Automatic Direction Finders: The grandfather of all radio navigation aids

Troubleshooting tips
NDB radio frequencies are relatively low and range is usually limited. NDB radio waves can be reflected off the ionosphere. Shorelines, mountains, or cliffs can also affect them. Lightning and atmospheric static also emit radio frequencies that are close to NDB frequencies and they can affect ADF accuracy. These are all normal and natural phenomena.

A pilot may complain that he can receive the station but the pointer wanders back and forth. It may be possible that there is nothing wrong with the ADF system. If you have the opportunity to debrief him, ask if there were mountains, cliffs, shorelines, or lightning in the area.

One last thing . . . just kidding about that Y2K comment earlier — those GPS satellites will be around for some time.