Active sonar

Tracking surveillance technology is varied in design and form. It ranges from simple beepers to sophisticated intelligent transportation systems. For example, there is radar to monitor over the horizon bi-static sensors for passive retrieval of emissions (cellular phones) or active sonar-hke capacity tagging systems that use projectiles to attach transmitters to moving objects illumination telescopic and detection systems. 

Active sonar consists of a transducer, which provides the acoustic energy beam, a transmit-... 

Sonar can also be used for underwater communication. including direct voice communication, where the human voice is transmitted as modulated ultrasonic energy by an active sonar device... 

In navigation and target detection, a scanning active sonar can be used to provide a realistic visual picture of sound-reflecting objects around the source. The scanning beam is rotated, and the reflected beams show the position of objects in a given plane as echo blips . Another display is required to show the depth/altitude of the object observed... 

Active sonar consists of a transducer, which provides the acoustic energy beam, a transmitting antenna and a receiver-transducer-amplifier which processes the returning beam. As in many types of radar, the sound beam is sent in pulses, so that range can be detd by timing the returning beam, identified by its place in the series of pulses... 

Transducer Device that converts one type of energy into another. In the case of active sonar, the transducer converts electrical power into sound waves generated into the water. 

To facilitate the transmission of sound into the water, shipboard sonars are mounted in large plastic bilges near the bow, requiring the relocation of the ship s anchors to avoid hitting them. Because of the distinctive sound, the return echo used on an active sonar beam to find a submerged object was known as pinging the target. 

The performance of sonar systems can be assessed by the passive and active sonar equations. The major parameters in the sonar equation, measured in decibel, are as follows ... 

The presence of surface, bottom, and volume reverberation (in active sonar)... 

FIGURE 17.42 Generic active sonar system. (Source Adapted from Knight, W., Pridham, R.G., and Kay, S.M. 1981. Digital signal processing for sonar. Proceedings of the IEEE 69( 11) 1454. IEEE 1981.)... 

In active sonar, the filters are matched to the known transmitted signals. If the object (acoustic reflector) has motion, it will induce Doppler on the reflected signal, and the receiver will be compKcated by the addition of a bank of Doppler compensators. Returns from a moving object are shifted in frequency by A/ = (2v/c)/, where v is the relative velocity (range rate) between the source and object, frequency of the source transmitter. 

Reverberation/clutter Inhomogeneities, such as dust, sea organisms, schools of fish, and sea mounds on the bottom of the sea, which form mass density discontinuities in the ocean medium. When an acoustic wave strikes these inhomogeneities, some of the acoustic energy is reflected and reradiated. The sum total of aU such reradiations is called reverberation. Reverberation is present only in active sonar, and in the case where the object echoes are completely masked by reverberation, the sonar system is said to be reverberation limited. 

FIGURE 3.43 Active sonar projectors (d) Tompilz projector and (b) Flextensional transducer. 

Sonar systems eanbe elassed as either active or passive. In active sonar an ultrasonie or acoustic pulse is transmitted... 

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