[amsat-bb] Re: bbsat ideas...

Rocky Jones 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...

Robert  WB5MZO

> 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  
> frequency.
> 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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