PhaseTroposphericDelayModifier.java
/* Copyright 2002-2022 CS GROUP
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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
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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.Field;
import org.hipparchus.CalculusFieldElement;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
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.models.earth.troposphere.DiscreteTroposphericModel;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.SpacecraftState;
import org.orekit.utils.Differentiation;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.ParameterFunction;
/**
* Class modifying theoretical phase measurement with tropospheric delay.
* The effect of tropospheric correction on the phase is directly computed
* through the computation of the tropospheric delay.
* @author David Soulard
* @author Bryan Cazabonne
* @since 10.2
*/
public class PhaseTroposphericDelayModifier implements EstimationModifier<Phase> {
/** Tropospheric delay model. */
private final DiscreteTroposphericModel tropoModel;
/** Constructor.
*
* @param model Tropospheric delay model appropriate for the current range measurement method.
*/
public PhaseTroposphericDelayModifier(final DiscreteTroposphericModel model) {
tropoModel = model;
}
/** Compute the measurement error due to Troposphere.
* @param station station
* @param state spacecraft state
* @param wavelength wavelength of the signal
* @return the measurement error due to Troposphere
*/
private double phaseErrorTroposphericModel(final GroundStation station, final SpacecraftState state, final double wavelength) {
// satellite position
final Vector3D position = state.getPVCoordinates().getPosition();
// elevation
final double elevation = station.getBaseFrame().getElevation(position,
state.getFrame(),
state.getDate());
// only consider measures above the horizon
if (elevation > 0) {
// delay in meters
final double delay = tropoModel.pathDelay(elevation, station.getBaseFrame().getPoint(), tropoModel.getParameters(), state.getDate());
return delay / wavelength;
}
return 0;
}
/** Compute the measurement error due to Troposphere.
* @param <T> type of the element
* @param station station
* @param state spacecraft state
* @param parameters tropospheric model parameters
* @param wavelength of the measurements
* @return the measurement error due to Troposphere
*/
private <T extends CalculusFieldElement<T>> T phaseErrorTroposphericModel(final GroundStation station,
final FieldSpacecraftState<T> state,
final T[] parameters, final double wavelength) {
// Field
final Field<T> field = state.getDate().getField();
final T zero = field.getZero();
// satellite elevation
final FieldVector3D<T> position = state.getPVCoordinates().getPosition();
final T elevation = station.getBaseFrame().getElevation(position,
state.getFrame(),
state.getDate());
// only consider measures above the horizon
if (elevation.getReal() > 0) {
// delay in meters
final T delay = tropoModel.pathDelay(elevation, station.getBaseFrame().getPoint(field), parameters, state.getDate());
return delay.divide(wavelength);
}
return zero;
}
/** Compute the Jacobian of the delay term wrt state using
* automatic differentiation.
*
* @param derivatives tropospheric delay derivatives
*
* @return Jacobian of the delay wrt state
*/
private double[][] phaseErrorJacobianState(final double[] derivatives) {
final double[][] finiteDifferencesJacobian = new double[1][6];
System.arraycopy(derivatives, 0, finiteDifferencesJacobian[0], 0, 6);
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
* @param wavelength wavelength of the signal
* @return derivative of the delay wrt station offset parameter
*/
private double phaseErrorParameterDerivative(final GroundStation station,
final ParameterDriver driver,
final SpacecraftState state,
final double wavelength) {
final ParameterFunction rangeError = parameterDriver -> phaseErrorTroposphericModel(station, state, wavelength);
final ParameterFunction phaseErrorDerivative =
Differentiation.differentiate(rangeError, 3, 10.0 * driver.getScale());
return phaseErrorDerivative.value(driver);
}
/** Compute the derivative of the delay term wrt parameters using
* automatic differentiation.
*
* @param derivatives tropospheric delay derivatives
* @param freeStateParameters dimension of the state.
* @return derivative of the delay wrt tropospheric 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 tropospheric parameters
final int dim = derivatives.length - freeStateParameters;
final double[] rangeError = new double[dim];
for (int i = 0; i < dim; i++) {
rangeError[i] = derivatives[freeStateParameters + i];
}
return rangeError;
}
/** {@inheritDoc} */
@Override
public List<ParameterDriver> getParametersDrivers() {
return tropoModel.getParametersDrivers();
}
/** {@inheritDoc} */
@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 range value
final double[] oldValue = estimated.getEstimatedValue();
// update estimated derivatives with Jacobian of the measure wrt state
final TroposphericGradientConverter converter = new TroposphericGradientConverter(state, 6, new InertialProvider(state.getFrame()));
final FieldSpacecraftState<Gradient> gState = converter.getState(tropoModel);
final Gradient[] gParameters = converter.getParameters(gState, tropoModel);
final Gradient gDelay = phaseErrorTroposphericModel(station, gState, gParameters, measurement.getWavelength());
final double[] derivatives = gDelay.getGradient();
// Update state derivatives
final double[][] djac = phaseErrorJacobianState(derivatives);
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 tropospheric parameter derivatives
int index = 0;
for (final ParameterDriver driver : getParametersDrivers()) {
if (driver.isSelected()) {
// update estimated derivatives with derivative of the modification wrt tropospheric 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, measurement.getWavelength());
estimated.setParameterDerivatives(driver, parameterDerivative);
}
}
// Update estimated value taking into account the tropospheric delay.
// The tropospheric delay is directly added to the phase.
final double[] newValue = oldValue.clone();
newValue[0] = newValue[0] + gDelay.getReal();
estimated.setEstimatedValue(newValue);
}
}