[Ground-station] Experimental Channels

[Phil Karn]

On 2/12/20 12:09, Paul Williamson via Ground-Station wrote:
>
> That scheme means that Phil's proposal to accommodate FM uplinks for
> users so inclined is much less interesting than it might first appear.
> The hypothetical FM users would have to also operate a digital uplink
> to set the channel up. It wouldn't replicate the experience of a plain
> analog FM transponder.
You could leave the FM channels on and turn them off only if they're
abused. Squelches would keep idle channels from consuming any downlink
capacity.
> Of course, even without the auth+auth scheme, there's no way a
> microwave GEO can replicate the key advantage of FM LEOs: the use of
> cheap off-the-shelf VHF/UHF FM transceivers and low-gain hand-waved
> antennas. The users already have to obtain specialized equipment to
> operate the DMT, and we are already planning to do our best to
> cost-optimize it; they might as well use it in the intended
> all-digital way and get all the advantages designed into the system.
Oh, I agree. I was just trying to anticipate the objections of the "FM
forever" crowd. Show them it's possible to do this, and maybe they won't
give you a hard time before they realize what you already know -- that
they should just go buy the appropriate digital ground station hardware.
>
> The experimental IQ relay channels are not about satisfying users who
> think they want an old-fashioned "analog experience". It's about
> experimenting with alternative uplink waveforms that the payload is
> not programmed to demodulate and multiplex onto the downlink.
> Presumably the goal would be to eventually implement the waveform in
> the payload, eliminating the gross inefficiency of downlinking raw IQ
> data. With a reprogrammable payload, that might be possible on the
> same mission. Depending on the details, it might only be possible in a
> follow-on mission with more powerful hardware on board.
You're right, of course; my reference to "old fashioned analog
experience" was meant to be tongue in cheek. Somebody (Scotty?) at the
meeting was making the same joke.
>
> I'm not so sure about that. Someone with more of the link budget
> figures at their fingertips will have to check me on this, but I'm
> guessing that normal uplink signals at appropriate power levels should
> be pretty hard to see on a waterfall.
Waterfalls can show some remarkably weak signals. I can easily see DRM
signals on HF that are way too weak to decode. But yeah, this requires
calculations.
> A full-bandwidth waterfall would only be useful for diagnosing loud
> interfering signals. Such a tool would not need to be always on. It
> might even encourage miscreants to transmit loud signals just to show
> up. The payload managers would need access to a waterfall
> occasionally, but it might be counterproductive to train users to
> watch it routinely.
>
>   -Paul KB5MU
>   
>
Could explain some of the QRM on QO-100, since the narrowband linear
transponder users are generally looking at a waterfall of the entire
passband.


--Phil