Modelling, Simulation and Control of Underwater Vehicles

Hội nghị toàn quốc về Điều khiển và Tự động hoá - VCCA-2011

VCCA-2011

Modelling, Simulation and Control of Underwater Vehicles

Mô hình hóa, mô phỏng và điều khiển phương tiện ngầm

Hung Duc Nguyen, Riaan Pienaar, Dev Ranmuthugala and William West

University of Tasmania / Australian Maritime College

e-Mail: [email protected]

Abstract Underwater vehicles have been developed over many

decades for exploration of seabed, discovery and

exploitation of marine resources. Maintaining control

of underwater vehicles for various missions at seas

requires a good understanding of the underwater

vehicle hydrodynamics and control characteristics. In

order to get students involved in the development of

control systems for underwater vehicles it is necessary

to have a working underwater vehicle with a fully-

functioned controller to do design and implement

missions. This paper presents the modelling,

simulation and control of a newly-built underwater

vehicle for academic and research purposes. A series

of small underwater vehicles have been designed and

built at the Australian Maritime College (University

of Tasmania) within the maritime engineering course

final year programmes. This includes the development

of mathematical models of these small underwater

vehicles for simulation and control design purposes.

This paper focuses on theoretical modelling,

simulation, control design and testings of the AMC

newly-built ROV/AUV.

Tóm tắt: Phương tiện ngầm đã được phát triển qua

nhiều thập niên dùng cho nhiều mục đích khác nhau

như thám hiểm đáy đại dương, thăm dò và khai thác

tài nguyên biển. Điều khiển duy trì phương tiện ngầm

làm các nhiệm vụ khác nhau trên biển đòi hỏi cần

phải hiểu rõ thủy động lực học và đặc tính điều khiển

của phương tiện ngầm. Nhằm để cho sinh viên phát

triển hệ thống điều khiển cho phương tiện ngầm cần

phải có một mô hình phương tiện ngầm hoạt động

được với một bộ điều khiển đầy đủ chức năng để thiết

kế và thực hiện nhiệm vụ. Bài báo này trình bày mô

hình hóa, mô phỏng và điều khiển một phương tiện

ngầm mới đóng để dùng cho mục đích giảng dạy và

nghiên cứu. Tại AMC (Đại học Tasmania) sinh viên

thiết kế và đóng một số phương tiện ngầm loại nhỏ

trong các chương trình cuối năm của khóa học công

nghệ hàng hải. Bài báo này bao gồm cả việc phát triển

mô hình toán của các phương tiện ngầm lọai nhỏ này

dùng cho mục đích mô phỏng và thiết kế điều khiển.

Bài báo này tập trung vào mô hình hóa lý thuyết, mô

phỏng, thiết kế điều khiển và tthử nghiệm phương tiện

ngầm mới đóng của AMC.

Nomenclature

Symbol Unit Meaning ν

Tu,v,w,p,q, rν

η T

n,e,d, , , η

Abbreviation DOF Degree of freedom

ROV Remotely operated vehicle

AUV Autonomous underwater vehicle

HIL Hardware in the loop

AMC Australian Maritime College

UTAS University of Tasmania

HAIN Hydroacoustic aided inertial navigation

1. Introduction Underwater vehicles require mathematical models to

describe behaviour and dynamics. Modelling

underwater vehicles usually has two aspects: one is

theoretical modelling and the other physical testing.

Around the world there are many institutes

developing underwater vehicles for various purposes.

AMC has developed a series of ROVs/AUVs for

academic uses. The goal is to build a virtual lab (a

HIL simulation program) of ROVs/AUVs that

interacts CFD software with a simulation program. A

virtual ROV/AUV will be controlled by a joystick

managed through an appropriate simulation program.

Possible applications of ROVs/AUVs are:

observe seabed conditions;

observe marine farms;

conduct underwater seismic survey for

discovery of oil and gas and exploitation of

marine resources; and

surveillance operation.

