[amsat-bb] Re: PSK v. APRS

Justin Pinnix justin at fuzzythinking.com
Tue Jun 7 07:55:42 PDT 2011


A packet satellite can also be operated simplex (TX and RX on the same
frequency).  That means a single antenna and no diplexer for both the bird
and ground station equipment.
On Tue, Jun 7, 2011 at 10:16 AM, Trevor . <m5aka at yahoo.co.uk> wrote:

> 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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