RangeRateTroposphericDelayModifier.java
/* Copyright 2002-2017 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
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package org.orekit.estimation.measurements.modifiers;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.errors.OrekitException;
import org.orekit.errors.OrekitExceptionWrapper;
import org.orekit.estimation.measurements.EstimatedMeasurement;
import org.orekit.estimation.measurements.EstimationModifier;
import org.orekit.estimation.measurements.GroundStation;
import org.orekit.estimation.measurements.RangeRate;
import org.orekit.models.earth.TroposphericModel;
import org.orekit.orbits.OrbitType;
import org.orekit.orbits.PositionAngle;
import org.orekit.propagation.Propagator;
import org.orekit.propagation.SpacecraftState;
import org.orekit.utils.Differentiation;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.ParameterFunction;
import org.orekit.utils.StateFunction;
/** Class modifying theoretical range-rate measurements with tropospheric delay.
* The effect of tropospheric correction on the range-rate is directly computed
* through the computation of the tropospheric delay difference with respect to
* time.
*
* In general, for GNSS, VLBI, ... there is hardly any frequency dependence in the delay.
* For SLR techniques however, the frequency dependence is sensitive.
*
* @author Joris Olympio
* @since 8.0
*/
public class RangeRateTroposphericDelayModifier implements EstimationModifier<RangeRate> {
/** Tropospheric delay model. */
private final TroposphericModel tropoModel;
/** Two-way measurement factor. */
private final double fTwoWay;
/** Constructor.
*
* @param model Tropospheric delay model appropriate for the current range-rate measurement method.
* @param tw Flag indicating whether the measurement is two-way.
*/
public RangeRateTroposphericDelayModifier(final TroposphericModel model, final boolean tw) {
tropoModel = model;
if (tw) {
fTwoWay = 2.;
} else {
fTwoWay = 1.;
}
}
/** Get the station height above mean sea level.
*
* @param station ground station (or measuring station)
* @return the measuring station height above sea level, m
*/
private double getStationHeightAMSL(final GroundStation station) {
// FIXME heigth should be computed with respect to geoid WGS84+GUND = EGM2008 for example
final double height = station.getBaseFrame().getPoint().getAltitude();
return height;
}
/** Compute the measurement error due to Troposphere.
* @param station station
* @param state spacecraft state
* @return the measurement error due to Troposphere
* @throws OrekitException if frames transformations cannot be computed
*/
public double rangeRateErrorTroposphericModel(final GroundStation station,
final SpacecraftState state)
throws OrekitException {
// The effect of tropospheric correction on the range rate is
// computed using finite differences.
final double dt = 10; // s
// station altitude AMSL in meters
final double height = getStationHeightAMSL(station);
// spacecraft position and elevation as seen from the ground station
final Vector3D position = state.getPVCoordinates().getPosition();
// elevation
final double elevation1 = station.getBaseFrame().getElevation(position,
state.getFrame(),
state.getDate());
// only consider measures above the horizon
if (elevation1 > 0) {
// tropospheric delay in meters
final double d1 = tropoModel.pathDelay(elevation1, height);
// propagate spacecraft state forward by dt
final SpacecraftState state2 = state.shiftedBy(dt);
// spacecraft position and elevation as seen from the ground station
final Vector3D position2 = state2.getPVCoordinates().getPosition();
// elevation
final double elevation2 = station.getBaseFrame().getElevation(position2,
state2.getFrame(),
state2.getDate());
// tropospheric delay dt after
final double d2 = tropoModel.pathDelay(elevation2, height);
return fTwoWay * (d2 - d1) / dt;
}
return 0;
}
/** Compute the Jacobian of the delay term wrt state.
*
* @param station station
* @param refstate spacecraft state
* @param delay current tropospheric delay
* @return Jacobian of the delay wrt state
* @throws OrekitException if frames transformations cannot be computed
*/
private double[][] rangeRateErrorJacobianState(final GroundStation station,
final SpacecraftState refstate,
final double delay)
throws OrekitException {
final double[][] finiteDifferencesJacobian =
Differentiation.differentiate(new StateFunction() {
public double[] value(final SpacecraftState state) throws OrekitException {
try {
// evaluate target's elevation with a changed target position
final double value = rangeRateErrorTroposphericModel(station, state);
return new double[] {value };
} catch (OrekitException oe) {
throw new OrekitExceptionWrapper(oe);
}
}
}, 1, Propagator.DEFAULT_LAW, OrbitType.CARTESIAN,
PositionAngle.TRUE, 15.0, 3).value(refstate);
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 delay current ionospheric delay
* @return derivative of the delay wrt station offset parameter
* @throws OrekitException if frames transformations cannot be computed
*/
private double rangeRateErrorParameterDerivative(final GroundStation station,
final ParameterDriver driver,
final SpacecraftState state,
final double delay)
throws OrekitException {
final ParameterFunction rangeError = new ParameterFunction() {
/** {@inheritDoc} */
@Override
public double value(final ParameterDriver parameterDriver) throws OrekitException {
return rangeRateErrorTroposphericModel(station, state);
}
};
final ParameterFunction rangeErrorDerivative =
Differentiation.differentiate(rangeError, driver, 3, 10.0);
return rangeErrorDerivative.value(driver);
}
/** {@inheritDoc} */
@Override
public List<ParameterDriver> getParametersDrivers() {
return Collections.emptyList();
}
/** {@inheritDoc} */
@Override
public void modify(final EstimatedMeasurement<RangeRate> estimated)
throws OrekitException {
final RangeRate measurement = estimated.getObservedMeasurement();
final GroundStation station = measurement.getStation();
final SpacecraftState state = estimated.getStates()[0];
final double[] oldValue = estimated.getEstimatedValue();
final double delay = rangeRateErrorTroposphericModel(station, state);
// update estimated value taking into account the tropospheric delay.
// The tropospheric delay is directly added to the range.
final double[] newValue = oldValue.clone();
newValue[0] = newValue[0] + delay;
estimated.setEstimatedValue(newValue);
// update estimated derivatives with Jacobian of the measure wrt state
final double[][] djac = rangeRateErrorJacobianState(station,
state,
delay);
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);
for (final ParameterDriver driver : Arrays.asList(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 += rangeRateErrorParameterDerivative(station, driver, state, delay);
estimated.setParameterDerivatives(driver, parameterDerivative);
}
}
}
}