobile docking system, consisting of a depressor wing, a capture
system, and a transponder, was designed with the intent of being
towed behind a manned surface vessel. A REMUS 100 AUV was
programmed to approach the docking cone, which was towed
at approximately 1 m/s. The docking cone then locked onto the
vehicle and, once sensed, the surface craft was instructed to
recover the AUV and dock assembly. The progress made by
Hydroid represented a major advance toward unmanned AUV
recovery from a mobile manned platform. However, the towable station proposed by Hydroid [24] still represents a type of
docking device that may not always be ideal for L&R. Towing
an entire docking station affects the performance of the USV,
in terms of increased payload and drag. Small USVs, such as
the WAM-V 16, have limited payload capacity and can only
output limited thrust. In addition, the physical size of a docking
station is usually too large to be carried on board a USV, meaning that it would have to be dragged during the entire mission.
Towing a system also puts limitation on operating in congested
environments.
Improvements to the standard designs for LARS or docking station have been proposed throughout the years [25], [26].
Specifically, Park et al. [25] demonstrates that the precision at
which an AUV approaches the dock can be dramatically im-
293
proved by adding a vision system on either the dock or the
vehicle itself. In [26], the use of an electromagnetic field as
alternative to acoustics is explored, for locating the docking station underwater. The use of a vision system to recover an AUV
at the surface from a USV is discussed in [27]. Specifically, a
hybrid coordinated maneuver for docking a USV on a torpedo
shaped AUV is introduced. This controlled maneuver is formulated based on visual information to estimate the AUV position
and attitude relative to the USV, with the intent of guiding the
USV to dock on the free-floating AUV at the surface. The approach here is to let the AUV complete its mission by sending
its GPS position at the surface to the USV. The USV would
then drive to that location and attempt to localize the AUV via
vision. The hybrid maneuver is then formulated and performed,
enabling the USV to dock onto the AUV. Docking on the sea
surface can be challenging in the presence of waves.
It is evident that none of the systems described in this literature review can be utilized to launch and recover REMUS
100 from the WAM-V 16. However, the difficulties encountered in the manned L&R of an AUV are somewhat similar to
the autonomous process and were given appropriate attention.
The same can be said about the similarity between designing a
docking station and implementing a LARS.
III. L&R SEA TRIALS AND RESULTS
The novel approach to L&R an AUV consists of utilizing a
station-keeping USV to run operations. Testing, via sea trials,
of L&R of an AUV from a USV, utilizing the custom designed
LARS shown in Fig. 2, were conducted in closed waters at North
and South Lake, Hollywood, FL, USA. These tests were necessary to validate the theoretical analysis presented in [7], utilizing
the subsystems developed for this research and introduced in [3],
[9], [10], and [28]. The results of the sea trials presented in this
section provide verification that a station-keeping USV can be
utilized to L&R an AUV autonomously. Station-keeping was
accomplished utilizing the station-keeping controller presented
in [28] and [29] on the USV and the USBL system presented in
[3] and [9] on the AUV.
Before the initiation of the any of the tests, the USV was
commanded to autonomously station-keep at different headings
within the area where the sea trials were conducted. Doing so
allowed the system to acquire knowledge about the operating
environment, enabling it to autonomously deduce the most appropriate approach for L&R: mobile versus station-keeping. If
the high level planner on the USV deduced that the ideal method
for L&R was station-keeping, a specific protocol was initiated
to begin the desired autonomous operation. L&R were tested
independently and their results are described in Section III-A
and III-B, respectively.
A. Autonomous Launch of an AUV From
a Station-Keeping USV
For this test, the AUV was mounted on the LARS on board
the USV, as shown in Fig. 3.
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294
Fig. 3.
IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 44, NO. 2, APRIL 2019
WAM-V 16 transporting REMUS 100 to launch site.
TABLE III
MEAN AND STANDARD DEVIATION OF APPARENT WIND SPEED, AND RELATIVE
WIND DIRECTION DURING LAUNCH
To quantify the disturbance sensed by the USV during the
sea trials, mean and standard deviation of wind speed and angle
of attack, over 140-s duration launching operation, are given in
Table III.
The estimates and the subsequent decisions generated by
the high level planner on the USV prior to starting the sea
trials are given in Table II. These included: ideal heading for
launching the AUV ψi , expected average station-keeping posiēψ for the USV during launch,
tion error ēr , and heading error ideal station-keeping controller and preferred method for
launch.
Once autonomous mode was initiated, the USV was commanded by the high-level system to station-keep at a specific position, with the ideal heading identified. After the USV reached
steady state, the launching routine was started within the LARS.
After a safety check, the AUV was therefore lowered into the
water and held at the surface for a few seconds [see Fig. 4(a)],
allowing for a fault check, and finally released [see Fig. 4(b)].
