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Operational Aspects of an Ocean-Going
USV Acting as Communication Node
Martin Kurowski, Erik Rentzow, Detlef Dewitz, Torsten Jeinsch, Bernhard P. Lampe,
University of Rostock, Rostock/Germany, martin.kurowski@uni-rostock.de
Sebastian Ritz, Robert Kutz, Florin Boeck,
TU Berlin, Berlin/Germany, sebastian.ritz@tu-berlin.de
Sergej Neumann, David Oertel,
Karlsruhe Institute of Technology, Karlsruhe/Germany, sergej.neumann@kit.edu
Abstract
This paper describes the basic characteristics of a six degrees of freedom dynamic model of an
innovative ocean-going unmanned surface vehicle. The model will be used in an explicit and implicit
way to ensure the operation of the autonomously acting vehicle, which serves as communication node
between surface and underwater parts of a complex deep-sea monitoring system. Properties of the
acoustic communication have been taken into account when designing the unmanned surface vehicle.
Finally, it has been built as a shallow submerged vehicle with water surface-piercing towers to assure
a reliable acoustic communication and positioning link up to a depth of 6000m even in heavy sea
states. As the vehicle motion has a decisive impact on its operation, the basic characteristics of the
motion of the vehicle in waves have been investigated from the quasi-static case using potential theory
to simpler dynamic models for the specific degree of freedom. Further, these models can be used to
predict the impact of the prevailing environmental disturbances during the vehicle operation.
1. Motivation
Worldwide increasing activities for exploring and surveying the seabed can be observed in the mari-
time industry. This is mainly due to the rising demands on winning raw materials from the deep-sea in
up to 6000 m depth. In order to comply with these demands, high-capacity monitoring systems have
to be developed. Within the research project SMIS (Subsea Monitoring via Intelligent Swarms, fund-
ed by the German Federal Ministry of Economics and Technology (BMWi - FKZ 03SX348)), differ-
ent unmanned underwater and surface vehicles will be designed and combined to an innovative sys-
tem for efficient monitoring of large-scale deep-sea areas. In that way, the project partners (IMPaC
Offshore Engineering GmbH, ENITECH Energietechnik – Elektronik GmbH, TU Berlin, Karlsruhe
Institute of Technology, Leibniz Institute for Baltic Sea Research Warnemünde, University of Ros-
tock) are developing specialized vehicles, including an unmanned surface vehicle (USV), autonomous
underwater vehicles (AUVs) and a seabed station (SBS), which operate as an intelligent team to ac-
complish various missions. The full autonomous deep-sea operations last several days, where the
AUVs can recharge at the SBS. For more details about the complexity of the SMIS system see Boeck
et al. (2014).
The central element of the SMIS system is the autonomously acting USV, which serves as communi-
cation node between the surface and the underwater parts. In that way, the vehicle provides geo-
references to the underwater vehicles via ultra-short baseline (USBL) acoustic link and sends states of
the complete system to an operator via satellite communication. The USV is an ocean-going vehicle,
which has been designed as a shallow submerged vehicle with water surface-piercing towers. This
concept assures a reliable acoustic communication and USBL-positioning even in heavy sea states
Ritz et al. (2014). Moreover, the vehicle is designed for long endurance operations up to seven days,
where continuous power supply is provided by a hybrid energy concept consisting of lithium polymer
batteries and a solid oxide fuel cell (SOFC). It will reduce the acoustic noise that would be generated
by conventional combustion engines. Certainly, the vehicle is equipped with various sensors for
measuring the position, attitude and velocities. For the purpose of enhancing the autonomy, the vehi-
cle utilizes an Automatic Identification System (AIS) and radar system and integrates itself into the
487
shipping traffic. Detailed information about the automation of the USV can be found in Rentzow et al.
(2015).
As the vehicle acts autonomously, it has to be equipped with a hierarchical system for guidance, navi-
gation and control (GNC) to fulfill the respective missions. The main application of the USV is to up-
date the positions of the underwater vehicles, even in heavy sea states when the communication capa-
bility is reduced due to large roll and pitch angles of the surface vehicle. The GNC system is based on
a dynamic model of the vehicle’s motion, which is e.g. used for predicting states, synthesis of hetero-
geneous low-level and high-level controllers and diverse mission planning tasks.
