PhaseIonosphericDelayModifier.java
/* Copyright 2002-2020 CS GROUP
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* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.estimation.measurements.modifiers;
import java.util.Arrays;
import java.util.List;
import org.hipparchus.RealFieldElement;
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 RealFieldElement<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);
}
}