.. 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.