[amsat-bb] Re: some exceedingly clever technology

Phil Karn karn at philkarn.net
Fri Aug 26 09:54:22 PDT 2011

On 8/24/11 8:11 PM, R Oler wrote:
> http://onorbit.com/node/3709  

Thrusters are necessary for orbit control, but for attitude control
(which I think we really need) you'd *really* prefer something that
doesn't consume a fuel.

These attitude control systems come in basically two types:
magnetorquers and reaction wheels. Magnetorquers exchange angular
momentum with the earth by acting on its magnetic field, but they tend
to be slow, imprecise and require complex control and sensing systems
including a magnetometer far enough from the spacecraft to get away from
its own magnetic fields.

Reaction wheels come in two kinds: momentum wheels and control moment
gyros. Momentum wheels change speed while control moment gyros don't.
The latter seem much more common in spacecraft but I'd investigate both.
They're good for quick, accurate pointing like you'd need to keep an
antenna pointed down or a solar panel pointed at the sun.

Having both a magnetorquer and a set of wheels can be advantageous as
the magnetorquer can be used occasionally to dump accumulated angular
momentum from the wheels should it build up from small external torques.

Controlling attitude also means measuring it, and for this I keep
thinking about small, cheap CCD video cameras. If we could build good
enough sunshades we could snap pictures of star fields and look them up
in a database. If you can match multiple stars in an image, then a
single image could fix the attitude of the spacecraft. But I'd want to
put a camera on each surface if possible. They could also sense the sun
and the earth, though that's not as simple as it might seem. The sun
usually overloads a CCD and produces streaks while the earth is very
large in LEO. It might be possible to recognize the limb of the earth
and get better precision that way. A lot would depend on the software
processing these images, and it would be a challenge to write.

-Phil, KA9Q

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