Calculation method ApsideDetector

Orekit Python Wrapper usage
1

Hello dear forum members,

I have a question regarding the ApsideDetector. For my work, I require a very precise determination of the time of the next occurring perigee or apogee. Therefore, my question is which calculation method is used to determine the apsides. Is it based on the short-term average method?

Kind regards,
Max

2

No, it is based on the dot product of position and velocity, hence it is zero when they are orthogonal at apsides. The sign change is from negative to positive at perigee and from positive to negative at apogee. It is therefore an instantaneous (or osculating) computation.

1 Like
3

Hello

To not state the obvious, but I had struggle in the past with that, being the dot product of osculating vectors, it might change sign multiple times for a near circular orbit, so to capture events very far from the theoretical argument of perigee concept.

Is there a new solution with the new average orbit methods since Orekit 13.x?

Thanks

Alberto

4

Well, for near circular orbits, apsides are inherently unstable.

5

Hello dear forum members,
Could someone tell me, regarding the numerical propagator setup below, what the minimum required eccentricity is so that all apogees and perigees are correctly captured?

def _build_core_propagator(
    self,
    initial_orbit: Orbit,
) -> NumericalPropagator:
    initial_state = SpacecraftState(initial_orbit, self.mass)

    tolerances = ToleranceProvider.getDefaultToleranceProvider(
        0.001
    ).getTolerances(initial_orbit, initial_orbit.getType())

    integrator = DormandPrince853Integrator(
        1e-3,
        60.0,
        JArray_double.cast_(tolerances[0]),
        JArray_double.cast_(tolerances[1]),
    )
    integrator.setInitialStepSize(60.0)

    propagator = NumericalPropagator(integrator)
    propagator.setOrbitType(initial_orbit.getType())
    propagator.setPositionAngleType(PositionAngleType.MEAN)
    propagator.setInitialState(initial_state)
    return propagator

Thanks a lot!

6

The answer really depends on the orbit type (LEO, MEO or GEO for example) and the force models. Typically, if you have a Low Earth Orbit that is also a Sun Synchronous Orbit with Earth phasing, then you will have a frozen perigee. However, the perigee is frozen only in mean orbit, not in osculating orbit! When you look at osculating orbit, the eccentricity vector moves a lot. You can look for example at figure 1 in Very Low Thrust Station-Keeping for Low Earth Orbiting Satellites.