For conventional operations of USV for hydrographic surveys or special applications like Search-and-
Rescue, Majohr and Buch (2006), Kurowski and Lampe (2014), it is sufficient to consider the motion
of the vehicle in three degrees of freedom (3DoF). In contrast to that, the total motion of the SMIS
USV has to be considered in 6DoF as it has been addressed in several publications, e.g. Krishna-
murthy et al. (2005). During the design phase, calculations based on potential theory and computa-
tional fluid dynamics (CFD) can be made to set-up or change significant design parameters of the ve-
hicle, Kutz (2015). Furthermore, they can be used to calculate extreme values in motion, which can be
applied in a predictive way in the guidance system of the vehicle. In practice, it is a cumbersome task
to identify the unknown parameters of such nonlinear models, due to strong couplings of the motion
variables, measurement noise and unknown disturbances. Hence, simplifications should be made to
identify the basic characteristics of the dynamic behavior of the vehicle, especially in roll and pitch
motion. In order to parameterize the resulting models, special maneuvers have to be carried out to de-
couple the motions and identify the corresponding parameters. Afterwards, these simpler models can
be used to update the mission settings and ensure the reliable operation as communication node.
2. Properties of the acoustic communication
This section starts with a short introduction to acoustic underwater communication and its physical
properties. Afterward some of the difficulties in this field are investigated, supported by results of
field experiments. The acquired insight is then used to discuss some design specifications of the USV.
Compared to terrestrial communication, the underwater channel is a harsh environment in terms of
data transfer. Electromagnetic waves, used in common radio communication, will be absorbed by
seawater within less than 100 m. Only with frequencies lower than 300 Hz it is possible to communi-
cate over larger ranges; however this requires big antennas and excessive transmission power, Peach
and Yarali (2013). The only operational solution for long range communication in underwater envi-
ronments is acoustics. Underwater acoustic systems however, yield some particularities that need to
be addressed.
2.1. Sound velocity
The sound velocity in water mainly depends on three factors: pressure, temperature and salinity.
While pressure primarily correlates with the water depth, temperature and salinity can vary widely
from one sea to another and changes over the water column. Due to the change of sound velocity,
acoustic signals will be deflected from their original propagation direction. A popular example of this
phenomenon is the SOFAR channel, where, through refraction effects, sound waves can propagate
thousands of kilometers before dissipating, Lerch et al. (2009). These refraction effects also concern
the communication capabilities of the USV. Fig. 1 shows how sound waves get deflected for different
radiation angle when transmitting a signal from the surface. Here, the computed sound ray refraction
is solely based on Snell’s Law, Jensen et al. (2011), neglecting all other possible influences like shad-
owing and air bubbles. The illustration utilizes a typical sound velocity profile for the Middle Atlantic
Ocean, showed in simplified terms on the left side. As shown by the diagram, sound waves that prop-
agate vertically from top to bottom are less affected by refraction, than those signals with a horizontal
angle of propagation. Considering this, the communication beam has a curved body. For simplifica-
488
tion reasons, this can be neglected when assuming a reduced radiation angle, which leads to the ap-
proximation of a communication cone.
Fig. 1: Simplified sound velocity profile for the Middle Atlantic Ocean (left), sound diffusion with
varying radiation angle (right)
2.2. Absorption
Due to divergence, acoustic signal intensity decreases quadratic with distance to the source. Further-
more, frequency dependent relaxation attenuation impairs signal energy and thus communication
range. In order to identify the maximal communication range under real conditions, multiple experi-
ments were conducted in the Middle Atlantic Ocean, Neumann et al. (2015). The experiments have
shown that communication was still possible with a slant range of more than 9000 m, however the
success ration of delivered packages dropped rapidly at ~8500 m slant range from sender to receiver,
Fig. 2.
Fig. 2: Delivered and failed packages due to the communication slant-range
489
2.3. Acoustic noise
Like in every communication channel, noise is almost ubiquitous. Natural acoustic noise is introduced
mainly at the water surface by wind, streams, rain and waves. In addition to the volume, air bubbles,
caused by waves, produce big signal absorption at the water surface. This reduces the communication
range of surface to surface transmissions, depending on the weather conditions. For a transceiver at
the water surface, this can even result in a total communication break down during heavy sea. How-
ever, the risk of communication loss decreases rapidly when placing the acoustic transceiver further
away from the surface. Also an acoustic baffle, that mutes the surface noise, can help to increase the
signal to noise ratio for a surface transceiver. These considerations where taken into account during
the design of the USV.
