Police or traffic radar uses the “Doppler” effect to measure speed. You have heard this effect as an approaching vehicle with a siren went flying by you. It seemed like the siren tone changed as it passed by, didn’t it? Now, you KNOW it didn’t change, but as the vehicle came at you, the sound waves of the siren were “speeded up” by the speed of the vehicle. If you “speed up” sound, it is the same as increasing its frequency or ‘pitch’. After it passed you, the sound waves were “slowed down” by the same movement. Your ears hear these changes as changes in pitch.

Radio waves can go through a similar process. Say officer Fife is sitting in his car alongside the road. His radar unit is throwing out a constant beam of microwave radio energy. (Think of "microwave radio energy" as a kind of sound wave... just for the moment, ok? Think of it like the siren in the example above.) This beam is reflected by the ground, trees, signs, etc. All these objects are stationary, so the reflected wave is of the same frequency as the wave that was sent out. The radar senses this reflected wave and uses it as a “ground reference”. With Fife’s car stationary, the ground reference plays only a small role, but it plays a fundamental role when he is moving. (More on that later.)

Now, along YOU come, doing 70 mph (which is a bad thing because the speed limit is 45). The radar is hitting you and bouncing back. Because you are moving toward the radar, the radio waves have picked up speed by the speed of your moving car that they bounced off of. This means they are shifted slightly higher in frequency. (In fact, with a Ka band radar operating at 34.2 gHz the waves bouncing off you are shifted up 7149 hertz.) The radar unit detects this shift and does some quick math and displays a “Target” speed of 70. But Fife is asleep.

Now, did you notice something? That 7149 hertz difference is in the audio range. Humans can hear a 7149 hertz sound. This 7149 hertz signal is processed and displayed as a speed of 70. But this signal is also amplified and sent to the radar’s speaker. This speaker comes alive with the SOUND of you speeding and Fife wakes up. But by the time he’s really “with it” you have seen him and slowed down to 45. He hears the tone going down in frequency (to 4600 hertz at 45 mph) and knows without looking at the radar that he has a target that sees him.

(We replace a LOT of speakers in radar units. They squeeze a little speaker into the radar head and then drive the heck out of it – so they do fail a lot. Now, in Georgia, officers are trained and REQUIRED to match the sound of the speeder, the sight of the speeder, and the display of the speed on the radar’s display. The idea is to make an absolute determination that the target is a real target and exactly WHO it is. If the speaker fails, the officer can’t testify that he followed procedure.)

As you pass by Fife, you keep a steady speed of 45. He switches to his rear-facing antenna. Since you are going away from him, the reflected waves are in effect slowed down by the speed of you moving away in much the same way as they were sped up before. They are now of a lower frequency than the original wave – but lower by the same amount as they were higher by before. This means that the SOUND in his speaker (the difference frequency) is the same – the difference frequency of 4600 hertz. And you are still under surveillance.


Let’s say that Fife is now moving along at 60 mph. Remember that “ground reference” reflection? Well, now IT is shifted up in frequency by the speed of the patrol car. What I mean is this: the radar now sees the entire world moving at it at the speed of the patrol car. Many radar units will recognize the change in the ground reference and automatically switch to “Patrol Mode”. The “patrol speed” display shows this speed. The “target” display is blank until…

Along you come, at 70 mph. The shift in your echo is now much greater, you are moving at him at the combined speed of your car and his. Your echo looks to the radar like a 130 mph echo. But the radar is in “Patrol Mode” so it does some easy math and subtracts the patrol speed (that is, the ground reference) from the indication it is getting from you. It displays your speed as the difference between the speed you and he are approaching each other at and the ground reference. The “Target” display shows 70 mph.

If you are coming up behind Fife’s car, and IF he switches to his rear antenna, THAT ground reference signal will show the world going away from the radar at 60 mph. But you come up doing 70. This means that the radar sees you approaching at 10 mph (the difference in your speed and the speed of the patrol car). But since the radar is in “Patrol Mode”, it adds the 60 mph of the ground reference and displays a “Target” of 70.

This leads us to an obvious conclusion: Science and mathematics haven’t been this interesting since you took that class in high school with the hot teacher!