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Re: PSK-31 Satellite Donwlink Opportunity?



OK, it will probably be impossible to get enough isolation at 10m to do 
this.   Also is 28.120 MHZ a satellite uplink band?

Doppler is a problem with PSK31, but maybe we can have a slightly widerband 
PSK that can tolerate some doppler-- maybe a psk400 with some FEC-- most of 
us have been able to copy AO-40 at one point it time.  How much harder would 
it be to make QSOs with a modem of that type?  Or, maybe a multitone FSK 
varient like MT63...

Fred W0FMS


>From: Bob Bruninga <bruninga@usna.edu>
>To: ka1lm@AMSAT.Org
>CC: MCGWIER ROBERT <rwmcgwier@home.com>, digitalradio@yahoogroups.com,   
>carcomm@egroups.com, psk31@bipt106.bi.ehu.es, amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] PSK-31 Satellite Donwlink Opportunity?
>Date: Sun, 25 Feb 2001 22:41:27 -0500 (EST)
>
>Yes, by making the UPLINK FM receiver have a widebandwidth of say 40 KHz,
>then even +/- 10 KHz of normal UHF doppler stays in the passband.  THus
>the recovered audio is unaffected by uplink doppler.  THis audio then
>simply goes into a normal 3 KHz SSB downlink.
>
>Again, this is only a quick-demo way of getting a low doppler multi-stream
>PSK-31 downlink to "test the waters".  THe ultimate objective for the
>"next" satellite would be the INBAND-INVERTING transponder so that then
>you can have 20 independent uplink QSO's... with only 45 HZ total
>doppler...
>
>I have posted the design of the INBAND-INVERTING transponder on
>http://web.usna.navy.mil/~bruninga/photos/PSKSAT.jpg
>
>Problem is, I havent figured out how much isolation I can get in such a
>tiny space... yet...
>
>bob
>
>  On Sun, 25 Feb 2001 ka1lm@amsat.org wrote:
>
> > On Sun, 25 Feb 2001, MCGWIER ROBERT wrote:
> >
> > > You have forgotten the contribution to Doppler caused by the uplink.
> > > Even with an inverting transponder, and practically no frequency
> > > tracking in most of the PSK-31 implementations <<that I know about>>,
> > > this will be a difficulty.
> >
> > Bob:
> >
> > Sticking my neck out, as I have *NO* experience designing receivers or
> > other flight hardware...
> >
> > WB4APR said:
> >
> > > > On this short fuse experiment, we would simply put 20 PSK-31 audio
> > > > streams into a single UHF FM uplink having an onboard wideband (no
> > > > doppler) receiver.  THus, the only doppler is the small 600 Hz
> > > > experienced on the 10m SSB downlink.  The PSK-31 software on the 
>ground
> > > > can EASILY track this, since ALL QSO's move together!
> >
> > I think WB4APR Bob's statement about doppler shift is correct *if* the 
>UHF
> > uplink receiver is designed as follows:
> >
> > The receiver is FM.  Use the discriminator output to drive an AFC 
>circuit.
> > Assume the UHF receiver and AFC circuit has sufficient bandwidth to
> > accomodate the audio stream and +/- 9 KHz of doppler shift.  Assume the
> > uplink has sufficient ERP to overcome the greater noise in an uplink 
>wide
> > enough to meet this requirement (say 20-30 KHz total).  Convert the 
>signal
> > to baseband audio before being fed back into the transmitter.
> >
> > I think this gives us a no-tune uplink.  The uplink receiver is handling
> > doppler tuning via AFC.
> >
> > I recall that the AO-27 uplink uses this approach.  Doppler shift on 145
> > MHz is +/-3 KHz, so the total receiver bandwidth can be smaller and
> > therefore provide a better SNR.  I **think** (8+ years is a long time) 
>the
> > Microsat uplinks use the same approach, albeit again on 145 MHz instead 
>of
> > 435 MHz.
> >
> > Bob (WB4APR), how do you intend to "process" the uplink - audio bent 
>pipe?
> > It seems to me that other than a wider receiver bandwidth due to doppler
> > and therefore greater noise in the uplink, the approach is sound.  What 
>do
> > you expect the bandwidth to be of the combined uplink signal? (no
> > experience with PSK31 here.)
> >
> > 73 Steve KA1LM
> >
> > 
>-------------------------------------------------------------------------
> > Stephan A. Greene                            sgreene@patriot.net
> > Amateur  KA1LM@amsat.org  Grid FM18hx        38 59'83.33"N 77 23'6.15"W
> > 
>-------------------------------------------------------------------------
> >
> >
>
>de WB4APR@amsat.org, Bob
>
>APRS LIVE pages     http://web.usna.navy.mil/~bruninga/aprs.html
>APRS SATELLITES     http://web.usna.navy.mil/~bruninga/astars.html
>MIM/Mic-E/Mic-Lite  http://www.toad.net/~wclement/bruninga/mic-lite.html
>
>
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