[aprssig] Realtime VHF Propogation using APRS-IS

[Steve Dimse]

On Aug 25, 2005, at 4:22 PM, A.J. Farmer (AJ3U) wrote:

> Here is a very interesting site I stumbled upon:
>
> http://www.mountainlake.k12.mn.us/ham/aprs/
>
There is a serious problem with using the APRS Internet System data  
for this purpose. The APRS IS was designed to move the data payloads,  
not the path information. Duplicate packets are filtered, which make  
ANY interpretation tricky, but in my opinion this site makes claims  
the data cannot support.

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  
received in this time frame that do not appear on the list. Many, if  
not most, interesting propagation events will be lost because of this  
filtering. Path modifications by digipeaters may place stations here  
not actually heard, though in the example I do not see any.

The site calculates hop distances between callsigns listed in the  
path...however, unless everyone runs trace, callsigns are not added  
or are dropped from the path in routine digipeater operation, so a  
packet may have traveled through multiple digis between the two  
identifiable endpoints.

The site depends on manual removal of HF stations from the analysis.  
Manual filtering will miss some HF stations containing bad data, and  
filter some good data. Stations transmitting on both HF an VHF are  
particularly problematic.

My analysis of the problems with this interpretation of the data is  
based on sketchy details on the web site, and some of the points I  
make may be wrong in a specific detail, there is no question that the  
raw data on the APRS IS simply does not contain the information  
needed to produce the output this web site contains. The charts are  
pretty though!

If you want to see propagation studies done right with modified APRS  
tools, see

http://w2ev.rochesterny.org/PropNET/

Ev has created a separate internet hub without filtering designed  
explicitly for propagation studies. Furthermore, the system primarily  
uses PSK31, resulting in a greater sensitivity to propagation events.  
It supports many bands, though almost all activity is 10 meters at  
this time.

findU has separate support for PropNET, here are the paths observed  
in the last day

http://propnet.findu.com/catch.cgi?last=24

There are animated GIFs for the last day and last week created each  
night

http://propnet.findu.com/yesterday.gif
http://propnet.findu.com/lastweek.gif

PropNET was a lot more work than the harvesting APRS IS data, and  
because effort is required from each of the participating stations,  
fewer people participate and PropNET produces much less data, but the  
data it does returns has real meaning. I'd encourage anyone  
interested in propagation to join Ev's effort, all he needs is more  
stations and this will become a serious tool.

Steve K4HG