UNDER WATER

COMMUNICATION

INTRODUCTION

Technique of sending and receiving message below water.

Most commonly employed using hydrophones.

Difficult due to factors like multi-path propagation, time

variations of the channel, small available bandwidth and

strong signal attenuation.

Underwater communication uses acoustic waves instead

of electromagnetic waves.

DEFICIENCY IN CURRENT COMMUNICATION

Future ocean environment will be increasingly

complicated.

Radio waves propagate under water at extremely low

frequencies (30Hz-300Hz) & require large antennae and

high transmission power.

Optical waves do not suffer much attenuation but are

affected by scattering.

Acoustic waves are the single best solution for

communicating Under water.

ABOUT Underwater Acoustics is the study of propagation of sound

in water & interaction of mechanical waves that constitute

with water & its boundaries.

Typical frequencies associated with Underwater Acoustics

are 10Hz to 1MHz

The propagation of sound in the ocean at frequencies lower

than 10 Hz is not possible.

Frequencies above 1 MHz are rarely used because they are

absorbed very quickly.

Underwater Acoustics is also known as

HYDROACOUSTICS.

PEOPLE WHO THOUGHT IT IS POSSIBLE

"If you cause your ship to stop and place the

head of a long tube in the water and place the

outer extremity to your ear, you will hear ships

at a great distance from you.“

In 1687 Isaac Newton wrote

his Mathematical Principles of Natural

Philosophy which included the first

mathematical treatment of sound.

THE MAIN INITIATIVE TO DEVELOP THE

TECHNOLOGY

BASIC ACOUSTIC COMMUNICATION MODEL

ACOUSTIC MODEM

Converts digital data into

special underwater sound

signals.

These signals are then

received by a second

acoustic modem and

converted back into

digital data.

Oceanographers use acoustics to

control underwater instruments

and acquire the data that they

collect remotely.

This technology can also be used

to control small, unmanned

submarines, called Autonomous

Undersea Vehicles (AUV's), and

get data back from them in real-

time.

HYDROPHONE

• Hydrophones are designed to be used underwater for

recording or listening to underwater sound.

• Hydrophones are based on a piezoelectric transducer that

generates electricity when subjected to a pressure change

• Transducers can convert a sound signal into an electrical

signal since sound is a pressure wave.

• From late in World War I until the introduction of

active sonar, hydrophones were the sole method for

submarines to detect targets while submerged

PIEZOELECTRIC TRANSDUCER• Piezoelectricity means electricity resulting from pressure.

• It is a device that transforms one type of energy to another by taking advantage

of the piezoelectric properties of certain crystals or other materials.

• Piezoelectric material is subjected to stress or force, it

generates an electrical potential or voltage proportional

to the magnitude of the force.

• This type of transducer ideal as a converter of

mechanical energy or force into electric potential.

A piezoelectric disk generates

a voltage when deformed

DIRECTIONAL HYDROPHONESFocused Transducers

• Uses a single transducer element with a dish or conical-shaped

sound reflector to focus the signals

• Can be produced from a low-cost omnidirectional type

• Must be used while stationary, as the reflector impedes its

movement through water

Array of Hydrophones

• Multiple hydrophones can be arranged in an array

• It will add the signals from the desired direction while subtracting

signals from other directions.

• Hydrophones are arranged in a "line array“, but may be in two- or

three-dimensional arrangements

Array of Hydrophones

Sound is transmitted by the ship and reflected off the submerged submarine.

The reflected sound reaches hydrophone A first, then hydrophone B, and

finally hydrophone C. The time-of-arrival-difference between the

hydrophones in the array is used to determine the direction to the submarine.

SOSUS HYDROPHONES

• The United States Navy's initial intent for the

system was for tracking Soviet submarines,

which had to pass through the gap to attack

targets further west.

• Sound Surveillance System, is a chain of underwater

listening posts located around the world in places such as

the Atlantic Ocean near Greenland, Iceland and the United

Kingdom — the GIUK gap, and at various locations in

the Pacific Ocean.

• Using the sounds made by the seismic event, scientists can tell if the event

is an earthquake or a volcanic eruption.

• NOAA uses the Navy's Sound Surveillance System (SOSUS) and

additional hydrophones to monitor the North Pacific Ocean and the North

Atlantic Ocean for seismic events.

• Hydrophones located around the Pacific Ocean monitor the ocean for

sounds of seismic events. The sounds made by a seismic event are also

used to accurately locate the event.

• Sonar (sound navigation and ranging) is a technology that

uses acoustical waves to sense the location of objects in the

ocean.

• The simplest sonar devices send out a sound pulse from

a transducer, and then precisely measure the time it takes for

the sound pulses to be reflected back to the transducer.

• The distance to an object can be calculated using this time

difference and the speed of sound in the water (approximately

1,500 meters per second).

SONAR

More sophisticated sonar systems can provide additional

direction and range information. Sonar was developed

during World War I as an aid in finding both submarines

and icebergs.

SONAR

Autonomous vehicles

working under the ice

can be controlled and

their data can be

transmitted to a

topside station using

underwater acoustic

links

Autonomous Underwater

Vehicle

ACOUSTIC LINKS ARE USED TO CONTROL UNDERWATER INSTRUMENTS AND

ACQUIRE THE DATA REMOTELY

APPLICATIONS OF AUV’S & OTHER DEVICES USING

ACOUSTIC SIGNALS

U.S. National Oceanic and

Atmospheric

Administration (NOAA)

Deep-ocean Assessment

and Reporting of

Tsunamis (DART)

program has installed

bottom pressure sensors

near regions with a

history of tsunami

generation, to measure

waves as they spread.

CLOSE-UP OF A DART II SURFACE BUOY

• An acoustic link transmits data from

the bottom pressure sensor to the

surface buoy.

• Then satellite links relay the data to

NOAA tsunami warning centres.

• Real-time data about tsunamis is given

to NOAA forecaster that could

potentially impact coastal areas.

Underwater data links can be

combined with satellite data links to

provide data in real-time from

instruments on the seafloor to

scientists ashore.

The AUV Designed By PIBHMC

This AUV has Been

Constructed by The U.S

Navy to Detect

underwater proximity

mines and approaching

torpedo's.

BlueFin -21 Also known as “Mine-

Hunter”

DETECTING UNDER WATER OBJECTS

• A robot crawler carries a modem, a

camera, and a digital signal-

processing unit.

• Traversing the seafloor, searches for

an object.

• When object found, sends an acoustic

signal to a ship or shore based station

• Can then be commanded to take a

still frame photo, compress it and

transfer the image to an acoustic

signal that is sent back to the

investigator

There is an increasing

interest in USWN

technologies and their

potential

applications. However,

there are several open

issues to solve in order to

provide an

efficient and reliable data

transport to the

applications.