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Devices able to emit and/or detect acoustic signals are known as SONAR (SOund NAvigation and Ranging). As described in par. 2.3 the activities performed in underwater environment are several and different. Moreover, the features of the employed hardware should be tailored to the require- ments of the considered application. In general, sonars fall into two main categories: passive and active.

Passive sonars (Fig. 3.5a) are realized by acoustic devices able to listen and record sounds originated from external sources. Typical transducers used in passive sonar systems are hydrophones (analogous to microphones for out-of-water applications) that convert the front pressure representing the acoustic wave into an electrical signal. Passive sonars find their use in scientific activities such as ocean monitoring and seismic event detection, but also in coastal surveillance and ships/submarines recognition that are typical tasks in the military field.

Figure 3.5: Example of passive (a) and active (b) sonar application [9]. Active sonars (Fig. 3.5b) are devices composed of a projector emitting a sequence of one or more acoustic pulses and a hydrophone measuring the echoes coming back from the surrounding objects and obstacles. By ana- lyzing the returning echoes it is possible to infer the distance and bearing of the detected target, information that turns out to be essential in navy actions. Furthermore, the features of the active sonars are exploited also for exploring activities, especially in the so called echo sounding concerning bathymetry measurements.

The sonar principle of operation is strictly related to the physical phe- nomena happening underwater. Introducing some parameters representing all those effects described in par. 3.1 allows the formulation of the sonar equation that summarizes the relationship between equipment, medium and target [10].

3.2.1 Passive sonar

The passive sonar captures the sound produced by a certain target (Fig. 3.6), whose radiated noise is quantified through the Source Level parameter SL measured at the distance of 1 yd. The signal propagating through the medium experiences absorption and attenuation expressed in terms of Trans- mission Loss T L. Moreover, a component of background noise, assumed to be isotropic, is summed to the target signal. However, the detected Noise Level N L is reduced by the Directivity Index DI of the sonar transducer (hydrophone). The sum of the parameters introduced above, expressed in

Sonar side Target side

Hydrophone

SL

SL-TL-(NL-DI)

t t

Figure 3.6: Time description of passive sonar signal detection. decibels, returns the following Signal-to-Noise Level LS/N:

LS/N = SL − T L − (N L − DI) [dB] (3.35)

The detection of a target occurs when the LS/N exceeds a certain threshold

DT , that is when:

SL − T L − (N L − DI) = DT (3.36)

The rearrangement of parameters in eq. (3.36) so that useful signal and noise components are separated leads to obtain the statement:

SL − T L = N L − DI + DT (3.37)

representing the passive sonar equation.

3.2.2 Active sonar

The case of active sonar (Fig. 3.7) is rather different from the passive one, since now the radiating source is the sonar itself. The signal emitted by the projector with Source Level SL reaches the target and, after reflection, it comes back to the sonar receiving transducer. Therefore, as the path of the signal doubles with respect to the passive sonar scenario, the Transmission

Loss T L has to be counted twice too. Furthermore, when the sound wave strikes a target, a portion of the incident energy is absorbed while the re- maining part is scattered. The ability of the target to reflect energy back in the sonar direction is defined as Target Strength T S. So considering a back-

Sonar side Target side

Projector Hydrophone SL SL-TL SL-TL+TS SL-2TL+TS-RL t t

Figure 3.7: Time description of active sonar signal detection.

ground Noise Level N L, the Directivity Index DI and setting a Detection Threshold DT , the active sonar equation is given by:

SL − 2T L + T S = N L − DI + DT (3.38)

It is woth noting that the active sonar, as an emitter of acoustic energy, causes an additive self-noise component due to the presence of objects and obstacles (bubbles, rocks, fishes) producing reflections that superimpose over those ones coming from the target. This other noise source is expressed in terms or Reverberation Level RL and it is usually introduced in place of N L − DI in eq. (3.38) that becomes:

SL − 2T L + T S = RL + DT (3.39)

Eqs. (3.38) and (3.39) refers to the monostatic active sonar where projector and hydrophone are represented by the same transducer. Sometimes trans- mitting and receiving transducers are instead separated, representing the so called bistatic sonar. In this latter case the transmission losses to and from the target are, in general, different, so the accuracy of eqs. (3.38) and (3.39) is limited.

Some parameters as the Source Level SL are employed in the description of both active and passive sonar behavior even if they refer to different elements. Therefore Table 3.1 gathers all the introduced parameters sorted by case (active or passive sonar) and by origin (sonar equipment, medium, target).

Table 3.1: Sonar equations parameters

Equipment parameters Medium parameters Target parameters Passive Hydrophone Directivity Index DI Transmission Loss TL

Target Source Level SL Sonar Detection Threshold DT Background Noise Level NL

Projector Source Level SL

Transmission Loss TL Active Hydrophone Directivity Index DI

Background Noise Level NL Target Strength TS Sonar Detection Threshold DT

Reverberation Level RL Self-Noise Level NL