254 lines
14 KiB
Plaintext
254 lines
14 KiB
Plaintext
ÜÜÜÜÜÜÜÜÜÜÜÜÜ ÜÜÜ ÜÜÜÜ
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ÜÛÛÛÛÛÛÛÛßÛßßßßßÛÛÜ ÜÜßßßßÜÜÜÜ ÜÛÜ ÜÛÛÛÛÛÛÛÛÜÜÜÜÜÛßß ßÛÛ
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ßÛÛÛÛÛÛÛÛÛÛÛÛÛÛÜ ßÛÛ ÜÛÛÛÜÛÛÜÜÜ ßÛÛÛÛÜ ßÛÛÛÛÛÛÛÜÛÛÜÜÜÛÛÝ Ûß
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ßßßÛÛÛÛÛÛÛÛÛÛÜ ÞÝ ÛÛÛÛÛÛÛÛÛÛÛßßÛÜÞÛÛÛ ÛÛÛÛÛÜ ßßÛÛÛÞß
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Mo.iMP ÜÛÛÜ ßÛÛÛÛÛÛÛÝÛ ÞÛÛÛÛÛÛÛÛÛ ÞÛÛÛÛ ÞÛÛÛÛÛÝ ßÛß
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ÜÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛÝ ÛÛÛ ÛÛÛÛÛÛ
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ÜÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛ ÞÛÛÛÛÛÛÛÛ ß ÞÛÛÛÛÛÛÜ ÜÛ
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ÜÛÛÛÛÛÛÛÝ ÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÞÛÛÛÛÛÛÛÛÛß
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ÜÛßÛÛÛÛÛÛ ÜÜ ÛÛÛÛÛÛÛÛÝ ÛÛÞÛÛÛÛÛÝ ÞÛÛÛÛÛÛßß
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ÜÛßÛÛÛÛÛÛÜÛÛÛÛÜÞÛÛÛÛÛÛÛÛ ÞÛ ßÛÛÛÛÛ Ü ÛÝÛÛÛÛÛ Ü
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ÜÛ ÞÛÛÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛ ßÛÜ ßÛÛÛÜÜ ÜÜÛÛÛß ÞÛ ÞÛÛÛÝ ÜÜÛÛ
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ÛÛ ÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛÛÜ ßÛÜ ßßÛÛÛÛÛÛÛÛÛß ÜÜÜß ÛÛÛÛÜÜÜÜÜÜÜÛÛÛÛÛß
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ßÛÜ ÜÛÛÛß ßÛÛÛÛÛÛÛÛÛÛÜ ßßÜÜ ßßÜÛÛßß ßÛÛÜ ßßßÛßÛÛÛÛÛÛÛßß
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ßßßßß ßßÛÛß ßßßßß ßßßßßßßßßßßßß
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ARRoGANT CoURiERS WiTH ESSaYS
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Grade Level: Type of Work Subject/Topic is on:
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[ ]6-8 [ ]Class Notes [Essay on what Radar is ]
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[ ]9-10 [ ]Cliff Notes [ ]
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[x]11-12 [x]Essay/Report [ ]
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[ ]College [ ]Misc [ ]
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Dizzed: 09/94 # of Words:2215 School: ? State: ?
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ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>Chop Here>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ
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What Radar is
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The word "radar" was invented by scientists of the United States Navy
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during World War II. The word comes from the first letters in the term
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"radio detection and ranging.""Detection," as used here, means finding an
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object or target by sending out a radio signal that will bounce back off
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the target as a radio echo. "Ranging" means measuring the distance to the
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target from the radar set(the device tat sends out the radio signal and
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picks up the returning echo).
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Radar set on the ground uses radio echoes to locate aircraft, ships,
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and other objects. Radar sets can locate artificial satellites and
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spacecraft thousands of kilometres from the earth. They can find such
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"targets" even in darkness, smoke, clouds, fog, or rain.
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Radar is also used in weather prediction to locate storm systems. An
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airplane can carry a radar set\et to aid in determining the airplane's
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ground speed. Ships can carry radar to detect icebergs, other ships, and
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aircraft.
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Radar can do more than find a target. It can tell how fast and in
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which direction the target is moving. This information can be used to
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direct the firing of guns and missiles to protect a country against attack.
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In peacetime, radar can help navigate ships, land planes in a fog, and
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guide astronauts. Radar can help control street traffic and assist the
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police in finding speeding automobiles.
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Radar sets come in many sizes. A small set, made for use in a guided
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missile, is not much larger than a coffeepot. The larger sets used to study
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distant planets may oupy a building many stories high. The size of a radar
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set depends on the job it is expected to do. But all radar sets, regardless
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of their size, use the principle of the echo.
