[amsat-bb] Re: some exceedingly clever technology
domenico.i8cvs at tin.it
Sat Aug 27 00:13:29 PDT 2011
Hi Phil, KA9Q
What you describes using both magnetorquers ,momentum wheels
and CCD video cameras for the attitude control system is what
was made on AO40 but it was a big satellite and not a microsatellite.
----- Original Message -----
From: "Phil Karn" <karn at philkarn.net>
To: "R Oler" <orbitjet at hotmail.com>
Cc: "Amsat BB" <amsat-bb at amsat.org>
Sent: Friday, August 26, 2011 6:54 PM
Subject: [amsat-bb] Re: some exceedingly clever technology
> 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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