What Is Radar?

When people use radar, they are usually trying to accomplish one of three things:

All three of these activities can be accomplished using two things you may be familiar with from everyday life — echo and Doppler effect, which is also called Doppler shift. These two concepts are easy to understand in the realm of sound because your ears hear echo and Doppler effect every day. Radar systems make use of the same techniques using radio waves.

In this article, we'll uncover radar's secrets. Let's look at the sound version first, since you may be familiar with this concept.

Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. In short, echoes are reflected radio waves. The length of time between the moment you shout and the moment you hear the echo is determined by the distance between you and the surface that creates the echo.

Here's how to understand Doppler effect. Let's say there is a car coming toward you at 60 miles per hour (mph) in a parking lot and its horn is blaring. You will hear the horn in one pitch as the car approaches, but when the car passes you, the pitch of the horn will suddenly drop. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler effect.

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 the car is standing still, it is exactly 1 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 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 toots its 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 effect 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've seen that the echo of a sound can be used to determine how far away something is, and we've also seen that we can use the Doppler effect 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:

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 effect 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 get an accurate distance measurement of the airplane. Using special signal processing equipment, the radar set can also measure the Doppler effect very accurately and determine the speed of the airplane.

In ground-based radar, there's a lot more potential interference from other radar signals than in air-based radar. When a police radar shoots out a radar pulse, it echoes off of all sorts of objects — fences, bridges, mountains, buildings. The easiest way to remove all of this clutter from the actual targets and improve radar performance is to filter it out by recognizing what's not Doppler-shifted. Police use dopler radars that only look for Doppler-shifted signals, and because the radar beam is a narrow beam, the radar detection only hits one car.

Some police are now using a laser technique to measure the speed of cars. This technique is called lidar, and it uses short pulses of light instead of short pulses of radio waves. See How Radar Detectors Work for information on lidar technology.

While we've focused on pulse radar, there's also CW radar. Also called continuous wave radar, CW radar uses electromagnetic waves without interruption instead of pulses. Both use radar antenna and electronic components, but the two types of radar differ in energy consumption. Since CW radar has a continuous signal, it's constantly working.