OrbitBlender.java

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package org.orekit.orbits;

import org.hipparchus.analysis.polynomials.SmoothStepFactory;
import org.orekit.errors.OrekitException;
import org.orekit.frames.Frame;
import org.orekit.propagation.Propagator;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.analytical.AbstractAnalyticalPropagator;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.PVCoordinates;

import java.util.List;

/**
 * Orbit blender.
 * <p>
 * Its purpose is to interpolate orbit state between tabulated orbit states using the concept of blending, exposed in :
 * "Efficient Covariance Interpolation using Blending of Approximate State Error Transitions" by Sergei Tanygin, and applying
 * it to orbit states instead of covariances.
 * <p>
 * It propagates tabulated values to the interpolating time using given propagator and then blend each propagated
 * states using a smoothstep function. It gives especially good results as explained
 * <a href="https://orekit.org/doc/technical-notes/Implementation_of_covariance_interpolation_in_Orekit.pdf">here</a>
 * compared to Hermite interpolation when time steps between tabulated values get significant (In LEO, &gt; 10 mn for
 * example).
 * <p>
 * <b>In most cases, an analytical propagator would be used to quickly fill the gap between tabulated values and recreate
 * a dense ephemeris</b>.
 * <p>
 * However, a fully configured and accurate numerical propagator can be used to recreate an even
 * more precise ephemeris in case the initial tabulated values were obtained from an external source.
 * <p>
 * Note that in the current implementation, the returned blended orbit is necessarily Cartesian.
 *
 * @author Vincent Cucchietti
 * @see org.hipparchus.analysis.polynomials.SmoothStepFactory
 * @see org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction
 * @see Propagator
 * @see AbstractAnalyticalPropagator
 *
 * @since 12.0
 */
public class OrbitBlender extends AbstractOrbitInterpolator {

    /** Propagator used to propagate tabulated orbits to interpolating time. */
    private final Propagator blendingPropagator;

    /** Blending function. */
    private final SmoothStepFactory.SmoothStepFunction blendingFunction;

    /**
     * Default constructor.
     * <p>
     * <b>In most cases, an analytical propagator would be used to quickly fill the gap between tabulated values and recreate
     * a dense ephemeris</b>.
     * <p>
     * However, a fully configured and accurate numerical propagator can be used to recreate an even
     * more precise ephemeris in case the initial tabulated values were obtained from an external source.
     *
     * @param blendingFunction
     * {@link org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction smoothstep function} used for
     * blending
     * @param blendingPropagator propagator used to propagate tabulated orbits to interpolating time
     * @param outputInertialFrame output inertial frame
     *
     * @throws OrekitException if output frame is not inertial
     * @see org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction
     */
    public OrbitBlender(final SmoothStepFactory.SmoothStepFunction blendingFunction,
                        final Propagator blendingPropagator,
                        final Frame outputInertialFrame) {
        super(DEFAULT_INTERPOLATION_POINTS, 0., outputInertialFrame);
        this.blendingFunction   = blendingFunction;
        this.blendingPropagator = blendingPropagator;
    }

    /** {@inheritDoc} */
    @Override
    protected Orbit interpolate(final InterpolationData interpolationData) {

        // Get first and last entry
        final List<Orbit> neighborList  = interpolationData.getNeighborList();
        final Orbit       previousOrbit = neighborList.get(0);
        final Orbit       nextOrbit     = neighborList.get(1);

        // Propagate orbits
        final AbsoluteDate interpolationDate = interpolationData.getInterpolationDate();
        final Orbit forwardedOrbit  = propagateOrbit(previousOrbit, interpolationDate);
        final Orbit backwardedOrbit = propagateOrbit(nextOrbit, interpolationDate);

        // Extract position-velocity-acceleration coordinates
        final PVCoordinates forwardedPV  = forwardedOrbit.getPVCoordinates(getOutputInertialFrame());
        final PVCoordinates backwardedPV = backwardedOrbit.getPVCoordinates(getOutputInertialFrame());

        // Blend PV coordinates
        final double timeParameter = getTimeParameter(interpolationDate, previousOrbit.getDate(), nextOrbit.getDate());
        final double blendingValue = blendingFunction.value(timeParameter);

        final PVCoordinates blendedPV = forwardedPV.blendArithmeticallyWith(backwardedPV, blendingValue);

        // Output new blended instance
        return new CartesianOrbit(blendedPV, getOutputInertialFrame(), interpolationDate, previousOrbit.getMu());
    }

    /**
     * Propagate orbit using predefined {@link Propagator propagator}.
     *
     * @param tabulatedOrbit tabulated orbit to propagate
     * @param propagationDate propagation date
     *
     * @return orbit propagated to propagation date
     */
    private Orbit propagateOrbit(final Orbit tabulatedOrbit,
                                 final AbsoluteDate propagationDate) {
        blendingPropagator.resetInitialState(new SpacecraftState(tabulatedOrbit));
        return blendingPropagator.propagate(propagationDate).getOrbit();
    }
}