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What is Doppler Radar? - Definition & Uses

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn how the Doppler effect is used to measure features of moving objects by radar, and how this technique is applied in real life. Complete a quiz on the topic.

Doppler Radar Definition

Radar is any system that sends out radio waves, which reflect off surfaces and return to be detected by the same device. Radar was originally short for radio detecting and ranging. The purpose is to figure out the presence, speed, distance or direction of objects, especially vehicles. For example, radar can be used to track airplanes. This is similar to the way that submarines track other underwater objects, except that underwater sonar uses sound instead of radio waves.

A Military Radar Image
A Military Radar Image

Doppler radar is specifically radar used to figure out the speed of an object'. Doppler radar works using the knowledge that waves produced by an object will be squashed closer together if it's moving towards you, or spread further apart if it's moving away. This is why a police siren sounds like it goes down in pitch as the car moves away from you: as each wave is produced, the car moves further and further away, increasing the distance between each wave front. On the other hand, if the car moves towards you, the car has a chance to catch up with the previous wave it produced, squashing the waves together. Compressed waves are higher frequency, and higher frequency is higher pitch. Spread out waves are lower frequency, and lower frequency is lower pitch. Therefore, a police car siren moving towards you will have a higher pitch siren, and a police car siren moving away from you will have a lower pitch siren.

Car sirens sound higher in pitch if the car is moving towards you, because of the Doppler effect
Car sirens sound higher in pitch if the car is moving towards you, because of the Doppler effect

Doppler radar works in a similar way. Doppler radar bounces a pulse of radio waves off the target object. Once the wave bounces, it acts just like the sound waves from the police siren. And by measuring how much the frequency of the radio waves changes after bouncing off the object, you can calculate the speed of that object.

Illustration of the Doppler Effect
Illustration of the Doppler Effect

But how is this used? When do we need to know the speed of an object?

Doppler Radar Uses

Doppler radar is used in many ways. The most common, everyday example is a speed camera or speed gun. If you're driving too fast, and pass a speed camera, it will take a picture of your car and you'll get a ticket. This happens because the speed camera is constantly sending out radio waves to calculate the speed of each object that passes. If you've ever seen a police officer pointing a radar gun, it works the same way.

Doppler Radar Gun
Doppler Radar Gun

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