In addition to the natural noise sources, artificial noise like motor noise leads to a significantly in-
creased noise pollution. The impact on noise level of a passing ship has been investigated experimen-
tally by measuring the noise with an acoustic modem that was attached to a GPS equipped buoy. De-
tails to the experimental set-up can be found in Neumann et al. (2015). The modem was located at a
depth of around two meters and the research vessel passed by several times with a constant velocity of
8 knots. Fig. 3 shows the impact of the passing ship on the noise level. Besides noise of other vehi-
cles, the propulsion system of the USV produces its own noise. These effects and their implication to
the communication capabilities of the USV could not be investigated so far and need further analysis.
Fig. 3: Impact of a passing ship on the noise level
3. The USV as communication node
During a SMIS team operation, the USV acts as a communication node between the underwater seg-
ments and the control station (operator), which can be placed on a ship in the operation area or on-
shore. Furthermore, it provides geo-references (global satellite based position data) to the underwater
vehicles by utilizing acoustic USBL transmissions to correct the resulting drift of the position due to
dead reckoning. The surface communication is realized by different redundant radio and satellite te-
lemetry links, depending on distance and data volume. While the underwater vehicles perform meas-
uring and monitoring tasks in up to 6000 m depth, the USV has to contact them in cyclic intervals to
490
transmit position references and control commands or to receive state information. Fig. 4 shows a
SMIS team, consisting of an USV, several AUVs performing a lawn-mowing maneuver and a SBS. In
addition, a cone indicates the acoustic communication and localization area. Based on the experiments
described in Section 2, the sound beam is approximated by a cone, whose lateral surface depicts the
maximal underwater communication range. The USV positions itself above the respective operation
area to assure the cyclic communication with the underwater vehicles.
Fig. 4: SMIS Team with USV as communication node
Depending on the depth of the respective communication partner, the communication area can be
defined to be the area at the base of the cone. Using the experimental determined maximum slant
ranges, the angle of beam spread and the radius of the base of the communication cone can be
calculated by trigonometrical relations, Fig. 5. Hence, the acoustical outshined volume or rather the
swept area is obtained by the circular surface. Table I shows the results of the experiments in the
Atlantic Ocean. The respective rows represent the data at the time of communication dropout for
several long distance measurements.
Table I: Experimental results of long-range communication and localization trials
Depth
[m]
Slant range
[m]
Angle of beam
spread [°]
radius of the
cone base,
[km]
swept area
[km²]
4800
8542
111.62
6.32
125.46
5000
8542
108.35
6.16
119.13
5000
9694
117.90
7.16
160.98
Fig. 5: Swept area and communication cone
491
In heavy sea states the reliable communication area shrinks to the intersection of all resulting sound
beams due to the pitch and roll motion of the USV. Transferred to the cone approximation, the angle
of beam spread is reduced by the pitch and roll angles and the swept area becomes an elliptic shape.
In that way, the vehicle used as deep-sea communication node has to be designed in a special way, as
described in Section 4. Furthermore, the automation system and the GNC system have to be devel-
oped to secure a reliable localization of the underwater vehicles even in case of harsh environmental
conditions. Therefore, the basic dynamic characteristics of the USV will be considered in Section 5.
4. Design background of the USV
The main requirements influencing the USV design are minimal motions in waves, self-righting
behavior after capsizing and minimal resistance. Furthermore, internal space for the equipment has to
be provided. In order to limit production costs and efforts, a simplified line arrangement is considered.
The requirement of a minimal response to waves and the fast recovery of stability after capsizing
seem to be inconsistent. Therefore, a submerged body with three surface piercing struts is considered
for the USV, Fig. 6. Moreover, this SWA-Concept (Small Waterplane Area) influenced design
reduces the excitation forces due to waves. As reaction to heeling due to lateral wind load, the struts
immerge, which generate additional buoyancy and counteract the heeling moment. Moreover, the
three surface piecing struts build the base for the antenna platform for terrestrial radio communication
and satellite data transfer and provide a ventilation trunk for the fuel cell. The operational components
and the payload are located underneath the waterline. The hull wraps the components and forms a
hydrodynamic beneficial shape. Most of the components are placed in flooded sections. This has the
advantage that heat is released into the environment, and possible gas leakages cannot lead to an
explosive mixture.
