[amsat-bb] Re: [Mep-dev] Re: Re: BBsat Call for ideas
bruninga at usna.edu
Mon Dec 1 17:14:30 PST 2008
> Okay,... What would be necessary to
> start/construct a ground station access
> point to route satellite images/data/
> telemetry to the internet?...
> Is that something desirable?
Yes! The following addressess the AX.25 packet downlinks:
If you look at a pass over the USA, it might only take 4 good
OSCAR class ground stations in the four corners of the USA to
provide pretty good coverage of all passes of all satellties...
BUT, tracking horizon-to-horizon requires beam antennas,
tracking and computer control plus doppler. Yes, this needs
pretty good automated "full OSCAR class" stations.
*However*, what we have been pushing for years is to simplify
this down to a simple receiver and OMNI vertical antenna so that
more people can contribute, and fewer stations have to wear out
their tracking systems and moving parts and all of that
complexity. With an OMNI vertical, there are all kinds of
1) No moving parts
2) No Tracking
3) No Doppler tuning (U only hear the center of pass)
4) No tower needed (doesn't need to see horizon)
5) No Beams, no elevation, no Azimuth concerns
6) Short coax and no pre-amp required usually
7) You still get "10+ dB gain" to the satellite!
8) Any Radio, and any TNC connected to any APRS client software
can then become just a permanent satgate 24/7/365 with no
The reason this works is that the Vertical Antenna is only
concerned with gain above 30 degrees and does not have to have
the high-gain antenna required to work the horizon where the
satellites are 6 to 10 dB weaker.
A 19.5" vertical 3/4 wave whip antenna over a ground plane at
roof or even backyard level that only needs to see the sky above
30 degrees will give you almost 7 dB gain at all angles over
about 25 to 30 degrees and can usually get to the shack with
less than 20' of coax. Plus the satellite is 6 to 10 dB closer,
so in effect you will hear satellites (above 30 degrees) just as
good as the OSCAR class stations (do at the horizon).
The downside that compensates for all these advantages is the
fact that LEO satellites are only above 25 degrees or so less
than 25% of the time. BUT, WE COMPENSATE for that by getting
MORE volunteer ground stations. But this is EASIER because all
it takes is a radio, a TNC, and a non-tower mounted vertical
whip (19" over a ground plane gives good gain on 2 meters and
almost 7 dBi gain on UHF above 25 degrees).
This is why we invite all small packet-relay satellites to share
our 145.825 MHz downlink (after coordination via the IARU of
course). The more ground stations, the better reliability, and
the the more satellites, the better continuity for packet
mobiles and portables.
These omni-SATgates all feed into the global APRS system which
feeds many live internet sites and also anyone who wants to mine
the downlink via a telnet connection. Each ground station only
needs to capture a few packets when it is under the satellite,
the other dozens of ground stations collect the packets over
other areas, and the continuous stream going into the internet
is available to all (with all dupes eliminated).
That's what feeds these web sites:
http://www.ariss.net for the ISS downlink
http://pcsat.aprs.org for the other APRS satellites
And many others such as findu.com, and oaprs.net and aprs.fi
The more the merrier!
Now for the upcoming 9600 baud downlink experiments by the ISS,
we will need these OMNI ground stations to set their radio TNC's
to 9600 baud for the duration of that experiment.
> ----- Original Message -----
> From: "Robert Bruninga" <bruninga at usna.edu>
> To: "'Michelle'" <w5nyv at yahoo.com>; "'Timothy J. Salo'"
> <salo at saloits.com>;
> <amsat-bb at amsat.org>
> Sent: Monday, December 01, 2008 11:41 AM
> Subject: [amsat-bb] Re: [Mep-dev] Re: Re: BBsat Call for ideas
> Def: MEP = Microwave Experimentation Project
> > One of the stated intentions of MEP
> > is to continue the digital ground station
> > work that was begun in support of ACP.
> > While MEP is intended to be deployed as a
> > terrestrial system, the model is based off of ACP.
> For what it's worth... the issue of high-gain antenna pointing
> among terrestrial users is being taken care of with an APRS
> packet that will indicate to everyone on the net, where each
> person's microwave antenna is pointing live. This can allow
> automatic antenna tracking during terrestrial ops. See
> > We preserve the idea of a multiple access payload in the
> > "groundsat", which will be a mountaintop repeater instead of
> > in orbit. We are using the same bands, with the only
> > difference being a shift off the satellite sub-bands. We
> > taken advantage of a greatly improved link budget by
> > orienting the system to high-definition video for
> > For satellite use, we would be able to support some number
> > voice channels, depending on the payload.
> > We are open invitation and welcome anyone at any level who
> > would like to participate.
> > Our current phase is exploratory. We're beginning to
> > transition to analyzing requirements, and then we'll design,
> > implement, test and verify.
> > -Michelle W5NYV
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of
> the author.
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