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[aprssig] The ideal APRS digi

Robert Bruninga bruninga at usna.edu
Sun Sep 26 23:18:30 UTC 2004


To clarify what I am proposing for APRS alt-input digis:

The ideal APRS digi should consist of:

1) RX/TX on 144.39 connected to good APRS style TNC
2) RX on alt input channel for locals using any RELAY-only TNC
3) External Carrier Detects cross connected between TNC's

See the drawing at: http://www.ew.usna.edu/~bruninga/digi.gif

The result will show a drastic improvement in reliability for all
locals 
in the vicinity of that digi because their packets will NOT have to
contend with any QRM from out of area.  The original APRS
system encouraged growth by simplicity of putting up digipeaters.
But the downside was that local traffic must contend with all DX
traffic on the same frequency.  And in most areas, there is 20
or more times more DX packet QRM on the channel than
local.

Separating the contention on the single 144.39 input channel between
locals and DX packets so that locals have their own channels will
improve local reliability by at least an order of magnitude.

We suggest checking on the availability of 144.99 in your area for
such use as an alternate input channel for APRS.  Even if that
frequency
is curently shared by a NODE or BBS or DX cluster, if they are
amenable
to sharing, it can be shown that the dozen or so 1 second APRS packets

per hour or so will not cause any substantial interference to existing

operations.  This is beacuse the APRS packets are only transmited
once on the alternate input channel from ground user stations only.
There is no digipeat, or further propogation on that frequency.  In
fact,
since ground users can only hear other stations direct within 5 to 10
miles depending on whether they are fixed or mobile, and under 
typical ham densities, there are on only a few other APRS stations
one can hear direct, then any typical user of the shared frequency 
will probably on hear an APRS packet once every 5 to 10 minutes
or less.  This is certainly a miniscule load on the shared local
channel.

Just some thoughts.

de WB4APR, Bob




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