Fig. 6: Sectional view of the SMIS-USV, 1 sensor-platform, 2 fuel cell, 3 LiPo-batteries, 4 watertight
casted components, 5 gas container
The gas containers supply the hybrid energy concept. The electric motor is supplied by waterproof
batteries with an energy capacity for some hours of operation. The SOFC reloads the batteries
permanently and extends the operational range up to one week. Dry compartments can be found at the
top end of the submerged body and in the center. The fuel cell for instance has to be placed into a dry
environment. At the bottom of the USV a modular payload keel is arranged. It enables the user to
customize the vehicle to different operational tasks. In the SMIS-arrangement, the acoustic USBL-
492
modem is covered by a streamlined hood, but also scientific sensors like an ADCP (Acoustic-
Doppler-current-profiler), a multi-beam sonar or a sub-bottom-profiler are possible payloads. The
spatial separation of the heavy elements as batteries and payload at the bottom and air filled
compartments at the top leads to a low center of gravity. The center of buoyancy is higher; thereby a
positive initial stability is accomplished. The open hull design with flooded compartments makes the
vehicle light weight and manageable on deck of an expedition ship, but at the same time inertial
enough in operation state to counteract wave and wind loads. The weight in air is about 500 kg but the
hull displaces 1250 liters. This feature makes the USV the perfect choice as communication node
between satellites and underwater vehicles such as AUVs and stationary bottom based vehicles. For
the underwater communication, it is essential to reduce the body motion and keep position above
specific coordinates. Every movement (rolling or pitching) would scatter the emission of the acoustic
cone over a distance of up to 6000 m. An ambitious task is to determine the hydrodynamic properties.
Due to unsteady flow pattern and thereby generated pressure distribution on the vehicles surface,
spontaneously increasing moments, mostly in pitch, can appear. This phenomenon, based on the so
called Munk-Moment, Munk (1979), and the effect of waves generated from a vehicle on or near the
surface, has to be estimated for the autonomous operation over the full speed range. This pitch-/heave
instability is also known from SWATH-ships or submarines in surface operation. Therefore, many
different variants of the USV (variation of bow geometry, positions of center of gravity, position of
struts and so on) were analyzed by unsteady CFD with focus on the dynamic motion behavior over the
complete speed range. The final version has an optimized stern to reduce the resistance and due to the
arrangement of the struts the pitch-/heave instability could be prevented over the complete speed
range with an option to extend the speed range by 50%, Ritz et al. (2014). The results are shown in
Fig. 7. There are still heave and pitch motions with raising velocity, but no immediate instability.
Fig. 7: Results of the hydrodynamic calculations, resistance, heave and pitch over speed; Kutz (2015)
Besides analytical and computer aided calculations, a lot of hands-on experience was considered in
order to find a suitable design. From several on site expeditions to the Baltic Sea and the Atlantic
Ocean on German research vessels, some improvements to existing research equipment can be found
in the USV design. The complete stern geometry is made of robust fiber-reinforced plastic and is
easily removable in order to increase the accessibility of the complex electronic components. New
materials such as lightweight aluminum-foam sandwich panels are used to enhance stiffness and
resistance against wave loads. Moreover, hatch covers are installed on deck, thereby quick access to
493
data loggers, gas containers and the fuel cell is guaranteed. A very important aspect of working with
research vehicles on site is the launch and recovery procedure. The seaman’s task to recover the
vehicle can be simplified by adding big lugs or handles to the superstructure. The USV has a
tightened cable rope between bow tip and first strut. On the aft deck big handles are provided.
Nevertheless, cost efficiency is a design criterion as well. The CFD resistance optimized vehicle
length reduces the operational cost or increases the operational radius respectively. The midship
section geometry is greatly influenced by the dimensions of the fuel cell. This box-shaped section
continues along the parallel central body then bends with single-curved alloy-sheets towards the bow
tip. The stern is molded, to support a harmonic propeller inflow. To improve the propulsion effi-
ciency, a steering duct was designed. Hence, the ship length could be reduced as the rudder stock is
located in the propeller plane and not behind it.
5. Basic models of the USV
Different model descriptions are used during the design process and the operation of marine vessels.
