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*To*: orekit-users@orekit.org*Subject*: [Orekit Users] Problem with Eckstein-Hechler propagation - 50km off from STK/J2 after 24h*From*: "MARTIN SAMPAYO Alvaro" <Alvaro.MARTIN-SAMPAYO@etu.isae.fr>*Date*: Thu, 02 Jun 2016 14:27:47 +0200*User-agent*: SOGoMail 2.3.10

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].

**Follow-Ups**:**Re: [Orekit Users] Problem with Eckstein-Hechler propagation - 50km off from STK/J2 after 24h***From:*Christophe Le Bris <chris.lebris@gmail.com>

**Re: [Orekit Users] Problem with Eckstein-Hechler propagation - 50km off from STK/J2 after 24h***From:*paulcefo <paulcefo@buffalo.edu>

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