RADAR Functioning DetailsRadar is something that is in use all around us, although it is normally invisible. Airports use radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings. Police use radar to detect the speed of passing motorists. NASA uses radar to map the earth and other planets, to track satellites and space debris and to help with things like docking and maneuvering. The military uses it to detect the enemy and to guide weapons. Weathermen use radar to track storms, hurricanes and tornados. You even see a form of radar at many grocery stores opening the doors automatically! Obviously radar is an extremely useful technology, and in this edition of How Stuff Works we will look at radar's secrets.
When people use radar, they are usually trying to accomplish one of three things:
Echo and Doppler Shift
Doppler Shift is also common and you also experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, 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 MPH 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.
Here's what is happening. The speed of sound through the air in the parking lot is fixed. 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 one mile away from you, and it toots 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 6 second delay (while the sound travels one mile at 600 MPH) followed by exactly one minute's worth of sound.
Doppler shift - The person behind the car hears
a lower tone than the driver because the car is
moving away. The person in front of the car hears a
higher tone than the driver because the car is approaching.
Now let's say that the car is instead moving toward you at 60 MPH. It starts from a mile away and toots it's horn for exactly one minute. You will still hear the 6 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 less 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.
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Understanding Radar (external link)
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