In the design process preferably quasi-static models are used in form of motion response amplitude
operator (RAO) to identify extreme values in the motion of the vehicle at certain wave frequencies
and wave heights. For guidance, navigation and control of marine vehicles, dynamical models of the
vessel are needed to be used during the operation. The complexity of the used model depends on its
application. For conventional operations of USV for hydrographic surveys in calm water, it is possible
to reduce the equations of motion to a lower order dimension. For instance, the given GNC task for
measuring USV allows limiting the motion to the nearly undisturbed water surface and neglecting the
motions in heave, roll and pitch. Dynamic modeling and parameterization for GNC of measuring
USV was described in Kurowski et al. (2015). Obviously, the attitude of the vehicle, especially in roll
and pitch, has to be considered with respect to accomplish applications as communication node in a
deep-sea monitoring system. On the one hand, the attitude information is fed into the communication
system to improve the acoustic localization. On the other hand, the control system uses these data to
predict the vehicle motion and adapts the mission control task itself.
5.1. Motion response amplitude operator (RAO)
Motion RAOs are used to determine the behavior of a ship operating in a given state of sea. They are
calculated by using potential theory to compute frequency-dependent added mass , potential
damping coefficients , restoring matrix as well as amplitudes and phases of the first-order wave
load between the vehicle and the waves for a given wave direction and frequency. This correlation can
be written in linear case as equation of motion
,
(1)
where is the position and attitude vector (6DoF) of the rigid body, is the mass and inertia of the
rigid body, is the wave frequency and is the harmonic excitation force proportional to the
wave height. The Motion RAO parameters for the SMIS USV have been computed using the
commercial program WAMIT. Furthermore, it puts out the magnitude and the phase of the Motion
RAO transfer functions
(2)
which represent the vehicles motion due to the wave elevation . The Motion RAOs have to be
calculated for different ship speeds and encountering wave angles ; see Fossen (2011) for more
details. Fig. 8 shows the Motion RAO of the USV in case of zero vehicle speed and waves abeam of
the ship, which is the worst case scenario because of the vehicle design. Scatter plots show the
statistic distribution of wave frequencies and wave heights in a certain area. The USV deployed as
494
communication node will act in the Atlantic Ocean, where the probable frequency range is given by
and the mean significant wave height is . That scenario
shows unfavorable extreme values in pitch motion despite of the wave direction. In this context, it
should be noticed that the motion angles of the vehicle are only valid up to 15° due to the assumed
linearity.
Fig. 8: Motion RAOs at zero speed for wave encountering angle 90°
The equation of motion (1) can be used to compute a time series of the motion of the vehicle in terms
of harmonic wave excitation. Thereby, a probable wave frequency is chosen to simulate the potential
operating conditions of the vehicle in a certain area. An extension to different standard wave spectra is
provided in the Marine Systems Simulator MSS (2010). In that case, the motion of the vehicle is
summarized about varying wave frequencies and wave directions around the defined mean values.
Thereby, the Motion RAOs are interpolated between the encountering wave angles.
Fig. 9: Time series of the USV motion
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Fig. 9 shows the simulation of the USV using significant wave height , wave encountering
range and wave frequency . Especially the pitch motion
shows a considerably oscillatory behavior. However, the mainly excited roll motion has only slight
oscillatory extent.
Normally, these time series cannot be used to predict the motion of the vehicle in practice due to
insufficient information about the prevailing environmental disturbances. Hence, a simpler dynamical
model has to be used to describe the basic characteristics of the motion of the vehicle relative to the
wave excitation.
5.2. Dynamic modeling
The primary control task of the USV is to maintain the communication and localization capability
even in heavy sea states. Therefore, the vehicle has to follow the underwater vehicles to keep them in
the communication cone or reduce its own motions in waves by changing its heading or speed. These
two different tasks have to be decoupled. Typically, the mission control task is realized using a GNC
system. Thereby, the low frequency motion is computed by decoupling it from the motion of the
vehicle in waves using standard filter techniques presented in Kurowski et al. (2015). In order to solve
the second task, an additional model of the vehicle motion in waves is needed. The supposed model
should be described using a simple structure, which can be updated online and used implicitly for the
control task. The simplest way to achieve that is to adapt the Motion RAO transfer function to a fixed
second-order-system for the specific degree of freedom. Doing so, the state space representation is
given by
,
(3)
defining the damping ratio , the undamped angular frequency , the gain factor , the roll rate ,
the roll angle and the excitation . Obviously, the model parameters change due to the mean
wave frequency, the wave height, the encountering direction and the speed of the vehicle.