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How Radar Works
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Radar sets produce radio signals. They radiate(send out) these
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signals into space with a transmitter.
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When a radio signal strikes an object such as an airplane, part of the
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signal is reflected back to the radar antenna. The signal is picked up
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there as a radar echo. A radar set changes the radar echo into an image
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that can be seen on a screen. A radar set also gives the direction of the
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target and its distance from the set. How Radar Began
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In 1900 a radio pioneer, Nikola Tesla, noticed that large objects can
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produce reflected radio signals that are strong enough to be picked up. He
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knew that reflected radio signals are really radio echoes. So he predicted
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that such echoes could be used to find the position and course of ships at
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sea. But nothing was dine about it until just before World War II. In
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1935, Robert A. Watson)Watt and other British scientists developed a
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system of radio echoes that could detect approaching aircraft. This later
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developed into the radar system that proved effective against German air
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raids on Britain in World War II.
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An important step in making radar possible had taken place in the
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United States in 1925. The new idea was to send out the radio signals in
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short bursts, called Pulses.
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This was so important because if you imagine that you are about to
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shout across a canyon to make an echo. If you shout a long sentence, the
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first words will come back before you can finish the last words. It would
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be impossible to hear the echo clearly because it would be mixed with your
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own speech. But suppose you shout a short word, such as "Hello" The echo
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comes back crisp and clear with no interference.
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Now suppose a radio signal is given off in a short burst, or pulse,
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and is reflected from an object. The echo comes back clearly. But if the
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radio signal lasts a long time, the echo comes back while the signal is
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still going out. The radar operator cannot detect the echo at all.
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By using echoes, you can find out how far away the reflecting wall of
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a canyon is. Sound travels through the air at a speed of about 335 meters
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(1,100 feet) a second. If the sound takes 1 second to hit the canyon wall
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and return, it must have gone 335 meters. But that is the distance of the
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round trip the wall and back. The wall must be half that far away, or
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167.5 meters(550 feet). To find the distance to an echo)making surface,
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count the seconds it takes for the echo to return. The multiply the number
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of seconds by 167.5 meters.
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A radar set works on the same principle. It sends out a very short
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radio signal. Then it counts the time it takes for the echo to cone back.
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Radio signals travel at a known speed which is 300,000 kilometres (186,000
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miles) a second (the speed of light). If the radio signal comes back in
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1/1000 second, then the round trip is 300 kilometres (186 miles). The
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target must be half that far, or 150 kilometres (93 miles), away.
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The location of the target in relation to the radar is found in a
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different way. The radar antenna sends ut radio pulses in a narrow beam,
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much like the beam of a flashlight. The antenna (and its beam) is slowly
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rotated through all possible directions, searching the entire sky for
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targets. An echo comes back, to strike the plane. When an echo comes
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back, it can be seen on a screen. This shows the radar operator where the
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radar beam hit the plane and ,therefore, the location of the plane.
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Between 1935 and 1939, a network of radar stations was built along the
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coast of Britain. These radar sets gave early warning of attacking planes
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an missiles. Germany had also developed radar ground stations before the
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beginning of World War II. The United States developed radar systems
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during the war and later created both early) warning (DEW) lines of radar
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extending the coverage of radar detection system. Later developments
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included the ballistic)missile)early warning system(BMEWS) and the
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combining of radar equipment with high)speed digital computers. Radar
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Systems
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A radar set, also called a radar system, has four main parts)a
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transmitter, and antenna, a receiver, and a indicator. The transmitter
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produces the short radio pulses. Each pulse lasts only about 1/1,000,000
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second. There are usually about 200 or 300 pulses produced each second.
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The same antenna is used both to send out the radio pulses and to pick up
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the echoes. The returning echoes are sent to the receiver, where their
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strength is increased. The echoes then go to the indicator, which shows
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the range and direction of the target to the operator. On the indicator
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the echoes appear as bright spots, called blips.
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The usual type of indicator is the plan position indicator, or PPI.
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It has a large tube, much like the picture tube in a television set. On
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the face of this tube, the operator sees a maplike picture of the
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surrounding region. This picture looks as if it were made liking down at
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the area from high above the radar set. The blips show where land areas
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are located. Blips also show the position of targets such as planes and
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ships. The radar operator can pick out these targets because they are
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moving, while the land areas are not. Uses of Radar
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Radar has both military and civilian uses. There are two main military
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uses of radar. One is called search radar. The other is called fire
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control radar. Search radar sets are the kind already discussed. They
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continually search the sky to find targets, and they help ships and
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aircraft to find other object. Fire)control radar sets help to aim a gun or
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missile so that it will hit the target when it is fired. These sets have
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to be more aurate than search radar sets. They must be able to pinpoint a
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target no large than a basketball as far away as 1,600 kilometres (1,100
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miles).
