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
= = =
Joe, I understand why we can see and decode one Doppler shifted signal, but
how do you explain 2 or 3 extra Doppler reflections from the same station?
BTW, congrats on working 5T5PA on 50.323 F/H. Johannes peaked at +11 at
16:36Z when I worked him and more than an hour later he is still at a
steady +4 to +6 dB here in VA. I chatted w/him using JTAlert and he said he
is having lots of fun working so much 6M DX in the F/H mode.
73,
Rich - K1HTV
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