| |
The substance through which sound travels is
called a medium. All types of matter are sound
mediums of varying efficiency. The denser the
medium, the more rapidly sound travels through
it. Therefore, steel is a better medium than water,
and water is a better medium than air.
Let us take a look at what happens to a sonar
impulse after it leaves the transducer (the
transmitting device in the water). The transducer
introduces the sound wave into the water by
converting the equipment’s electrical energy into
sound vibrations. The impulse travels at a rate
of between 4,700 and 5,300 feet per second,
depending on the temperature, salinity, and
pressure of the water. The rate of travel of the
impulse is four or five times faster than the speed
of sound in air. However, the hazards of travel
take their toll on its speed and signal strength.
Current, bubbles, and wakes absorb some of the
sound. As the impulse passes through foreign
matter such as seaweed, silt, and animal life in
the water, it scatters and becomes even weaker.
As the sound wave travels away from the
transducer, it spreads out like a searchlight beam.
The further away it travels from the transducer,
the weaker it becomes.
Once the wave strikes an object such as a
submarine, that portion of the impulse which is
at a right angle to the object reverberates toward
the sonar receiver. Again absorption, scattering,
and spreading will affect the strength of the
impulse. However, it will still signal a possible
target unless multiple reflections, or echoes,
such as reverberations, self-noise, and a high
surrounding noise level, drown it out.
Multiple reflections, or echoes, can come from
many sources. Sound waves bouncing off small
objects such as fish or air bubbles produce small
echoes. Sound waves reflected from the sea
surface and bottom also cause echoes, and the
sea mass itself causes reverberations. These
reverberations appear on video and audio
receivers. Reverberations from nearby points may
be so loud on the audio receiver that they interfere
with, or completely mask, the returning echo from
the target.
SHIPBOARD ASW ORGANIZATION
Sonar control is the major shipboard ASW
station. Other stations are the bridge, the
combat information center (CIC), and the ASW
weapons batteries. On most ships this organization
is integrated into the combat systems department.
Sonar control is the ASW station that
maintains a continuous underwater search for
submarines. From the bridge, the officer of the
deck conns the ship, keeping other control
stations informed of the ship’s maneuvers.
The combat information center is the key
station for coordinating search/attack operations
within the ship and betweens ships and/or
aircraft. Personnel in CIC plot, display, evaluate,
and disseminate all air, surface, and subsurface
contact information and recommend search plans
to the commanding officer.
In modern ASW ships, the captain and the
tactical action officer (TAO) often direct the
attack from CIC. However, the CO may choose
to remain on the bridge. When that happens,
repeaters duplicate information from CIC for the
captain’s use while phone talkers relay amplifying
information to him. That enables the captain (in
conjunction with the TAO in CIC) to evaluate
critical elements of the attack from his position
on the bridge. After evaluating elements such as
the target’s course and speed, the captain can then
authorize delivery of the necessary ASW weapons.
AMPHIBIOUS WARFARE
Amphibious warfare encompasses many
different types of ships, aircraft, weapons, and
landing forces used in a concerted military effort
on a hostile shore. An amphibious operation is
an attack launched from the sea by naval and
landing forces. The landing forces, transported
by afloat landing craft and helicopters, may
include Army and Marine Corps troops. During
such operations, both surface ships and aircraft
usually bombard the hostile shore immediately
before the landing.
Amphibious operations are conducted to
establish a landing force on a hostile shore to do
all of the following actions: to prosecute further
combat operations; to obtain a site for an
advanced naval or air base; and to deny the use
of an area or facility to the enemy.
The principle type of amphibious operation
is the amphibious assault. The amphibious assault
follows a well-defined pattern. The general
sequence consists of planning; embarkation;
rehearsal; movement to the objective; and finally,
assault and capture of the objective.
PLANNING
The planning phase of an amphibious assault
reflects the collected intelligence data on enemy
12-15
|
