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| To: | [email protected] |
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| Subject: | Re: LF: OL9TIT on WSPR?? |
| From: | Andy Talbot <[email protected]> |
| Date: | Wed, 30 Dec 2015 21:18:07 +0000 |
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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 <[email protected]> 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! |
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