Re: [Mep-dev] Re: Re: BBsat Call for ideas
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 advantages:
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 wear-and-tear
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.
Bob, Wb4APR
----- Original Message ----- From: "Robert Bruninga" bruninga@usna.edu To: "'Michelle'" w5nyv@yahoo.com; "'Timothy J. Salo'" salo@saloits.com; amsat-bb@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
for
automatic antenna tracking during terrestrial ops. See http://www.aprs.org/info/microwave-exp-proj.txt
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
have
taken advantage of a greatly improved link budget by orienting the system to high-definition video for
terrestrial
use.
For satellite use, we would be able to support some number
of
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
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Robert Bruninga