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Re: cube sat



Hi Nick,

you wrote:
>The questions for the group is what kind of pay load 
>could you suggest to fly

Short answer:
A "linear" transponder with an associated digital signal processor chip
because of the number of DIFFERENT things such a satellite could become
after it is in orbit.

Long answer:
First, I've wanted to see someone implement a linear transponder with
spectral equalization.  This is an idea I first heard of from G3RUH.
It is a "linear" transponder with a digital signal processor between 
the input and the output that "equalizes" the power distribution 
among all the signals in the bandpass.  The results is that everyone 
gets a fair share of the downlink power regardless of how much power 
that station uses for their uplink.  This prevents a single high power
uplink station (an "alligator") from unfairly consuming most of the 
downlink power at the expense of stations with lower uplink power.

G3RUH's original article on this topic is available at:

http://www.amsat.org/amsat/articles/g3ruh/102.txt

Second, as a bonus...
If the onboard digital signal processor is reprogrammable from
the ground (please!), then numerous other functions could be 
implemented in software in the digital signal processor, so that 
the satellite could "become" an APRS digipeater, a slow scan 
TV transmitter/repeater, a regular linear transponder, an FM bent pipe,
a flying mailbox, a FEC downlink, a PSK31 satellite, or whatever 
you want, just by loading different software into the digital signal 
processor.

Bonus points: use a direct digital synthesis (DDS) chip to set 
the frequencies of the transponders on the satellite.  This 
would let the transponders MOVE to different frequencies in the 
same band to avoid ground interference (read: taxis), for example.
Command uplink receivers still have to be crystal controlled, I 
think.  Limited movement of transponder frequencies, within the 
limits of the DSP analog-to-digital and digital-to-analog 
converters could also be achieved by the DSP chip itself.  
Either method would let you keep the different functions of the 
satellite on different uplinks so that when the satellite changes 
modes (say, from FM bent pipe to linear transponder to PSK31) you 
would not have stations uplinking on the same frequency with the 
wrong mode causing interference. Different modes would be on 
different uplink frequencies.

Douglas KA2UPW
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