[amsat-bb] Re: PSK v. APRS
bruninga at usna.edu
Tue Jun 7 07:00:27 PDT 2011
> My pet hate is AX.25 1200bps FM [on] satellites...
> ...But the reason AX.25 1200bps FM is still used
> for new satellites is because there's so much
> infrastructure, TNC's, PC software etc, already
> inplace, it's the easy option.
Actually, there is another very important reason. Link budget.
With AX.25 one can have a 2W transmitter on a cubesat which has enough power
to hit an HT on the ground with a whip antenna. The same cubesat running
BPSK might be restricted to only 200mw.
The difference is the Duty Cycle. The AX.25 packet only lasts 1 second say
once every 10 seconds for an average power of 200mw. Which is the same
power as the continuous power of the BPSK (200 mw) which is all there is
available from the cubesat bus.
But the advantage of the AX.25 BURST mode is 10 TIMEs the RF power to the
user on the ground.
For FM mobile-to-mobile satellite communications on OMNI antennas, that is
why we use AX.25, to extend satellite links to mobiles.
There are of course other issues and BPSK does have better performance for
the SAME power, but as you say, there are not that many mobiles with SSB and
all the hardware necessary to auto-tune the radio to track the Doppler.
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