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| Sunday, 10 July 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Radar - extremely useful technology
You would have heard the word 'radar' at least once in your life. Some of you may not know what radar is, while some will know what it is all about. Radar is something that is in use all around us, although it is normally invisible. Air traffic control uses radar to track planes both on and off the ground and also to guide planes to land smoothly. NASA uses radar to map the Earth and other planets, to track satellites and space debris. The military uses it to detect the enemy and to guide weapons. Meteorologists use radar to track storms, hurricanes and tornadoes. You even see a form of radar at many stores and offices when the doors open automatically! Obviously, radar is an extremely useful technology. People usually use radar to accomplish one of three things: detect the presence of an object at a distance - usually the "object" is moving, like an airplane, but radar can also detect stationary objects buried underground. In some cases, radar can identify an object as well; for example, it can identify the type of aircraft it has detected. Radar can also detect the speed of an object. This is the reason why police use radar. Another reason is to map something - space shuttles and orbiting satellites use what is called Synthetic Aperture Radar to create detailed topographic maps of the surface of planets and moons. All three activities can be accomplished using two things you may be familiar with, in everyday life: echo and Doppler shift. These two concepts are easy to understand in the realm of sound because your ears hear echo and Doppler shift every day. Radar makes use of the same techniques using radio waves. Echo and Doppler shift When you shout into a well, the sound of your shout travels down the well and is reflected (echoes) off the surface of the water at the bottom of the well. If you measure the time it takes for the echo to return, and if you know the speed of sound, you can calculate the depth of the well fairly accurately. Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment you hear the echo is determined by the distance between you and the surface that creates the echo. Doppler shift is also common. You probably experience it daily (often without realising it). Doppler shift occurs when sound is generated by, or reflected off, a moving object. Doppler shift in the extreme creates sonic booms.
Here's how to understand Doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 miles per hour and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you, the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler shift. You can combine echo and doppler shift in the following way. Say, you send out a loud sound toward a car moving toward you. Some of the sound waves will bounce off the car (an echo). Because the car is moving toward you, however, the sound waves will be compressed. Therefore, the sound of the echo will have a higher pitch than the original sound you sent. If you measure the pitch of the echo, you can determine how fast the car is going. Understanding radar We have seen that the echo of a sound can be used to determine how far away something is, and we have also seen that we can use the Doppler shift of the echo to determine how fast something is going. It is therefore possible to create a "sound radar", and that is exactly what sonar is. Submarines and boats use sonar all the time. You could use the same principles with sound in the air, but sound in the air poses some problems: * Sound doesn't travel very far - maybe a mile at the most. * Almost everyone can hear sounds, so a "sound radar" would definitely disturb the neighbours (you can eliminate most of this problem by using ultrasound instead of audible sound). * Because the echo of the sound would be very faint, it would probably be hard to detect. Radar therefore uses radio waves instead of sound. Radio waves travel far, are invisible to humans and are easy to detect even when they are faint. Let's consider a typical radar set designed to detect airplanes in flight. The radar set turns on its transmitter and shoots out a short, high-intensity burst of high-frequency radio waves. The burst might last a microsecond. The radar set then turns off its transmitter, turns on its receiver and listens for an echo. The radar set measures the time it takes for the echo to arrive, as well as the Doppler shift of the echo. Radio waves travel at the speed of light, roughly 1,000 feet per microsecond; so if the radar set has a good high-speed clock, it can measure the distance of the airplane very accurately. Using special signal processing equipment, the radar set can also measure the Doppler shift very accurately and determine the speed of the airplane. In ground-based radar, there's a lot more potential interference than in air-based radar. The police in developed countries use radar to track speeding motorists. When a police radar shoots out a pulse, it echoes off all sorts of objects - fences, bridges, mountains or buildings. The easiest way to remove such clutter is to filter it out by recognising that it is not Doppler-shifted. A police radar looks only for Doppler-shifted signals, and because the radar beam is tightly focused, it hits only one car. Police in developed countries now use a laser technique to measure the speed of cars. This technique is called lidar, and uses light instead of radio waves. Source: Internet |
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