- Slides: 10
• SONAR (an acronym for Sound NAvigation and Ranging) is an acoustic equipment that works with the principle of underwater sound propagation like echosounder. It is useful to navigate the vessel safely and to find the fish shoal in the distance from a vessel. Also it can measure the echo characteristics of "targets" in the water. Originally it was called as “ASDIC” which stands for “Allied Submarine Detection Investigation Committee” developed for the naval welfare as a device to detect sub marine during the World War I. In World War II, the Americans used the term SONAR for their systems. Later, this is used for finding fishes and become indispensable equipment for the successful and profitable fishing operation. Principle is similar to echo sounder. In case of echo sounder, the ultrasonic sound waves are transmitted vertically downward, but in SONAR, the sound beam can be directed to any angle by turning the transducer to any bearing around the vessel to 360º. and has the same four main components the transmitter, the transducer, the receiver and the recorder or display.
Main components of SONAR has the four basic components : • 1) Transmitter • 2) Transducer • 3) Recorder • 4) Display
Transmitter • Transmitter is in the case of echo sounder, the transmitter generates pulses of electrical energy and pass on to the transducer.
Transducer • SONAR is able to train(rotate) the transducer through 360 o and also be tilt able
• RECEIVER • The SONAR receiver is similar in operation to that of echosounder. The very weak signals that are returned from fish shoal or any target to the transducer are amplified to a sufficient magnitude and then sent to the display unit.
RECORDER • display of target on the sonargram • able to indicate distance and direction is the Plan Position Indication (PPI) on a CRT, which is similar to a radar display. The ship is the centre of the scope and the sonar beam can be seen travelling away from the ship on the bearing selected. If a target is detected the echo is shown on the screen, giving an indication of its range and bearing. The approximate depth of the target can be calculated from the distance and the angle of tilt. The audible indication of the echoes by loudspeaker is quite essential for the identification of various targets.
Fishing with sonar • The echosounder indicates the bottom of the sea and fish under the vessel, the sonar can search a whole area, horizontally, in all directions. • Used for pelagic fishing methods such as purse seining and mid water trawling. It is also quite useful for assessing bottom conditions and configurations, which are of interest for bottom trawling and other bottom fishing techniques
Net sonde • Net sonde is a special acoustic instrument which collects the operational information of trawl net. It has the same working principle of echosounder , but for the position of the transducer. The transducer of the net sonde is fixed in the head rope of the trawl net. It monitors the operation of trawl net and its catch and transmits it for display in the wheel house during trawling. Net sonde is also called as Trawl eye or Net sounder or Trawl Monitoring System.
• Initially developed for mid water trawling • Provides a much better estimate of catching efficiency and amount of catch by showing the fish in and above the net opening. Net sonde equipment requires a direct connection between fishing gear and wheel house a vessel which can be either by cable or by a wireless link.