[amsat-bb] Re: bbsat ideas...
orbitjet at hotmail.com
Mon Nov 24 21:20:41 PST 2008
What I think would be a good cubesat is just an SSTV transmitter...
> From: kf6kyi at gmail.com
> To: amsat-bb at amsat.org
> Date: Mon, 24 Nov 2008 20:56:44 -0800
> Subject: [amsat-bb] bbsat ideas...
> Nice thought provoking question, Joanne. Here's an off-the-cuff
> idea. I haven't tried to work out any of the numbers, it's just some
> idle musing.
> It has occurred to me that we are in the midst of a minor revolution
> in amateur radio (which will undoubtedly be followed by a more major
> revolution). In the last 10 years, cheap computing and sound cards
> have caused a great deal of experimentation with "sound card
> modes" (the minor revolution), and will ultimately lead to the major
> revolution (widespread use of SDR). We have a kind of flexibility
> that we couldn't think of even a few years ago, a flexibility that we
> gain from Moore's law.
> So here's an idea: let's do away with the need for Doppler correction
> entirely. It's not like we don't have decent orbital elements for the
> satellites that we use. It's not like our ground stations don't have
> accurate timing information available to them. Even if we didn't, we
> could still output a (coded?) carrier that our
> soundcard modem could lock onto, and then transmit relative to that
> 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
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