DSSTTesseralContext

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package org.orekit.propagation.semianalytical.dsst.forces;

import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.orekit.forces.gravity.potential.UnnormalizedSphericalHarmonicsProvider;
import org.orekit.frames.Frame;
import org.orekit.frames.StaticTransform;
import org.orekit.propagation.semianalytical.dsst.utilities.AuxiliaryElements;

/**
 * This class is a container for the common parameters used in {@link DSSTTesseral}.
 * <p>
 * It performs parameters initialization at each integration step for the Tesseral contribution
 * to the central body gravitational perturbation.
 * </p>
 * @author Bryan Cazabonne
 * @since 10.0
 */
public class DSSTTesseralContext extends ForceModelContext {

    /** Retrograde factor I.
     * <p>
     *  DSST model needs equinoctial orbit as internal representation.
     *  Classical equinoctial elements have discontinuities when inclination
     *  is close to zero. In this representation, I = +1. <br>
     * To avoid this discontinuity, another representation exists and equinoctial
     *  elements can be expressed in a different way, called "retrograde" orbit.
     *  This implies I = -1. <br>
     *  As Orekit doesn't implement the retrograde orbit, I is always set to +1.
     *  But for the sake of consistency with the theory, the retrograde factor
     *  has been kept in the formulas.
     * </p>
     */
    private static final int I = 1;

    /** A = sqrt(μ * a). */
    private double A;

    // Common factors for potential computation
    /** &Chi; = 1 / sqrt(1 - e²) = 1 / B. */
    private double chi;

    /** &Chi;². */
    private double chi2;

    /** Central body rotation angle θ. */
    private double theta;

    // Common factors from equinoctial coefficients
    /** 2 * a / A . */
    private double ax2oA;

    /** 1 / (A * B) . */
    private double ooAB;

    /** B / A . */
    private double BoA;

    /** B / (A * (1 + B)) . */
    private double BoABpo;

    /** C / (2 * A * B) . */
    private double Co2AB;

    /** μ / a . */
    private double moa;

    /** R / a . */
    private double roa;

    /** ecc². */
    private double e2;

    /** Keplerian mean motion. */
    private double n;

    /** Keplerian period. */
    private double period;

    /** Ratio of satellite period to central body rotation period. */
    private double ratio;

    /**
     * Simple constructor.
     *
     * @param auxiliaryElements auxiliary elements related to the current orbit
     * @param centralBodyFrame           rotating body frame
     * @param provider                   provider for spherical harmonics
     * @param maxFrequencyShortPeriodics maximum value for j
     * @param bodyPeriod                 central body rotation period (seconds)
     * @param parameters                 values of the force model parameters
     */
    DSSTTesseralContext(final AuxiliaryElements auxiliaryElements,
                        final Frame centralBodyFrame,
                        final UnnormalizedSphericalHarmonicsProvider provider,
                        final int maxFrequencyShortPeriodics,
                        final double bodyPeriod,
                        final double[] parameters) {

        super(auxiliaryElements);

        final double mu = parameters[0];

        // Keplerian Mean Motion
        final double absA = FastMath.abs(auxiliaryElements.getSma());
        n = FastMath.sqrt(mu / absA) / absA;

        // Keplerian period
        final double a = auxiliaryElements.getSma();
        period = (a < 0) ? Double.POSITIVE_INFINITY : 2.0 * FastMath.PI * a * FastMath.sqrt(a / mu);

        A = FastMath.sqrt(mu * auxiliaryElements.getSma());

        // Eccentricity square
        e2 = auxiliaryElements.getEcc() * auxiliaryElements.getEcc();

        // Central body rotation angle from equation 2.7.1-(3)(4).
        final StaticTransform t = centralBodyFrame.getStaticTransformTo(
                auxiliaryElements.getFrame(),
                auxiliaryElements.getDate());
        final Vector3D xB = t.transformVector(Vector3D.PLUS_I);
        final Vector3D yB = t.transformVector(Vector3D.PLUS_J);
        theta = FastMath.atan2(-auxiliaryElements.getVectorF().dotProduct(yB) + I * auxiliaryElements.getVectorG().dotProduct(xB),
                auxiliaryElements.getVectorF().dotProduct(xB) + I * auxiliaryElements.getVectorG().dotProduct(yB));

        // Common factors from equinoctial coefficients
        // 2 * a / A
        ax2oA = 2. * auxiliaryElements.getSma() / A;
        // B / A
        BoA = auxiliaryElements.getB() / A;
        // 1 / AB
        ooAB = 1. / (A * auxiliaryElements.getB());
        // C / 2AB
        Co2AB = auxiliaryElements.getC() * ooAB / 2.;
        // B / (A * (1 + B))
        BoABpo = BoA / (1. + auxiliaryElements.getB());
        // &mu / a
        moa = mu / auxiliaryElements.getSma();
        // R / a
        roa = provider.getAe() / auxiliaryElements.getSma();

        // &Chi; = 1 / B
        chi = 1. / auxiliaryElements.getB();
        chi2 = chi * chi;

        // Ratio of satellite to central body periods to define resonant terms
        ratio = period / bodyPeriod;

    }

    /** Get ecc².
     * @return e2
     */
    public double getE2() {
        return e2;
    }

    /**
     * Get Central body rotation angle θ.
     * @return theta
     */
    public double getTheta() {
        return theta;
    }

    /**
     * Get ax2oA = 2 * a / A .
     * @return ax2oA
     */
    public double getAx2oA() {
        return ax2oA;
    }

    /**
     * Get &Chi; = 1 / sqrt(1 - e²) = 1 / B.
     * @return chi
     */
    public double getChi() {
        return chi;
    }

    /**
     * Get &Chi;².
     * @return chi2
     */
    public double getChi2() {
        return chi2;
    }

    /**
     * Get B / A.
     * @return BoA
     */
    public double getBoA() {
        return BoA;
    }

    /**
     * Get ooAB = 1 / (A * B).
     * @return ooAB
     */
    public double getOoAB() {
        return ooAB;
    }

    /**
     * Get Co2AB = C / 2AB.
     * @return Co2AB
     */
    public double getCo2AB() {
        return Co2AB;
    }

    /**
     * Get BoABpo = B / A(1 + B).
     * @return BoABpo
     */
    public double getBoABpo() {
        return BoABpo;
    }

    /**
     * Get μ / a .
     * @return moa
     */
    public double getMoa() {
        return moa;
    }

    /**
     * Get roa = R / a.
     * @return roa
     */
    public double getRoa() {
        return roa;
    }

    /**
     * Get the Keplerian period.
     * <p>
     * The Keplerian period is computed directly from semi major axis and central
     * acceleration constant.
     * </p>
     * @return Keplerian period in seconds, or positive infinity for hyperbolic
     *         orbits
     */
    public double getOrbitPeriod() {
        return period;
    }

    /**
     * Get the Keplerian mean motion.
     * <p>
     * The Keplerian mean motion is computed directly from semi major axis and
     * central acceleration constant.
     * </p>
     * @return Keplerian mean motion in radians per second
     */
    public double getMeanMotion() {
        return n;
    }

    /**
     * Get the ratio of satellite period to central body rotation period.
     * @return ratio
     */
    public double getRatio() {
        return ratio;
    }

}