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


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