Чтение общенаучной литературы. Кытманова О.А. - 38 стр.

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39
to know the height of aircraft, so that those on the same course but flying at
different levels can be kept apart . To give the controller this information, a second
radar, called a "secondary surveillance radar", is used. This works differently and
needs the "help" of the aircraft. It sends out a sequence of pulses to an electronic
"black box", called a "transponder", fitted on the aircraft. The transponder is linked
to the aircraft altimeter (the device that measures the plane's altitude or height) to
transmit back to the radar a coded message identifying the aircraft and its altitude.
Military aircraft use a similar radar system with secret codes to identify "friend"
from "foe". A "hostile" aircraft does not know what code to transmit.
To work out the height of unidentified aircraft, special height-finding radars are
used. There are two methods of height-finding: one called "nodding" radar and the
other "three-dimensional" radar A nodding height-finder is like an ordinary radar
standing on its side. When the controller wants to find the height of a target
displayed on the screen, the height-finder is rotated to the bearing of the aircraft,
whereupon the radar nods up and down. By measuring the angle of the antenna at
the point when the beam strikes the target, the height of the aircraft can then be
worked out, There are also what are called "3-D" radars. Instead of using two
radars, the two functions are combined in a single antenna system Here the "nod"
is an electronic signal instead of a mechanical movement of the antenna.
There are many other uses for these radars. Most missiles are guided by radar.
Quite often the radar is mounted in the nose of the weapon Radars are also fitted
on board some aircraft to warn the pilot of air turbulence and thunderstorms Radars
now play an important part in weather forecasting and are also found on hoard
spacecraft, mapping the surface of the Earth below. Radars using continuous wave
transmission (rather than pulses) are fitted in devices such as the proximity fuse,
which causes a missile or shell to explode when close to its target. Also, as some
car drivers discover to their cost, radar can be an effective way to catch the
motorist who exceeds the speed limit.
RADIO
In nature there exists a vast range or spectrum of electromagnetic radiation. At the
top end of the scale fin terms of the number of oscillations or vibrations per
second] are cosmic rays, followed by gamma rays. X-rays, ultraviolet rays, visible
light, infra-red rays and, at the bottom of the scale, radio waves.
Radio communication makes use of the "act that electromagnetic waves can carry
electrical copies of sounds when the sounds have been transformed into electrical
oscillations of the same frequencies. An electrical oscillation is an electric current
which instead of flowing steadily in one direction reverses its direction of flow at
regular intervals. A cycle is one complete sequence and the number of times it
repeats itself every second is called the ''frequency''.
Radio waves cover a range of frequencies from about 30 million kHz to 10 kHz
(kHz=a kiloHertz, an oscillation of 1.000 cycles a second). These waves travel
through space at a uniform speed of 300,000 kilometers 1186,000 miles) a second.
This article outlines the history of radio communications. The article

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