[Skimmertalk] Duplicate Spots

[Bob Wilson, N6TV]

Without getting into the weeds, the main thing to avoid is reports from the
same skimmer location using the same RBN callsign but with two totally
different Rx antennas covering the same frequencies, so spots only a few
minutes apart, or within the same minute, from the same skimmer, with the
same frequency, show two completely different signal reports.

Each individual SNR report by each Skimmer is very accurately calculated by
the SkimSrv software.  Though they may vary a lot over time, the MEDIAN of
all reports is usually very stable.  But calculating a median report when
each duplicate report skews too much because of totally different antennas
leads to meaningless statistics since it's like reports from two completely
different skimmer locations even though they are at the same location.

What you want to avoid it seeing reports like this from the Spots Analysis
Tool caused by duplicate posts (graphic may be deleted by this reflector,
but see the archives for the attachment.

[image: image.png]

73,
Bob, N6TV


On Mon, Jan 27, 2025 at 3:04 PM Jeff Blaine <KeepWalking188 at ac0c.com> wrote:

> Bob,
>
> In the context of multiple antennas & skimmers, I'm curious about this
> data analysis use with skimmers.
>
> Maybe it's been discussed before, but the RBN reports of the same station
> from the same skimmer vary quite a lot over time in my observation.  I've
> noticed this in many contests where I may have two screens of the RBN
> website feed up - one looking at my signal and another looking at another
> guy who I'm interested in comparing too, to see how we look at a single
> given skimmer site.  In this case, these reports can vary by 10+ db very
> easily, especially on the lower bands.
>
> Why I'm thinking about that flows from your comment on the data analysis
> and it reminds me that I have seen some guys talking on the reflectors
> about using RBN reports for fine tuning antennas where they are considering
> changes that are in the few-dB range and using RBN as the measurement
> method.  I don't understand how that can be done with precision given the
> variability the atmosphere provides in cases excluding ground wave prop.
> It seems like the noise in the data would swamp the relative compare value
> for this objective.
>
> Skimmer reports SNR so both the noise floor skimmer sees as well as the
> signal(s) of interest needs to be stable over time so that Skimmer is able
> to provide a repeatable report (at least within a short measurement
> window).  Otherwise it seems like a guy would need to have two very
> different antennas (for example one skimmer tied to his TX vertical and
> another one tied to a low-noise directional RX) so that the SNR difference
> would be significant enough to overcome the prop.
>
> Maybe there is a statistical method that addresses this? Or maybe I'm
> missing a key point here that is otherwise obvious?
>
> 73/jeff/ac0c
> alpha-charlie-zero-charliewww.ac0c.com
>
> On 1/27/2025 1:21 PM, Bob Wilson, N6TV wrote:
>
> If you have two skimmer instances monitoring the same frequencies, they
> should never post under the same Skimmer callsign, otherwise the reports
> will be quite useless for data analysis, because the signal reports will be
> bouncing all over the place and you can't tell which report to use.
> Eliminating dupes alone doesn't solve this problem, because the
> same station could be reporting with two different signal strengths every
> few minutes, depending on which skimmer heard it first.
>
> If the two SDRs or two Skimmers are monitoring the same band segments, use
> 3V8SS-1 for one and 3V8SS-2 for the other.  You need to run two independent
> copies of Aggregator to make this work right, since the posted callsign is
> determined by the first SkimSrv.ini, not by Aggregator.
>
> For more details explaining how to do this, please review this post by
> Aggregator author W3OA:
>
> https://groups.io/g/RBN-OPS/message/4938
>
> 73,
> Bob, N6TV
>
>
> On Sun, Jan 26, 2025 at 6:12 AM Ashraf Chaabane <ash.kf5eyy at gmail.com>
> wrote:
>
>> Hi All,
>>
>> I use two receivers with separate receiving antennas. Both are skimming
>> to Aggregator. How can I ensure Aggregator doesn't send the same spot
>> twice if it happens that they come from both receivers?
>>
>> 73 Ash 3V8SS
>>
>>
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>
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