It does, Joe.
Thanks for the first convincing explanation that I have seen.
Regards
Conrad PA5Y
-----Original Message-----
From: Joe Taylor <j...@princeton.edu>
Sent: 10 July 2019 01:41
To: WSJT software development <wsjt-devel@lists.sourceforge.net>
Subject: Re: [wsjt-devel] FT8 ghost signals
Hi Conrad,
On 7/8/2019 18:29, Conrad PA5Y wrote:
> In fact Martin, sometimes they clearly AP reflections are but often they are
> not. They have no discernible doppler slant at all and appear to be a fixed
> offset and have me completely puzzled. I regularly see 2 or 3 'ghost' signals
> between 6 and 20dB lower in amplitude than the main signal on 2m from
> stations with 100W to a 9ele or more. Stations can be several hundred km away
> from me. The offsets vary between 70-150 Hz and can be positive or negative.
> In order to have a doppler shift something must be moving and with an offset
> of 100Hz or more quite quickly. You would think therefore that the frequency
> offset would change and it often does not but remains static for several
> periods and then just disappears. How on earth can this happen? I have no
> reasonable explanation for it but I assure you it is real.
>
> Aircraft scatter on 6m occurs of course but not so often, however during
> intense Es events sure enough I also see these ghosts on 6m and again no
> shift. It really is very puzzling indeed.
Evidently what's puzzling you is Doppler shifts that remain nearly constant for
a minute or so. Aircraft at 10,000 meters can be line-of-sight visible at
distances up to several hundred km. With typical aircraft speeds, in such
cases the geometry for bi-static reflections will not change much over a
minute. So the Doppler shift can remain nearly constant. Does this not make
sense?
-- Joe, K1JT
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