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[aprssig] RE: timeslotting on HF ? was > 15. APRS trackers on 10m (Robert Bruninga)

Robert Bruninga bruninga at usna.edu
Sun Nov 27 16:28:14 UTC 2005


>>> scott at opentrac.org 11/26/05 3:26 PM >>>
>I have little faith in 1200 bps FSK on 10 meters.

Ah, but the practical problem for under-the-seat trackers
on 10m is not "best-modulation" but frequency stability.  
There is just no innexpensive way to get anything close 
to the frequency accuracy required for PSK and even the
200 Hz shift used on HF has also been abandoned
in the past due to the impracticality of getting long
term under-the-seat frequency accuracy without 
extrordinary cost.

So the 10m FSK with 1200 baud 1000 Hz shift is 
the best way to go for cheap kit-built project with
an under-the-seat black-box objective.  Especially
since ANY AX.25 TNC can decode it.

> Each slot could be defined by a two byte code,
> and if something like the Pocket Tracker or Open
> Tracker came onboard with firmware prewired to support
> this type of time slotting this could really
> dramatically change how APRS could work on HF.  

I think it is much simpler just to have the 10m<=>144.39
digipeaters do the normal carrier-sense.  When the channel 
clears, then inject the packet.    I think there is little need
for time-slotting on the 28 MHz channel since the number 
of these things (in any  single digipeater input area) would
never approach even the aloha 19% limit.

But, since a PIC is involved, I guess,...  Why not?  The
onboard PIC does know the callsign of the owner, so it 
could then assign a slot based on an algorithm based on 
the call (including SSID in the algorithm).  Then in the 
unlikely event that there was a conflict, the stations 
affected could resolve it by changing one of  their SSID's.

We had proposed these kind of under-the-seat black
box trackers back in the mid 90's  for the 30m APRS
channel, but the frquency stability for the 200 Hz
shift on HF turned out to be just too hard.  It was
very tempting because 100W XTAL controlled FSK
transmitters could be built with no more than about
5 transistors total, using some of the Class E
MOSFET designs...

But for large numbers of trackers, the 300 baud
slow speed would never do.  Thats why this 10m
1200 baud LOCAL idea is a fresh start and a good
idea... along witht he freeby CB antennas... etc

de Wb4APR, Bob





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