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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 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.


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