[amsat-bb] Re: PSK v. APRS

Trevor . m5aka at yahoo.co.uk
Tue Jun 7 07:16:47 PDT 2011

Hi Bob,

I accept what you are saying about burst mode but it's possible to develop a BPSK system that does burst mode too. 

A BPSK system with Forward Error Correction can offer a 16 dB advantage over AX.25 1200 FM. 

As you say there's loads of FM rigs out there so receiving AX.25 is simple and that's the reason everyone still uses it but for the future we should be looking to more efficient means of communication.

73 Trevor M5AKA

--- On Tue, 7/6/11, Bob Bruninga <bruninga at usna.edu> wrote:
> > 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.
> Bob, Wb4APR

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