As the first step to realize such a virtual lab for

ROVs/AUVs, it is necessary to develop mathematical

models for vehicles. The main purpose of this paper is

to:

describe the AMC ROV/AUV;

model the ROV/AUV using relevant theory;

simulate the ROV/AUV;

design a controller for the ROV/AUV

preliminarily; and

design captive test for the estimation of the

hydrodynamic coefficients and validation of

the assumed model.

The paper is organized as follows: Section 1

introduction, Section 2 reference frames and

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equations of kinematics and kinetics, Section 3 brief

description of the AMC ROV/AUV, Section 4 control

algorithms and design of experiments, Section 5

model scaled experiments and Section 6 conclusions.

Additional information is given in Appendix.

2. Reference Frames and Equations Two reference frames for underwater vehicles are

shown in Fig. 1. NED is the earth-fixed reference

frame and XYZ is the body-fixed reference frame.

Fig. 1 Reference frames for underwater vehicles

2.1 Kinematics

Referring to Fig. 1 the 6-DOF kinematic equations in

the NED (north-east-down) reference frame in the

vector form are [3][4],

η J η ν (1)

where

n

b 3 3

3 3

R Θ 0J η

0 T Θ (2)

with 3 3S η and 3ν . The angle rotation

matrix n 3 3

b

R Θ is defined in terms of the

principal rotations,

x,

1 0 0

0 c s

0 s c

R , y,

c 0 s

0 1 0

s 0 c

R and

z,

c s 0

s c 0

0 0 1

R (3)

where s =sin(.), c = cos(.) using the zyx-convention,

n

b z, y, x,: R Θ R R R (4)

or

n

b

c c s c c s s s s c c s

s c c c s s s c s s s c

s c s c c

R Θ

(5)

The inverse transformation satisfies,

1n b T T T

b n x, y, z,

R Θ R Θ R R R (6)

The Euler angle attitude transformation matrix is:

1 s t c t

0 c s

0 s / c c / c

T Θ

1

1 0 s

0 c c s

0 s c c

T Θ o90 (7)

It should be noted that T Θ is undefined for a

pitch angle of o90 and that 1 T

T Θ T Θ .

2.2 Kinetics

The 6-DOF kinetic equations in the body-fixed

reference frame in the vector form [3] are therefore,

0 wind wave Mν C ν ν D ν ν g η g τ τ τ (8)

where

M = MRB+MA: system inertia matrix (including added

mass)

C ν = RB AC ν C ν : Coriolis-centripetal matrix

(including added mass)

D ν : damping matrix

g η : vector of gravitational/buoyancy forces and

moments

0g : vector used for pretrimming (ballast control)

τ : vector of control inputs

windτ : vector of wind-induced forces and moments

waveτ : vector of wave-induced forces and moments

2.3 Mathematical Model with Environmental

Disturbances

In order to improve performance of the control

systems for underwater vehicles it is necessary to

consider effects of external disturbances on

underwater vehicles, which include wind, waves and

currents. According to Fossen [3], for control system

design it is common to assume the principle of

superposition when considering wind and wave

disturbances. In general, the environmental forces and

moments will be highly nonlinear and both additive

and multiplicative to the dynamic equations of

motion. An accurate description of the environmental

forces and moments is important in vessel simulators

that are produced for human operators.

With effects of external disturbances Equation (8) is

rewritten as [3][4],

RB RB A r A r r r r

0

M ν C ν ν M ν C ν ν D ν ν

g η g τ w (9)

where wind wave w τ τ and

r c ν ν ν (where

6

c ν is the velocity of the ocean current expressed

in the NED). Further information on modelling

environmental disturbances can be found in [2][3].

3. Brief Description of AMC’s

ROV/AUV-3 3.1 Dimensions of AMC ROV/AUV-3

The 3rd

generation of AMC ROV/AUV is named

AMC ROV/AUV-3. The main particulars of the

vehicle are given in Table 1. Fig.2 shows the AMC

ROV/AUV-3 which has been tested for watertight

N

E D

O

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integrity to a depth of 40 metres. Fig. 3 and Fig. 4

show the arrangement of its sensors and actuators.

Two boxes named Box 1 and Box 2 are provided for

electronics and batteries.