Nevertheless, this simple model can be updated online due to the characteristics of the prevailing state
of sea and the vehicle operating point. In order to validate the structure and to obtain the parameters
for the acoustic relevant degree of freedom in roll and pitch, negative step response trails have been
carried out in the towing tank of the Technical University of Berlin. The parameters have been
determined from the step responses by using the prediction error method. Fig. 10 illustrates this test in
case of exciting the roll motion.
Fig. 10: Photo series from the roll maneuver in steps of 1 s
Due to the floating position of the vehicle, nonlinear effects occur if the hull emerges from the water.
496
In order to handle this effect, the nonlinear model
(4)
has been considered, which is adapted from Perez (2005). In that description the parameters
(5)
are functions of the model states. Fig. 11 shows the identification results of both models. The dashed
line depicts the measurement. As can be seen, the amplitudes of the linear model (solid line) cannot be
fitted to the roll motion of the vehicle, especially if larger roll angles occur. The nonlinear model
(dotted line) shows satisfactory results even in cases of larger roll angles.
Fig. 11: Identification results of the roll motion
5.3. Operation
Most of the operation time, the USV will move at slight speed, even in cases of significant vehicle
motion due to waves. The resulting swept area can be calculated as described in Section 3, using the
beam spread reduction angle , Fig. 5. Certainly, this angle is estimated from the motion of the
vehicle. The resulting swept areas due to varying beam spread reduction angles are given in Table II.
Table II: Reduction of swept area assuming constant depth (5000 m), Slant range (8500 m) and Angle
of beam spread (100°)
Beam spread reduction angle
Radius of the cone base
swept area
5°
6.01 km
113.49 km²
10°
5.46 km
93.78 km²
15°
4.88 km
74.67 km²
20°
4.25 km
56.75 km²
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In that way, the acoustic node will operate in a kind of dynamic positioning task and moves primarily
against wind, current and wave drifts. Due to the vehicle design and the actuators, the heave, roll and
pitch motion cannot be controlled actively. Therefore, the disturbances have to be measured or
estimated as accurate as possible to find an optimal heading and speed, which ensures a minimum
motion of the vehicle in waves and secures a minimal influence on the acoustic relevant degrees of
freedom. Wind and current are measured by a weather sensor and respectively by a Doppler log.
Against that, the motion of the vehicle in waves has to be estimated from measurements of the Atti-
tude-heading-reference-system using simple dynamic models to be used online during the operation
of the vehicle.
6. Conclusion and further work
The paper describes operational aspects of an ocean-going USV, which acts as communication node
within a complex deep-sea monitoring system. In that way, the characteristics of the acoustic
communication has been carried out and adapted to the operation of the USV. The vehicle has been
designed as semi-submerged and partly flooded vehicle to satisfy the requirement of being a low noise
vehicle with reduced motions in waves. Nevertheless, the roll and pitch motion of the vehicle have a
decisive impact of the mission control system of the vehicle. Hence, the basic characteristics of the
motion of the vehicle in waves have been investigated from the quasi-static case to the simple linear
and nonlinear dynamic models of the uncoupled motion of the vehicle in roll and pitch. Simulations of
the vehicle motion as described in Section 5 are further used to design superior control schemes to
adapt the mission control task in advance. Additionally, they can be used to parameterize and test the
controllers. The simple dynamic models are used to predict the impact of the prevailing environmental
disturbances. Moreover, these calculations can be used to feed some kind of extremum-seeking
controller to minimize the motion or maximize the swept area of the acoustic communication cone,
which is essential for the localization of the underwater vehicles operating in deep-sea areas.
Currently towing tests are carried out in the towing tank of TU Berlin. After that, the vehicle is fully
equipped in Rostock and Sassnitz, where the vehicle was built by REAN shipyard. Experimental trials
in the port of Rostock and at the Baltic Sea should demonstrate the robustness and reliability of USV.
As part of deep-sea trials at the Atlantic or Pacific Ocean, the USV will show its capabilities as
communication node at high sea states and worse environmental conditions within the complex SMIS
monitoring system.
Acknowledgment
We would like to thank the German Federal Ministry of Economics and Technology (BMWi) and the
Project Management Jülich for supporting the SMIS project under registration number FKZ 03SX348.
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