Hello!
411A: NDB's are not 'calibrated' in the sense that a VOR might be, however the signal can be distorted by ground objects...or...a poorly functioning receiver/indicator in the airplane.
I know, Sir. But I didn't want to write a whole page of text book here, if all I wanted to say could be described with one simple word. And there are calibration flights for NDB approaches too, so the word "calibrated" is not too far off anyway. The signal seems to be distorted by ground objects in this case, as the described error shows with all our training aircraft.
Greetings, Max
BTW: Did I insult you in any way, so that you need to ridicule me?