Underwater Acoustic Positioning Systems

A system for the tracking and navigation of

underwater vehicles or divers by means of

acoustic distance and/or direction measurements,

and subsequent position triangulation

By Gwo-Shyh Song

Underwater acoustic positioning systems are generally

categorized into three broad types

LBL: Long baseline systems, use a sea-floor baseline transponder network.

Ultra short baseline (USBL) systems and the related super short baseline

(SSBL) systems, rely on a small (ex. 230 mm across), tightly integrated

transducer array that is typically mounted on the bottom end of a strong,

rigid transducer pole which is installed either on the side or in some cases

on the bottom of a surface vessel.

Short baseline (SBL), systems use a baseline consisting of three or more

individual sonar transducers that are connected by wire to a central

control box.

GPS Intelligent Buoys (GIB) systems are inverted LBL devices where

the transducers are replaced by floating buoys, self-positionned by GPS.

SSBL - Super Short Base Line

The calculation of positioning is based on

range and on vertical and horizontal angle

measurements, from a single multi element

transducer. The system provides three-

dimensional transponder positions relative

to the vessel.

USBL (Ultra-short baseline, sometimes known as SSBL for

Super Short Base Line)

1. A complete USBL system consists of a transceiver, which is mounted on a

pole under a ship, and a transponder/responder on the seafloor, a towfish, or

on a ROV. A computer, or "topside unit", is used to calculate a position from the

ranges and bearings measured by the transceiver.

2. An acoustic pulse is transmitted by the transceiver and detected by the

subsea transponder, which replies with its own acoustic pulse. This return pulse

is detected by the shipboard transceiver. The time from the transmission of the

initial acoustic pulse until the reply is detected is measured by the USBL

system and is converted into a range.

3. Angles are measured by the transceiver, which contains an array of

transducers. The transceiver head normally contains three or more transducers

separated by a baseline of 10 cm or less. A method called “phase-differencing”

within this transducer array is used to calculate the angle to the subsea

transponder.

4. Additional sensors including GPS, a gyro or electronic compass and a vertical

reference unit are then used to compensate for the changing position and

orientation (pitch, roll, bearing) of the surface vessel and its transducer pole.

The disadvantage is that positioning accuracy and robustness is not as good as for LBL

systems. The reason is that the fixed angle resolved by a USBL system translates to a larger

position error at greater distance. Also, the multiple sensors needed for the USBL transducer

pole position and orientation compensation each introduce additional errors.

USBL manufacturers

ORE LXT, Trackpoint II

Nautronix RS7, Hydrostar, RS900, 910 Series

Kongsberg Simrad HPR 400P, HPR 410, HiPAP

Sonardyne USBL

Tritech Micron Nav - USBL Tracking System

Frequency Band 20-28KHz

Tracking Range 500m (1,640ft) typical

Horizontal, 150m (492ft) typical Vertical

Range Accuracy +/- 0.2 meters typical

Bearing Accuracy +/-3 degrees

Position Update Rate 0.5 Seconds –

10 Seconds

Targets Tracked 1 responder and up to

15 transponders

• Advantages

• Low system complexity makes USBL an easy tool to use.

• Ship based system – no need to deploy transponders on the seafloor.

• Only a single transceiver at the surface – one pole/deployment machine.

• Good range accuracy with time of flight systems.

• Disadvantages

• Detailed calibration of system required - usually not rigorously completed.

• Absolute position accuracy depends on additional sensors - ship's gyro and

vertical reference unit.

• Minimal redundancy – only a few commercial systems offer an over-determined

solution.

• Large transceiver/transducer gate valve or pole required with a high degree of

repeatability of alignment.

USBL/SSBL

SBL - Short Base Line

The calculation of position is based on

range, and vertical and horizontal angle

measurements from a minimum of three

hull mounted transducers. The baselines

are between transducers on the vessel. A

transponder is positioned relative to the

vessel. Accuracy is < 0.5% of slant range.

The system provides three-dimensional

transponder positions relative to the vessel.

SBL Acoustic Positioning System suitable for tracking underwater targets from boats or

ships that are either anchored or under way.

SBL positioning accuracy improves with transducer spacing. Thus, where space permits,

such as when operating from larger vessels or a dock, the SBL system can achieve a

precision and position robustness that is similar to that of sea floor mounted LBL systems,

making the system suitable for high-accuracy survey work.

Short baseline systems determine the position of a tracked target such as a ROV by

measuring the target's distance from three or more transducers that are, for

example, lowered over the side of the surface vessel from which tracking operations

take place. These range measurements, which are often supplemented by depth

data from a pressure sensor, are then used to triangulate the position of the target.

