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Underwater acoustic communication is a technique of sending and receiving message below water. There are several ways of employing such communication but the mostcommon is using hydrophones . Under water communication is difficult due to factorslike multi-path propagation , time variations of the channel, small available bandwidthand strong signal attenuation , especially over long ranges. In underwater communication

there are low data rates compared to terrestrial communication, since underwater communication uses acoustic waves instead of electromagnetic waves . Electromagnetic transmission is more difficult in water Acoustic transmission is better suited to water than air Speed of sound in water ~ 1500m/secSpeed of sound in air ~ 340m/sec

Why using sound as communication medium in UW-ASN??

#Radio waves propagate at long distances through conductive sea water only at extra lowfrequencies (30-300 Hz), which require large antennae and high transmission power.

#Optical waves do not suffer from such highattenuation but are affected by scattering. Moreover, transmission of optical signalsrequires high precision in pointing the narrow laserbeams.UW-ASN::Underwater Acoustic Sensor Network

Deploy underwater sensors to record data during the monitoring mission, and thenrecover the instruments. This approach has the following disadvantages :#Real time monitoring is not possible.#No interaction is possible between onshore control systems and the monitoringinstruments.#If failures or misconfigurations occur, it may not be possible to detect them before the

instruments are recovered.#The amount of data that can be recorded during the monitoring mission by every sensor is limited by the capacity of the onboard storage devices (memories, hard disks, etc).

Underwater acoustic communication is a technique of sending and receiving message below water. There are several ways of employing such communication but the mostcommon is using hydrophones.

CHALLENGES:#Battery power is limited and usually batteries can not be recharged because solar energy cannot be exploited.#The available bandwidth is severely limited.#Channel characteristics, including long and variable propagation delays, multi-path andfading problems.#High bit error rates.#Underwater sensors are prone to failures because of fouling, corrosion, etc.#A unique feature of underwater networks is that the environment is constantly mobile,naturally causing the node passive mobility.

http://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Radio_propagationhttp://en.wikipedia.org/wiki/Radio_propagationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Acoustic_wavehttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Radio_propagationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Acoustic_wavehttp://en.wikipedia.org/wiki/Electromagnetic_waves

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#The ocean can be as deep as 10 km.

WHY IS VECTOR SENSOR:

@A vector sensor is capable of measuring important non-scalar components of theacoustic field such as the wave velocity, which cannot be obtained by a single scalar

pressure sensor.@They have been mainly used for underwater target localizationand SONAR applications.@Earlier underwater acoustic communication systems have been relying on scalar sensors only, which measure the pressure of the acoustic field. Vector sensors measurethe scalar and vector components of the acoustic field in a single point in space, thereforecan serve as a compact multichannel receiver @In general, there are two types of vector sensors: inertial and gradient. Inertial sensorstruly measure the velocity or acceleration by responding to the acoustic medium motion,whereas gradient sensors employ a finite-difference approximation to estimate thegradients of the acoustic field such as velocity and acceleration.@{{{in fig.Vector sensor communications with three channels the pressure channel p ,represented by a straight dashed line, and two pressure-equivalent velocitychannels pz and py , shown by curved dashed lines./]]]@In the example of vector sensor communications shown, there is one transmitter

pressure transducer, shown by a black dot, whereas for reception we use a vector sensor,shown by a black square, which measures the pressure and the y and z components of thevelocity. This is a 13 single-input multiple-output (SIMO) system. With more pressuretransmitters, one can have a multiple-input multiple-output (MIMO) system also.

Active sonar uses a sound transmitter and a receiver. When the two are in the same placeit is monostatic operation. When the transmitter and receiver are separated it is bistaticoperation. When more transmitters (or more receivers) are used, again spatially separated,it is multistatic operation. Most sonars are used monostatically with the same array often

being used for transmission and reception

Principle of an Active SONAR

http://en.wikipedia.org/wiki/SONARhttp://en.wikipedia.org/wiki/SONARhttp://en.wikipedia.org/w/index.php?title=Single-input_multiple-output&action=edit&redlink=1http://en.wikipedia.org/wiki/Multiple-input_multiple-outputhttp://en.wikipedia.org/wiki/Multiple-input_multiple-outputhttp://en.wikipedia.org/wiki/SONARhttp://en.wikipedia.org/w/index.php?title=Single-input_multiple-output&action=edit&redlink=1http://en.wikipedia.org/wiki/Multiple-input_multiple-output

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Active sonar creates a pulse of sound, often called a "ping", and then listensfor reflections (echo ) of the pulse. This pulse of sound is generally created electronicallyusing a sonar Projector consisting of a signal generator, power amplifier and electro-acoustic transducer/array. A beamformer is usually employed to concentrate the acoustic

power into a beam, which may be swept to cover the required search angles

To measure the distance to an object, the time from transmission of a pulse to reception ismeasured and converted into a range by knowing the speed of sound. To measurethe bearing , several hydrophones are used, and the set measures the relative arrival timeto each, or with an array of hydrophones, by measuring the relative amplitude in beamsformed through a process called beamforming .

Passive sonar listens without transmitting. It is often employed in military settings,

although it is also used in science applications, e.g., detecting fish for presence/absencestudies in various aquatic environments..

Noise limitationsPassive sonar on vehicles is usually severely limited because of noise generated by thevehicle. For this reason, many submarines operate nuclear reactors that can be cooledwithout pumps, using silent convection , or fuel cells or batteries , which can also runsilently. APPS:

Seismic monitoring.

Pollution monitoringOcean currents monitoringEquipment monitoring and controlAutonomous Underwater Vehicles (AUV

http://en.wikipedia.org/wiki/Pulsehttp://en.wikipedia.org/wiki/Pulsehttp://en.wikipedia.org/wiki/Reflection_(physics)http://en.wikipedia.org/wiki/Echo_(phenomenon)http://en.wikipedia.org/wiki/Sonar_Projectorhttp://en.wikipedia.org/wiki/Sonar_Projectorhttp://en.wikipedia.org/wiki/Bearing_(navigation)http://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Fuel_cellhttp://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/Pulsehttp://en.wikipedia.org/wiki/Reflection_(physics)http://en.wikipedia.org/wiki/Echo_(phenomenon)http://en.wikipedia.org/wiki/Sonar_Projectorhttp://en.wikipedia.org/wiki/Bearing_(navigation)http://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Fuel_cellhttp://en.wikipedia.org/wiki/Battery_(electricity)