This module explains the method of active deceptive jamming and what effects it has on a radar.
Active, deception jamming:
A radar detector detects radar, and then transmits a frequency corresponding to a given (legal) speed. This frequency is read by the radar detector, which mistakes it for the Doppler shifted echo off of the car, and gives it a fake reading of whatever speed the jammer prefers. This method requires that the jammer knows the EXACT frequency that the cops are transmitting.
Advantages
It is not detected by the radar gun as jamming.
Works very well if one knows the frequency which the cops are using (which is a given for the X and K bands). Radar detectors operating in the Ka band can use any number of different frequencies. Some deception jammers are actually sophisticated enough to read a (fixed) Ka frequency and then transmit the appropriate jamming frequency; however, due to constant FCC crackdowns on jammer manufacturers, these are very difficult to find.
Disadvantages
Still just as illegal as all active jammers, with the same penalties
Once again, if the radar gun is well built and faster than the jammer device, it can get a reading of a car’s speed before the jammer turns on.
Many modern day Ka band radar guns switch rapidly between many different frequencies, foiling all deception jammers since they cannot determine which frequency to transmit
Example
Say we want to send the signal sent to read that we were actually driving a (legal) 55 mph. Then our jammer outputs a false peak at a frequency corresponding to 55 mph:
This causes the radar gun’s matched filter to incorrectly output 55 mph:
Thus, the radar reads the 100mph car’s speed as actually 55 mph.
