Re: [amsat-bb] Re: New baseline for GEO (rev-1)

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

> 1) get some orientation to the bird, so it can use a small beam.  
> Wasn't
> there a microsat years ago that used a weighted boom to orient itself?
> The boom could double as the beam's center strut.

It's called gravity gradient stabilization, and it's one of my favorite 
attitude control methods .. but it doesn't work well at all when you 
get out to GEO altitudes because the earth's tidal gradient is 
negligible compared with how strong it is at LEO.  You'd need an 
impractically long boom at GEO.

Another drawback is that you would not have much control over which end 
faced in and which end faced out -- you either need some sort of 
propellant system on board to turn it around if it ends up backwards, 
or antennas on both ends and some way to detect which end is Earth 
facing.  Which ties in, somewhat indirectly, to ..

> 2) if you know which way Earth is, couldn't the satellite choose from
> among a set of elements, by phase, to keep its gain aimed at the 
> planet?

Tricky problem.  Not the antenna switching, which is pretty 
straightforward theoretically, but figuring out which way you want your 
gain to go.  The best indicator of which direction is the right one is 
where your control uplink is coming from, but if you don't have that, 
and haven't heard the control uplink in a while, do you hang out and 
wait for other uplinks, or do you go into a search mode and start 
scanning around until you find it again?  Lot of tradeoffs in that 
decision.  Good idea, though .. just lots of tedious engineering 
involved in making it work.

"Many that live deserve death. And some that die deserve
life. Can you give it to them? Then do not be too eager to deal out
death in judgement. Even the very wise cannot see all ends." -- Gandalf

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