A few seconds after release, the AUV carriage structure was retracted and the USV commanded to leave the deployment site,
leaving the AUV behind [see Fig. 4(c)].
To easily identify the time at which the AUV was launched,
the USV pitch angle θW is plotted in Fig. 5. A 5◦ offset can in
fact be spotted at the instant when the AUV is deployed.
In the controlled environment with low winds (< 2 m/s) and
no waves, REMUS 100 was effectively launched from the
station-keeping WAM-V 16, without running in any failure.
x, y, and z position of the systems during launch are shown
in Fig. 6. As it can be seen, the AUV always sits within 1 m
of the USV, even after it is deployed. The z position of the
AUV, which is maintained almost constant before and after de-
ployment, highlights that the external disturbance was almost
irrelevant. As a result, the station-keeping controller was able to
perform very well and REMUS 100 was able to settle steadily
right in-between the USV’s pontoons without colliding. The excellent performance of the station-keeping controller can also
be observed in Fig. 7, which shows the vehicles’ heading during
the sea trials. The USV’s heading during launch is in fact kept
within 10◦ of its desired value, while the AUV’s remains almost
constant after deployment.
The results from the launch test confirmed that in the presence
of disturbance, it becomes very difficult to effectively launch the
AUV with the USV station-keeping. This is because, once the
AUV is deployed, there is no way to control it until it starts its
mission.
B. Automated AUV Docking to a Station-Keeping USV
The docking test started with the vehicles separated in the
water, as shown in Fig. 8. Since the REMUS 100 pressure vessel
developed as part of the LARS, as shown in Fig. 2, had not been
implemented at the time the sea trials were conducted, a pair
of brackets was used in place to enable the AUV to establish a
connection with the docking line underwater.
To quantify the disturbance sensed by the USV during the
sea trials, mean and standard deviation of wind speed and wind
angle of attack, over the day during which recovery was tested,
are given in Table V (recovery duration is 50 000 s).
The estimates and the subsequent decisions generated by the
high-level planner on the USV prior to starting the sea trials are
given in Table VI. These included: ideal heading for recovering
ēr ,
the AUV ψi , expected average station-keeping position error and heading error ēψ for the USV during launch, ideal stationkeeping controller and preferred method for recovery.
Once autonomous mode was initiated, the USV was commanded by the high-level system to station-keep at a specific
position (see HOME in Fig. 9) with the deduced ideal heading.
The depressor wing, docking line, and transponder assembly
(see Fig. 2) were then lowered into the water at a depth of 4 m
by the USV. The transponder on the docking line periodically
transmitted acoustic pings, constituting a homing beacon. In the
meantime, the AUV was commanded to attempt docking by following the preprogrammed path shown in Fig. 9, at a constant
speed of 1 m/s, and altitude of −1 m.
For this test, the AUV was manually launched at the deployment site marked by the green buoy in Fig. 9. The AUV initiated
its docking routine by acquiring a GPS fix, using it as a reference point to dead reckon to waypoint 1 (WP1) in Fig. 9. When
the AUV reached WP1, more accurate navigation was enabled
through engagement of the DUSBL-APS, which monitors and
homes in on the acoustic pings transmitted by the transponder mounted on the docking line connected to the USV [see
Fig. 10(a)]. While the AUV navigated, the USV kept station
maintaining heading and position on the surface at HOME in
Fig. 9. Using the DUSBL-APS, REMUS 100 was therefore able
to adjust its trajectory, correcting all accumulated errors and finally home to the docking line connected to the WAM-V 16 [see
Fig. 10(b)]. The docking attempt ended when the AUV reached
HOME in Fig. 9, thus rising to the water surface in-between
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SARDA AND DHANAK: LAUNCH AND RECOVERY OF AN AUV FROM A STATION-KEEPING UNMANNED SURFACE VEHICLE
Fig. 4.
Snapshots from station-keeping launch. (a) AUV is lowered in the water. (b) AUV is deployed. (c) AUV carriage structure is retrieved.
Fig. 5.
WAM-V 16 pitch angle (◦ ) at launch.
Fig. 6.
x position (m), y position (m), and z position (m) during launch.
Fig. 7.
295
Heading (◦ ) during launch.
Fig. 8. WAM-V 16 station-keeping with depressor wing assembly deployed,
and REMUS 100 equipped with docking brackets, attempting to dock to the
line underwater.
