Sonar (Sound Operated Navigation And Ranging) uses sound propagation to detect, navigate or communicate under the surface of water. This post will discuss in detail the meaning of Sonar, it’s types, architecture, how it works, applications, advantages and disadvantages.
What is Sonar
Sonar is the acronym for Sound Operated Navigation And Ranging and this system makes use of underwater Acoustic energy to determine the presence and nature of the target under water.
Fig. 1 – Introduction to Sonar
Sound pulses are transmitted through water by Sonar System and it receives the reflected sound waves which are recorded and processed for various purposes. The received signal might be in the form of echoes or the acoustic noise of marine vessels (Ships or any other watercraft), sounds from marine animals etc. Sonar is specifically used to prepare Nautical Charts and for Seafloor Mapping as shown in the Fig. 2.
Fig. 2 – Image of Seafloor Mapping
Types of Sonar
The two types include:
- Active Sonar
- Passive Sonar
Active Sonar
Active Sonar operates by transmitting a well-defined signal termed as “ping”. Transducers made of piezoelectric or magneto strictive material, convert Electrical pulses into sound signals which are transmitted into the water. If the target object is in the path of the transmitted signal, the acoustic signal bounces off the target object and returns an “echo” to the Receiver and Transducer converts it back to electrical signal.
The time between the emission of the sound pulse and its reception helps in determining the range and orientation of the object. The schematic representation of the same is shown in the Fig. 3 (a). The received signal undergoes Spatial Sampling, Filtering and Time-Frequency processing to obtain the final output which is in visual format.
Fig. 3 – (a) Active Sonar (b) Passive Sonar
Passive Sonar
This is based on the detection of sounds produced by a moving object or marine vessel. Aquatic animals, Ships, Submarines produce distinct sounds. These sounds emanating from different Target Objects is used for detection and estimation of target object. Passive Sonar cannot determine the range of the target object unless it is used in conjunction with other devices.
The received signal is processed using techniques like LOFAR (Low-Frequency Analysis and Recording) and DEMON (Demodulation of Envelope Modulation On Noise). Passive detection is widely used in the study of marine life.
Architecture of Sonar
It consists of components like:
- Transmitter
- Receiver
- Transducer
- Synchronizer
- Control Unit
- Display Unit
Fig. 4 – Components of Sonar System
Transmitter
It excites the sensors with electrical signals. These signals are converted in to sound energy and the waves are radiated under water. The transmitted signals strike the target object and reflects back.
Receiver
The reflected signal called ‘echo’ is captured by the Receiver and converts the sound waves back to electrical signals. This unit also filters the received signal for further processing. Pulse Compression techniques are incorporated to improve range resolution.
Transducer
It is the key component of the system. Transducer is made up of Piezoelectric material which has the ability to generate electric potential. When the voltage is applied to the Transducer, it oscillates creating an acoustic pulse.
Conversely, when the pressure is applied on the Transducer, it produces electrical signal. The pressure on the Transducer is created by the received signal. The process of converting electrical energy to sound energy and back is called ‘Transduction’.
Synchronizer
It helps in the coordination of Transmission and Reception to operate a system in unison.
Control Unit
It controls the entire system. This unit helps in transmission, processing and reception of the signal.
Display Unit
It displays the processed data in a visual format. The display is either a scanned image or PPI (Planned Position Indicator) image.
How does Sonar Work
Firstly, let us consider the operation of Active Sonar. In case of Active Sonar, Acoustic signal is radiated in the water by the Transmitter. When the signal strikes the target it reflects back to the Receiver unit. The signal is then processed and the range of the ‘target’ is estimated.
Eg. If the time period between transmission of sound wave and reception of the ‘echo’ is 6 sec, it is estimated that sound has taken 3 secs to travel to the target object and 3 seconds to return. The average speed of sound in the water is 1,500 meters per second which implies that the object is 3 sec x 1,500 m/sec or 4,500 meters away.
Fig. 5 – Passive Detection Using Hydrophones
Passive Sonar system consists of several Hydrophones which act as receiving sensors. These sensors captures the sounds of the target object. Each sensor records the intensity of the sound wave along with the time delay in reception of the sound wave.
The recorded data is analysed and the sensor which records the highest amplitude with least time delay will be considered in close proximity with the point of reflection of the sound wave. The performance of Passive detection is largely dependent on underwater environment. Fig. 5 (above) shows schematic representation of Passive Detection using Hydrophones.
Applications of Sonar
The applications of Sound Operated Navigation and Ranging include:
- This technology is used for Bathymetry study which includes sea floor mapping.
- It is mainly used for underwater surveillance.
- It is widely used in Military applications.
- It is also used by Fishing industry.
- Underwater communication is easier with this technology.
- Used for weather forecasting and Geophysical research.
Advantages of Sonar
The advantages of Sound Operated Navigation and Ranging are:
- Attenuation of sound waves is less in water.
- Implementation of the system is not expensive.
- Reliable and Accuracy is high.
Disadvantages of Sonar
The disadvantages Sound Operated Navigation and Ranging include:
- Scattering is the major source of Interference.
- Poses threat to Marine life.
- Poor directional resolution occurs due to the high beam of Sonar.
- Impact of reverberation affects the systems performance.
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