Baseline transducer (A) sends a signal, which is received by a transponder (B) on

the tracked target. The transponder replies, and the reply is received by the three

baseline transducers (A, C, D). Signal run time measurements now yield the

distances B-A, B-C and B-D. The resulting target positions are always relative to the

location of the baseline transducers. In cases where tracking is conducted from a

moving boat but the target position must be known in earth coordinates such as

latitude/longitude or UTM, the SBL positioning system is combined with a GPS

receiver and an electronic compass, both mounted on the boat. These instruments

determine the location and orientation of the boat, which are combined with the

relative position data from the SBL system to establish the position of the tracked

target in earth coordinates.

SBL Manufacturers

MORS (Oceano)

Nautronix (Honeywell) RS5D

Sonardyne SBL

Advantages

• Low system complexity makes SBL an easy

tool to use.

• Good range accuracy with time of flight

system.

• Spatial redundancy built in.

• Ship based system – no need to deploy

transponders on the seafloor.

• Small transducers/gate values.

• Disadvantages

• System needs large baselines for accuracy

in deep water (>40m).

• Very good dry dock/structure calibration

required.

• Detailed offshore calibration of system

required - usually not rigorously completed.

• Absolute position accuracy depends on

additional sensors - ship's gyro and vertical

reference unit.

• >3 transceiver deployment poles/machines

needed.

LBL - Long Base Line

The calculation of position is based on

range measurements only. The ROV, a

subsea module and the vessel are

positioned relative to a calibrated array of

transponders.

LBL Acoustic Positioning System used to track underwater vehicles and divers.

LBL systems use

networks of sea-floor

mounted baseline

transponders as

reference points for

navigation. These are

generally deployed

around the perimeter

of a work site. The

LBL technique results

in very high

positioning accuracy

and position stability

that is independent of

water depth. It is

generally better than

1-meter and can reach

a few centimeters

accuracy.

These range measurements, which are often supplemented by depth data (Z)

from pressure sensors on the devices, are then used to triangulate the

position of the vehicle or diver. A diver mounted interrogator (A) sends a

signal, which is received by the baseline transponders (B, C, D). The

transponders reply, and the replies are received again by the diver station (A).

Signal run time measurements now yield the distances A-B, A-C and A-D,

which are used to compute the diver position by triangulation or position

search algorithms. The resulting positions are relative to the location of the

baseline transducers. These can be readily converted to a geo-referenced

coordinate system such as latitude/longitude or UTM if the geo-positions of

the baseline stations (B, C, D) are first established.

LBL Acoustic Positioning Method

LBL systems are generally applied to

tasks where the required standard of

positioning accuracy or reliability exceeds

the capabilities of USBL and SBL

systems. LBL Transducer

LBL Manufacturers

Benthos Edgetech (EG&G)

Imetrix MORS (Oceano)

Nautronix (Honeywell) 906, 916 Sonatech

Kongsberg Simrad 408 Sonardyne L/USBLPAN, Compatt.

Kongsberg Simrad 418 Combined LBL/USBL

The Multi-User Long

Base Line (MULBL

by Konsberg), is a

hydroacoustic

positioning system.

Several in dividual

vessels and

Remotely Operated

Vehicles (ROVs) can

position themselves

using the same

seabed transponder

array.

Advantages

• Very good position accuracy independent of water depth.

• Observation redundancy.

• Can provide high relative accuracy positioning over large

areas.

• Small transducer – only one deployment machine/pole.

• Disadvantages

• Complex system requiring expert operators.

• Large arrays of expensive equipment.

• Operational time consumed for deployment/recovery.

• Conventional systems require comprehensive calibration

at each deployment.

LBL Acoustic Positioning Method

Remotely Operated Vehicles (ROVs)

This term has been fairly well received around the industry,

however, the debate will likely expand regarding LCAUVs

(LCROVs–Low Cost ROVs (add a “V” and you get Very low

cost)) as the academic community advances the field toward

smaller and cheaper autonomous (self-running) vehicles. And,

in the extreme, are all the systems discussed really vehicles?

Thus, the question of which term to choose: ROV, AUV, UUV,

RCV, RPV, LCROV, VLCROV, LCAUV... And, they are not just

“undersea” vehicles, they are working the inland waters at an

ever increasing pace. ROVs will primarily pertain to tethered (繫纜) systems, including expendable fiber optic tethers; AUVs

(Autonomous Untethered Vehicles) will deal with untethered

systems, to include non-tethered communication links such as

acoustics; UUVs (Unmanned Undersea Vehicles) will remain

with the US Navy and the military; LCROVs will remain

inexpensive vehicles.