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IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 44, NO. 2, APRIL 2019
TABLE IV
STATION-KEEPING ERROR ESTIMATES, PREFERRED STATION-KEEPING
CONTROLLER, AND OPERATING METHOD
TABLE V
MEAN AND STANDARD DEVIATION OF APPARENT WIND SPEED, AND RELATIVE
ANGLE OF ATTACK DURING RECOVERY
TABLE VI
STATION-KEEPING ERRORS ESTIMATES, PREFERRED STATION-KEEPING
CONTROLLER, AND OPERATING METHOD DEDUCED FOR RECOVERY
Fig. 9. AUV predefined path for recovery. The deployment site was marked
with a green buoy, WP1 was marked with a red buoy, and the AUV homing
location (HOME) was marked with a yellow buoy.
the USV’s pontoons, connected to the docking line and ready to
be extracted from the water. For a successful run, REMUS 100
needed to be connected to the line [see Fig. 10(c) and (d)]. The
missed connection between the two vehicles represented a failed
attempt. A sequence of eight runs were completed, leading to 3
successful docks and 5 failed attempts.
During each run, REMUS 100 attempted to dock to a stationkeeping WAM-V 16 as described before. The path of the AUV
for each run, and the position of the station-keeping USV
recorded throughout the entire day are shown in Fig. 11. As
can be seen from the figure, REMUS 100 consistently followed
the preprogrammed path (see Fig. 9). In addition, the WAM-V
16 successfully maintained position over the course of the sea
trials. Furthermore, the AUV achieved almost the same heading
as the USV anytime it attempted to dock, as can be seen from
Fig. 12.
The recorded surge velocity of the AUV throughout each
run shown in Fig. 13 also confirms that the AUV performed as
expected, reaching the desired speed of 1 m/s and maintaining
it until it reached WP1. Since the vehicle is then forced to
make a steep turn of more than 90◦ , its surge velocity drops
momentarily to ∼0.6 m/s. After that, the AUV picks up speed
again, reaching a steady speed of 0.9 m/s, which is less than the
desired value. This is because when the DUSBL-APS engaged at
WP1, it resulted in the control effort on the AUV being devoted
to making heading and pitch adjustments at the cost of the speed.
Overall, the speed controller performance of the AUV can be
considered sufficient for purpose of this test; however, the same
cannot be said about the AUV altitude control. Fig. 14 shows
the depth of the AUV throughout each run. The goal was for
the AUV to dock at z = −1 m under water. In Fig. 14, it can
be seen that the AUV does not maintain the desired depth of
−1 m. Thus, the depth at which the AUV attempts to dock is
very unpredictable, since it constantly oscillates between −0.5
and −1.5 m. Unfortunately, the poor controller performance
cannot be compensated in anyway, since the control system
on the REMUS 100 AUV is not accessible by the user, and it
cannot be modified. The inability of REMUS 100 to maintain
a steady depth when docking is the likely reason why the AUV
was not able to consistently dock. Apart from the deficiency in
maintaining steady depth zR and correspondingly a steady pitch
angle θR (see Fig. 15), the AUV performance was consistent
in all runs for all other parameters. Since a few differences are
indefinable between successful and unsuccessful runs, two plots
are shown: RUNs 1, 5, 6, 7, and 8 where the vehicle did not dock
(see Fig. 15, top subplot) and RUN 2, 3, and 4 where REMUS
100 successfully docked (see Fig. 15, bottom subplot).
As can be seen, the AUV pitch angle variations are similar
throughout each run, with deep troughs at the start and at the
end. While the troughs at the beginning are associated with the
diving motion of the AUV, the final troughs were unexpected;
for the runs when the docking did not occur, the final AUV
pitch angle just before reaching the docking line was ∼ −28◦
and may have been the cause for the AUV not docking. This is
because the significant negative pitch of the AUV at the end of
the run creates a scenario where the vehicle attempts to establish
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SARDA AND DHANAK: LAUNCH AND RECOVERY OF AN AUV FROM A STATION-KEEPING UNMANNED SURFACE VEHICLE
297
Fig. 10. Snapshots from docking tests. (a) Line and transponder assembly viewed from GoPro mounted on the depressor wing underwater. (b) REMUS 100
docking to the line underwater. (c) and (d) REMUS 100 docked.
Fig. 11. Vehicles’ path (m) during docking. The AUV trajectories for attempted docking Runs 1–8 are shown; the station-keeping USV position is
marked with an “x.”
Fig. 12.
Heading (◦ ) during each run of recovery.
Fig. 13.
REMUS 100 surge velocity (m/s) during each run of recovery.
Fig. 14.
REMUS 100 z position (m) during recovery.
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298
Fig. 15.
IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 44, NO. 2, APRIL 2019
REMUS 100 pitch angle (◦ ) during recovery.
a connection with the docking line at an angle. It is very probable
that the changes in pitch angle at the end of the run are caused by
the AUV attempting to rapidly correct for the error in its depth
from the desired value. The runs during which REMUS 100 was
able to successfully dock are the ones that show minimal change
in pitch angle at the end of the run.