Table 1 Main particulars of AMC ROV/AUV-3

Length over all 830 mm

Width of frame 285 mm

Overall width 445 mm

Height 265 mm

Height with light 323 mm

Weight in the air 17.1 kg

Fig. 2 The 3rd generation of AMC ROV/AUV-3

Fig. 3 Body-fixed reference frame of AMC ROV/AUV-3

Fig. 4 Arrangement of thrsuters of AMC ROV/AUV-3 (ui, i

= 1 to 3, are the voltage inputs of thrusters)

AMC ROV/AUV-3 is equipped with the following

sensors and actuators (see Fig. 5):

sensors: 6-DOF IMU, pressure/depth sensor

actuators/thrusters: 3 Seabotix thrusters

(Model BTD150);

servo motor to control the forward camera; and

three lights.

3.2 4-DOF Mathematical Model (block-shaped

ROV)

In order to derive the differential equations governing

the dynamics of the vehicle, it is assumed that:

the origin of the body-fixed reference frame is

at the centre of gravity where the vertical

thruster is located;

the body has an equivalent block shape; and

the rolling and pitch motion can be neglected.

Fig. 5 Input and output variables of the AMC ROV/AUV-3

Thus, the 6-DOF model in Equation (9) is simplified

to a 4-DOF model as follows [2][3][4].

Kinematics:

η J η ν (10)

Kinetics:

Mν C ν D ν g Bu (11)

where:

x

y

z

η ;

c s 0 0

s c 0 0

0 0 1 0

0 0 0 1

J ;

u

v

w

r

ν ;

u

v

w

z r

m X 0 0 0

0 m Y 0 0

0 0 m Z 0

0 0 0 I N

M ;

v

u

v u

0 mr 0 Y v

mr 0 0 X u

0 0 0 0

Y v X u 0 0

C ν ;

u u

v v

w w

r r

X u 0 0 0

0 Y v 0 0

0 0 Z w 0

0 0 0 N r

D ν ;

0

0

0

0

g ;

k 0 k

0 0 0

0 0 k

kl 0 kl

B ; and

1

2

3

u

u

u

u

Numerical values of the coefficients in Equations (10)

and (11) are given in Table 2 in Appendix.

4. Control Algorithms and Design of

Experiments In order to design a controller for missions at sea, the

automatic control system as a whole is illustrated in

Fig. 6 showing the signal flow of guidance,

navigation and control systems.

G

u2

u1

u3

Thruster 1

Thruster 2

Thruster 3 Box 1 Box 2

Torch 1

Camera

house

Torch 2

Pressure

sensor

Y

Z

X

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Guidance system: to receive prior information,

predefined inputs and waypoints and generate

desired trajectory including desired speed,

depth (heave), yaw and position. A joystick

may be used to generate reference signals

[3][4][7].

Navigation system: equipped with GNSS/INS

receivers and other sensors to provide

measurement of speed, depth, yaw and

position [3][4][7]; and

Control system: to detect error by comparing

speed, depth, heading angle and position with

desired values and calculate control signals

and send them to the controller allocation

devices (actuators) [3][4][7].

Fig. 6 Guidance, Navigation and Control signal flow [3]

Fig. 7 shows an arrangement of sensors, actuators and

target PC (onboard equipment) and their connection

to a host PC with software.

Fig. 7 Arrangement of sensors, actuators and connection of

the target PC to the host PC

In general controls of a ROV/AUV include:

heading control:

speed, depth (heave) and pitch control;

roll, surge and sway; and

position control.

As the first step to realize a hardware-in-the-loop

system, computer simulation programs are developed

using the mathematical model in Equations (10) and

(11). A number of tests are carried out for the

simulation programmes including:

open-loop system tests;

manoeuvring tests; and

closed-loop system tests.

In the simulation programs for closed-loop control

systems (including depth and course keeping, pitch

and roll control and position control) the conventional

PID control law was used:

P I D

d tu K e t K e t dt K

dt (28)

4.1 Open-Loop System: Straight ahead and

Turning Circle Manoeuvres

With different values of voltage inputs of two

thrusters at a certain depth, the following were tested

with the simulation programmes:

u = [12 12 0] straight ahead (Fig. 8)

u = [12 -12 0] left turn (Fig. 9)

u = [-12 12 0] right turn (Fig. 10).

