Cosine Effect and Radar Accuracy
If the target is in a direct line (collision course) with the police radar or sports radar gun the measured speed will be exact. As the angle of incidence increases, if you move either right or left of this direct line, the accuracy of radar guns will decrease. The measured speed will decrease as you move off this centerline. This phenomenon is called the Cosine Effect. It is called this because the measured speed is directly related to the cosine of the angle between the radar gun and the target’s direction of travel. As a quick reference to radar accuracy, remember to keep your targets direction of travel in a direct line with you, and not perpendicular.
The Cosine Effect refers to the angle of the target vehicle in relation to the patrol vehicle where the radar is mounted. The traffic radar should be operated as parallel as possible to the targets, although it is hardly possible to do perfectly. When the angle between the radar beam and target becomes too significant, the relative speed will be less than the true speed producing a lesser speed reading than what the vehicle is actually traveling. Thus, the cosine effect is always in the favor of the motorist. The greater the angle the lesser the speed will be recorded compared to the actual speed of the moving target.
In the radar moving mode of operation care needs to be taken to make sure the radar antenna is pointed at a less than 100 degree angle to the roadway. Since the Target Speed is calculated by taking the Closing Speed and subtracting the Patrol Speed, if the patrol speed is incorrect, then an incorrect Target speed reading could occur. Proper aiming and positioning of the antenna, proper tracking of the target vehicle, proper observation and proper checking of the patrol vehicle's speed displayed in the radar 's Patrol Speed window against the Patrol Vehicle's speedometer should make it apparent to the operator if the reading is incorrect.
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