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Re: Brief msgs via 9600 Pacsats?



    IMHO, "APRS like" operations really aren't feasible on 
    these birds, particularly with the configuration you're 
    proposing.  The mechanics of keeping a yagi hand pointed
    at the sky, while tuning an HT with the other hand
    (and typing on the computer with a third) really isn't
    feasible for any type of "real" use.
						-- al lawler

Agreed.  As a regular AO-27 user, working with a hand-held antenna was
difficult at best unless i had a table next to me to log with, and even
then it was marginal.  (And obviously no tables in the backcountry;
the local construction project was kind enough to leave a giant wire 
spool where i like to work from, and that's nice but just lucky.)  It 
only became practical for me when i took the suggestion of another 
experienced satellite operator and started making audio cassette records 
of the passes, and then transcribing them later.  That works great!

Now, to extend that concept to 'pacsat' operations.  Now, if i'm running
packet mobile, i can have the laptop switch memory channels, but i'll
most likely have to do that by hand if operating portable.

    That depends.  On the 9600 FSK satellites, if you have a wide
    enough IF filter, you can operate very nicely with no Doppler
    tuning at all.  You suffer some signal-to-noise degradation by
    using a wider filter than would have been necessary, but that
    can be easily made up with antenna gain.
						 -- Paul, KB5MU

That's what AO-27 does on the 2m uplink, and it also has a very sensitve
receiver to make up for said 'degradation'.  But 70cm is another story
entirely.  My experience with AO-27 is at least +/- 10 KHz, in addition 
to the bandwidth of a doppler-corrected link.  But i operate about 30% of
the pass at 436.805, with very little time actually spent at the nominal
downlink frequency. (Ever heard a train whistle go off just as the train
passes you?  But it's comparatively quiet up in orbit, so 'signal-to-noise
degradation' is much more an issue for ground stations.

On the 70cm vs. 2m question, to reiterate, we'd be going from approximately
10 KHz to 20 KHz bandwidth at VHF, but from 10 KHz to 30-40 KHz at UHF
(depending on how much you want to account for potential VFO inaccuracies 
on both ends).  n.b. These numbers are off-the-cuff guesses, and many
(most?) folks on this list may know these numbers better than i do.

But perhaps more importantly, if we compensate for doppler in the receiver
rather than the transmitter on 2m, then the FM operations (J-FM or JD) 
take much less room in the satellite sub-band, which is much smaller at 
2m than 70cm.

    A small full duplex radio, and some form of high gain 
    omnidirectional antenna (or electronically switched 
    array) which could passively listen for most of a pass 
    would probably prove to be  far more useful, and 
    wouldn't be limited to very short 'aprs like' 
    transmissions - you could send and recieve 
    anything you wanted...
						-- al lawler

Indeed, and a few HTs qualify.  But using a hand-held antenna, as one
might in the backcountry, really does preclude keyboard-to-keyboard
operations, except possibly with a Pilot or other PDA.  Such a device
should be able to run enough AX25 to support APRS, but i don't think
that's the best approach, as i would speculate that for most people,
CW is comparable in speed to current handwriting recognition software
and at that point, we may as well send CW and have the Pilot decode
it for those of us who can't get CW by ear...

The better approach would be to prepare messages in advance and uplink
them during the pass.  It could watch for others listing messages and
if none are started soon enough, make such a request itself.  If that
finishes in time, then request those messages.  During the pass, the
operator would be stuck with the task of aiming the antenna and also
adjusting the doppler.  The Pilot should be able to say when, so the
operator doesn't have to do that by ear (feasible on J-FM, not so
good in mode JD), and even display the correct heading and elevation
(which could be computed in advance, perhaps on a larger computer,
so as not to interfere with packet reception).

But for backcountry operation, this really does imply an integrated
TNC inside the HT and suitable software for the Pilot or other PDA.
Maybe this is just something for the future...

    Interesting point.  Especially interesting in light of the fact that
    they were originally *supposed* to have such a function, and
    in fact a much more general selective-listing feature.  Never
    got implemented.  Sigh.
						 -- Paul, KB5MU

Is that in ROM in the birds?  Or is that something we can fix on at
least some of those that are up there? 

    This was implemented when the Pacsats were first launched, and then
    was removed when they went to the broadcast protocol for the downlink.

    Originally you could connect to the satellite and send a command to list 
    directory entries that were selected by an equation that you sent to the 
    satellite.  The problem was that there were limited number of connections 
    supported.

So maybe its just a matter of software on the receiving end.  The question
might be whether those broadcasts will be heard soon enough in the pass to
be useful and/or if there's a way to elicit them.  Being able to get that
information in a timely fashion one way or another  would make backcountry
or Third World operation more effective, if someone handheld could
pick up messages for themself and then upload those already prepared.
It does sound like we're getting closer to where that's feasible
without alot of expensive equipment.

			-- KD6PAG  ('Networking Old-timer, RF newbie')
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