IV. CONCLUSION
At sea testing, and demonstration of an automated L&R of
an AUV from a station-keeping USV have been carried out. A
methodology for L&R of a REMUS 100 AUV from the WAMV 16 has been identified. According to the approach considered
here, the USV station-keeps heading and position at the surface,
while the AUV is lowered in the water during launch, and as the
AUV approaches the USV to dock underwater during recovery.
The docking system is composed of a line connected to an
acoustic transponder beacon, and a depressor wing that keeps
the assembly taut.
Automated L&R sea trials of an AUV, using a station-keeping
USV, highlighted important aspects of the concept of operations.
Specifically, the results lead to six major conclusions.
1) L&R of an AUV from a station-keeping platform can
be a useful alternative to a mobile approach, when the
operating conditions allow it. Furthermore, cutting down
the number of autonomous systems in motions from two
to one, the architecture scheme is simplified, thus reducing
room for error.
2) A complex motion planner is no longer needed for the
USV, since the vehicle simply needs to station-keep at a
desired heading and position at the point of AUV recovery.
The AUV is therefore the only vehicle required to plan and
follow a trajectory for homing onto the docking line.
3) Reliable communication at a defined update rate, which is
very difficult to achieve via acoustics, is no longer crucial,
since only one single message is necessary to initiate the
recovery routine. This simply consists of a desired recov-
ery pose for the USV, where the AUV will be recovered.
Furthermore, communication takes place only after the
vehicles are physically connected or, in case of failure,
to initiate new recovery attempts. In the case of mobile
recovery, communication between the vehicles is a major
bottleneck, since the two vehicles need to continuously
share their state in real time. This is extremely difficult,
since the USBL performance is dependent on the distance
between the vehicles, the water depth, and the proximity
to the sea bottom.
4) L&R of an AUV from a station-keeping USV also enables the operation to be carried out over a confined region, which may be important when space for docking
maneuvers is limited.
5) The presence of waves during launch can compromise the
operations when utilizing a station-keeping USV as platform for L&R. The alternative approach [2] involving the
USV in motion during the launching process is preferred
under such operating conditions.
6) Presence of cross currents can compromise launching operations when utilizing a station-keeping USV as platform
for L&R. Currents aligned at an angle to the vehicle can
induce rotation of the AUV after it is deployed, possibly
forcing it to collide with the USV, if the latter is keeping
station.
7) The USV needs to be in motion once the AUV docks,
towing the AUV behind it as it is extracted onto the USV.
8) Identifying an ideal vehicle heading under given environmental conditions can dramatically improve the performance of the low-level controllers, thereby reducing the
possibility of failure during L&R.
The results of the sea trials presented here show that three out
of eight recovery attempts were successful, thus demonstrating
that using a station-keeping USV to recover an AUV can be a
strong alternative to the mobile approach. Sea trials [3], [10]
and simulations [8] of AUV recovery using a moving USV have
in fact shown that the recovery task can be compromised if each
individual subsystem (e.g., USBL, high level planner, trajectory
tracking controller, and LARS) does not perform perfectly.
Since two distinct methodologies for L&R can be carried out
using the same systems, a new algorithm could be developed
for making an appropriate selection based on the prevailing
environment. The sea trials described here were conducted in
a protected area in the absence of any significant waves. Additional data, representative of more adverse ocean conditions
would improve the L&R capability as well as the performance
of low-level controllers under such conditions. In addition, use
of the latching system on the AUV shown in Fig. 2 would significantly improve the success rate for docking. Finally, the vehicles
can be programmed to reattempt the task, whenever the AUV
misses the docking line.
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Edoardo I. Sarda received the B.S. degree in mechanical engineering from Lake Superior State University, Sault Ste. Marie, MI, USA, in 2012, and
the Ph.D. degree in ocean engineering from Florida
Atlantic University, Dania Beach, FL, USA, in 2016.
He is currently an R&D Engineer in the field of collaborative and mobile robotics. His research interests
include autonomous vehicles, launch and recovery,
human robot collaboration, automated systems and
control.
Dr. Sarda is a member of the IEEE Oceanic Engineering Society.
Manhar R. Dhanak received the B.Sc. degree (honors) in mathematics from Imperial College, University of London, London, U.K., in 1976, and the Ph.D.
degree in applied mathematics from the University of
London, London, U.K., in 1980.
He is currently a Professor of Ocean Engineering
and the Director of the Institute for Ocean and Systems Engineering (SeaTech), Florida Atlantic University, Dania Beach, FL, USA, where he has been
since 1990, having previously been a Senior Research Associate with the University of Cambridge,
Cambridge, U.K. (1989–1990) and a Research Scientist with Topexpress Ltd.,
Cambridge, U.K.
Prof. Dhanak is an Associate Fellow of the American Institute of Aeronautics
and Astronautics.
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