[Lowfer] 474.2 kHz WSPR false decodes

[craig wasson]

Since my profession is data networking - WSPR frustrates me.  It has
forward error correction, but that does not really prevent decoding bad
packets.  It reduces the likelihood somewhat, but that is offset by making
a totally unreadable packet somewhat readable - so you still get bad
decodes.

>From reading the protocol specs there appears to be no error detection at
all.  With only 50 bits of payload, there is not much room for a
traditional checksum - or even a simple parity check.  I have not been able
to figure out how the decoder knows when a potential packet is present as
opposed to just random noise.  If anyone has any insight as to how the
presence of a potentially valid WSPR packet is detected I'd appreciate the
pointer.  I have a feeling it's buried in the FEC logic.

It would be nice if a future WSPR packet format included some sort of error
detection, but I'm at a loss as to where those extra bits may come from if
you want to maintain backwards compatibility with existing formats.  About
all I can think of is taking more advantage of the fact that the format of
a valid call sign is somewhat specific and use this to shift a few bits
around and create some sort of a checksum.  But the call is already fairly
highly compressed - and in some cases replaced by a hash, so not a lot of
room there.

Craig - N6IO



On Mon, Nov 23, 2015 at 11:41 AM, John Andrews <w1tag at charter.net> wrote:

> Garry,
>
> Well, for one thing, the WG2XXO callsign is bogus for 630 meters, as it
> belongs to a company with a grant up in the 2 GHz range.
>
> From some other recent activity, such as you mentioned, there are
> suspicions of "pirates" in our midst, probably to be expected as the
> possibility of a ham band gets more likely. Of course, U.S. call signs that
> lack the "X" as the first letter in the suffix are definitely not real.
>
> Looking at the database, it appears that WG2XXM was sending on 475.710
> during the periods that "WG2XXO" was decoded. That may be part of the
> puzzle.
>
> John, W1TAG
>
>
> On 11/23/2015 9:30 AM, Garry and Linda Hess wrote:
>
>> WSPR decodes are quite reliable in general. When false decodes occur
>> they're usually not hard to identify - the callsign is bogus, the power
>> level ridiculous, etc. However, recently I've seen a few decodes that
>> appear reasonable by themselves, but when you look at others that have
>> similar decodes the frequency is all over the map. For example, consider
>> WG2XXO by searching the database as
>>
>> http://www.wsprnet.org/olddb?mode=html&band=2190&limit=200&findcall=WG2XXO&findreporter=&sort=date
>> .
>> Supposedly I decoded this station on Nov 19 from FN30. Strange no NE
>> listeners reported it. The callsign is real but the FCC FORM 442
>> indicates an address in NC not New England. Others have reported decodes
>> for the same callsign but some of them are for 2 frequencies
>> simultaneously and overall the frequency given varies wildly.
>>
>> Other suspect callsigns are WH2XKW, WH2WYT, and WH2WSX. Perhaps there
>> are more.
>>
>> If anyone knows these stations are for real I'd like to hear about it.
>> To me these "reasonable" decodes look like the result of RX overload/IM.
>> Not sure what the other reporters are using for receive but my eprobe,
>> despite impressive IM2 and IM3 specifications, produces IM products that
>> can be ripe for misinterpretation. The problem may also relate to
>> different decoding software between WSPR and WSPR-X. I've been using the
>> latter.
>>
>> 73, Garry, K3SIW, EN52ta, Elgin, IL
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