Sonar
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   Sonar is a remote sensing technique based on the 
echolocation of sound waves in water.  The
name Sonar is short for SOund Navigation And
Ranging and is closely related to the technology
of RADAR which stands for RAdio Detection And Ranging.
The basic principles behind both technologies are
the same.  RADAR employs the transmission and reflection
of radio waves in air to detect objects in the atmosphere.
Similarly, SONAR employs sound waves rather than radio
waves to detect underwater objects.  Sound waves are
preferable in underwater applications since radio
waves loose too much energy when they propagate through
the water.  Likewise, the propagation of sound waves in
air is also inefficient.

   SONAR was originally motivated by the desire to detect
icebergs after the sinking of the Titanic in 1912.  
World War I subsequently stimulated the development of
further SONAR technology in order to detect enemy submarines.
However SONAR now has tremendous civilian as well as
Naval applications.  Such applications include determining
water depth, finding fish, mapping the ocean floor, and
locating various objects such pipelines, wellheads, and
shipwrecks.  It can also be used to study water currents,
and determine characteristics of ocean floor sediments.
Although a human invention, it can be argued that the
principles behind SONAR were discovered far earlier by
nature.  Some marine mammals use sound waves
to navigate and find food in much the same way that the
Navy searched for submarines in World War I.  Also
echolocating bats (Eptesicus fuscus) are experts at
navigating using only waves in the 25 to 100  
kilohertz (kHz) range.

   Most SONAR systems fall into two broad categories:
active (echolocation) and passive.  Passive SONAR
is the simplest category and it consists of systems
that essentially do nothing but listen for sound
vibrations in the water.  Active SONAR systems are
more complicated and involve the projection of
short pulses of sound which propagate through the
water in a narrow beam at about 1500 m/s.  These pulses
then reflect off possible targets in the beam.  A portion
of the reflected beam is later detected at the beam's 
original source.  The distance to the target can be
calculated as half the time between transmission and
detection of the beam times the velocity of the beam.  
The direction of the target can also be determined from
the relative orientation of the reflected beam.  In passive
SONAR the direction of a target can be determined as
well from the direction in which sounds waves are detected,
but range detection is much harder.  However passive SONAR
systems are far prefereable in military applications, where
it is crucial to detect enemy vessels without divulging
one's location.

   In active SONAR the echolocation beam is first transmitted by 
an electroacoustic transducer which converts an electrical
signal of certain duration and frequency into a sound
signal which radiates into the water.  Typically the 
transducer is reciprocal in the sense that it can also
be used to detect the returning echoes.  This can be
accomplished by using a piezoelectric ceramic such
as barium titanate.  In the case of transmission, an
alternating voltage applied to the ceramic causes it
to vibrate which sets of sound waves in water.  In
the case of detection, the alternating pressure received
from the echo causes the piezoelectric substance 
to generate a similar electrical signal which, after 
amplification, yields the required range and direction
information about the target.

   Most practical transducers are formed from a large number
of small piezoelectric elements which are packed into a 
transducer array.  A large number of elements allows sound
waves emitted to constructively and destructively interefere
in such a way that the emitted beam has a highly directional
nature.  Beams emitted from arrays can be as narrow as 0.1
angstroms and can give high resolution sonar images of the
target.  

   There are three basic transducer orientations.  In the usual
depth sounder, the sound beam is directed downward from a transducer
that hanging below the water from the keel of a ship.  Such configurations
are useful for measuring ocean depth and detecting any fish that
may be below.  Another configuration is the side scan, where a beam
is transmitted on either side of the ship, usually perpendicular
to the direction of travel.  Such a configuration is useful in scanning
large areas and in mapping the ocean floor while moving at constant
speed.  The third and most popular employs a rotating sound beam which 
can scan a sector of water surrounding a usually stationary sonar platform.

Typical frequencies used in underwater sonar are dictated
by the absorption spectrum of sound energy in 
water.  Roughly, the higher the frequency, the greater
the absorption of sound energy, at a rate proportional
to the frequency squared.  Thus passive sonar used to
detect submarines operate between 3.5 to 35 kHz which
yields a detection range of about 10 km (6 mi).  Active
SONAR used to detect much smaller objects must operate
at higher frequencies, typically between 100 kHz to
1.0 MHz yielding ranges of a few hundred meters or less.