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Date: Wed, 30 Dec 2015 21:18:07 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: OL9TIT on WSPR??
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This is a well known phenomena - surely you've seen it discussed on the LF
groups over the years.

All efficient communications schemes first of all remove all redundancy
from a message, before adding in tehe controlled redundancy for the forward
error correction (FEC).   One inevitability of this is that when the very
occasional burst of random noise or interference passes all the
requirements of the FEC in the decoder, the resulting completely random
message chucked out looks perfectly valid.

WSPR compresses callsigns and locators is such as way that any random
version of that compressed bit pattern regerates to something that looks
valid.   So any randomly generated message will be a callsign and a locator.

See http://www.g4jnt.com/Coding/WSPR_Coding_Process.pdf for a description
of how WSPR encodes callsigns and locators.

Any strong FEC will most of the time only chuck out correct messages, or
nothing.  But just once in a while - bearing in mind that each reception
session with just pure noise / interference present is actually thorwing
thousands of possible solutions at the decoder.  So by the simple laws of
probability, you know for certain that occasionally somethign will get past.

Of course, what everyone then ignores is that there is a third level of
error correction applied - the intel of teh operator.   You look at the
result, and see that 0L9 is not in locator square AA, so you know its
false.   I suppose it is possible that the software could incorporate a
lookup table of valid call vs. Loc. but its easier to just let teh operator
filter out those false messages.

So, the fact the "... software is free .." is nothing to do with the false
decodes.  Its a mathematical inevitability of efficicient coding and heavy
FEC.

BTW, trawling through the WSPR database to try to find what you reported,
it's [zero]L9 not [oscar]L9
ie 0L9 not OL9    which is an even more unlikely call prefix, not Czech at
all


Andy  G4JNT

On 30 December 2015 at 20:10, LineOne <dhchurch@lineone.net> wrote:

> Glancing back at the WSPR reception report list at 1434 today I noted one
> from OL9TIT on 137.404 kHz at -32dB s/n. Thinking this was some sort of
> Titanic beacon from the Czech Republic and that was a fair level for the
> distance I looked at the location and it indicated about 17,000 km away in
> Antarctica. I think not!
> So who or what is OL9TIT and why did no-one else pick this up? I know
> there are a few spurious reports on Opera (it is free after all so no
> complaints from me) but spurii using WSPR?
> Otherwise the Rx system is working well here.
>
> Hugh, M0DSZ.
>
>

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<div dir=3D"ltr">This is a well known phenomena - surely you&#39;ve seen it=
 discussed on the LF groups over the years.<div><br></div><div>All efficien=
t communications schemes first of all remove all redundancy from a message,=
 before adding in tehe controlled redundancy for the forward error correcti=
on (FEC). =C2=A0 One inevitability of this is that when the very occasional=
 burst of random noise or interference passes all the requirements of the F=
EC in the decoder, the resulting completely random message chucked out look=
s perfectly valid.</div><div><br></div><div>WSPR compresses callsigns and l=
ocators is such as way that any random version of that compressed bit patte=
rn regerates to something that looks valid. =C2=A0 So any randomly generate=
d message will be a callsign and a locator.</div><div><br></div><div>See=C2=
=A0<a href=3D"http://www.g4jnt.com/Coding/WSPR_Coding_Process.pdf">http://w=
ww.g4jnt.com/Coding/WSPR_Coding_Process.pdf</a> for a description of how WS=
PR encodes callsigns and locators.</div><div><br></div><div>Any strong FEC =
will most of the time only chuck out correct messages, or nothing.=C2=A0 Bu=
t just once in a while - bearing in mind that each reception session with j=
ust pure noise / interference present is actually thorwing thousands of pos=
sible solutions at the decoder.=C2=A0 So by the simple laws of probability,=
 you know for certain that occasionally somethign will get past.</div><div>=
<br></div><div>Of course, what everyone then ignores is that there is a thi=
rd level of error correction applied - the intel of teh operator. =C2=A0 Yo=
u look at the result, and see that 0L9 is not in locator square AA, so you =
know its false. =C2=A0 I suppose it is possible that the software could inc=
orporate a lookup table of valid call vs. Loc. but its easier to just let t=
eh operator filter out those false messages.</div><div><br></div><div>So, t=
he fact the &quot;... software is free ..&quot; is nothing to do with the f=
alse decodes.=C2=A0 Its a mathematical inevitability of efficicient coding =
and heavy FEC.</div><div><br></div><div>BTW, trawling through the WSPR data=
base to try to find what you reported, it&#39;s [zero]L9 not [oscar]L9</div=
><div>ie 0L9 not OL9 =C2=A0 =C2=A0which is an even more unlikely call prefi=
x, not Czech at all<br></div><div><br></div><div><br></div><div>Andy =C2=A0=
G4JNT</div><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On 30 =
December 2015 at 20:10, LineOne <span dir=3D"ltr">&lt;<a href=3D"mailto:dhc=
hurch@lineone.net" target=3D"_blank">dhchurch@lineone.net</a>&gt;</span> wr=
ote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border=
-left:1px #ccc solid;padding-left:1ex">Glancing back at the WSPR reception =
report list at 1434 today I noted one=C2=A0 from OL9TIT on 137.404 kHz at -=
32dB s/n. Thinking this was some sort of Titanic beacon from the Czech Repu=
blic and that was a fair level for the distance I looked at the location an=
d it indicated about 17,000 km away in Antarctica. I think not!<br>
So who or what is OL9TIT and why did no-one else pick this up? I know there=
 are a few spurious reports on Opera (it is free after all so no complaints=
 from me) but spurii using WSPR?<br>
Otherwise the Rx system is working well here.<br>
<br>
Hugh, M0DSZ.<br>
<br>
</blockquote></div><br></div></div>

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