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Re: [Orekit Users] Fwd: Sprite orbit-guesser on the web using Orekit?

Hi Michael,

Michael Turner <michael.eugene.turner@gmail.com> a écrit :


On Fri, Jan 31, 2014 at 10:20 AM, Hank Grabowski
<hank@applieddefense.com> wrote:

Sounds interesting.  What sort of assistance would you be looking for?


Between now and launch, I believe Zac has his hands full with the
ground station software. I don't want to distract him too much. I
think we only need basic starting-point data from him, e.g., about
PPOD deployment (timing, altitude, direction, etc.) and then Sprite
deployment (timing, relative velocities w.r.t. KickSat). For purposes
of predicting orbital decay, perhaps also ballistic coefficient of
Sprites at various angles.

Translating all that into orbit propagation is where I'm clueless. I'm
not an Orekit user. I've only contributed to the wiki at the level of
copy editing.


I think the following could be done easily:

 - start with the initial orbit of KickSat
 - at the prescribed time of sprites release, compute position and
   attitude of KickSat using any orbit propagator, and a Sun pointing
   attitude (i.e. using CelestialBodyPointing for the attitude provider),
 - supposing KickSat will release n sprites uniformly spread with some
   known radial velocity in its plane orthogonal to Sun pointing, you can
   compute the sprites initial positions and velocities as follows:

    SpacecraftState kicksat = kickSatPropagator.propagate(releaseDate);
    Transform satToInertial = kickSat.toTransform().getInverse();
    List<PVCoordinates> spritesPV = new ArrayList<PVCoordinates>(n);
    for (int i = 0; i < n; i++) {

      // each sprite has a relative position of 0, 0, 0 wrt KicSat
      // and a relative velocity at some angle in the release plane
      // orthogonal to KickSat Z axis
      double alpha = i * MathUtils.TWO_PI / n;
      PVCoordinates relativePV =
          new PVCoordinates(Vector3D.ZERO,
                            new Vector3D(dv * FastMath.cos(alpha),
                                         dv * FastMath.sin(alpha),
                                         0.0));

      // convert the relative PV (i.e. position velocity expressed
      // in spacecraft frame) into an absolute PV
      spritesPV.add(satToInertial.transformPV(relativePV));
    }
Then, one can create one propagator for each sprite and compute their trajectory after release from KickSat. In order to take drag into account for orbit decay, one should us a numerical propagator. Some of these propagators could be configured with elevation detectors to automatically compute the Acquisition Of Signal/Loss Of Signal events from a specified ground point (I would say the four propagators that correspond to sprites release towards velocity, across track to the left, across track to the right and in the rear direction, in order to have an idea of how the sprites cloud expand).
The propagators could also generate an ephemeris so some fancy display could be created (or people could download it). 

Does this seem achievable?

best regards,
Luc




Zac is concerned that time might be too short to do anything useful.
I'm not so sure, but getting basic requirements worked out should help
with the estimate. I think he envisions a tasty graphical display --
e.g., a real-time graphic showing the globe and positions of Sprites
(and KickSat). My thought was more like this: tell people what times
of day would be best for trying to get reception, depending on where
they are. Where reception is successful, the results might be used to
update the orbital elements on a per-Sprite basis. At this point, I
don't know how much dispersion is expected in the Sprite fleet.

Regards,
Michael Turner
Executive Director
Project Persephone
K-1 bldg 3F
7-2-6 Nishishinjuku
Shinjuku-ku Tokyo 160-0023
Tel: +81 (3) 6890-1140
Fax: +81 (3) 6890-1158
Mobile: +81 (90) 5203-8682
turner@projectpersephone.org
http://www.projectpersephone.org/

"Love does not consist in gazing at each other, but in looking outward
together in the same direction." -- Antoine de Saint-Exupéry


On Fri, Jan 31, 2014 at 10:20 AM, Hank Grabowski
<hank@applieddefense.com> wrote:

Sounds interesting.  What sort of assistance would you be looking for?


On Thu, Jan 30, 2014 at 10:05 AM, Michael Turner
<michael.eugene.turner@gmail.com> wrote:


See forwarded e-mail below, from Zac Manchester, the lead on KickSat.

  http://www.kickset.net

I have Java web hosting (metawerx.net) and a little experience with
getting the demo code running, but I'm not an orbital mechanics expert
by any means. The SpaceX CRS 3 / ELaNa 5 launch is now scheduled for
March 1st, so there's about one month to do something here with
Orekit.

I think it's an interesting problem -- a few hundred flat,
spin-stabilized femtosats, with high ballistic coefficents, deployed
radially from a sun-pointing, spin-stabilized cubesat. Being able to
predict their orbits even approximately might help those with ground
stations. Deviations from Orekit trajectory predictions might count as
exosphere density data (for all I know, which isn't much.)

 If anyone wants to help out with this, please write me. Everybody
involved with KickSat Sprites would greatly appreciate it.
Contributors will be duly credited.

Regards,
Michael Turner
Executive Director
Project Persephone
K-1 bldg 3F
7-2-6 Nishishinjuku
Shinjuku-ku Tokyo 160-0023
Tel: +81 (3) 6890-1140
Fax: +81 (3) 6890-1158
Mobile: +81 (90) 5203-8682
turner@projectpersephone.org
http://www.projectpersephone.org/

"Love does not consist in gazing at each other, but in looking outward
together in the same direction." -- Antoine de Saint-Exupéry



---------- Forwarded message ----------
From: Zac Manchester <zacinaction@gmail.com>
Date: Thu, Jan 30, 2014 at 4:03 AM
Subject: Re: Sprite orbit-guesser on the web using Orekit?
To: Michael Turner <michael.eugene.turner@gmail.com>


Hi Michael,

Good to hear from you - hope things are going well. First off, the
launch date has slipped once again to March 1, and I'm told that date
is "very soft" and likely subject to further (possibly significant)
delays.

Right now I'm working on getting my radio demodulator/decoder online
so that hams can just record audio files and upload them to be decoded
in the cloud. Ideally we could then plot all of the ground station
contacts on a google maps overlay or something.

As for some kind of astrodynamics thing using Orekit, the obvious
thing to do is some kind of orbit propagation/estimation using the
tracking data we get. The issue I think is that we're likely to run
out of time with the launch coming up so quickly. If this is what you
want to focus on, I'd recommend trying to pipe the Orekit orbit
propagator output into a google earth or google maps visualization to
be displayed online. If you can take care of the graphics/web UI
stuff, I can handle the astrodynamics end.

Have fun at the Brown conference - I've been to a couple and they're
always interesting. I think this one will be right up your alley.

Best,
Zac










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