Police Traffic Radar: General Operation

Take the light from a flashlight, this is very much like the invisible light output from a radar. Microwave beams behave very similar to normal light beams. Both types of beams spread in a straight line and reflect in the same manner. Metallic objects like cars, trucks, guard rails, and overpasses make the most effective reflectors, sending glints of microwaves around in irregular directions just like glints of light.

Of course, the main difference in these two light beams is that we can’t perceive the microwave beams. Due to the exact frequency of microwaves, they can be easily detected by a tuned radio receiver. In fact, this is the idea that today’s traffic radars are based upon.

Traffic radar works by shining its microwave searchlight down the road.

When an approaching target comes into range, the radar beam reflects off the target and gets received by the radar.

Traffic radar calculates speed from the reflections it receives. And once these beams are captured the radar employs the Doppler theory to calculate speed. You may be familiar with how the Doppler effect works on sound frequencies. You can experience the sound-Doppler effect at any railroad crossing. As the train approaches, you hear the sound at a set pitch. As the train passes, you will start to hear a lower pitch. The train itself is making the same sound both coming and leaving, but a stationary listener, the velocity of the train adds to the pitch of its sound as it approaches, and subtracts as it departs. This shift from true pitch is called the Doppler shift and the magnitude of the change depends only upon the speed of the train.

Traffic radar applies this Doppler principle to microwaves.

Specifically, the Doppler principle compares the originally transmitted frequency to the phase-shifted reflected frequency and calculates speed from this difference assuming the radar is functioning properly. Yet, in some instances the rader could give wrong results causing you to search for the services of a radar repair facility.

Just as with the flashlight/spotlight instance, the emitted beams only travel so far. The light beams will travel further if the light is brighter. Radar beams act the same way.

Traffic radar’s low power means that it has limited reach. Remember, as the signal propagates through air the littler it gets giving it less energy when impacting a target. For example, the radar operator may spray your automobile with microwaves while you’re still a mile away. This is a 2 mile length that the reflected beams have to travel. If the signal strength is too weak once it makes it back to the radar it cannot be read to calculate the object speed. If this is the situation, the vehicle is out of reach.

The radar’s reach is dependent on it’s output power and how reflective the target.

The maximum output power is fixed and cannot be changed. If you think the range is not sufficient anymore you should have it checked-out by a reputable radar repair shop. But reflectivity varies with each target.

For highway vehicles, radar reflectivity is typically a matter of size and shape. For little targets, the reflection is smaller meaning the reach on smaller vehicles is shorter. So if a radar can find a large vehicle at a mile, say, it may not be capable to detect a smaller vehicle because the reflections are smaller.

It is a common practice to send the unit to a radar repair facility so they can do a series of tests to insure accurate readings.