[amsat-bb] Re: Transponder

Phil Karn karn at philkarn.net
Sat Aug 6 12:29:09 PDT 2011


On 8/6/11 8:23 AM, George Henry wrote:

> It doesn't appear that the satellite has any active or passive attitude control 
> system. 

Correct.

> I have heard that the spin should eventually settle around the same
> axis as the antenna, but is there any reasonable hope that that would happen 
> during the satellite's operational lifetime?

Good question. Theory says it will *eventually* happen, but how fast
depends on the mechanisms to dissipate mechanical energy in the
spacecraft. Because the frame and the modules inside are so rigid, I
think the flexible 2m antenna will probably do most of the work.

One way to follow this is to measure the satellite's spin period very
accurately over time. Assuming no external torque (e.g., atmospheric
drag, which will eventually become significant) then the spacecraft must
conserve its original angular momentum. But as it dissipates excess
energy and converts to a spin around the preferred axis (i.e., shifts
its angular momentum to the body axis with the greatest moment of
inertia), the spin rate will decrease.

E.g., if the moment of inertia around the axis of initial spin is, say,
1, and the moment of inertia around the preferred (antenna) axis is 3,
then after it settles down the spin rate will be 1/3 of its original value.

So if we can follow the spin rate over time we can predict how long
it'll take to reach a stable spin around the antenna axis. That does
require knowledge of the moments and products of inertia, which I assume
were measured before launch.

73, Phil


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