G0OQK (98km) has just spotted me on 6m WSPR with a large 4Hz Doppler, suggesting reflection/scatter from aircraft somewhere along the path. I am surprised that the software decoded with this much Doppler. My signal was not strong at -28dB S/N.
I hope G0OQK comes on TX again soon as I'd like to see his trace on my software to see if he can be decoded by pure tropo. It seems to me that planes have to be around for the path to "work" and even then the Doppler has to be low enough, ideally much lower than 4Hz. There was a successful decode of G0OQK on 6m a few days ago with 0Hz Doppler but I think that was an aircraft crossing our path and not pure tropo. I'd like to think the path between us can be worked without the help of planes.
UPDATE 1750z: Apart from a single spot of me by G0OQK, all recent 6m WSPR spots have been by local G4IKZ (18km). I may go back to 10m.
UPDATE 1840z: Returned to 10m but it is very quiet with no Es, no GDX and no locals being copied on WSPR. Earlier JT65 attempts on 10m (called CQ) were no good. I had success with JT9-1 on 20m.
UPDATE 2112z: 10m "woke up" briefly here at 2012z with an Es opening to OE3ODW. Just a couple of 2 minute WSPR slots and he was gone.
As G0LRD has pointed out, drift on 6m (and higher) is as likely to be TX drift as aircraft reflection. Looking for multiple lines in any 2 minute slot is the only sure way to know if there was Doppler, and this only works with several planes with differing Doppler shifts. What we can never know is if a single line, drifting maybe, is TX drift on a direct (tropo) path or aircraft reflection.