Eckstein Hechler Propagator Returned Values

Orekit Python Wrapper usage
1

Hello!

I’m working with the Eckstein Hechler propagator and I wanted to know whether the propagate method returns osculating or mean elements.

Thanks in advance,

Verónica

2

Hi @VeronicaSastre

They are oscillating.

Regards,
Bryan

3

Thank you for the fast reply!

If they are osculating I have a question.

I’m propagating a TLE with the EH propagator as follows:

# Get the keplerian orbit from the TLE
tle_propagator = TLEPropagator.selectExtrapolator(mytle)
tle_orbit_cart = tle_propagator.getInitialState().getOrbit()
tle_orbit_kep = OrbitType.KEPLERIAN.convertType(tle_orbit_cart)

# Propagation with Eickstein Hechler Propagator
propagator_eh = EcksteinHechlerPropagator(tle_orbit_kep, 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)

print(1, tle_orbit_kep)
print(2, propagator_eh.getInitialState().getOrbit())
print(3, OrbitType.KEPLERIAN.convertType(propagator_eh.propagate(mytle.getDate()).getOrbit()))

The output is:

1 Keplerian parameters: {a: 6806442.630251932; e: 0.0017767159957931275; i: 2.4679562062536506; pa: -24.3674662951819; raan: 132.2416001288443; v: 24.36755660511185;}
2 Keplerian parameters: {a: 6806442.630251932; e: 0.0017767159957931275; i: 2.4679562062536506; pa: -24.3674662951819; raan: 132.2416001288443; v: 24.36755660511185;}
3 Keplerian parameters: {a: 6806748.12718845; e: 0.0018174975987912372; i: 2.467985753857901; pa: -23.775377104485703; raan: 132.2416001299259; v: 23.77546741333503;}

The output is the keplerian elements at the TLE epoch obtained in differents ways. As can be seen, the parameters in case 3 are different from the ones in cases 1 and 2. Do you know where this difference comes from?

Many thanks again,

Verónica

4

That’s because the dynamical models are different.

Orbit (1) is an osculating orbit computed through SGP4 model
Orbit (2) is the initial osculating orbit of Eckstein-Hechler (ECK) model. Since any propagation is performed, that’s normal that (1) and (2) are identical
Orbit (3) is an osculating orbit computed through ECK model.

The models are as followed:

  • ECK → Zonal harmonics from J2 to J6
  • SGP4 → Zonal harmonics from J2 to J4, non-conservative accelerations (mainly drag with the B* parameter) and luni-solar terms for “deep space satellites” (deep space satellite are, according the SGP4 model satellites for which the period is greater thant 225 minutes).

So, I think the differences can be explained by the two models which are different.

I don’t think it’s a good idea to propagate a TLE using ECK model. I think you shall use the TLEPropagator in that case.