Radar 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
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.
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.
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.
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.
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.
|While we're here on the topic of sound and motion, we can also understand
sonic booms. Say the car was moving toward you at exactly the speed of
sound - 600 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 600 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 LOUD! That is a sonic boom.
The same phenomena happens when a boat travels through water faster than waves travel through the water (waves in a lake move at a speed of perhaps 5 MPH - all waves travel through their medium at a fixed speed). The waves that the boat generates "stack up" and form the V-shaped bow wave (wake) that you see behind the boat. The bow wave is really a sonic boom of sorts. It is the stacked up combination of all of the waves the boat has generated. The wake forms a V shape, and the angle of the V is controlled by the speed of the boat.
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 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:
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 tell how fast the airplane is going.
The radar sends out a short, high-power pulse of radio waves
at a known frequency. When the waves hit an object they echo off
of it and the speed of the object Doppler shifts the echo. The
same antenna is used to receive the much-weaker signals that return.
In ground-based radar the problem is a little harder because of all the clutter. When a police radar shoots out a pulse, it echoes off of all sorts of objects - fences, bridges, mountains, etc. The easiest way to remove all of this sort of clutter is to filter it out by recognizing that it is not Doppler shifted. A police radar looks only for Doppler-shifted signals, and because the radar beam is tightly focused it only hits one car.
Police are now using a laser technique to measure the speed of cars. This technique is called lidar. It uses the same principles with light that radar uses with radio waves.
Sonar, radar and lidar all use echo and Doppler shift to do the same things - they measure the distance and speed of objects by their reflections.