# What is Cosine Error?

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What is Cosine Error?

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The accuracy of all Doppler speed radars depends on the angle between the radar beam and the moving object. This is based on the physics of how Doppler radar works. The accuracy can be calculated based on the Cosine formula. For instance, if you are within 8 degrees of angle from the line of travel of the object, you will be within 1% accuracy. If you are within 18 degrees you will be within 5% error and if you are within approximately 25 degrees you will be within 10% error. If you are at 45 degrees of angle, the error gets up to 30%. The speed will always read lower than the actual speed for angles off the direct line of travel. If you are exactly perpendicular (90 degrees), to the line of travel, the radar will not be able to make a speed reading. This is why the police cannot sit down a side street and catch you on radar as you go across in front of them. They need to be in line with your car. On the other hand, they donâ€™t have to worry if they are off angle by a few degrees. Becau