Sonar for Practising Engineers, (with solutions) 3rd edition

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xviii Introduction
Basic passive systems, however, give no information about the range of a target;
a signal may belong to a close, quiet target or a noisy, distant target. More complex
passive systems estimate range by the following methods:
Triangulation: measuring the bearings of a target from two well-separated
arrays.
Horizontal direct passive ranging (HDPR): based on the measurement of
wavefront curvature using three well-separated arrays.
Yertical directpassive ranging (VDPR): measuring the vertical arrival angles of
signals arriving at the same array via multiple paths as well as measuring the
time differences between them.
All of these methods are fundamentally dependent on the accuracy of the bearing
measurements and therefore demand large arrays and large separations to achieve
useful range estimates.
Active sonar uses a projector (an underwater loudspeaker) to generate a pulse
of sound which travels through the water to a target and is returned as an echo to a
hydrophone, often the same device as the projector and in this context more
commonly known as a transducer. The echo now has to be detected against a
background of noise and reverberation (unwanted echoes from the sea surface
and sea bed and from scatterers within the volume of the sea). Because the time
between transmission of a pulse and reception of an echo can be measured and the
speed of sound in the sea is known, the range of the echoing target is simply
calculated. Active sonars are sometimes known as echo ranging systems.
To survive in sonar, newcomers must become familiar with the decibel. They
will hear old hands discussing the design and performance of sonar systems in
exchanges where almost every other word seems to be ‘deebee’ (dB or decibel).
What is this decibel?
First of all, the be1 is inconveniently large so it has been divided by 10 to
become the decibel. It simply compares the power or intensity of the sound at one
point in a system with that at another. The decibel defined:
Power gain = 10logl0 (2) (dB) I
The decibel is of course also used in electronics, communications, radar and
airborne sound, and will be familiar to most engineers. But it is in sonar where it
really thrives. Long expressions such as the sonar equations are assembled with

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