Radar (Part III): Doppler Shifts

Radar is a fascinating technology: it uses energy we can’t see to form a picture we would otherwise miss. If you’re reading this, you’ve probably finished parts I (how radar works) and II (history of radar). If not, you don’t need to read them, but radar is interesting, so you might want to check ‘em out!

Moving on. To understand why the Doppler shift is necessary, let's take an example. Let's say there’s an air traffic controller and she sees two planes coming in at the same time. If they come in 2 minutes apart, there’s enough time to move the first plane and land the second one. If they land in less than that amount of time, the first plane won’t be out of the way before the second crashes in. How can she tell whether the planes need to be turned around?

To solve this, you have to know how fast the planes are moving. You could try taking a second radar scan and doing the math to figure out how far they traveled in that time to find the mph of each plane. Or, you could use Doppler radar.

Doppler is a specific category of radar that explores whether the objects you are looking at are moving. How can radar tell this? To understand that, you need the basics of frequency.

As you know, energy travels in waves, and radio waves are no exception. All waves have the same structure, a never-ending up and down line (like this: ~), that always goes up to the same height, turns around, and goes down to the same height, and repeats all over again. Some waves go up and down much faster. We call this “having a higher frequency” because the ups and downs happen more frequently in a given amount of time. Frequency is something a radar can detect, and so can you! Higher pitched sounds have a higher frequency, so something like a whistle would have a high frequency, while a foghorn would have a very low one. Now that you understand frequency, you can move on to Doppler shifts.

Say you had a string on your desk in the shape of a wave, going up and down and up and down. (you can try this out at home to get a clearer picture if you'd like) Now put your hand on the right slide of the “wave” and move it to the left, slowly. What happens to the frequency of the wave?

It goes up! By moving forward, you are squishing the wave together, giving it a higher frequency and a higher pitch.

Now put one finger on the right end of the string and drag it to the right. What happens to the frequency?

It goes down. By moving backward, the wave is being stretched apart, leaving a lower frequency and pitch.

These “shifts” up and down in frequency because of motion are known as Doppler shifts.

So, to put it into practice, the radar knows the frequency of the signals it sends out. So, if it gets back anything higher, it knows the object is moving closer. If it gets back anything lower, it knows the object is moving farther away.

Now, let’s get back to the air traffic controller from before. She can check the doppler shift of both planes to figure out their speed and calculate how long it will take each plane to reach the airstrip using the basic speed = distance * time formula. Then, all she has to do is subtract the times and find out if they’re more than 2 minutes apart.

Congratulations! You just avoided a plane crash, all thanks to Doppler Radar.

Illustration of waves from a moving light source.

Picture Credit: imagine.gsfc.nasa.gov