[OZAPRS] HF Observations Sunday 16th

[Robert Bruninga]

>Has anyone tried HF messaging ?

It can work very well...  Here's a tip...

Just remember that ACKS are twice as impacted by the
failure rate of the channel than the message itself.  
So the lack of an ACK does not necessarily mean the 
message did not go through.

If the channel success rate is 30%, then the chance of
an ACK is less than 10%.  The original APRS accounted
for this in many ways.

1) 30 second delayed ack.  For any ack sent, a second
one was scheduled 30 seconds later, this gave time for
the band to change and doubled the probabilty of the
ack, making up for the reduced probability of the channel.

2) In a dialog, an additional ACK is  embedded in out-
going messages.  This not only doubles the ACK
probability, but then the TX retries of the outgoing
message further multiply the ack probabilty.

Some later programs did not include these details, so
just be aware that the lack of an ACK does not necessarily
mean the message did not get there.  

*** And always send a human reply to any incoming message.  
This then does forced retries from your end to let the original 
sender get some confirmation that his message got 
through even if he didnt get an ack.  If your software does
embedded ACKS (APRSdos, Xastir, APRS+SA, APRSscs) then 
this will greatly multiply his ACK as well.

Bob, WB4APR
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