[aprssig] conflicting APRS intructions

Gary Dezern gary at garyndenise.org
Sat Oct 24 00:16:03 EDT 2009

On Oct 23, 2009, at 8:29 PM, Robert Bruninga wrote:

> hundreds of APRS transmitters in the region.  Down in the black
> hole, the Mobile hears almost none of this QRM and can hear the
> big digi much better than it can hear him.

Okay, this is a difference of how we interpret a "dead zone" (or black  
hole.)  My interpretation is geographical, meaning that the due to  
physical interaction between buildings, landscape, etc - there is no  
RF coverage from existing digi's.  This is extremely common in the  
hilly area I live in.  In this case, the mobile (or even home) station  
is unable to "hear" the existing digi's any better than the digi can  
"hear" the mobile station.

> That is not a direct conclusion.  It serves ALL Mobiles either
> one-way or two way to get out.  And it does no NEED to do
> anything to help the mobile in many cases.  Sure there are dead
> zones that are so dead, that the mobiles cannot decode the big
> digis, and in this case, then a full function APRS digi may be
> needed.

This was my conclusion for my area - so I ended up attaching my home  
VHF radio to my always-on linux machine, installing soundmodem,  
digi_ned, and javaprssvr...

I have to admit to never seeing a mobile "dead zone" that was dead  
only as a result of QRM.  Yes, there are occasions when my mobile  
radio doesn't "hear" it's outgoing beacon being repeated due to QRM,  
but those situations are always a result of timing, not of physical  

> But by definition, a FILL-IN is only placed in those areas to
> help the mobiles get out and compete with the hundreds of other
> signals that are hitting the big digis much closer to them.

And this is where I get confused all over again (but for a slightly  
different reason.)  If a "full function" digi should be installed in  
physical areas that are completely unserved by digi's (such as  
described above), and a "fill-in" is only placed to boost the signal  
from mobiles in order to compete with QRM, then it stands to reason  
that these "fill-in" digis as described here really should be placed  
in high QRM areas instead of "dead zones", and then can be doing more  
harm than good - as they have no way to know if the major WIDE2 digi  
heard the mobile beacon or not, and therefore can be adding to the  
overall QRM!

This, again, argues in favor of this "viscous digipeating" concept for  
fill-ins, as the aprx digi would LISTEN before automatically (and  
blindly) repeating every WIDE1-1 packet... and if aprx heard the major  
WIDE2 digi repeating the packet, it wouldn't do anything at all.   
Granted, it might not hear the major WIDE2 digipeating even when it  
does (due to collisions), but this presents a situation no worse than  
a dumb WIDE1-1 fill-in that just digi's every single WIDE1-1 it hears.

Take care
Gary / K3WOW

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