[amsat-bb] Re: bbsat ideas...
alexc at phys.au.dk
Tue Nov 25 11:39:47 PST 2008
On Mon, 24 Nov 2008 20:56:44 -0800
Mark Vandewettering <kf6kyi at gmail.com> wrote:
> Given the relatively limited amount of power that we are likely to
> have in a cubesat, the question then becomes what is the best way to
> use that power? It seems unlikely that any kind of linear
> transponder will allow more than just a couple of users meaningful
> access. I'd suggest it might make more sense to do some kind of
> digital transponder. I'm imagining a satellite which monitors a
> chunk of spectrum roughly the size of a current SSB signal, say
> 2.4khz. Imagine that space was divided into (say) 10 channels,
> each 240hz wide. We could easily fit a PSK63 signal (or a similar
> FSK signal, pick your poison) in that space. You could use a bent
> pipe crossband transponder, or potentially do a simplex repeater (say
> the sat listens for 10 seconds, then re-echoes for 10 seconds) on the
> same uplink frequency. If you are a downlink station, you know what
> you sent, and can tell if your signal got collided with, and if so,
> you can switch to another of the 10 slots. In the mean time, you
> can easily monitor all of the other slots as well, and try to pick
> an unoccupied one. While it might be difficult for a
> power-efficient controller to actually _decode_ each of the 10
> channels, it probably could determine which channels are busy itself
> by monitoring power in each of the channels. Maybe we can fill
> unused slots with telemetry? Or can we actually get enough DSP
> power into a cubesat to decode 10 channels of PSK (or some similar
> protocol), which would help a lot (the bird only transmits stuff,
> and what it transmits is free from noise/errors). If not 10
> channels, then how 'bout 5? 2? Even one? Then, we basically have a
> simple digipeater, which can obviously be done, given the existance
> of 1200 baud modems based upon PIC microcontrollers.
> Just some lunacy...
> Mark KF6KYI
Mark, I fully support your lunacy. In fact, I was having something
similar in mind when I first saw the SuperBrowser in DM780 by
Simon Brown HB9DRV. So, half of the software is already there
running on the desktop :-)
Personally, I think it should be a full duplex real-time repeater (not
just a bent pipe).
Moreover, if using SDR it would be easy to switch between various
digital modes, i.e. one week PSK, next week SSTV and so on.
I'm really not worried about computing power. Already today you can get
the processing power of a laptop in the size of a PDA.
If somebody wants to work on with this idea I'd love to get involved. I
might even be able to help establish some launch opportunities in the
2011-2012 time frame.
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