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Re: bbsat ideas...




On Nov 25, 2008, at 11:39 AM, Alexandru Csete wrote:

> On Mon, 24 Nov 2008 20:56:44 -0800
> Mark Vandewettering <kf6kyi@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.


It's not the size that is worrisome to me.  It's the power budget.    
The normal
guideline for cubesats is 1W total power.  Every milliwatt you use on  
DSP
and the like is power you don't get to use for the downlink.  It's not  
hard
to chew up your budget with cpus.   For instance, an ATMEGA168 (the chip
used on the arduino) is 250 microamps at 1.7v and 1Mhz clock rate.  It  
can
go even lower in sleep mode.  Yet these chips are actually quite  
capable of
doing a reasonable amount of work.  A dsPIC chip might go from a few
milliwatts to nearly 100mw when churning out 20 MIPS.    It's not clear
to me how much compute power we need, but if we are in the 100mw range,
we are spending 10% of our power budget.

(Note: I know nothing about the viability of any of these chips in the
space environment.  I picked a couple of chips that I know something  
about
as typical examples of what we can get on the power/curve.  I also tend
to think in terms of software, since I'm a software engineer by  
training.
Hence, software defined radios have some appeal.  It might be more
reasonable to consider a less computerized approach.)


> 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.
>
> 73
> Alex OZ9AEC

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