[aprssig] Realtime VHF Propogation using APRS-IS

[Steve Dimse]

On Aug 25, 2005, at 6:47 PM, A.J. Farmer (AJ3U) wrote:

> On 8/25/05, Steve Dimse <steve at dimse.com> wrote:
>
>> Just as an example, the page you mention
>>
>> http://www.mountainlake.k12.mn.us/ham/aprs/path.cgi?call=AJ3U-5
>>
>> is not really "Stations Received By AJ3U-5". Rather, it is those
>> stations sending packets received by AJ3U-5 which got to this web
>> site before those received via other IGates. There may be many others
>
> Hmm..  Well, AJ3U-5 is my station, and what is listed on this guy's
> web page definitely reflects with accuracy the stations that are heard
> direct by AJ3U-5...
>
It certainly is possible at any given time, that what that site  
claims and my statement about what I think it actually shows, are the  
same. However, the two statements are NOT the same, and for another  
station, or for your station at another time, they may well be  
different. In particular, the rare propagation event, meteor scatter,  
Es, or whatever, is very likely to be missed. The only way you can be  
sure it isn't is if you are actually looking at the output of your  
own station, BEFORE it gets to the APRS IS.
>
>> Duplicate packets are filtered, which make
>> ANY interpretation tricky, but in my opinion this site makes claims
>> the data cannot support.
>
> How are they filtered?  My understanding is this guy is connecting
> directly to the APRS-IS feed and processing the raw data.  I wouldn't
> think the raw feed would be filtered.

Duplicate data payloads are filtered. For example, say my home  
stations send a beacon,

K4HG>APRS,WIDE2-2:any data payload

it is picked up by a local IGate, say WT4X-1

K4HG>APRS,WIDE2-2,q..,WT4X-1*:any data payload

thanks to a meteor scatter, Es, or other propagation event, it is  
also heard by you:

K4HG>APRS,WIDE2-2,q..,AJ3U-5*:any data payload

A client of the APRS IS will see one of these packets, but NO client  
will see both.

Which of these two paths is seen on findU or any other client  
receiving the data from the APRS IS depends on things like RF  
digipeater delays, serial port speed in the IGate, internet latency,  
and which hubs the IGates and clients are connected to, and how the  
hubs are interconnected.

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.

Steve K4HG