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.
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