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          ARRoGANT                CoURiERS      WiTH     ESSaYS

Grade Level:       Type of Work           Subject/Topic is on:
 [ ]6-8                 [ ]Class Notes    [Essay on what Radar is  ]
 [ ]9-10                [ ]Cliff Notes    [                        ]
 [x]11-12               [x]Essay/Report   [                        ]
 [ ]College             [ ]Misc           [                        ]

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What Radar is

     The word "radar" was invented 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 tat sends out the radio signal and
picks up the returning echo).

     Radar set on the ground uses radio echoes to locate aircraft, ships,
and other objects.  Radar sets can locate artificial satellites and
spacecraft thousands of kilometres from the earth.  They can find such
"targets" even in darkness, smoke, clouds, fog, or rain.

     Radar is also used in weather prediction to locate storm systems.  An
airplane can carry a radar set\et to aid in determining the airplane's
ground speed.  Ships can carry radar to detect icebergs, other ships, and
aircraft.

     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 oupy a building many stories high. The size of a radar
set depends on the job it is expected to do. But 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
there as a radar echo.  A radar set changes the radar echo 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 dine about it until just before World War II.  In
1935,  Robert A. Watson)Watt 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.

     This was so important because if you imagine that you are about to
shout across a canyon to make an echo.  If you shout a long sentence, the
first words will come back before you can finish the last words.  It would
be impossible to hear the echo clearly because it would be mixed with your
own speech.  But suppose you shout a short word, such as "Hello"  The echo
comes back crisp and clear with no interference.

     Now suppose a radio signal is given off in a short burst, or pulse,
and is reflected from an object. The echo comes back clearly.  But if the
radio signal lasts a long time, the echo comes back while the signal is
still going out.  The radar operator cannot detect the echo at all.

     By using echoes, you can find out how far away the reflecting wall of
a canyon is.  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 canyon wall
and return, it must have gone 335 meters.  But that is the distance of the
round trip the wall and back.  The wall 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.  The 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 cone back.
Radio signals travel at a known speed which is 300,000 kilometres (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 kilometres (186 miles).  The
target must be half that far, or 150 kilometres (93 miles), away.
     
     The location of the target in relation to the radar is found in a
different way.  The radar antenna sends ut 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.  An echo comes back, to strike the plane.  When an echo comes
back, it can be seen on a screen.  This shows the radar operator where the
radar beam hit the plane 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
an 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)a
transmitter, and antenna, a receiver, and a 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 maplike picture of the
surrounding region.  This picture looks as if it were made liking down at
the area from high above the radar set.  The blips show where land areas
are located.  Blips 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.  One is called search radar.  The other is called 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 aurate than search radar sets.  They must be able to pinpoint a
target no large than a basketball as far away as 1,600 kilometres (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 groundcontrolled 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 ours at a point that corresponds to the distance of
the reflected object.  Thus, with a range of 93 miles, and 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 centre of the oscilloscope screen and
radiates outward at a constant rate.  When the beam reaches its maximum
radial length, it quickly returns to the centre.  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 aurate 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
harbour 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.


Work's 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.
