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[aprssig] Realtime VHF Propogation using APRS-IS

Steve Dimse steve at dimse.com
Fri Aug 26 14:04:27 UTC 2005


On Aug 26, 2005, at 9:21 AM, A.J. Farmer (AJ3U) wrote:

> On 8/26/05, Steve Dimse <steve at dimse.com> wrote:
>
>> In particular, the rare propagation event, meteor scatter,
>> Es, or whatever, is very likely to be missed.
>>
>
> The site may be presented as a propogation prediction site, but I
> agree with you that PropNET is a much better tool for that job.  My
> main interest was in the site's ability to show, even if not
> absolutely 100% accurate, the stations heard direct by another
> station.  To me, that is a very useful tool in determining the state
> of the APRS network in a certain area.

For some stations, it could be nearly 100% incorrect. For example,  
another IGate near you has a slower internet connection and is  
hearing all those stations you are, but his reports arrive on the  
APRS IS after yours, NONE of these calls will appear on that  
station's list. Other stations may represent a mix of the two  
situations, and be 50% inaacurate.

The only way to know what stations someone is hearing is to analyze  
their data BEFORE it gets to the APRS IS. Wishing it wasn't so or  
drawing pretty charts does not change the fact that the underlying  
data is unsuitable for this analysis!
>
>
>> The net effect is that no payload data is filtered, one copy always
>> gets through, but a LOT of path information is filtered. This
>> filtering is essential, the APRS IS would need to carry perhaps 10
>> times as much data were it not done.
>>
>
> I think I get it now.  The APRS-IS server that an IGate connects to
> (or perhaps a tier 1 server up the chain) does the filtering based on
> what it has already seen?  I didn't realize any filtering was done at
> that level.  I understand that filtering happens as you described it,
> I was just under the impression that it is done a little "further up
> the chain".  I thought if you connected to what I would consider the
> raw feed at a Tier 1 server, then you would see everything coming in,
> totally unfiltered, which would allow this website to work as
> described.  Where does the filtering occur?

Filtering occurs at every hub, and in some IGates.

Steve K4HG





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