PhaseIonosphericDelayModifier

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 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
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package org.orekit.estimation.measurements.modifiers;

import java.util.Arrays;
import java.util.List;

import org.hipparchus.CalculusFieldElement;
import org.hipparchus.analysis.differentiation.Gradient;
import org.orekit.attitudes.InertialProvider;
import org.orekit.estimation.measurements.EstimatedMeasurement;
import org.orekit.estimation.measurements.EstimationModifier;
import org.orekit.estimation.measurements.GroundStation;
import org.orekit.estimation.measurements.gnss.Phase;
import org.orekit.frames.TopocentricFrame;
import org.orekit.models.earth.ionosphere.IonosphericModel;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.SpacecraftState;
import org.orekit.utils.Constants;
import org.orekit.utils.Differentiation;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.ParameterFunction;

/**
 * Class modifying theoretical phase measurement with ionospheric delay.
 * The effect of ionospheric correction on the phase is directly computed
 * through the computation of the ionospheric delay.
 * @author David Soulard
 * @author Bryan Cazabonne
 * @since 10.2
 */
public class PhaseIonosphericDelayModifier implements EstimationModifier<Phase> {

    /** Ionospheric delay model. */
    private final IonosphericModel ionoModel;

    /** Frequency [Hz]. */
    private final double frequency;

    /** Constructor.
     *
     * @param model  Ionospheric delay model appropriate for the current range measurement method.
     * @param freq frequency of the signal in Hz
     */
    public PhaseIonosphericDelayModifier(final IonosphericModel model,
                                         final double freq) {
        ionoModel = model;
        frequency = freq;
    }

    /** Compute the measurement error due to ionosphere.
     * @param station station
     * @param state spacecraft state
     * @return the measurement error due to ionosphere
     */
    private double phaseErrorIonosphericModel(final GroundStation station,
                                              final SpacecraftState state) {

        // Base frame associated with the station
        final TopocentricFrame baseFrame = station.getBaseFrame();
        final double wavelength  = Constants.SPEED_OF_LIGHT / frequency;
        // delay in meters
        final double delay = ionoModel.pathDelay(state, baseFrame, frequency, ionoModel.getParameters());
        return delay / wavelength;
    }

    /** Compute the measurement error due to ionosphere.
     * @param <T> type of the element
     * @param station station
     * @param state spacecraft state
     * @param parameters ionospheric model parameters
     * @return the measurement error due to ionosphere
     */
    private <T extends CalculusFieldElement<T>> T phaseErrorIonosphericModel(final GroundStation station,
                                                                         final FieldSpacecraftState<T> state,
                                                                         final T[] parameters) {

        // Base frame associated with the station
        final TopocentricFrame baseFrame = station.getBaseFrame();
        final double wavelength  = Constants.SPEED_OF_LIGHT / frequency;
        // delay in meters
        final T delay = ionoModel.pathDelay(state, baseFrame, frequency, parameters);
        return delay.divide(wavelength);
    }

    /** Compute the Jacobian of the delay term wrt state using
    * automatic differentiation.
    *
    * @param derivatives ionospheric delay derivatives
    * @param freeStateParameters dimension of the state.
    *
    * @return Jacobian of the delay wrt state
    */
    private double[][] phaseErrorJacobianState(final double[] derivatives, final int freeStateParameters) {
        final double[][] finiteDifferencesJacobian = new double[1][6];
        for (int i = 0; i < freeStateParameters; i++) {
            finiteDifferencesJacobian[0][i] = derivatives[i];
        }
        return finiteDifferencesJacobian;
    }


    /** Compute the derivative of the delay term wrt parameters.
     *
     * @param station ground station
     * @param driver driver for the station offset parameter
     * @param state spacecraft state
     * @return derivative of the delay wrt station offset parameter
     */
    private double phaseErrorParameterDerivative(final GroundStation station,
                                                 final ParameterDriver driver,
                                                 final SpacecraftState state) {
        final ParameterFunction phaseError = parameterDriver -> phaseErrorIonosphericModel(station, state);
        final ParameterFunction phaseErrorDerivative =
                        Differentiation.differentiate(phaseError, 3, 10.0 * driver.getScale());
        return phaseErrorDerivative.value(driver);

