Quote:
Originally Posted by jdong
I'm speaking beyond my scope of knowledge, but I believe it listens for multiple doppler shifted signals, the strongest of which is the target's speed and some other signal mixed in there represents the patrolcar's speed?
I'm not the professor, just a radar geek who knows less than him, but I'll try and answer the question the best I can until he happens to come across this thread...
Actually, I think how most guns work is they look for multiple doppler returns. The "lower" speed return is the patrol car and the "higher" speed is the target + patrol speed. The gun subtracts the patrol speed from the target speed to get the actual target speed.
Newer guns with "same-lane moving mode" use a more sophisticated algorithm, or simply reverse the roles of the two doppler signals (higher speed return = patrol, lower = target).
Some guns can be electrically connected to the patrol's speedometer, but this is just used for verification that the correct doppler return is used for the patrol speed.
As for weaker than normal alerts during traffic stops, this is caused by either (a) a handheld gun being left on and sitting on the patrol car's seat/floor, or (b) a dash-mounted gun that is no longer optimally aimed down the roadway since the patrol car moved in order to pull over its victim. Also, one may often come up to one of these cars in the same direction as the radar is aimed (e.g. forward facing antenna, coming up on rear of cruiser), so the signal will be weak and then get stronger after passing the cruiser.
Another variation on (b) is if the cruiser was parked perpendicular to traffic while clocking (with the dash-mounted antenna aimed out the side window), then the LEO pulls someone over so the cruiser is now parallel to traffic, but the antenna is now perpendicular, that can cause a weak reading until you're just passing the cruiser.