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Re: LF: MF OPERA BAD

To: <[email protected]>
Subject: Re: LF: MF OPERA BAD
From: "Markus Vester" <[email protected]>
Date: Sat, 10 Jan 2015 12:18:46 +0100
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Hi Graham,
 
as far as I know, Stefan's RF power is actually lower than that of many other LF stations, eg. those running ex-Decca modules. But maybe he does have a more efficient antenna than most ;-)
 
I have indeed been wondering about those two simultaneous WD2XNS deep-search detections the other night. The statistical probability of simultaneous false positives would indeed be very low if were only due to noise. The problem is however that falses rarely pop out of the pure noise but are often triggered by real or spurious signals, eg. other intended transmissions or interference (like DCF39 crossmodulation). These may be in common view of several receivers. If the waveform of any of such signals happens to bear similarity to one of the sequences in the search list, they are likely to pop up as false correlations.
 
Having an accurate frequency readout would help to diagnose the source of the problem. For example, I got a number of reproducable opds falses for R7NT, which turned out to be reproducably triggered by one PA station who transmittied WSPR-15 sequences at these times. The pattern on one of the WSPR frequencies code just happened to bear similarity to R7NT's Opera sequence. This one was relatively easy to sort out from to the out-of-band frequency and the quarter-hour timing. Here's the excerpt from https://dl.dropboxusercontent.com/u/26404526/opds32_archive.txt :
 
2014-01-26 11:00:39 R7NT    2060km 137602.968Hz   3mHz -42.2dBOp  94% 15.2dB ?
2014-01-26 09:00:39 R7NT    2060km 137602.969Hz   2mHz -42.3dBOp  63% 15.1dB ?
...
2014-01-25 06:15:39 R7NT    2060km 137602.969Hz   3mHz -41.9dBOp  79% 15.4dB ?
2014-01-25 05:45:39 R7NT    2060km 137602.969Hz   4mHz -45.6dBOp  79% 15.2dB ?
...
2014-01-24 13:30:38 R7NT    2060km 137602.968Hz   3mHz -44.3dBOp  64% 15.7dB ?
2014-01-24 09:00:42 R7NT    2060km 137602.968Hz   2mHz -45.6dBOp  95% 15.0dB ?
...
2013-09-11 21:30:43 R7NT    2060km 137604.888Hz  10mHz -42.2dBOp  91% 15.0dB ?
 
Best 73,
Markus (DF6NM)

From: Graham
Sent: Saturday, January 10, 2015 1:43 AM
Subject: Re: LF: MF OPERA BAD

Yes  the  quality  check  is  performed  by  the  web linked  server  .. as  is  the  dynamic  call  list .   unique  decoded  calls  are  not  added  to  the  list .  only  if  2  from  independent  stations ...or   If using  opera  to  Tx , then  the  web  beacon  is directly  added
 
So your  local  decode  will  only  show  as  to  what  your   right/wrongly  detected  , for  dynamic  false  detections  will  all ways  be a  valid  call  , but the  system  automatically  validates  it  ,  and  will  show  in the  web-linked  spots  
 
The  probability  of  TWO   spontaneous   detections  occurring  at  or  close  to  the  same  time  are  quite  astronomic  , possibly  larger than  Stefan's   RF power on 136   ?
 
73-G,
 ...
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