Re: [Orekit Users] picosatellite biomagnetorquer experiment - request for answers/ideas/comments

[Michael Turner <michael.eugene.turner@gmail.com>]

On Thu, Jun 14, 2012 at 10:18 PM, Luc Maisonobe <Luc.Maisonobe@c-s.fr> wrote:
> Le 14/06/2012 13:03, Michael Turner a écrit :

[Repeating sources for background:]
[snip...]
>>    http://en.wikipedia.org/wiki/KickSat
>>    http://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space

[and one more, citing magnetometer and gyroscope plans:]

       http://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space/posts/247545

[snip]

[Repeating project details for background:]
>> My specific project involves attaching a lightly magnetized strip to
>> the sprite before launch, then measuring and transmitting the amount
>> of magnetorquing the sprite undergoes before it deorbits. For two
>> reasons, I'd like to be very sure of the theoretical *minimum* magnet
>> strength that could cause any measurable change in orientation.

[later, Luc:]
> Does KickSat provides some attitude-related information in its telemetry?

I believe it *can* - sprites will be equipped with some kind of
MEMS-scale magnetometer and gyroscope. See below.

[Constraints:]
>> (1) a magnet that's too strong might be disqualified as potentially
>> interfering with other sprites in KickSat;
>>
>> (2) I hope to use biologically derived biomaterials (bacteria or algae
>> containing magnetosomes) to make the strip a magnet; the maximum
>> achievable strength might be quite low.

I should probably add a third constraint:

  (3) Since a recent revision of the Sprite has a magnetometer, it
would probably be good if the magnetized strip's field strength a lot
weaker than the Earth's magnetic field; otherwise, the magnetometer
might be useless for determining attitude. There will also be an
onboard gyroscope. A gyroscope might be enough, but possibly not. Two
data sources on attitude is better than one. I'd also hoped to get a
reading of solar intensity on the PV cells, but I'm not sure that's
going to happen.

[me]
>> [snip] .... it's important to be able to filter out any
>> attitude changes that are due only to increasing atmospheric drag.

[Luc:]
>  Without attitude control, it will probably be tumbling and both drag
> force and radiation pressure will change a lot in amplitude and
> direction, so geometric information would be really important to
> evaluate all the forces and torques.

Yes, tumbling is expected. Detumbling is, in fact, one of the proposed
applications for "biomagnetorquing." For now, measuring some degree of
detumbling (however small) is the hope.

I expect radiation pressure to be a negligible factor. But I haven't
done any calculations, and this is a very light spacecraft, so maybe
it could be a factor in some circumstances. (Such as the
interplanetary missions considered for Sprites. But with a flight time
of at most a few weeks in LEO? That's an environment cluttered with
much larger forces.)

> Will it also be possible to get some orbit measurement for your specific
> sprite (or at least the swarm globally) once it is launched ?

KickSat is slated to go up with a NASA ELaNa flight

  http://www.nasa.gov/offices/education/centers/kennedy/technology/elana_feature.html

Presumably, the encasing Cubesat will be released at a fairly definite
point in ascent, and ejected with a known force. The encased sprites
themselves should be ejected with a known force. So there will be
enough information to derive approximate *initial* orbital elements.
Cumulative atmospheric drag on sprites may depend somewhat on initial
tumbling, which is probably not controllable at all. (Imagine, for
example, a sprite that is initially tumbling so that it slices
precisely edgewise through the upper atmosphere. It'll probably
deorbit much later than any of the others.)

[me]
>> Orekit appears to have models for atmospheric drag (though not much
>> wiki documentation yet on this?), and even the beginnings of a
>> tutorial about geomagnetic field modeling. In exchange for help with
>> questions, I would be more than happy to help flesh out any Orekit
>> documentation that already exists on these two topics, as needed. I
>> would also be happy to contribute to the Orekit documentation any
>> tutorial examples that come out of my own project.

[Luc]
> The current Orekit surface force models are more oriented towards center
> of gravity motion (i.e force) rather than attitude (i.e torques).
> Implementing complete torques models would imply evaluating forces on
> each facet individually, which could be done by a specialized version of
> the BoxAndSolarArraySpacecraft class.

Yes, I haven't seen much about torque in Orekit. But it is, after all,
an orbital dynamics kit.

I doubt that the sprite gyro will be able to provide any orientation
torque; but Zac et al. would know.

> ... As KickSat seems to have a very
> regular shape (basically a single flat board without any appendages
> except for you additional strip), this should be simple but getting the
> absorption/reflection coefficients for the various components may be
> difficult. We would also need the inertia matrix and center of gravity
> position.

I hope these can be derived accurately enough from readily-available
information: the (open-sourced) circuit board layout, the data sheets
on its components, and basic material properties of the plastic board
itself.

> So we could probably set up some model, but to be fair I am
> not sure they would be realistic.

Certainty is impossible anyway. As someone once said, "All models are
wrong; some are useful."

> If you have specific questions, do not hesitate to ask them on the list,
> we will try to help as much as we can.

I'll try not to burden the list with too many clueless-newbie
questions. I'm an experienced programmer, though I have no
professional experience with Java (closest language: C++.) But it's
been a long time since I studied the relevant college physics. And
there's not much time to get up to speed -- KickSat will probably
launch sometime next year. But Zac says this is the first of a series,
and his group is open-sourcing the design anyway, which improves the
odds of subsequent attempts. So there might be other chances

Cautionary note:

U.S. satellite technology exports are under ITAR. Last I checked,
satellite control systems come under ITAR. And measuring satellite
attitude changes might be considered part of satellite control.

The danger is negligible for some of us. Orekit is in Europe. I'm in
Japan. But Zac and his crew are in the U.S., at Cornell. ITAR is
becoming less stringent, but it's important to be sure that we don't
even *accidentally* put anybody at Cornell across the line into
illegal behavior. Some U.S. universities have been fined enormous
amounts of money for ITAR violations that students and faculty didn't
even realize they were committing.

I think there are no ITAR problems if an open-source satellite design
documents physical parameters that could be used as input for attitude
measurement software written, if the software is written outside the
U.S. But, to be on the safe side, contributing to such code is
probably where Zac et al. would need to be most careful. If their
contributions are already described in the open literature, ITAR
permits the use of them. That's a definite possibility in this case:
Zac's group at Cornell has published interesting work on "magnetically
pinned" spacecraft, and I might be looking at this literature for
ideas. But code patches to Orekit concerning spacecraft attitude
measurement and control might be very problematic, if the patches come
from the U.S. Personally, I think much in ITAR is ridiculous, when it
comes to student-project satellites. It's still the law, though.

Regards,
Michael Turner
Project Persephone
Tokyo