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[aprssig] Help with Interfacing a GPSto Kantronics KPC-3

Robert Bruninga bruninga at usna.edu
Wed Jan 3 23:03:34 UTC 2007


> The transmitting of raw NMEA strings over the air... 
> ... is highly discouraged in California.  Raw NMEA 
> ... occupy up to TEN times more air time than the 
> most compact encoding format for APRS (Mic-E).

A little caution and explanation with that advice...

1) True, it is 10 times longer, but when you include the packet
overhead this is more like only a factor of 2 longer.

2) The KPC-3+ TNC tracker that sends raw NMEA can actually
produce less QRM, because it can be set to PROPORTIONAL PATHING.

Here is an example in an area where each digi can hear 5 other
digis.  Lets compare a Mic-E tracker at one minute and a KPC-3+
properly set for proportinal pathing at 1 minute and compare the
total packets after 6 minutes:

Mic-E generates , 6 direct, 30 at first hop, 90 at second hop
for a total of 126 packets

A KPC-3+ TNC set for proportional pathing at a 1 minute rate
will generate 6 direct, 4 at first hop and 1 at second hop for a
total of 11 packets.  Now these are TWICE as long due to the
long NMEA string, so lets say these take the equivalent air time
of 22 packets if they were Mic-E.

SO bottom line is that a raw NMEA tracker running Proportional
Pathing will actually produce 5 times LESS qrm than a much
shorter Mic-E tracker while giving exactly the same performance
locally.

I agree, that NMEA packets are LONG, but since they are mostly
from KPC-3+ trackers that are the ONLY devices that can
implement Proportional Pathing, they more than make up for it in
reduced QRM to everyone else.  I'd recommend using a KPC-3+
tracker with proportional pathing in most routine applications
if it is going to be a dumb tracker.  Of course, other trackers
may also be efficient by using smart beaconing as implemented in
the HAM-HUD, but that is another topic since that is not a dumb
tracker.

See the web page on setting up the KPC-3+ tracker for
proportinal pathing:
http://www.ew.usna.edu/~bruninga/aprs/P-Pathing-kpc3.txt

Bob, WB4APR





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