ADF and Doppler
 

Scientists have recently found an answer to the problem of station passage over an NDB, which is often part of a question in commercial pilots' exams. Many people have been confused, when working with a VOR as well, on finding that their position has not actually been over the NDB at all.

The reason has been found to be Doppler effect, in that, when going towards or away from the beacon, the radio frequency shifts slightly, up and down, respectively, so the ADF equipment is never really tuned properly, and you are therefore never really over the beacon.

The only aircraft known to have successfully done this are those with the old "coffee grinder" tuning method, which can thus tune upwards slightly on the way to the beacon and detune after passage.

Phil

paco, can you give a reference to this scientific report?

All I can say is it was issued on the 1st April ;)

Phil

Yes, I was just going to say that the relative speed of a helicopter with respect to that of a radio wave isn't real quick!:)

Don't worry too much about the ADF anymore. In Western Europe they are removing more and more ADF's, so in a few years the problem will be solved anyway.
:)