What is SONAR?

Sonar systems can be both, passive (only receiving) or active (transmitting and receiving), while most are active systems. The principal design layout of sonars, see Fig. 1, comprises a control unit usually equipped with a display, a transmitter, a transducer, and a receiver. The transducer converts electric energy into acoustic energy and vice versa via piezo-electric elements (or similar elements). The transmitter provides an oscillating electric field (e.g. square waves) at certain frequencies. The receiver amplifies the electric signal and converts the analog signal into processable data. The control unit is responsible for data processing and filtering. While there usually is only one control unit, the transducers may be arranged as a phased array in order to achieve beamforming.

Figure 1. Schematic design of sonar devices

Application of Sonars

There is a wide variety of applications for sonars:

• Military,

• Fishing,

• Oil & Gas,

• Oceanography,

• Archaeology.

In military applications, they are used to identify and locate enemy underwater vehicles such as submarines and also to spot underwater mines or other potential threats.

In the fishing industry sonars are used to find shoals of fish.

In the oil & gas industry and in marine archaeology they are usually used to provide acoustic images of the seafloor, and in oceanography, the water properties are investigated acoustically.

Depending on the application different types of sonars are used. The most basic sonar is used for ranging purposes, i.e. measuring a distance between the sonar platform and the target (Downward Looking Sonar (DLS), Upward Looking Sonar (ULS), and Forward-Looking Sonar (FLS)). For acoustic imaging Single-beam Echo Sounders (SBESs), Multi-beam Echo Sounders (MBESs), Side-scan Sonars (SSSs), Synthetic Aperture Sonars (SASs), and sub-bottom profilers are available. Doppler Velocity Logs (DVLs) and Acoustic Doppler Current Profilers (ADCPs) provide information on the relative motion between the platform and the acoustic target which can either be the seafloor or the water column itself. In the latter case, ocean currents are analyzed. For wireless underwater communication, acoustic modems can be used. However, they only offer limited data rates and ranges with comparably high latency (Sendra et al., 2016). Sonar devices are typically mounted on underwater vehicles and surface vessels. For the latter sonars usually provide information on water depth and potential obstacles but they indeed are also used for tracking of fish and seismic exploration. The second - and a probably largest - group of carriers are underwater vehicles such as Autonomous Underwater Vehicles (AUVs) and Remotely-operated Vehicles (ROVs) but also towed vehicles and buoys.

Two types of technology share the name "sonar": active and passive. A passive SONAR system provides monitoring of the undersea environment without sending energy through the water. On the other hand, an active SONAR system can act the same as RADAR using responses from signals sent towards targets. Underwater signals obtained from passive SONAR contain valuable clues for source identification even in high noise environments. Attempts on detection/classification of acoustic signals based on spectral characteristics have met little success in the early era of SONAR system development (Urick 1963). In addition, finding the rules to classify objects underwater is more difficult than those for surface vessels.

Active sonar

Following are the features of the Active Sonar system.

• It consists of transmitter and receiver both.

• Active sonar transmits sound waves towards the object and receives reflected waves from it. Active Sonar sounds are emitted in pulse forms and it listens for the echo after transmission.

• The reflected waves are used to detect the object and measure its distance.

• As active sonar transmits sound waves in the sea, it is considered to be harmful to marine life.

• Active sonar has the capability to detect vessels that are quiet and are difficult to be detected by passive sonar as explained below.

• Active sonar can detect marine mammals in shipping lanes or in high sound pressure zones.

Passive Sonar

Following are the features of the Passive Sonar system.

• It consists of the receiver part only.

• It does not transmit sound waves but receives sound waves emitted by sea animals used for communication. It also receives other vibrations. Basically, passive sonar is used for the detection of noise made by others (engines, propellers, animals, etc.).

• Passive Sonar keeps a large sonic database. Moreover, the sonar operator classifies signals by use of a computer and uses stored databases in order to identify classes of ships and take action accordingly.

• As it does not transmit waves, it is considered to be safe for sea animals compared to the active sonar type.