    }

    /** Compute the derivative of the delay term wrt parameters using
    * automatic differentiation.
    *
    * @param derivatives ionospheric delay derivatives
    * @param freeStateParameters dimension of the state.
    * @return derivative of the delay wrt ionospheric model parameters
    */
    private double[] phaseErrorParameterDerivative(final double[] derivatives, final int freeStateParameters) {
        // 0 ... freeStateParameters - 1 -> derivatives of the delay wrt state
        // freeStateParameters ... n     -> derivatives of the delay wrt ionospheric parameters
        final int dim = derivatives.length - freeStateParameters;
        final double[] phaseError = new double[dim];

        for (int i = 0; i < dim; i++) {
            phaseError[i] = derivatives[freeStateParameters + i];
        }

        return phaseError;
    }

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

    @Override
    public void modify(final EstimatedMeasurement<Phase> estimated) {
        final Phase           measurement = estimated.getObservedMeasurement();
        final GroundStation   station     = measurement.getStation();
        final SpacecraftState state       = estimated.getStates()[0];

        // Old phase value
        final double[] oldValue = estimated.getEstimatedValue();

        // Compute ionospheric delay (the division by the wavelength is performed)
        final IonosphericGradientConverter converter =
                        new IonosphericGradientConverter(state, 6, new InertialProvider(state.getFrame()));
        final FieldSpacecraftState<Gradient> gState = converter.getState(ionoModel);
        final Gradient[] gParameters = converter.getParameters(gState, ionoModel);
        final Gradient gDelay = phaseErrorIonosphericModel(station, gState, gParameters);
        final double[] derivatives = gDelay.getGradient();

        // Update state derivatives
        final double[][] djac = phaseErrorJacobianState(derivatives, converter.getFreeStateParameters());
        final double[][] stateDerivatives = estimated.getStateDerivatives(0);
        for (int irow = 0; irow < stateDerivatives.length; ++irow) {
            for (int jcol = 0; jcol < stateDerivatives[0].length; ++jcol) {
                stateDerivatives[irow][jcol] -= djac[irow][jcol];
            }
        }
        estimated.setStateDerivatives(0, stateDerivatives);

        // Update ionospheric parameter derivatives
        int index = 0;
        for (final ParameterDriver driver : getParametersDrivers()) {
            if (driver.isSelected()) {
                // update estimated derivatives with derivative of the modification wrt ionospheric parameters
                double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                final double[] dDelaydP    = phaseErrorParameterDerivative(derivatives, converter.getFreeStateParameters());
                parameterDerivative -= dDelaydP[index];
                estimated.setParameterDerivatives(driver, parameterDerivative);
                index = index + 1;
            }

        }

        // Update station parameter derivatives
        for (final ParameterDriver driver : Arrays.asList(station.getClockOffsetDriver(),
                                                          station.getEastOffsetDriver(),
                                                          station.getNorthOffsetDriver(),
                                                          station.getZenithOffsetDriver())) {
            if (driver.isSelected()) {
                // update estimated derivatives with derivative of the modification wrt station parameters
                double parameterDerivative = estimated.getParameterDerivatives(driver)[0];
                parameterDerivative -= phaseErrorParameterDerivative(station, driver, state);
                estimated.setParameterDerivatives(driver, parameterDerivative);
            }
        }

        // Update estimated value taking into account the ionospheric delay.
        // The ionospheric delay is directly subtracted to the phase.
        final double[] newValue = oldValue.clone();
        newValue[0] = newValue[0] - gDelay.getValue();
        estimated.setEstimatedValue(newValue);
    }

}