By Jacalyn Sticha
Minnesota State Patrol
Police radar works on the Doppler principle; a change in the frequency of a signal in direct proportion to the speed of an object. The Doppler principle was discovered in 1842 by an Austrian physicist named Christian Johann Doppler, and has been used to measure speed since 1948. The police radar transmits a signal at the target at a known frequency and the return signal is converted into miles per hour.
Once a speed-reading is delivered, which is about the same time the radar detector is alerting the driver, it is likely already a done deal. We can often tell who has a detector because of the rapid decline in the speed of the targeted vehicle.
My experience has shown that motorists who use radar detectors may be stopped more frequently and possibly may have lengthier driving records. A false sense of security, initially, has them speeding more often and at higher rates. A radar detector in a vehicle is not as common and you may think; drivers who use(d) them seem to get rid of them quickly.
The notes written on a speed citation by an officer will have the information about radar detector use. These are the notes available to the court, attorneys and the judges. They will know that the violator had a device that they believed would help them break the law and remain undetected.
Another way of calculating speed is using time and distance. This can be done with aircraft and a trooper in a squad. The speed is calculated using the time is takes a vehicle to travel between two measured marks. No frequency signal is used therefore no detectors are set off.
These two methods are extremely accurate when properly used by well-trained law enforcement personnel.