Glen Hood, age 9, of Calgary, Alberta, Canada, for his question:
How does an airport use radar?
A visit to an airport is as exciting as a circus. You watch a plane take off and before it is lost in the wide blue sky, another is ready to land. The pilots seem to know just where and when to go and this is because they can depend on the airport's radar system to guide them safely to and fro.
We may watch the planes landing and leaving, but we may not visit the control tower where the air traffic is directed. This traffic office is a high room with glass walls that give a wide view of the airfield and the sky above it. If we were allowed inside, we would see a number of round radar screens. Each one is watched by an expert operator able to think fast and keep his mind on his job. Visitors are not allowed because they would disturb these experts. The radar screens show just where all the planes are for miles around the airport. Planes fly fast and the traffic picture changes. Operators must keep up with this changing scene and also know the whereabouts of all the planes and vacant runways on the ground. The pilots do not have all this information and they depend on traffic instructions from the control tower.
The control tower has a whole system of radar equipment that works together with radio and telephones, microphones and light signals. The antennas of the radar system are outside the control room, usually on top of the tower. They send out invisible beams of radio waves that fan out at about 186,000 miles a second. When the radio beams travel through empty sky they go on in straight lines. When they strike a solid object such as a plane, they bounce back to the antenna somewhat like an echo.
An ordinary echo brings back the sound of your voice. An airport radar antenna brings back an imprint of a solid plane in the sky. Electronic equipment relays the signal down to a screen inside the control room. The signal is a bright spot called a blip. As an eagle eyed operator watches his screen, he sees a needle shaped indicator sweep around and around. As it sweeps, it may show one or several blips. Each blip represents an airplane and the operator must be able to read just where it is. These fast moving radar signals give the control room a complete picture of what is going on in the sky.
The control room has phones and microphones, radios and light switches to contact the pilots in the planes. The pilots must remain in contact with the control tower for many miles around the airport. The operators plan a safe course for each plane and send the instructions to the pilot. Radar works in darkness and its bright blips pierce the clouds. On a foggy foggy day, the control tower may instruct the pilot to stop guiding his plane. Radar shows just where he is and electronic gadgets are trig¬gered to land the plane safely without the help of human hands.
There are many tricky radar operations and experts in the control tower call them by letters, no doubt in order to save time. GCA means Ground Control Approach which is the tricky operation used to land a plane in a dense fog. RHI means Range Height Indicator. It is a radar screen that shows, for example, that a plane is 65 miles away and flying 10,000 feet above the ground. PPI is a Flan Position Indicator. This radar screen has a special compass to show the exact distance and direction of the blips.