-80-60

-40-20

020

-1

-0.5

0

0.5

1-101

-100.5

-100

-99.5

-99

z p

os.

x pos.y pos.

Fig. 8 Straight ahead (z(0) = 100 m)

-2

0

2

4

6

-5

0

5

10-101

-100.5

-100

-99.5

-99

z p

os.

x pos.y pos.

Fig. 9 Left turn (z(0) = 100 m)

-2

0

2

4

6

-10

-5

0

5-101

-100.5

-100

-99.5

-99

z p

os.

x pos.y pos.

Fig. 10 Right turn (z(0) = 100 m)

Estimated

position and

velocities

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4.2 Open-loop System: Depth Control Manoeuvres

Depth control (including driving and surfacing) of the

AMC ROV/AUV-3 was done by the computer

simulation as follows:

u = [12 12 12]: diving (Fig. 11)

u = [12 12 -12]: surfacing (Fig. 12).

-80-60

-40-20

020

-1

-0.5

0

0.5

1-160

-150

-140

-130

-120

-110

-100

x pos.y pos.

z p

os.

Fig. 11 Diving (z(0) = 100 m)

-80-60

-40-20

020

-1

-0.5

0

0.5

1-100

-90

-80

-70

-60

-50

-40

x pos.y pos.

z p

os.

Fig. 12 Surfacing (z(0) = 100 m)

4.3 Depth and Yaw Control (Zigzag Manoeuvres,

Course/Depth Keeping and Changing)

In order to design automatic multitask mission

manoeuvring systems for the ROV/AUV, zigzag tests

(depth), depth control and course keeping and

changing control were carried out as shown below.

Zigzag tests (depth) (Fig. 13);

-50

0

50

100

150

-1

-0.5

0

0.5

1-115

-110

-105

-100

-95

-90

-85

x pos.y pos.

z p

os.

Fig. 13 Zigzag test (u3 = 10 V, change in z = 10 m)

Depth control with PID controller (Fig. 14);

0 50 100 150 200 250 300-115

-110

-105

-100

-95

-90

-85Depth Control - 2D Plotting

Time [s]

Heave [

m]

(a) 2-D plotting

-150

-100

-50

0

50

-1

0

1

2

x 10-14

-115

-110

-105

-100

-95

-90

-85

x pos.

Depth Control - 3D Plotting

y pos.

z p

os.

(b) 3-D plotting

Fig. 14 Depth control with a PID controller

Course keeping/changing with PID controller

(Fig. 15);

0 50 100 150 200 250 300-20

-10

0

10

20Course Keeping and Changing

Inpu

t 1

[V]

0 50 100 150 200 250 300-20

-10

0

10

20

Time [s]

Inpu

t 2

[V]

0 50 100 150 200 250 3000

20

40

60

80Course Keeping and Changing

Yaw

(de

g)

0 50 100 150 200 250 300-10

0

10

20

Time (s)

Yaw

rate

(de

g/s)

Fig. 15 Course keeping and changing manoeuvres

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5. Experiments for AMC ROV/AUV-3 Before conducting experiments with model-scaled

ROV/AUV, it is important to design the experiments

using the mathematical model-based simulators

described in Section 4.

At the AMC experiments to test the above control

algorithms with AMC ROV/AUV-3 can be conducted

in the Circulating Water Channel (CWC), the Model

Test Basin (MTB) and the Survival Pool. The CWC is

the best option with a 2.5 m depth as it is possible to

observe the vehicle during experiments. Fig. 16

shows the CWC and its arrangement.

Fi.g 16 The CWC and its arrangement

It is planned to install a PC\104 target PC and

electronics on the AMC’s vehicle. The target

computer is connected to the onshore host computer

via an Ethernet cable. The host PC is installed with

control programmes developed using software such as

MATLAB / Simulink / Real-time Workshop and RT-

LAB software.

