What is SONAR
SONAR stands for ‘Sound Navigation and Ranging’. As the name suggests, it is a technology that utilizes acoustic waves for underwater detection and ranging. Some of the possible applications of this technology are:-
It is used for detection of underwater vessels such as a submarine along with ranging it. It is used by marine biologists for detection of various species under water. It is used to study the various characteristics of the ocean surface.
Then the question is that how a SONAR system utilizes acoustic waves for all these applications? Well the principle of a SONAR system is very simple. It transmits sound waves in the form of pulses in water and then listens for Echoes. Echoes are sound waves reflected back in the direction of the SONAR transponder from various underwater objects.
These echoes can then be used for two purposes:-
Firstly these can be used to detect the distance of the object from the transponder. This is done by multiplying the speed of sound in water with the time it took in the reception of the echoes after the transmission of the acoustic pulses and then dividing the whole by a factor of 2. Secondly the echoes received can be used to identify the object in question. This can be done by a spectral analysis of the acoustic echoes received. This is based on the fact that for different objects, echoes are received with different spectral peaks.
Thus analyzing the spectrum of the acoustic echoes received by the transponder along with the time that it takes for these waves to bounce back, an accurate mapping can be done of the object in terms of its nature and its distance from the transponder.
Besides the above mentioned technology, there is also a passive technology used in SONAR systems in which no pulses are transmitted. Only echoes are heard for and from the analysis of these echoes various objects are detected.
SONAR technology can also be used to detect the motion of an object underwater by studying the effects of Doppler shift in the subsequent echoes received which causes the frequency of the received echoes to increase or decrease when the object is moving towards or away from the SONAR instrument respectively.
There are a few factors though that should be compensated for in a SONAR system. One of them is the variation of the speed of sound by factors like salinity of water, water density, temperature etc. Another one is the scattering of sound waves by small underwater objects and by the sea surface to counter which the SONAR transponders use narrow beam transmissions.