Page 345 - NS-2 Textbook

340 NAUTICAL SCIENCES

plotting the bearings of the hydrophone noise from the
three ships and seeing where the bearings crossed.
Today's sonar gear is much more sophisticated. It
provides highly accurate ranges and bearings to the sub-
merged submarine. Analysis of Doppler data provides
accurate courses and speeds for the submarine. The
sonar information is normally presented visually on a
CRT screen rather than by sotmd as the early devices did.
In addition, very sophisticated sonars for use by heli-
copters and fixed-wing aircraft have been developed.
There are two basic modes of operation of sonar sys-
tems employed for the detection of targets. They are re-
ferred to as active and passive.
Active sonars transmit underwater sound pulses that
A Navy helicopter lowering a dipping sonar during an undersea
strike targets and return in the form of echoes. The re-
warfare (U5W) exercise.
turned echoes indicate the range and bearing of the target.
Surface undersea warfare (USW) ships usually employ the
Dippiilg Sonar and SOllobuoys. Sonar equipment called
active (pinging) mode when seeking out submarines.
dippillg SOilar can be used by helicopters to detect sub-
Active sonar is also used by submarines and ships to ana-
merged submarines. The helicopter can hover and lower
lyze shorelines, bottom characteristics, and ocean depths.
a hydrophone or pinging transducer into the sea to a
Submarines can switch to active modes to locate ships or
depth of about 400 feet. The sonar searches a 360-degree
other submarines, but this is rarely done because it would
area. After searching, the helicopter hauls in the cable
give away the transmitting sub's location.
and goes to another spot quickly. When a submarine is
Passive sonars do not transmit sound. They only lis-
detected, the helicopter can attack it with homing torpe-
ten for sounds produced by the target to obtain accurate
does 01' bring in other USW units to assist.
bearing and estimated range information. Target detec-
Radio SOitobuoys are small, expendable floating hy-
tion is achieved at great ranges through the use of highly
drophone units that are dropped in the area of a sus-
sensitive hydrophones. Passive sonar is normally used
pected submarine by aircraft. They are usually dropped
mainly by submarines, but surface ships can employ pas-
one at a time in a circular pattern around the contact
sive modes in addition to their active sonar. Submarines
area. By analyzing the radio signals received from each
use passive sonar to analyze the noise of passing ships.
sonobuoy, the location and direction of movement of the
USW aircraft, helicopters, and shore stations also use
submarine can be determined. It can then be attacked by
passive sonar.
the aircraft itself or by other available USW forces. III
Until recently most shipboard sonar systems were
mounted in domes underneath the ship's bow, and were -_ .. _--
therefore called lIul/-mounted systems. In the past few
years a new type of passive sonar system called a towed CRITICAL THINKING
array has been installed in increasing numbers of USW
1. Compare and contrast the similarities and differences
surface ships and submarines. This consists of a semi-
between a mechanical sound 'wave and an electro-
buoyant tube of a length of several thousand feet or more
magnetic wave like light or radio.
that is fitted with numerous hydrophones. The tube is
2. In this chapter the Doppler Shift principle was ex-
unreeled and towed behind the ship. Such an array is ex-
plained relative to the behavior of sound. Research
tremely sensitive and can pick up noise generated by
how the same principles can be used with electromag-
submarines operating many miles away.
netic radiations such as radar and light to determine
Most ships also have aboard a fat/tometer (echo
the same kinds of information.
sounder) for detelmining water depth under the hull.
A sound pulse is h'ansmitted toward the bottom, and -_ .•. _--
its echo is received back. The fathometer is normally used
as a navigational aid, particularly when entering shallow Study Guide Questions
water. It also is used regularly in oceanographic research
to determine the contour of the sea bottom. Most Navy 1. A. What kind of a wave is sound?
ships keep their fathometer on continuously to have an B. What form do sound waves have?
accurate recording of the water depths on their course. 2. How does sound spread through a uniform medium?
The information can be displayed numerically or auto- 3. A. What is the minimum intensity in watts/m that
2
matically recorded by a stylus on a roll of graph paper. a sound must have in order to be heard?