The main problem I see with the 18:40 BFO is, that it occurs in the C-Channel and not in the R-Channel as all other BFO measurements. Yet the BFO for the 23:14 ground to air call also is in the C-Channel and aligns nicely with the "neighboring" R-Channel measurements, so a big difference between R- and C-Channel BFO doesn't seem to exist. How deterministic this behavior is for this particular terminal we don't know (inmarsat probably does).