[amsat-bb] APRS to Moon orbit?

[Robert Bruninga]

You may have missed the key element.  The antenna on the spacecraft for
this specific comm link must be omni directional to be considered a
"backup" comm system.  And by definition, omni antennas have 0 dB gain.

Also one cannot tap into an FM IF before the limiter for linear modes
because it does not have the AGC circuitry necessary to maintain linearity
through all stages.  BPSK needs linear stages on both transmit and
receive.  But I do agree that a linear narrowband mode is more likely the
optimum solution for the downlink, but I just wanted to see what was
possible.
Bob

-----Original Message-----
From: AMSAT-BB < Ross Whenmouth
Subject: Re: [amsat-bb] APRS to Moon orbit?

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:
> From: Robert Bruninga<bruninga at usna.edu>
>
> 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