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[aprssig] Airborne APRS

Robert Bruninga bruninga at usna.edu
Fri Oct 28 17:41:40 UTC 2005


>> If the aircraft is going to digipeat the ... balloon, 
>> make SURE that it can ... IGNORE DCD so it will 
>> transmit... independent of other QRM on channel.
>
>My _experience_ has been that even operating at 
>12,000 feet over the DFW area... is the exact
>opposite.   I operated a D7 with DCD turned on and 
>it transmitted at altitude. 

Yes, could be.  Dallas though is surrounded by large
areas of low usage.  In some other areas like the
East coast, when you get higher, then the next big
city digis' are heard, and then the next and the
next.  So around here it is a necessity to disable
DCD at altitude.  Granted, it may work in some areas 
but why risk it for brief events.  

Also, your observation was for a D7 probably sending
one packet a minute, not acting as a digi.  Yes, I assume
it will eventually find a silent time to transmit.  ALso with
all the other RF interference and intermod at altitude
the receiver front end may often be blocked such that
DCD goes low too.  So I agree, there can be some silent
periods...

But if the intent of flying  a TNC is to digipeat, then
one must consider whether it will hear enough
silence to be able transmit the packets piling up.

> I think your continual encouragement of transmitting 
>in the blind causes more QRM (_interference_) than 
>what exists on the channel already.

Maybe I should rephrase it to encourge flyers of these
payloads to consider the environment they will be in at 
altitude.  In many areas, if they wait for a clear channel 
they will possibly not be heard.  Same for balloons.  THe 
higher they go, the less they may ever hear a clear channel.

Bob






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