Page 339 - NS-2 Textbook

334 NAUTICAL SCIENCES

Refraction is the bending of radio waves over the horizon, extending their range to receivers beneath Earth's curvature.

ing wave. Thls type of modulation is used for FM com-
mercial radio broadcasts and the sound portion of televi-
sion broadcasts, with cruTier wave frequencies in the
MHz range.
Pulse lliodulatioll is different from either amplih.de or
frequency modulation in that there is usually no im-
pressed modulating wave. In this type of modification,
the continuous ·wave is broken up into very short bursts,
Trapping. The duct acts as a wave guide.
or "pulses," separated by relatively long periods of si-
lence during which no wave is transmitted. This is the
type of transmission used by most types of radars.
RADIO

As mentioned above, radio waves are electromagnetic RADAR
waves transmitted in the range from about 5 kHz to
30 GHz. On Earth most of these waves are intentionally Radar (short for radio detection and ranging) was devel-
generated by electronic devices such as radio and TV oped originally as a means for detecting and ranging on
transmitters, though some are by-products of other elec- targets in warfare. But it has also been adapted to an in-
tronic gear. There are also a number of natural radio creasing number of other applications ranging fTOm
transmitters, such as interstellar gas and star systems in speed detection devices to storm tracking. Radar is based
space. Artificial radio transmissions use a series of elec- on the principle that electromagnetic waves can be
tromagnetic waves transmitted at constant frequency formed into a beam, and that part of waves so transmit-
and amplitude called continuous waves. Because an un- ted will be reflected back if the wave encmmters an ob-
modified continuous wave cannot convey much infor- ject in its path.
mation, the wave is normally modified or modulated in Navy radars are grouped in three general categories:
some way. When this is done, the basic continuous wave search, fiTe control, and special. Searcll radars are of two
is referred to as a carrier wave. categories: air search and surface search. These are used
In practice there are three methods by which a carrier for early warning and general navigation. Search radars
wave may be modulated to convey information. These detect targets at maximum range, 'while sacrificing some
are amplitude, frequency, and pulse modulation. In am- detail. Fire cOl1irol radars are important parts of gun and
plitude modulatioll, abbreviated AM, the amplitude of the missile fire control systems. They are used after a target
carrier wave is modified in accordance with the ampli- has been located by search radar. Special radars are used
tude of a modulating wave, such as voice or music. In the for specific purposes, which include grolmd-controlled
radio receiver the signal is demodulated by removing the approach (GCA) radar at airfields, carrier-controlled ap-
modulating wave, which is then amplified and related to proach (CCA) radar, and height-finding radar.
the listener by means of a speaker. Thls type of modula- Radar operates velY much like a sound V\'ave or echo
tion is widely used in the commercial radio broadcast reflection. If you shout in the direction of a cliff, you will
band, with carrier wave frequencies in the kHz range. hear your shout return from the direction of the cliff. What
In frequellcy modulatioll, abbreviated FM, the fre- actually takes place is that the sound waves generated by
quency of the carrier wave instead of the amplihlde is the shout travel through air lmtil they strike the cliff. There
altered in accordance with the frequency of the modulat- they are reflected, and some return to the originating spot,