[aprssig] Airborne Digis

[Stephen H. Smith]

Gregory A. Carter wrote:
> Hello All,
>
>
>
> My thoughts were perhaps to set WIDE1-1,WIDE2-1,SAR1-1 and turning on 
> preempting on our airborne digi setting it to only digi on SAR1-1.  My 
> question here is, if the airborne digi preempts, hitting on SAR1-1 
> what happens on the RF network with the WIDE1-1,WIDE2-1, will they be 
> skipped or will they be processed?
>
> DISCLAIMER:  I am not looking for a debate on whether or not an 
> airborne digi should be used, I'm just looking for an honest reply to 
> my questions or suggestion on how to make it work with the least 
> amount of negative impact on the RF network.
>
> Thanks!
>
> Greg
>
> NV6G
> OpenAPRS.Net

What do you mean by " if the airborne digi preempts, hitting on SAR1-1"  
?     

Path hops are processed SEQUENTIALLY from left to right.    Assuming you 
mean some kind of modified firmware that will let the airborne digi 
respond to "SAR1-1" out of sequence, even if the first two hops are not 
yet "used up",  it will depend on how the path is retransmitted.   If it 
comes out of the digi as:  

    WIDE1-1,WIDE2-1,SAR1-1*    

then potentially dozens and dozens of digis on the ground over a radius 
of hundreds of miles will simultaneously respond to the "WIDE1-1" hop.  
[Both low-level home fill-ins AND true high-level WIDES respond to 
WIDE1-1.]   Then all these retransmissions over a huge area will trigger 
all the real WIDEs again on the second (WIDE2-1) hop.  

If the airborne firmware traps and modifies the path string to output 
something like:

   WIDE1-1*,WIDE2-1*,SAR1-1*     

or chops off the first two hops entirely to output just

   SAR1-1*   

the airborne digi will, at least, not re-trigger the ground-based 
digis.   However this STILL won't prevent ground-based digis from 
responding independently and simultaneously to the first two hops while 
the airborne digi is responding to the last hop. 

Further, on each digipeat by a ground-based digi, the airborne one will 
hear yet another "un-used"  SAR1-1 and act on it again if the dupe table 
isn't big enough and dupe time isn't long enough.   You will be creating 
huge numbers of simultaneous packets that will "step on each other" over 
a very large area.  

The disruption caused by "WIDE1-1,WIDE2-1" paths on balloons at altitude 
has been discussed many times on this list.   The impact on the 144.39 
network of a digipeater at altitude, rather than just a originating 
transmitter, will be at least an order of magnitude worse.



If your user's devices could be set to alternately beacon on two 
different paths (i.e. the "proportional pathing" of the D710),  and you 
set the airborne digi to respond exclusively to SAR1-1 only, the impact 
on 144.39 would be VASTLY reduced.    The users would alternately beacon 
"WIDE1-1,WIDE2-1" (to utilize the terrestrial digipeater network only), 
and SAR1-1 (to utilize the airborne digi with a guaranteed single hop 
only).     


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Stephen H. Smith    wa8lmf (at) aol.com
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