[CQ-Contest] Summarizing the Skimmer Accuracy Thread

[VR2BrettGraham]

N4ZR said:

> Bret, I am a little puzzled by your complaint. When you refer to
> "wrong-frequency spots" are you referring to spots that vary by 0.1-0.2
> KHz, or to imagesequidistant from the center frequency of a given
> receiver? If the former, you should be aware that each receiver on the
> RBN is separately calibrated, and we notify the operators any time we
> see spots that are more than that amount off one of a number of
> reference stations.

It is no surprise to me that you are puzzled Pete, as I have made no 
complaint.

I have merely pointed out that RBN has a lot of wrong-freq spots.

The example of UA9CDC might have been Igor CQing on three different 
bands over just a few minutes time - or maybe not.  It was just a 
coincidence that one of my queries found him when I went back to look at 
the data to find something to add to my reply to his post about another 
explanation for same-band-wrong-freq RBN spots (station with fecal 
signal actually on multiple frequencies).

I suggest you re-read what I had previously posted.  This isn't about 
spots spread over a few hundred cycles.

> It is true that I/Q receivers feeding sound cards for digitization are
> prone to images spaced equidistant from the center frequency, but this
> is not generally true with direct down-conversion designs such as the
> QS1R. We had a couple of exceptions during ARRL CW, and we are trying
> to understand what made them different from the other 70 or so identical
> receivers that did not have the problem.

That is only one of the mechanisms behind wrong-freq RBN spots that I 
can see.  I/Q inbalance has nothing to do with, for example, some 
skimmers spotting the same call on a completely different band than 
other skimmers.  And based on what some skimmers are said to be using, 
DS has some of the same problems of QSD-based receivers.  All receivers 
have spurious responses.  Skimmer doesn't know better & spots them all.

Here is some more data.  On 2012-09-21, there were 151 RBN spots of the 
same call but on another frequency >3 kc away on the same band at 
exactly the same time (to the second) as another skimmer spot.  For this 
query, I can avoid "dupes", so opening that up to spots of the same call 
 >3 kc apart on the same band within about 9 seconds of each other finds 
369.

That is just one mechanism.  Spots of same call on different band on the 
very same second look to have been 140.  That's already 0.3% of RBN 
spots that day & that will exclude a LOT of wrong-band spots.  Add 
busted call spots & who-knows-how-many busted call wrong-band or 
wrong-freq spots & then scale all this up by some factor as activity is 
much higher on weekends (I'm working with weekday data here, as it makes 
it easier to find wrong-band spots as no multi-multis on then) - I 
believe RBN could be blowing 1-2% of what it spots, perhaps more.

GL, ex-VR2BG/p.