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Re: [Orekit Users] Problem with Eckstein-Hechler propagation - 50km off from STK/J2 after 24h
Hi, thank you for the answers.
*@Christophe*:
I would like to test J2 only in orekit, but as far as I know Eckstein-Hechler directly considers 6 harmonics and I think there is no way to make it work as a J2, but I might be wrong.
I will look into the differences in orbit definitions, thanks for the hint.
*@Paul*:
My goal is to do a preliminary analysis of coverage, revisit times, etc on a time-frame of up to 30 days. I am interested in 'nominal' orbits, design orbits such as repeating ground tracks, etc. Hence I thought a J2 model was ideal as it is the typical reference for the design of these orbits as far as I know. Usually other smaller perturbations are adjusted for by orbit control, so I thought very accurate models would be counterproductive (for this reason and for their longer computational time, as I am also interested in having a quick propagator).
While I knew Eckstein-Hechler would be slower than J2, I assumed the impact of the extra computations would be small and EH and J2 would not diverge that much. So my current situation is that I don't know whether my EH propagator is really diverging that much from a J2 or if my definition of orbits in STK/orekit is not consistent. I considered DSST unnecessarily complex but I am no expert.
Thanks both for your help,
Alvaro
Le Jeudi 2 Juin 2016 15:56 CEST, paulcefo <paulcefo@buffalo.edu> a écrit:
> Alvaro,
>
> May I ask you to give an approximate discussion of the accuracy goal and
> the time spans that you are requiring of the Orekit orbit propagator?
>
> The Orekit DSST orbit propagator may be on interest to you.
>
> Paul
>
> --
> Dr. Paul J. Cefola
> Consultant in Aerospace Systems, Spaceflight Mechanics, & Astrodynamics
> Adjunct Faculty, Dept. of Mechanical and Aerospace Engineering,
> University at Buffalo (SUNY)
>
> 4 Moonstone Way
> Vineyard Haven, MA 02568
> USA
>
> 508-696-1884 (phone on Martha's Vineyard)
> 978-201-1393 (cell)
>
> paulcefo@buffalo.edu
> paul.cefola@gmail.com
>
> On 06/02/2016 8:27 am, MARTIN SAMPAYO Alvaro wrote:
> > Hi all,
> >
> > This is my first post as I have recently started using orekit for my
> > master thesis, I hope the community can provide some advice, and I
> > thank you in advance for that.
> >
> > I am planning on propagating using Eckstein-hechler (EH) propagator,
> > my intention is to have a fast propagation with custom stephandlers
> > which I have already implemented. I could make-do with a simple J2,
> > but in orekit I found that EH was the simplest that included Earth
> > oblateness effect (which I definitely need to consider).
> >
> > I run Orekit from Matlab, and have tested my orbit propagation using
> > Keplerianpropagator against STK successfully. However, when I cross
> > check the results of orekit's EH propagator with STK's J2 or J4, I see
> > a huge discordance that I cannot attribute to nominal differences in
> > the propagators margins of error. I am talking of about 51 km of error
> > (ground track projection) after just 24 hours.
> >
> > Since the 2-body problem matches the results of STK, I assume my model
> > and my synchronization of orekit and STK are correct. I assume I am
> > having issues with orekit's EH propagator but I cannot see why or
> > where. There's some more info below. *Any suggestions will be
> > appreciated!*
> >
> > Thank you!
> >
> > Alvaro
> >
> > ------------------------------------
> >
> > I am attaching two images with the ground tracks after 5 days:
> >
> > - EH.png : the propagation of orekit/EH vs STK/J2.
> > - Kepler.png : the propagation of orekit/Kepler and STK/2-body
> > propagation
> >
> > Here are the parameters of my orbit.
> >
> > Propagation for 5 days starts on 2004-jan-01 at 01:30:00 UTC.
> >
> > semimajor axis 7578137 m.
> > eccentricity 0.
> > inclination 98.8 deg.
> > argument of perigee 90 deg.
> > RAAN 0.
> > true anomaly 0.
> >
> >
> > My initializations (note matlab's syntax to use java objects):
> >
> > initialDate = AbsoluteDate(2004, 01, 01, 01, 30, 00.000,...
> > TimeScalesFactory.getUTC());
> >
> > inertialFrame = FramesFactory.getEME2000();
> >
> > orbit = KeplerianOrbit(a, e, i, omega, raan, lv,...
> > PositionAngle.TRUE,...
> > inertialFrame, initialDate, ...
> > Constants.EIGEN5C_EARTH_MU);
> >
> > EcksteinHechlerPropagator(orbit,...
> > Constants.EIGEN5C_EARTH_EQUATORIAL_RADIUS,...
> > Constants.EIGEN5C_EARTH_MU, ...
> > Constants.EIGEN5C_EARTH_C20,...
> > Constants.EIGEN5C_EARTH_C30, ...
> > Constants.EIGEN5C_EARTH_C40,...
> > Constants.EIGEN5C_EARTH_C50,...
> > Constants.EIGEN5C_EARTH_C60);
> >
> >
> > and my transformations to ECEF (these happen in the fixedstephandler,
> > coded in java):
> >
> > this.ECEFframe = FramesFactory.getITRF(IERSConventions.IERS_2010, true)
> >
> > ...
> >
> > Transform fromJ2000toITRFtransform =
> > currentState.getFrame().getTransformTo(this.ECEFframe,
> > currentState.getDate());
> >
> > // Obtain satellite position vector
> > Vector3D satelliteVector = fromJ2000toITRFtransform.transformVector(
> > currentState.getPVCoordinates().getPosition());
> >
> > satLatitudes.add(satelliteVector.getDelta());
> > satLongitudes.add(satelliteVector.getAlpha() > FastMath.PI ?
> > satelliteVector.getAlpha()-2*FastMath.PI :
> > satelliteVector.getAlpha()); // To change representation from [0, 2pi]
> > to [-pi, pi].