VARE Industries, Roselle, New Jersey, developed a maneuverable underwater camera

system–a Mobile Underwater Vehicle System. Serial No. six–was delivered to the Naval

Ordnance Test Station (NOTS) in Pasadena, California, in 1961. Called the VARE vehicle, or

XN-3, it was equipped with a clamshell claw and operated by the Navy laboratory.

The PAP-104s, manufactured by Societie Eca, Meudon, France, were flowing off a high-

speed production line 1975–relatively speaking. The small mine neutralization vehicle, which

could inspect and identify explosive ordnance with its onboard CCTV, had the capability of

delivering an explosive charge to the site, which would then be detonated from the surface.

The pioneer of the ROV industry -- Drew Michel. While working with Taylor

Diving in the Gulf of Mexico, he demonstrated the potential of the small vehicles

and led their introduction into the oil industry.

Perry RECON IV

SCORPIO

Perry Offshore in Florida picked up the US

Navy’s design of the NAVFAC SNOOPY

vehicle and soon had a line of vehicles

called RECON on the market.

AMETEK, Straza Division, in San Diego,

developed the Deep Drone for the US

Navy and soon turned that into their

SCORPIO line of vehicles.

ROVs in early 1980s’ (Adolescence stage)

Immaturity Stage (Late 1980s): The offshore industry is experiencing a

growth spurt in the early eighties and the first conference dedicated to ROVs

is held in San Diego in 1983.

Canadian entrepreneur (Jim

McFarlane) bought into the

business with a series of low cost

vehicles–DART, TREC and

TROV– and International

Submarine Engineering in

Vancouver, British Columbia

became a competitor. Not initially

known for their reliability, they

never the less made a foothold

and provided whatever the user

wanted, becoming a world leader

in the production of reliable

industrial grade ROVs.

With the collapse of oil prices in 1986,

the number of manufacturers of large

ROV systems began to shrink, the

developers of the LCROVs began to

emerge. The technological advances

had finally been made to miniaturize

the vehicles to a point where they were

"easily" portable and available at a cost

that academic institutions and civil

organizations could afford. The first to

break through was the MiniRover,

developed by Chris Nicholson, soon

followed by Deep Ocean Engineering’s

Phantom vehicles. Other variations

began to appear around the world,

however, DOE, and Benthos (which

picked up the MiniRover line), together

cornered the lion’s share of the market.

The Low Cost ROV (LCROV)

~ US$ 30000-50000

The controls for the Phantom 300 are very simple and user friendly

making it easy to use by inexperienced researchers. The Phantom 300

has a Sony zoom color camera on a tilt platform and fixed lights. The

cable is 250 feet long allowing access to depths of 200 feet seawater.

Maturity Stage (1990+)

The US Navy reached the magic 20,000-foot (6,096-meter) barrier in 1990–

twice. The first time was with the CURV III vehicle that once again morphed into

a deeper, 20,000-foot (6,096- meter) plus configuration.

Operated by Eastport International for the US Navy’s Supervisor of Salvage,

CURV III reached a depth of 20,105 feet (6,128 meter).

Less than a week later, that record was broken by the Advanced Tethered

Vehicle’s record dive to 20,600 feet (6,279 meters).

Japan stormed onto center stage with a series of excellent vehicles topped

by the Kaiko. The Kaiko not only took over the record for the deepest dive,

but also obliterated it, reaching the deepest point in the Mariana Trench–

35,791 feet (10,909 meters).

Today, a quick estimate indicates there are over

100 vehicle manufacturers, and over 100

operators using approximately 3000 vehicles of

various sizes and capabilities.

Deep Ocean Engineering Phantom S2 ROV

Phantom HD2 ROV

Observation Class ROVs

Work Class ROVs

High Capability Electric ROVs

A new class of ROV is not necessarily low cost and can approach the $500,000

mark. These new vehicles feature the latest in technology from Brushless DC

motors (thrusters) to PC-based control systems and fiber optic telemetry systems.

The ability to do heavy work is still not possible with the electric ROVs, primarily

limited by the needed electro-hydraulic design nature of modern manipulator and

work systems.

Perry Tritech's VOYAGER Deep Sea Systems International's MaxROVER

Medium Sized Vehicles The electro-hydraulic vehicles ranging from 20-100

horsepower typically, which can only carry moderate

payloads and have limited through-frame lift capability.