Fi.g 17 Target and host computers and software

Control hardware and software will be developed in

two stages as shown below:

Stage 1: ROV (PC\104, Ethernet connection);

Stage 2: AUV (Microcontroller, Ethernet or

Wireless connection).

The following experiments are planned for each stage:

depth zigzag test (yaw is kept constant);

depth control test;

course keeping and changing tests;

yaw zigzag test (depth is kept constant);

yaw turning circle test (depth is kept constant);

and

trajectory tracking control tests.

6. Conclusions The paper has described the:

reference frames for description of ROV/AUV

kinematics and kinetics;

development of mathematical models (4-DOF

and 3-DOF) of the AMC ROV/AUV-3 based

on relevant theory;

development of simulation programs and

design of experiments for various scenarios;

including: open-loop manoeuvres and closed-

loop control manoeuvres with PID control law;

AMC experimental facilities; and

computer simulation results showing the

feasibility of the control algorithms for various

manoeuvres of the AMC ROV/AUV.

The following recommendations are proposed for

future work:

conduct experiments in the CWC, Survival

Pool or Model Test Basin;

analyse data from the experiments and verify

the mathematical models;

use CFD simulation method for modelling;

use experimental system identification

methods and experimental data for estimation

of hydrodynamic coefficients;

determine coefficients of the vehicle; and

develop 3D trajectory tracking control

systems.

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MTS/IEEE/TECHNO-OCEAN'04 (OTO’04),

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Biography

Dr. Hung Nguyen is a lecturer

in Marine Control Engineering

at National Centre for

Maritime Engineering and

Hydrodynamics, Australian

Maritime College, Australia.

He obtained his BE degree in

Nautical Science at Vietnam

Maritime University in 1991,

then he worked as a lecturer

there until 1995. He

completed the MSc in Marine Systems Engineering in

1998 at Tokyo University of Marine Science and

Technology and then the PhD degree in Marine

Control Engineering at the same university in 2001.

During April 2001 to July 2002 he worked as a

research and development engineer at Fieldtech Co.

Ltd., a civil engineering related nuclear instrument

manufacturing company, in Japan. He moved to the

Australian Maritime College, Australia in August

2002. His research interests include guidance,

navigation and control of marine vehicles, self-tuning

and optimal control, recursive system identification,

real-time control and hardware-in-the-loop simulation

of marine vehicles and dynamics of marine vehicles.

Mr. Riaan Pienaar is a fourth

year engineering student. He

has a special interest in

Subsea Engineering and hence

decided to study Ocean

Engineering at the Australian

Maritime College. He also has

a keen interest in UUVs and

for this reason chose to

complete a final year project

entitled “Simulation and

Modelling of ROVs and AUVs”. Riaan is now about

to graduate and enter into the offshore engineering

industry.

Dr Dev Ranmuthugala is the

Associate Dean, Teaching &

Learning, and Associate

Professor in Maritime

Engineering at the Australian

Maritime College, University

of Tasmania. He has also

served as Head of Department

in Maritime Engineering and

Vessel Operations over the

past 15 years. Prior to joining

AMC, he worked as a marine engineer and in the

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design and sales of piping systems. His research

includes: experimental and computational fluid

dynamics to investigate the hydrodynamic

characteristics of underwater vehicles, behaviour of

submarines operating near the free surface, stability of

surfaced submarines, towed underwater vehicle

systems, and maritime engineering education.

Mr. William West jointed the

Australian Army as a Fitter

and Turner when awarded

Apprentice of the Year by

BHP and Ansett Australia in

1979. He worked on several

projects as: commissioning

HMAS Tobruk and Marine

Engineering. On discharge he

began work with Caterpillar

(South Australia) as an

Industrial Engines Technician where he assembled

and maintained diesel powered generators for the oil

& gas sector. In 1986 he returned to Western

Australia; employed as a Mechanic, Maritime Aids

(Australian Maritime Safety Authority) upgrading,

repairing and surveying lighthouses. On completing

his engineering diplomas’ in Mechanical and

Industrial Fluid Power, he took employment with

EMS Services (WA) as a specialist in naval

hydraulics. In 2005 he commenced study at the

Australian Maritime College (AMC) toward his

degree in Engineering (Marine and Offshore

Systems). Graduating in 2009 he took casual work

with AMC to design and build the ROV/AUV used in

this paper for the purposes of observation and

academic research.

