[amsat-bb] another question

Daniel Kekez va3kkz at amsat.org
Fri Oct 14 01:50:20 UTC 2016

On Thu, Oct 13, 2016 at 03:46:05PM -0400, skristof at etczone.com wrote:
> My question: We can get data about voltages coming from X, Y, and Z
> solar panels. I understand that X, Y, and Z are three spatial axes. 
> But solar panels are planes. Are the panels oriented in a particular
> plane (i.e. is the X panel in the X-Y plane or the X-Z plane) or do they
> rotate around their particular axis? 

The X panel's normal vector is aligned with the X axis of the
satellite. So the X panel is parallel to the Y-Z plane.

> If they are fixed in one plane, is it standard for most satellites, or
> does it vary from satellite to satellite? If it varies, how can we tell
> which plane the solar panel is in? 

Coordinate axes are arbitary for each satellite. The designer of the
satellite will include an axis triad on solid model drawings so others
will know how the axes are labeled.

In fact, various components on the satellite, like attitude sensors (sun
sensors, Earth horizon sensors, star trackers), will have their own axes
in their own frames of reference. Some math is then required to map their
measurements into the satellite's frame of reference, which is called
the body frame.

> Are the axes always in the same orientation (e.g. is the Z axis always
> pointed towards the Earth?)? 

It depends on the attitude control system of the satellite. In the
absence of control, the satellite could be pointed anywhere at any given
time. If the satellite has magnets, it can align with the Earth's
magnetic field, like a compass needle does. Earth observation satellites
use reaction wheels or control-modement gyros to precisely control their
pointing and keep their camera on one face of the satellite percisely
pointed at the Earth.

> If there is a book or other reference for these kinds of things, please
> let me know.  

One of the common engineering texts that give an overview of all satellite
subsystems and various design trades is _Space Mission Analysis and
Design_ by Larson and Wertz. The latest edition of this book is now
called _Space Mission Engineering_ by Wertz. But there are many others.

-Daniel VA3KKZ

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