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38
so that the radar beam sweeps through 360° round the ship to a range of about 90
kilometers (56 miles).
In all radars it is vital that the transmission (sending out) and reception of radio
waves in the transceiver are in close harmony. Everything depends on accurate
measurement of the time between the transmission of the pulse and the return of
the echo. About 1,000 pulses per second are transmitted. Short pulses are best for
short-range work, longer pulses are better for long-range work.
An important part of the transceiver is the modulator circuit. This "keys" the
transmitter so that it oscillates, or pulses, for exactly the right length of time. The
transmission power is generated in a device called a magnetron, which can handle
these very high frequency oscillations and very short pulses.
Between each pulse the transmitter is switched off and isolated. The very weak
echoes from the target are picked up by the antenna and fed into the receiver,
amplified (made stronger), and then passed to the display unit.
The display unit usually carries all the controls necessary for the operation of the
whole radar. It has a cathode ray tube similar to that used in a television set, but
specially adapted for radar work. In the neck of the tube is an "electron gun" which
shoots a beam of electrons at a chemically coated screen at the far end. The
chemicals on the screen glow when they are hit by the electrons and. although the
screen is on the inside wall of the tube, the resulting spot of light can be seen
through the glass face exactly as you view a television picture. Unlike a television
tube, however, a radar tube has a circular screen marked off ("calibrated") in
degrees around its edge The electron beam travels out from the center to the edge.
This radial motion of the electron beam, known as the "trace", is matched with the
rotation of the antenna. So when the trace is at 0° on the tube calibration, the
antenna is pointing straight ahead The beginning of each trace corresponds exactly
with the moment at which a pulse of radar energy is transmitted. When an echo is
received it brightens up the trace for a moment. This is a "blip", and its distance
from the center of the tube corresponds exactly with the time taken for the radar
pulse to travel to the target and return. So the "blip" on the screen gives the range
and bearing of the target As the trace rotates, a complete picture is built up from
the "afterglow" of the chemical coating of the tube. This type of display is called a
Plan Position Indicator (PPI) and is the most common form of presenting radar
information.
Computers and Radar
From this simple description, you can see that timing is important to the way
radar works. By using computers, various "tricks" can make the display easier to
read and also get rid of unwanted echoes. For example, in an air traffic control
radar (controlling the movement of civil and military aircraft) it is important to be
able to see only the aircraft. Other echoes, such as rain or high ground, only
confuse the display A computer can be programmed to display only those targets
which are moving fast enough to be aircraft. Also, the computer can "draw" the
radar picture and show targets as crosses, squares, or triangles, more easily read
than the old-fashioned blip. Whether the air traffic controller sees blips or
computer-generated symbols, he must be able to identify the targets. He also needs
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