Perry Tritech's (originally AMETEK's) Scorpio

These ROVs range in

weight from 1,000-

2,200 kg with typical

payload capacities in

the 100-200 kg range.

This class was

developed to perform

work, carrying one or

two manipulators, in

high current conditions,

capable of working in

3-knot or greater

currents.

Large Sized Vehicles The vehicles range in weight from about 2,000-6,500

kg, used for current deep water operations to 2,500

meters ranging from 100-250 horsepower. These

vehicles may stand over 8 ft (2.4 m) tall.

Perry Tritech's TRITON XL

Large Class ROVs There is a significant increase in difficulty when extending the

operating depth from 2,500 m to 3,000 m. The weight of an

armored umbilical becomes critical, having to support its own

weight suspended from the surface.

Tritech's Triton ST

Only a few of

these vehicles

have been

completed.

Ultra-Deep Vehicles This class of vehicle is represented by

those special-built ROVs with depth

capabilities of 4,000 meters and beyond.

Monterey Bay Aquarium Research Institute's (MBARI) Tiburon

It is lower

in power to

keep

umbilical

sizes small

and is used

primarily

for deep

ocean

research,

search and

salvage

missions.

Mitsui's Kaiko

3.1x2.0x2.3m

11000m

5600 KG

Many of the ultra-deep systems are designed for science. A

scientist can observe life in the very deep ocean for extended

periods of time with the use of this type of ROV.

The French Epaulard built by ECA

was the first acoustically controlled

AUV to dive to 6,000 m. The vehicle

weighed nearly 3,048 kg and was 13

ft (4 m) long, 3.6 ft (1.1 m) wide and

6.6 ft (2 m) high. Built in 1981, this

AUV was well ahead of its time. The

vehicle is now retired and on display

at IFREMER.

AUVs Its attend is to get rid of the umbilical and

lett the mature vehicle go its own way.

SIRENE, developed by IFREMER, is designed to land a benthic station with high accuracy

in depths to 4,000 m. The AUV is tele-supervised through a bi-directional acoustic link. It is

an 4 tons free swimming shuttle positioned via a hybrid acoustic long baseline, dead

reckoning system aboard the vehicle itself.

Hugin, in its commercial role

as a survey and inspection

tool, for the tasks such as

deep water seabed mapping

and pipeline inspection, and at

a fraction of the cost of ROV

systems. The main advantage

of the AUV is that it can get a

sonar closer to the seabed

resulting in much higher

resolution data, while

eliminating the umbilical

greatly increases its survey

speed and maneuverability.

The developers consisted of FFI

(Norwegian Defense Research

Establishment), Kongsberg Simrad

and the Norwegian Underwater

Technology Center (NUTEC).

US Defense Advanced Research Projects Agency's (DARPA) AUVs

In the US, Woods Hole has

ABE, Florida Atlantic

University has the Ocean

Explorer and the Ocean

Voyagers, and MIT has the

Sea Squirt and Odyssey. In

Canada, ISE has added the

Dolphin and Theseus

vehicles. The IMTP in Russia

is enjoying privatization,

developing the Tunnel Sea

Lion for the US and the CR-

01A for China–they also

unveil their Typhlonus

vehicle.

Towed Systems

W.S. Ocean Systems Ltd.'s U-TOW

Typical sensors used aboard these vehicles are CTDs,

transmissometers, flourometers, nephelometers,

bioluminescence and irradiance meters, optical plankton

recorders, dissolved oxygen, pH, chlorophyll and others.

Cable/Pipeline Location Systems

This class of vehicle is specifically

designed to locate cables or pipeline

either buried or unburied on the

seabed. The vehicles are normally

either a conventional towed body, or

a sled. The sleds are commonly

used in shallow water and are

dragged along the seafloor by the

surface support craft. Both types

carry either a magnetometer or

fluxgate gradiometer for locating

metallic objects. A very unique

design by Seatec, incorporated

spinning rotors on the tow body that

allow the vehicle to be steered along

a pipeline.

Seatec's Towed Vehicle

The system has the capability of flying via its thrusters

in a neutrally buoyant state or becoming heavy and

crawling on the seabed. The system is fitted with a

jetting tool for cable or flexible pipe burial.

Soil Machine Dynamics Ltd. Plough Plows come in all sizes and

configurations, weighing up to 80

tons, resisting tow forces to 250

tons and capable of shallow water

work to depths of 1,500 m.

Not all plows are used to dig trenches; some specialty plows

are built just as back-fill systems for filling in trenches dug by

other plows. End of Class