Re: [amsat-bb] squint angles and P3D

[Paul Williamson <kb5mu@xxxxxxxxx>]

> >The whole idea of 3-axis stabilization is to keep the antennas pointing at
> >the sub-satellite point.  Thus, the squint angles should always be low and
> >not worth calculating.

This three-dimensional stuff is hard for me to get my mind around, but it seems to me that the antennas can't always be kept exactly nadir-pointing on P3D. If you control the attitude that way, there are certain seasons of the year when the sun angle on the solar panels is poor most of the time. It seems like the power budget would suffer greatly. We'd probably be better off to compromise antenna pointing to improve sun angle.

> > Remember, though, that P3D will occasionally go into spin mode to dump the
> >momentum wheel energy, etc.  During these times, squint angles can be
> >calculated just as for AO-13.

That's the first I've heard of this. I know P3D will be spinning for the first many months, prior to the final burn of the 400N thruster. My understanding was that after that burn, P3D will be despun and its panels deployed, and will never spin again (until the reaction wheels fail).

I don't see why it needs to spin to dump the momentum wheels. The magnetorquers produce a torque against the Earth's magnetic field. To make this work on a spinning spacecraft, you have to (electrically) rotate the coils to despin them. On a non-spinning spacecraft, you don't even have to do that. You might have to re-orient the spacecraft (away from nadir-pointing again) to get the torque you need, but you shouldn't have to spin it. Am I confused about this?

What other reasons are hidden in the "etc" above?

73  -Paul
kb5mu@amsat.org


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