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Re: bbsat ideas...



I think an SSTV beacon sat would be really cool.

On Nov 24, 2008, at 11:20 PM, Rocky Jones <orbitjet@hotmail.com> wrote:

>
> What I think would be a good cubesat is just an SSTV transmitter...
>
> Robert  WB5MZO
>
>> From: kf6kyi@gmail.com
>> To: amsat-bb@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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