On 7 Jul 2019 at 15:02, Rich Zwirko - K1HTV wrote: > Martin, > The reason why I believe that the ghost FT8 signals are not being caused > by aircraft is because of the amount of Doppler being observed on the > waterfall display. In the case of the aircraft reflections that you have > observed, there is a clearly discernible slant on them. In relationship to > the observer, the relatively large delta change in the azimuth heading of > the reflecting aircraft is producing the waterfall slant that you have seen. > > However, the waterfall patterns of Es reflected FT8 signals that I and > others in the Mid-Atlantic area of the States have observed are basically > parallel to the main FT8 waterfall signal. They do NOT show the > characteristic slant observed on signals reflected off aircraft. > > Does anyone have any idea as to why are these observed single or multiple > FT8 backscatter ghost signals are so relatively frequency stable?
Hi again Rich, if you're interested in following up on this, there was a discussion thread (initiated by me, just over a year ago) about this topic; you'll find it at: https://groups.io/g/RSGBTechnical/topic/21562277 As you'll see, there's ample evidence, eg. the figures provided by Neil G4DBN, to confirm that the multiple FT8 decodes are most probably the result of reflections from aircraft. The spectral plot provided in the aforementioned discussion thread by Martin G4FUI is pretty convincing. As you'll see from the discussion thread, the topic of scattering of VHF signals by aircraft was investigated and analysed by G3BGL way back in 1966! In the scenario you mention, ie. the 'ghost' signals appearing to be parallel to the primary signals on the waterfall, I suspect that if it were possible to analyse the 'ghost' signals with sufficient resolution you'd find that there was some Doppler shift on them but it's not immediately apparent on the waterfall at least in part because of the multi-tone nature of the FT8 signal. I've observed that 'ghost' signals from a greater range, eg. from stations 100-200 miles away from me, don't exhibit the same degree of slope as signals from 10 miles away. I've even (albeit very infrequently) seen what appeared to be multiple decodes from much more distant stations, eg. up to 1000 miles away, and the non-powerline frequency secondary signals did appear to be parallel to the primary signal. Perhaps the only way to verify whether or not the signals you and others observe do exhibit any Doppler shift would be to conduct some tests using single-tone signals and potential reflectors at different ranges - it would be very much easier to do fine-grain analysis on those using a tool such as G3PLX's SBSpectrum software (which I've used, with results that confirm my hypothesis). What we need is for someone to do the math(s) to calculate the Doppler shift on a 50MHz or 144MHz signal from moving reflectors at different ranges and angles, and with different velocities. One of the postings in the discussion thread from last year refers to something called Airscout which appears to be an aircraft scatter prediction tool created by DL2ALF - perhaps that'll provide the answers. --- Martin, G0HDB --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