Appendix A1. Numerical values of the 4DOF Mathematical

Model for AMC ROV/AUV-3

Table 2 Numerical values of ROV/AUV parameters

m [kg] 17.1 Iz [kgm2] 24.7

l [m] 0.2225 g [m/s2] 9.81

uX 26.0 vY 186.4

wZ 65.95 rN 3.704

u uX 139.2

v vY 53.5

w wZ 79.3

r rN 0.0

k 0.107

A2. 3-DOF Model

Assumptions for modeling AMC ROV/AUV-3 are

[20]:

the ROV/AUV operates at low speeds;

there are no couplings between the six degrees

of freedom;

the vehicle does not develop an angle of trim

or roll during any manoeuvres;

when manoeuvring the sway velocity is

negligible; and

the influence from disturbances such as current

or waves are negligible.

The 3-DOF model of AMC ROV/AUV-3 is

summarized as follows [2][6][20]:

Mν Dν τ (12)

where τ Bu ;

u

w

r

ν ;

u

w

z r

m X 0 0

0 m Z 0

0 0 I N

M ;

u u

w w

r r

X u 0 0

0 Z w 0

0 0 N r

D ;

k k 0

0 0 k

lk lk 0

B ;

and T

1 2 3v v vu .

A3. An Overview of Acoustic Underwater

Positioning and Navigation Systems

This appendix outlines hydroacoustic positioning and

navigation systems as recommended by the reviewers.

One of the great challenges in control and operation

of ROVs/AUVs is the difficulty in underwater data

communication, positioning and navigation. Radio

frequency (RF) wave and wireless transmission

underwater is very weak, so RF navigation systems

like GNSS/D-GNSS and wireless communication

systems are not applicable in underwater vehicles.

Underwater acoustic positioning and navigation

methods help to control and operate ROVs/AUVs.

The main elements of a hydroacoustic positioning and

navigation system as shown in Fig. A1 include a

transmitter (transducer), receiver (transponder), signal

processing and corrections, incorporation of

peripheral data, display of position and some form of

noise and interference mitigation.

Fig. A1 Illustration of hydroacoustic principles (courtesy of

Kongsberg)

A signal (pulse) is sent from the transducer, and is

aimed towards the seabed transponder. This pulse

activates the transponder, which responds

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immediately to the vessel transducer. The transducer,

with corresponding electronics, calculates an accurate

position of the transponder relative to the vessel

[20][21]. Transmission and reception of acoustic

pulses are to track or position a limited number of

objects, both static and mobile [27].

According to Kongsberg Maritime [20], there are

several typical problems for underwater positioning

and navigation. Sound waves do not follow a straight

path. Deflection occurs when the sound passes

through different thermo clines in the sea. Thermo

clines are a result of differences in temperature and

salinity. The velocity of sound varies accordingly to

these factors, and shadow zones can occur. Another

problem with sound in water is noise generated from

the vessel itself and surrounding objects.

A3.1 Operating Principles

Underwater acoustic positioning and navigation

systems use different principles for measurements and

calculations below:

super short baseline (SSBL);

short baseline (SBL);

long baseline (LBL);

multi-user long baseline (MULBL); and

combined mode system.

A3.1.1 SSBL - Super Short Baseline

The calculation of positioning is based on range, and

on vertical and horizontal angle measurements, from a

single multi element transducer. The system (as

shown in Fig. A2) provides three-dimensional

transponder positions relative to the vessel [21].

Fig. A2 Super short baseline principle [24]

A3.1.2 SBL - Short Baseline

The calculation of position is based on range, and

vertical and horizontal angle measurements from a

minimum of three hull mounted transducers. The

system provides three-dimensional transponder

positions relative to the vessel [21] (see Fig. A3).

A3.1.3 LBL - Long Baseline

The calculation of position is based on range

measurements only. The vessel is positioned relative

to a calibrated array of transponders [21] as shown in

Fig. A4.

