Radar
 
Radar is an electronic device that can "see" hundreds of
miles despite fog, rain, snow, clouds, and darkness. It can
find and accurately located missiles, adrift, ships cities,
rainstorms, and mountains. Radar uses radio waves instead o
flight waves, which the human eye uses in seeing.
 
The word "radar" was coined by scientists of the United
States Navy during World War II. The word comes from the
first letters in the term "Radio Detection and Ranging."
Detection, as used here, means finding an object or target
by sending out a radio signal that will bounce back off the
target as a radio echo. Ranging means measuring the
distance to the target from the radar set (the device that
sends out the radio signal and picks up the returning echo).
 
A radar set on the ground uses radio echoes to locate
aircraft, ships, and other objects at far greater distances
than the eye can see, even in perfect weather. Radar sets
can locate artificial satellites and spacecraft thousands
of kilometers from the earth. 

Radar is also used in weather prediction to locate storm
systems. Many airplanes have a radio altimeter which is a
radar that tells the pilot his altitude above the earth.
Ships have radar to detect icebergs or other ships in fog,
rain, or snow and prevent collisions.
 
Radar can do more than find a target. It can tell how fast
and in which direction the target is moving. This
information can be used to direct the firing of guns and
missiles to protect a country against attack. In peacetime,
radar can help navigate ships, land planes in a fog, and
guide astronauts. Radar can help control street traffic and
assist the police in finding speeding automobiles.
 
Radar sets come in many sizes. A small set, made for use in
a guided missile, is not much larger than a coffeepot. The
larger sets, used to study distant planets, may occupy a
building many stories high. The size of a radar set depends
on the job it is expected to perform; all radar sets,
regardless of their size, use the principle of the echo.
 
How Radar Works
 
Radar sets produce radio signals. They radiate (send out)
these signals into space with a transmitter. When a radio
signal strikes an object such as an airplane, part of the
signal is reflected back to the radar antenna. The signal
is picked up as a radar echo and then changed into an image
that can be seen on a screen. A radar set also gives the
direction of the target and its distance from the set. 

How Radar Began
 
In 1900, a radio pioneer, Nikola Tesla, noticed that large
objects can produce reflected radio signals that are strong
enough to be picked up. He knew that reflected radio
signals are really radio echoes, so he predicted that such
echoes could be used to find the position and course of
ships at sea. But nothing was done about it until just
before World War II. In 1935, Robert A. Watson and other
British scientists developed a system of radio echoes that
could detect approaching aircraft. This later developed
into the radar system that proved effective against German
air raids on Britain in World War II.
 
An important step in making radar possible had taken place
in the United States in 1925. The new idea was to send out
the radio signals in short bursts, called Pulses.
 
By using echoes, the distance of an object can be detected.
Since sound travels through the air at a speed of about 335
meters (1,100 feet) a second, if the sound takes 1 second
to hit the object and return, it must have gone 335 meters.
But that is the distance of the round trip, therefore, the
object must be half that far away, or 167.5 meters(550
feet). To find the distance to an echo making surface,
count the seconds it takes for the echo to return, then
multiply the number of seconds by 167.5 meters.
 
A radar set works on the same principle. It sends out a
very short radio signal. Then it counts the time it takes
for the echo to come back. Radio signals travel at a speed
of 300,000 kilometers (186,000 miles) a second (the speed
of light). If the radio signal comes back in 1/1000 second,
then the round trip is 300 kilometers (186 miles). The
target must be half that far, or 150 kilometers (93 miles),
away.
 
The location of the target in relation to the radar is
found in a different way. The radar antenna sends out radio
pulses in a narrow beam, much like the beam of a
flashlight. The antenna (and its beam) is slowly rotated
through all possible directions, searching the entire sky
for targets. When an echo comes back, it can be seen on a
screen. This shows the radar operator where the radar beam
hit the object and, therefore, the location of the plane.
 
Between 1935 and 1939, a network of radar stations was
built along the coast of Britain. These radar sets gave
early warning of attacking planes and missiles. Germany had
also developed radar ground stations before the beginning
of World War II. The United States developed radar systems
during the war and later created both early) warning (DEW)
lines of radar extending the coverage of radar detection
system. Later developments included the ballistic missile
early warning system (BMEWS) and the combining of radar
equipment with high speed digital computers. 

Radar Systems
 
A radar set, also called a radar system, has four main
parts: transmitter, antenna, receiver, and an indicator.
The transmitter produces the short radio pulses. Each pulse
lasts only about 1/1,000,000 second. There are usually
about 200 or 300 pulses produced each second. The same
antenna is used both to send out the radio pulses and to
pick up the echoes. The returning echoes are sent to the
receiver, where their strength is increased. The echoes
then go to the indicator, which shows the range and
direction of the target to the operator. On the indicator
the echoes appear as bright spots, called blips.
 
The usual type of indicator is the plan position indicator,
or PPI. It has a large tube, much like the picture tube in
a television set. On the face of this tube, the operator
sees a map-like picture of the surrounding region. This
picture looks as if it were made looking down at the area
from high above the radar set. The blips show where land
areas are located. They also show the position of targets
such as planes and ships. The radar operator can pick out
these targets because they are moving, while the land areas
are not. 

