Not that I've ever used a radar gun, but I'm a physics student, soon to be a physics minor. Only if you are travelling directly at the (stationary) radar gun will it give your true speed reading. This cosine error (if I'm understanding correctly that it refers to the error produced by varying angles) will only give a lower speed than the car is actually traveling. For example, if you are clocked going exactly perpendicular to the radar, it will show a speed of zero, because you are not moving in reference to the radar. So even if you are clocked close the officer, it can be proven with simple physics that you were going even faster than the gun showed.
Food for thought I guess.
