To my understanding, radar works by shooting an invisible beam that bounces off the object and returns to the gun and times it. So my question is, how does that work while the cop is moving?
Surely if the cop was driving at 45mph, and tested a stationary sign, the sign would read back as traveling at 45mph, no? So if that’s the case, when the officer is on the road driving the speed limit and he measures another cars speed, how does it work?
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The radar system knows the cops speed, that’s easy to verify. So it just factors that into the equation when computing another cars speed.
If the cop is going 55, and another car passes them at 90mph, instead of only reading 35mph (since that’s the difference between their speeds), it’ll add the cops speed and report 90.
Simple. By taking into account the speed of the police car as well. This is how radars in planes also work. Otherwise, yes, it would be as you say.
There are 2 way that know of that radar systems use to compensate for the vehicle speed. One way is that the police car speedometer is calibrated to the standard of +- 2 mph. This speed is electronically reported to the radar system. The second way is that the radar receives a reflection from the ground and can read its own speed that way.
Police usually do radar readings when they are stopped. In a speed trap, usually there will be separate cars doing the radar measurements, and a chase car to pull people over.
Radar systems can be used while moving, it is just connected to either the police car’s speedometer, or to a GPS. They can measure the difference in speed between your car and the police car, then add in the speed of the police car to get your speed. But cops don’t really need to use radar to know how fast you are going in a chase. If they are keeping pace with you, and they are doing 90 MPH, then it means you are doing 90 MPH as well. If they are driving at 65 MPH, and you speed by them, it means you are going way over 65 MPH.
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