Uses of Radar
 
Radar has both military and civilian uses. There are two
main military uses of radar called search radar and fire
control radar. Search radar sets are the kind already
discussed. They continually search the sky to find targets,
and they help ships and aircraft to find other object. Fire
control radar sets help to aim a gun or missile so that it
will hit the target when it is fired. These sets have to be
more accurate than search radar sets. They must be able to
pinpoint a target no large than a basketball as far away as
1,600 kilometers (1,100 miles).
 
One big problem in radar is still unsolved. Engineers call
it discrimination. The target on a radar screen is not a
true picture but a blip of light. All blips look the same.
If a country fires a missile at another country, the
missile can be made to drop harmless pieces of metal, or
decoys. Both the decoys and the missile show up as blips on
radar, so it is hard to discriminate between them.
Scientists are trying to solve this problem.
 
In civilian use, radar sets are most often used to help
navigate ships and planes. The radar sets, carried on a
ship or plane, pick up echoes from other ships and planes
and help prevent collisions. On ships they also pick up
echoes from buoys in channels when the ships enter or leave
port.
 
Radar sets are widely used to help airplanes land when the
weather is bad and pilots cannot see the ground. The ground
controlled approach, or GCA, radar is placed near the end
of the runway. An indicator in the control tower shows the
operator where the plane is at all times. The operator then
talks to the pilot by radio during the landing of the
plane, giving the pilot instructions on just how to follow
a safe course while landing.
 
Radar sets can also be used to get echoes from raindrops,
snowflakes, weather fronts, and cloud formations. Weather
forecasters use such radar, normally combined with optical
radar light detection and ranging, or lidar, to study
storms and find the location of hurricanes and blizzards.
Such radar can also track the migrations of birds and
insects. In astronomy, scientists use radar to map distant
planets that are almost impossible to map by other means.
 
The police use small radar sets to help catch speeding
automobiles. A set placed by the side of the road or held
in the operator's hand measures the speed of passing cars.
When a speeding driver goes by, the operator radios ahead
to a waiting police car, which picks up the speeder. Other
radar sets can count the number of cars on busy streets.
This information can then be used to adjust traffic signals
during rush hours or bad weather.
 
Radar plays a major part in tracking artificial satellites,
space probes, and spacecraft. Astronauts landing on the
moon used radar to tell them how high they were and how
fast they were descending toward the moon's surface. 

Making the Radar Image Visible
 
There are a number of electronic methods for converting
reflected pulses into visible symbols. They may be divided
into range indicators and plan position indicators. Some
radar systems use a combination of both types of indicators.
 
One type of indicator, the A-scope, has an electron beam
which sweeps across the oscilloscope screen once in the
interval between pulses. This sweep is made when the
antenna is receiving reflected waves. The line of light
formed by the sweep is called a time base. The length of
time base corresponds to the range of the radar system.
Thus, if pulses are emitted 1/1000 of a second apart, the
time base corresponds to a range of 93 miles.
 
Repeated sweeps of the electron beam maintain the straight
line on the screen. A reflected wave causes the line to
spurt upward in a narrow peak called a pip. The pip outs at
a point that corresponds to the distance of the reflected
object. Thus, with a range of 93 miles, an object 31 miles
away produces a pip one third of the distance along the
line.
 
In a plan position indicator system (PPI), the antenna's
movement is tracked by the trace of an oscilloscope tube.
The position of the trace on the scope corresponds to the
direction of the beam from the antenna. A reflection
appears as a bright spot on the oscilloscope. The scanning
is radial. A sweep starts from the canter of the
oscilloscope screen and radiates outward at a constant
rate. When the beam reaches its maximum radial length, it
quickly returns to the center. The direction of the line on
the screen matches that of the antenna's radio beam. The
position of the spot on the screen bears a direction
relation to the distance and direction of the object.
 
A A-scope produces an enlarged image of a part of a PPI
picture and projects it on a screen bisected by a
horizontal range line. The PPI system is accurate in the
measurement of the direction of objects. However, for exact
measurement of distance, an A-scope or a B-scope is needed.
 
Today, a sea captain can guide his ship safely through a
crowded harbor in dense fog, and a pilot can land his plane
through a thick overcast. An electronic device called radar
makes this possible.
 
A radar unit can pierce fog, storm, or black night as far
as the horizon. Within its range it can show an observer
ships, planes, storm clouds, small islands, coastlines, and
prominent landmarks. It also measures the distance to these
objects. Radar can even measure the distance to the moon. 
 
Works Cited:
 
Bender, Lionel. The World Of Science. Southside Publishers
Ltd. Copyright, Equinox, 1989
 
Bram, Leon L. Funk & Wagnalls new Encyclopedia. Funk &
Wagnalls, Inc., New York. Book No.16, pg 45)49
 
Lawson, Donald E. Comptons Encyclopedia. Curtis Publishing
Co., 1955., Book No. 19, pg 76)80.