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@amsat.org wrote:

From: Robert Bruningabruninga@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