Long Range NDB's also suffer from a night effect (if my memory is correct)
i.e. The reciever will pick up two signals,
one the normal ground signal
and two the signal which skips of the ironised layer in the atmosphere.(The layer is lower in the atmosphere at night.)
The skipped signal is polerised 90 deg.and the two signals combine to give false indication (I Think)
Just to add confusion
regards
Steamhead