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Re: [Orekit Users] Problem with Eckstein-Hechler propagation - 50km off from STK/J2 after 24h


MARTIN SAMPAYO Alvaro <Alvaro.MARTIN-SAMPAYO@etu.isae.fr> a écrit :


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. 

You can use a constructor with explicit values for the zonal terms
and simply set the terms after J2 to 0.

Beware that the constructor uses the un-normalized coefficients
c20, c30, ... These coefficients are the *opposite* of J2.
The amplitude is the same because they are un-normalized here,
just the sign has to be set properly.

Luc


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)
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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].