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One big problem in radar is still unsolved. Engineers call it
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discrimination. The target on a radar screen is not a true picture but a
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blip of light. All blips look the same. If a country fires a missile at
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another country. The missile can be made to drop harmless pieces of metal,
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or decoys. Both the decoys and the missile show up as blips on radar, so
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it is hard to discriminate between them. Scientists are trying to solve
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this problem.
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In civilian use, radar sets are most often used to help navigate ships
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and planes. The radar sets, carried on a ship or plane, pick up echoes
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from other ships and planes and help prevent collisions. On ships they
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also pick up echoes from buoys in channels when the ships enter or leave
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port.
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Radar sets are widely used to help airplanes land when the weather is
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bad and pilots cannot see the ground. The groundcontrolled approach, or
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GCA, radar is placed near the end of the runway. An indicator in the
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control tower shows the operator where the plane is at all times. The
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operator then talks to the pilot by radio during the landing of the plane,
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giving the pilot instructions on just how to follow a safe course while
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landing.
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Radar sets can also be used to get echoes from raindrops, snowflakes,
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weather fronts, and cloud formations. Weather forecasters use such radar,
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normally combined with optical radar(light detection and ranging, or
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lidar), to study storms and find the location of hurricanes and blizzards.
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Such radar can also track the migrations of birds and insects. In
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astronomy, scientists use radar to map distant planets that are almost
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impossible to map by other means.
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The police use small radar sets to help catch speeding automobiles. A
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set placed by the side of the road or held in the operator's hand measures
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the speed of passing cars. When a speeding driver goes by, the operator
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radios ahead to a waiting police car, which picks up the speeder. Other
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radar sets can count the number of cars on busy streets. This information
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can then be used to adjust traffic signals during rush hours or bad
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weather.
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Radar plays a major part in tracking artificial satellites, space
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probes, and spacecraft. Astronauts landing on the moon used radar to tell
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them how high they were and how fast they were descending toward the moon's
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surface. Making the Radar Image Visible
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There are a number of electronic methods for converting reflected
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pulses into visible symbols. They may be divided into range indicators and
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plan)position indicators. Some radar systems use a combination of both
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types of indicators.
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One type of indicator, the A)scope, has an electron beam which sweeps
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across the oscilloscope screen once in the interval between pulses. This
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sweep is made when the antenna is receiving reflected waves. The line of
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light formed by the sweep is called a time base. The length of time base
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corresponds to the range of the radar system. Thus, if pulses are emitted
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1/1000 of a second apart, the time base corresponds to a range of 93 miles.
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Repeated sweeps of the electron beam maintain the straight line on the
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screen. A reflected wave causes the line to spurt upward in a narrow peak
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called a pip. The pip ours at a point that corresponds to the distance of
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the reflected object. Thus, with a range of 93 miles, and object 31 miles
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away produces a pip one third of the distance along the line.
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In a plan position indicator system(PPI), the antenna's movement is
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tracked by the trace of an oscilloscope tube. The position of the trace on
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the scope corresponds to the direction of the beam from the antenna. A
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reflection appears as a bright spot on the oscilloscope. The scanning is
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radial. A sweep starts from the centre of the oscilloscope screen and
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radiates outward at a constant rate. When the beam reaches its maximum
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radial length, it quickly returns to the centre. The direction of the line
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on the screen matches that of the antenna's radio beam. The position of
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the spot on the screen bears a direction relation to the distance and
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direction of the object.
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A A)scope produces an enlarged image of a part of a PPI picture and
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projects it on a screen bisected by a horizontal range line. The PPI
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system is aurate in the measurement of the direction of objects. However,
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for exact measurement of distance, an A)scope or a B)scope is needed.
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Today a sea captain can guide his ship safely through a crowded
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harbour in dense fog, and a pilot can land his plane through a thick
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overcast. An electronic device called radar makes this possible.
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A radar unit can pierce fog, storm, or black night as far as the
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horizon. Within its range it can show an observer ships, planes, storm
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clouds, small islands, coastlines, and prominent landmarks. It also
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measures the distance to these objects. Radar can even measure the distance
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to the moon.
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Work's Cited
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Bender, Lionel. The World Of Science. Southside Publishers Ltd.
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Copyright, Equinox, 1989
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Bram, Leon L. Funk & Wagnalls new Encyclopedia. Funk &
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Wagnalls, Inc., New York. Book No.16, pg 45)49
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Lawson, Donald E. Comptons Encyclopedia. Curtis Publishing Co.,
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1955., Book No. 19, pg 76)80.
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