[amsat-bb] APRS to Moon orbit?

[Ross Whenmouth]

I strongly suggest using a modulation other than 1200 baud AFSK packet 
(AX.25) over NBFM. The path you propose is severely power limited, and 
AFSK over NBFM is at about a 7 to 10 dB penalty compared to other 
modulations (like BPSK or direct FSK): https://www.rowetel.com/?p=3799

 From my perspective, it would be much easier to connect a PC soundcard 
to an SSB transceiver to produce say 25W BPSK / MSK / etc than it would 
be to "upgrade" my station to include a 250W amplifier + PSU, etc. Also, 
a "9k6 ready" FM transceiver could be used for a direct FSK uplink (1200 
bps direct FSK @ 50W ?), and the IF of an FM transceiver can be tapped 
before the limiter, mixed down from 455 kHz to say 12 kHz and fed into 
the line in of a PC soundcard, permitting a BPSK etc downlink to be 
received with an FM rig and demodulated in software on the PC...

AX.25 packet doesn't have any forward error correction, thus a single 
bit error results in the whole packet being dropped (we can do better!): 
http://www.stensat.org/docs/FX-25_01_06.pdf


Assuming that the bird was in an "earth-synchronous" polar orbit around 
the moon, it would always have line of sight to the surface of the 
earth, and a gain antenna pointed perpendicular to the plane of the 
polar orbit would always face the earth. I believe that getting mass to 
lunar orbit is horrendously expensive, so would it make more sense to 
use a gain antenna on the satellite? a 100W UHF amplifier (for an omni 
antenna) requires quite a bit of power and cooling to operate, and I bet 
that cooling a 100W amplifier in a vacuum and microgravity is an 
"interesting" engineering challenge...


Dare I say it, but the path loss to / from geostationary earth orbit is 
significantly less, the coverage area is still really good (like a 1/3rd 
of the earths surface from a single satellite) and we've probably got 
far more chance of getting a payload there (es'hail-2...).


73 ZL2WRW
Ross Whenmouth


On 26/10/18 10:59 AM, amsat-bb-request at amsat.org wrote:
> Message: 8
> Date: Thu, 25 Oct 2018 17:46:23 -0400
> From: Robert Bruninga<bruninga at usna.edu>
> To:amsat-bb at amsat.org
> Subject: [amsat-bb] APRS to Moon orbit?
> Message-ID:<9916263926d9b8f446280d0b78967759 at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Here's a crazy idea for APRS to moon orbit (omni antenna).
>
> Unless I did something wrong, I think an Oscar class long yagi, and 250W
> amp and 1200 baud FM packet can be received out near the moon with 3 dB
> margin?  A key factor is that the nearly 16 dB of assumed noise floor we
> have here on the ground at VHF does not exist in deep space.  And VHF
> gives the largest receive aperture for a crossed dipole on the spacecraft.
>
> The omni UHF downlink is a bear.  But with quad UHF long yagis and a 100W
> transmitter at the moon distance we still come up about 12 dB short.  Can
> we make up for that with distributed processing gain from a minimum of 16
> Oscar class ground stations all phased together with the internet and GPS
> precise timing and a little software?
>
> If we could, then we have APRS out to moon orbit, with the downlink
> visible to all via the APRS internet system.  And in digipeating mode, we
> have half-earth coverage half the time when the moon is up.
>
> What did I do wrong?
>
> Bob, Wb4APR