[aprssig] APRS servers and space data

[Steve Dimse]

On Apr 22, 2006, at 4:34 AM, Andrew Rich wrote:

> Gudday
>
> Hey I am wondering if the aprs-is servers reject those packets that  
> are the same within a period of time ?

Absolutely, dup removal has been a key component of the APRS IS from  
the beginning, and is the reason why the APRS IS is not suitable for  
propagation studies.
>
> Is it possible for a satellite ground station to in fact receive  
> very close to each other the same packet and the aprs-is reject the  
> second one ?

Yes, the APRS IS bandwidth would be several times larger than it  
already is were dup checking not done.
>
> In the case of the satellite downlink station you would kind of  
> expect to capture and relay all data if possible.

It does capture and relay it all, but the APRS IS removes the dups.  
The focus of the APRS IS is the payload of the packet.
>
> Also how does it get on when two stations hear the same packet at  
> the same time ? Which one wins ?
>
Unpredictable, based on the topology of the APRS IS and any  
propagation delays. Dup checking is done at every stage, including  
twice at findU (once in the javAPRSrvr and once in the parser).  
Packets from different IGates may be heard at different points in the  
APRS IS.

For example, two IGates hear the same packet, N1XYZ is connected to  
first.aprs.net, W9ABC is connected to third. Without unusual internet  
propagation delays, a user connected to first will see the packet  
from a different IGate than a user at third. A user at second might  
see either one depending on small difference in internet propagation,  
and if there are multiple packets heard by both IGates, this user  
might see successive packets from the same IGate or the other IGate  
as these small delays change. What is important is all users will see  
the same payloads, and as quickly as possible.

Steve K4HG