[amsat-bb] Re: some exceedingly clever technology

[i8cvs]

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.

73" de

i8CVS Domenico

----- 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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