Fig. A3 Short baseline principle [24]

Fig. A4 Long baseline principle [24]

Advantages and disadvantages of SSBL, SBL and

LBL methods are given in Table 2.

Table 2 Advantages and disadvantages of SSBL, SBL

and LBL systems [27]

System Advantages Disadvantages

SSBL Good potential accuracy

Requires only a single

subsea pinger or

transponder

One time calibration

Highest noise

susceptibility

Accuracy dependent on

shipboard VRU (vertical

reference unit)

SBL Good potential accuracy

Requires only a single

subsea pinger

One time calibration

Accuracy dependent on

shipboard VRU and

heading sensor/gyro

compass

Multiple hydrophones

required through the hull

LBL Highest potential

accuracy

Accuracy preserved over

wider operating area

One hydrophone needed

Redundant data for

statistical testing/quality

control

Requires multiple

subsea/seabed

transponders

Update intervals long

compared to SBL/SSBL

systems

Need to redeploy and

recalibrate at each site

A3.1.4 Combined Mode Systems

Any combination of the three principles above secures

flexibility as well as a high degree of redundancy and

accuracy [21]. Combined systems come in many

varieties below:

long and super short baseline;

long and short baseline;

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short and super shot baseline; and

long, short, super short baseline.

A3.1.5 Multi-user Long Base Line System

The long base line system is extended to multi-users.

A transponder array is deployed and calibrated using

subsea baseline measurements, or run time

calibration. The transponder array must be deployed

in such a way that one of the transponders in the array

has communication with all the other transponders in

the array. This transponder is used as a Master in the

positioning phase. The other transponders are called

Slaves. See Fig. A5.

The Master transponder acts as a beacon. It starts a

positioning sequence by performing the steps below

[25]:

1. the Master interrogates the Slaves in the array

by transmitting the common LBL interrogation

channel to them;

2. after “a turn-around” delay from its own

interrogation, the Master transmits the

individual transponder channel to be received

by the vessels/ROVs/AUVs positioned in the

array; and

3. each Slave transponder receives the

interrogation from the Master beacon, and

transmits its individual reply channels after a

turn-around array.

Fig. A5 Multi-user long baseline principle [21]

If the Slave misses an interrogation from the Master,

it will still reply because it knows the position update

rate. The same principle may be used to save battery

for the Master. The Master may be programmed to

send an interrogation with lower rate, and the Slaves

will use this interrogation to adjust its timing and still

send pulses at the position update rate [25].

The calculation of the position is based on the

measured differences in range between the

transponders in the array. In addition, any measured

angles towards the transponder will be used. Together

with the known coordinates of each transponder, this

is enough to calculate position. Compared to the

standard LBL, the MULBL needs one more

transponder in the array. All vessels that are going to

use the MULBL array need the coordinates of the

transponders and the channel numbers. These data are

distributed on a file [25].

A3.2 Hydroacoustic Aided Inertial Navigation

System

There are many position reference systems that can be

used for marine vehicles. But when a vessel is alone

in the open ocean far way from shore it is only the

satellite based GNSS and the seabed transponder

based hydroacoustic position reference system that

can give reliable reference position [22]. Fig. A7

shows various position reference systems that can be

used for a vessel.

It is ideal to combine acoustic and inertial positioning

principles because they have complementary

qualities. The underwater acoustic positioning and

navigation system itself is characterised by relatively

high and evenly distributed noise and no drift in the

position, while inertial positioning has very low short-

term noise and relatively large drift in the position

over time [22].

Based on the combined acoustic and inertial

positioning principles a hydroacoustic aided inertial

navigation (HAIN) system has been proven its highly

reliable reference position. Main advantages of the

HAIN system are:

Fig. A7 Various position reference systems for a marine

vehicle [22][26]

improved acoustic position accuracy

higher position update rate

extends operational depth capabilities

longer transponder-battery lifetime; and

position update during acoustic drop-out.

Slave 1 Slave 2

Slave 3

Slave 4

Master

ROV/AUV

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