G3WKW (134km) was spotted yesterday on 6m WSPR. I am pretty sure I can only get a path to Bob via aircraft reflections and I think it was aircraft approaching London Heathrow. Although I copied Bob's 5W I did not get spotted by him, although I am using a lower ERP and my signal was probably in the noise at his end, if indeed he was listening/looking. Bob's 5W was around -24dB S/N with low Doppler. Of course, with aircraft, the WSPR software may not have decoded a stronger signal with more Doppler or more multi-path. It is not uncommon to see reflections from many aircraft in a single WSPR burst. Often decodes are impossible unless aircraft flight patterns result in low Doppler.
In this regard, WSPR may not be the best mode. Also WSPR TX bursts are nearly 2 minutes long and this is a long time with passing planes. Other digital modes with shorter TX bursts are probably better. In this regard, passing planes are more like MS bursts so modes well suited to MS would be better.