# [amsat-bb] another question

skristof at etczone.com skristof at etczone.com
Fri Oct 14 17:36:48 UTC 2016

```Thank you. That makes a bunch of sense. For some reason my brain got
stuck on thinking about extended solar panels like on the ISS, instead
of on the side of a cube (Cubesat!).

Steve

On 2016-10-13 21:50, Daniel Kekez wrote:

> 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.
>
> Regards,
> -Daniel VA3KKZ
```