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)
>> 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].