OnBoardAntennaPhaseModifier.java

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package org.orekit.estimation.measurements.modifiers;

import java.util.Collections;
import java.util.List;

import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.estimation.measurements.EstimatedMeasurement;
import org.orekit.estimation.measurements.EstimationModifier;
import org.orekit.estimation.measurements.gnss.Phase;
import org.orekit.frames.Transform;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeStampedPVCoordinates;

/** On-board antenna offset effect on phase measurements.
 * @author David Soulard
 * @since 10.2
 */
public class OnBoardAntennaPhaseModifier implements EstimationModifier<Phase> {

    /** Position of the Antenna Phase Center in satellite frame. */
    private final Vector3D antennaPhaseCenter;

    /** Simple constructor.
     * @param antennaPhaseCenter position of the Antenna Phase Center in satellite frame
     */
    public OnBoardAntennaPhaseModifier(final Vector3D antennaPhaseCenter) {
        this.antennaPhaseCenter = antennaPhaseCenter;
    }

    /** {@inheritDoc} */
    @Override
    public List<ParameterDriver> getParametersDrivers() {
        return Collections.emptyList();
    }

    /** {@inheritDoc} */
    @Override
    public void modify(final EstimatedMeasurement<Phase> estimated) {
        // the participants are spacecraft at emission, ground station at reception
        final TimeStampedPVCoordinates[] participants = estimated.getParticipants();
        final AbsoluteDate               emissionDate = participants[0].getDate();
        final Vector3D                   pReception   = participants[1].getPosition();

        // transform from spacecraft to inertial frame at emission date
        final SpacecraftState refState          = estimated.getStates()[0];
        final SpacecraftState emissionState     = refState.shiftedBy(emissionDate.durationFrom(refState.getDate()));
        final Transform       spacecraftToInert = emissionState.toTransform().getInverse();

        // compute the geometrical value of the range directly from participants positions.
        // Note that this may be different from the value returned by estimated.getEstimatedValue(),
        // because other modifiers may already have been taken into account
        final Vector3D pSpacecraft = spacecraftToInert.transformPosition(Vector3D.ZERO);
        final double rangeUsingSpacecraftCenter = Vector3D.distance(pSpacecraft, pReception);

        // compute the geometrical value of the range replacing
        // the spacecraft position with antenna phase center position
        final Vector3D pAPC = spacecraftToInert.transformPosition(antennaPhaseCenter);
        final double rangeUsingAntennaPhaseCenter = Vector3D.distance(pAPC, pReception);

        // get the estimated value before this modifier is applied
        final double[] value = estimated.getEstimatedValue();

        // modify the value
        value[0] += (rangeUsingAntennaPhaseCenter - rangeUsingSpacecraftCenter) / estimated.getObservedMeasurement().getWavelength();
        estimated.setEstimatedValue(value);
    }

}