Re: LF: RE: Wierd WSPR spots

[Andy Talbot <andy.g4jnt@googlemail.com>]

You may be referring to this, I posted a few weeks ago.

 

IF, and this is a big IF, a chunk of interference has managed to get past the convolutional / Viterbi  data decoder in the WSPR decoding routine and if this interference has sidebands, then there is a probability all its sidebands will wobble together.   If this wobble is interpreted as valid FSK that passes the decoder test, then all will result in the same set of bits being decoded.
 
Now, the source encoding and removal of all input redundancy means than just about any random set of 72 bits that get generated will end up with a valid looking callsign, locator and a power level.   And of course, all sidebands will give the same result if they've wobbled together.
 
There are probably far more decodes of the interference that get rejected, its just those that the laws of probability say must happen occasionally, with their sidebands, that generate the multiple hits.

 

So there's some broadband unstable interference with sidebands that manages to occasionally get through

and each sideband, as it wobbles in frequency with the others, decodes to a similar result.

Andy
www.g4jnt.com

On 16 October 2010 14:29, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

John,

the "fake twins" occur regulary.
Jim (or someone else) has offered an explanation some time ago, But I'm afraid I deleted that mail.

73, Rik  ON7YD - OR7T

________________________________________
Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens gm4slv [gm4slv@sighthound.demon.co.uk]
Verzonden: zaterdag 16 oktober 2010 15:16
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: Wierd WSPR spots

Jim, LF,


I just noticed 2 spots, in the same minute, from Jim on 136kHz

2010-10-16 12:24 HV6KDM 0.137463 -22 3 GB36 0.001 M0BMU IO91vr 14488
2010-10-16 12:24 HV6KDM 0.137553 -19 3 GB36 0.001 M0BMU IO91vr 14488

The same mis-decoded Callsign & Locator - 2 different frequencies &
different S/N ratios.

How's that done then?

John
GM4SLV