[aprssig] Airborne Digis

[Stephen H. Smith]

Gregory A. Carter wrote:
> Hi Stephen,
>
> When I pictured that scenario what I really was getting at is, 
> assuming the WIDE1-1 got fill in repeated first and we're now up to 
> the digi level where it would see WIDE2-1.  In fact, my question would 
> be more appropriate here:
>
> Tracker TXs: Packet A (WIDE2-1,SAR1-1,WIDE2-1)
> Airborne Digi Sees SAR1-1, retransmits using up WIDE2-1*,SAR1-1* 
> leaving WIDE2-1: Packet B
> Mountain Digi Sees original (packet A), digis on first WIDE2-1
> Mountain Digi Sees last unused WIDE2-1 from Airborne digi (packet B) 
> and Digis a second time.
>
> Greg
>

Again:

Home digis blindly (no dupe checking) responds to *WIDE1-1* only.

Mtn top WIDEn-N responds to BOTH    *WIDE1-1*    and    *WIDE2-n*

The first hop (WIDE1-1) can be and will be processed by ALL home digis 
within earshot AND all big guns within earshot simultaneously.  Then the 
second tier of true WIDE big guns only will process the second hop.  
BOTH hops are likely to get used, before the ground-based digis get 
tripped up on the non-standard SAR1-1 hop in the path .   

At the same time, the air-borne digi is pre-emptively responding to the 
SAR1-1 hop only, and then the big guns will respond  to the trailing 
WIDE2-1 a second time.  

*IF* the TXD is set correctly in all the digis involved (i.e. NONE), the 
ground-based WIDE1-1 hop, and the airborne SAR1-1 hop will happen in the 
same time frame.   Then the ground-based WIDE2-1 and the air-launched 
trailing WIDE2-1 should happen simultaneously.     

The real question is:   Will any one of the simultaneous transmissions 
have positive capture at the receive location for the next hop or at the 
igate or end receiving station.  Unless one signal arrives at the 
destination receiver at least 6 dB stronger (and preferrably 12 dB 
better to allow a margin for mobile flutter during the transmission)  
than the others, you will just get a howling, squealing "double" at the 
destination receiver with no capture and no data recovered.  

For ground-based digis, the differences in distance usually ensure that 
nearer digis will dominate over more distant ones, since the path loss 
over non-line-of-sight paths is considerable.    The airborne digi 
however, with it's perfect minimum-loss path through free-space may 
"compete" to a standoff with even distant receivers on the ground.  

Further, you are trusting that all the firmware and software in all the 
devices involved (old TNCs, Tracker 2s, TT4s, KPC3s, software running 
TNCs at home, etc) will have the TXD set correctly so that all this 
stuff transmits in the proper time windows. 


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Stephen H. Smith    wa8lmf (at) aol.com
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