[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

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
  
_______________________________________________
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb



AMSAT Top AMSAT Home