Sonar History

• Developed after World War I in Britain and the United States, sonar technology was kept secret until World War II, when it was used very effectively to counter the devastating attacks of the German U-Boats.

• During the Cold War, improvements in sonar technology were critical to tracking the quieter, more sophisticated Soviet submarines.

• These large submarines were most commonly tracked in deep ocean areas, and the Navy invested a great deal of money and resources in understanding the physics of sound transmission in the open ocean.

What is sonar?

• Sonar (Sound Navigation And Ranging) is the generic name of the technology that is used to locate objects underwater.

• Sonar systems are of two basic types - active and passive.

• In active sonar, the system emits a pulse of sound and then the operator listens for echoes.

• In passive sonar, the operator listens to sounds emitted by the object one is trying to locate.

Active Sonar

• An active sonar system is an apparatus used for obtaining information about underwater objects and events by transmitting sound waves and observing the return echoes.

• When a sound signal is sent into the water, part of it will be reflected back if it strikes an object or "target".

• The distance to the object can be calculated by measuring the time between when the signal is sent out and when the reflected sound, or echo, is received.

Cont…………..

• For Example, if four seconds elapse between the emission of the outgoing sound and the return of its echo, the sound has taken two seconds to travel to the object and two seconds to return.

• The average speed of sound in the water is 1,500 meters per second. So if it takes two seconds for sound to reach the object, we can assume the object is 2 sec x 1,500 m/sec or 3,000 meters away.

Cont…………..

• Experienced sonar technicians are often able to tell the difference between echoes produced by a submarine, a rock outcrop, a school of fish, or a whale.

Passive Sonar Systems

• An apparatus used only for receiving the sounds generated by underwater objects is called a passive sonar system, which can be utilized in marine biology for detecting sounds generated by fish and other aquatic animals.

• Ships, submarines, marine mammals, and fish all make noise, and this noise can be used by passive sonar systems to locate them, in much the same way humans use their ears to locate someone speaking in a room.

• Passive sonar systems can be arrays of hydrophones towed behind a ship or submarine, or a fixed system of hydrophones cabled to shore or attached to a mooring.

How is sound used to locate fish?

• Some sonar systems are especially designed to locate fish.

• Fish finding sonar units send and receive signals many times per second.

• They concentrate sound into a beam that is transmitted from a transducer.

• .

• These units include visual displays that print the echoes.

• The bottom appears as a continuous line drawn across the display.

• In addition, any objects that are in the water between the surface and the bottom may also be displayed.

• Fish finders detect the presence of fish primarily by detecting the air in their swim bladders.

• The air conserved in the swim bladder changes the sound path and reflects energy back.

• The fish finder detects this reflected energy and converts it into fish images on the screen.

• Fish finders operate at high frequencies of sound, approximately 20-200 kHz (20-200,000 cycles per second).

• This helps define targets and can even display two fish as two separate echoes.

How is sound used to navigate underwater? • A basic underwater navigation system

uses a hydrophone and an acoustic tracking beacon. The hydrophone is attached to a boat, below the water and works like an antenna.

• The beacon, somewhere below the boat, transmits a signal that is picked up by the hydrophone.

• The system calculates the range (distance) and bearing (horizontal direction) to the beacon.

• The position of the beacon is determined relative to the boat. I

• f the exact location of the boat is known, then the exact location of the beacon can be calculated.

How position is calculated

• Underwater positioning is based on the basic principle:

• Distance = Speed x Time, • where Speed is the speed of sound in water.• The system measures the amount of time

between the initial ping (sent from the transducer) and the return ping (received from the transponder) and calculates the distance from that transponder: