Radar guns for a long time have been the conventional method of an officer tracking your car's speed to determine whether or not to pull you over. If this gun's measurement says you are over the speed limit, that alone can get you a ticket and cause you problems. What is the radar gun's accuracy is challenged? What then happens to the ticket?

In most jurisdictions police officers are required to get their radar gun calibrated at least every 6 months. This time frame varies from place to place. The reason is because these guns aren't always completely accurate. But the standing rule in most places is that if the gun has been calibrated then it's considered accurate enough for court.

There was a case where someone was told they were going 62 miles an hour when they were confident he was actually traveling 45 miles an hour. In order to prove his innocence he had a satellite based device or GPS that could show that there was no way that the vehicle was actually traveling 62 miles per hour. There can be user error on part of the officer using the radar gun.

Outside of some sort of GPS proof, look up your local jurisdiction's rules to find out what requirements the officer has to have on the radar gun or training hours for the gun. If either of these are out of date, then the radar's evidence can't be admitted to the court as evidence which means the case will be thrown out.