Cobra SLR 600 Operating Instructions Manual page 10

You will only need to calibrate the compass once. However, if
you move your SLR-600 to a different location in the vehicle, or
to a different vehicle, it is best to calibration again.
Note:
If your compass is unable to detect the magnetic field of
the Earth it cannot give the correct reading. This may happen if
you are under a bridge, inside a parking garage or any building
constructed with metal framework. Other nearby vehicles with
a lot of metal may also cause your compass to give inaccurate
readings. Your compass will work again correctly as soon as you
get away from these. There is no need to calibrate your compass
again after this.
For additional information, please visit:
www.cobra.com