[amsat-bb] Re: BBsat Call for ideas

Greg D. ko6th_greg at hotmail.com
Thu Nov 27 18:16:40 PST 2008

Ah, but how does one know that a time slot is not being used?  With many stations on the ground, there is a good chance that multiple stations will pick the same "empty" slot to be theirs, and we're back to collisions.

So, this is what I was talking about when I suggested, as essentially you do, to work on a prototype of the ACP messaging system that is being proposed for Eagle.  There are two significant technical challenges that I see, and the protocol used to facilitate fair and resilient access to the satellite by all the ground stations it overflies is one.  Fair is making sure that everyone has an equal share of the satellite, and resilient is that as the load on the satellite goes up, the whole thing doesn't degrade to being unusable.  I haven't checked, but has the Eagle team published their protocol for doing this?  Perhaps we can start with that, simplifying as much as we can to make it fit the limited cubesat environment.

The other challenge is the modulation system, which will be an interesting study in engineering tradeoffs.  As one poster noted, the more power you put into the electronics, the less you have for the downlink transmitter.  Better modulation schemes can compensate for a lower transmit power, but if it takes more power to do that modulation, its a wonder if you come out ahead...

Greg  KO6TH

> Date: Wed, 26 Nov 2008 09:52:19 -0800
> From: cathrynm at junglevision.com
> CC: amsat-bb at amsat.org
> Subject: [amsat-bb] Re: BBsat Call for ideas
> How about this.
> 1.  On the satellite there's a 9600 FSK regenerating repeater.
> If it hears a bit, it sends a bit without any other smarts.
> 2m up, 440 down. 
> 2.  Also there's a clock.  It puts out a tick 8 times
> per second.
> 3.  For the ground stations, we design a simple
> TDMA system.  Each user gets a time slot for
> transmitting into the satellite. Let me see, 9600
> baud is 960 characters a second, so we break
> that into 8 blocks of 100  character or so every second and
> for up to 16 users for a 2 second time block. Enough
> bytes to do a small ax25 packet, or maybe we just
> stew up something new. Callsign + checksum + data.
> 3.  Then we just make it a big chat room in the sky. The
> thing flies over, you type on your computer and then
> your software uses an unused time slot for
> sending the data.  Everyone can read your message,
> and can chat back to you.
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