How They Work:“The Radar” cont’d

Here’s what happens. The speed of sound through the air in the parking lot is fixed. For simplicity of calculation, let’s say it’s 600 mph (the exact speed is determined by the air’s pressure, temperature and humidity). Imagine that the car is standing still, it is exactly 1 mile away from you and it hoots its horn for exactly one minute. The sound waves from the horn will propagate from the car toward you at a rate of 600 mph. What you will hear is a six-second delay (while the sound travels 1 mile at 600 mph) followed by exactly one minute’s worth of sound. Now let’s say that the car is moving toward you at 60 mph. It starts from a mile away and hoots it’s horn for exactly one minute. You will still hear the six-second delay. However, the sound will only play for 54 seconds. That’s because the car will be right next to you after one minute, and the sound at the end of the minute gets to you instantaneously. The car (from the driver’s perspective) is still blaring its horn for one minute. Because the car is moving, however, the minute’s worth of sound gets packed into 54 seconds from your perspective. The same number of sound waves are packed into a smaller amount of time. Therefore, their frequency is increased, and the horn’s tone sounds higher to you. As the car passes you and moves away, the process is reversed and the sound expands to fill more time. Therefore, the tone is lower. 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. We can use the sound and motion to help us understand the working or radar, the understanding of sonic booms will bring the point home. Say the car was moving toward you at exactly the speed of sound -- 700 mph or so. The car is blowing its horn. The sound waves generated by the horn cannot go any faster than the speed of sound, so both the car and the horn are coming at you at 700 mph, so all of the sound coming from the car “stacks up.” You hear nothing, but you can see the car approaching. At exactly the same moment the car arrives, so does all of its sound and it is sonic boom. 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 has a couple of 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 is likely that it would 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 take 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. When a police radar shoots out a pulse, it echoes off of all sorts of objects -- fences, bridges, mountains, buildings. The easiest way to remove this entire sort of clutter is to filter it out by recognizing that it is not Doppler-shifted. Police radar looks only for Doppler-shifted signals, and because the radar beam is tightly focused it hits only one car. Police are now using a laser technique to measure the speed of cars. This technique is called lidar, and it uses light instead of radio waves. eddie@afrowebs.com