AbstractDragForceModel.java
/* Copyright 2002-2020 CS GROUP
* Licensed to CS GROUP (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.
*/
package org.orekit.forces.drag;
import org.hipparchus.RealFieldElement;
import org.hipparchus.analysis.differentiation.DSFactory;
import org.hipparchus.analysis.differentiation.DerivativeStructure;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.forces.AbstractForceModel;
import org.orekit.frames.Frame;
import org.orekit.frames.Transform;
import org.orekit.models.earth.atmosphere.Atmosphere;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.FieldPVCoordinates;
/**
* Base class for drag force models.
* @see DragForce
* @see TimeSpanDragForce
* @author Bryan Cazabonne
* @since 10.2
*/
public abstract class AbstractDragForceModel extends AbstractForceModel {
/** Atmospheric model. */
private final Atmosphere atmosphere;
/**
* Constructor.
* @param atmosphere atmospheric model
*/
protected AbstractDragForceModel(final Atmosphere atmosphere) {
this.atmosphere = atmosphere;
}
/** {@inheritDoc} */
@Override
public boolean dependsOnPositionOnly() {
return false;
}
/** Check if a field state corresponds to derivatives with respect to state.
* @param state state to check
* @param <T> type of the field elements
* @return true if state corresponds to derivatives with respect to state
*/
protected <T extends RealFieldElement<T>> boolean isDSStateDerivative(final FieldSpacecraftState<T> state) {
try {
final DerivativeStructure dsMass = (DerivativeStructure) state.getMass();
final int o = dsMass.getOrder();
final int p = dsMass.getFreeParameters();
// To be in the desired case:
// Order must be 1 (first order derivatives only)
// Number of parameters must be 6 (PV), 7 (PV + drag coefficient) or 8 (PV + drag coefficient + lift ratio)
if (o != 1 || (p != 6 && p != 7 && p != 8)) {
return false;
}
// Check that the first 6 parameters are position and velocity
@SuppressWarnings("unchecked")
final FieldPVCoordinates<DerivativeStructure> pv = (FieldPVCoordinates<DerivativeStructure>) state.getPVCoordinates();
return isVariable(pv.getPosition().getX(), 0) &&
isVariable(pv.getPosition().getY(), 1) &&
isVariable(pv.getPosition().getZ(), 2) &&
isVariable(pv.getVelocity().getX(), 3) &&
isVariable(pv.getVelocity().getY(), 4) &&
isVariable(pv.getVelocity().getZ(), 5);
} catch (ClassCastException cce) {
return false;
}
}
/** Check if a field state corresponds to derivatives with respect to state.
* @param state state to check
* @param <T> type of the field elements
* @return true if state corresponds to derivatives with respect to state
*/
protected <T extends RealFieldElement<T>> boolean isGradientStateDerivative(final FieldSpacecraftState<T> state) {
try {
final Gradient gMass = (Gradient) state.getMass();
final int p = gMass.getFreeParameters();
// To be in the desired case:
// Number of parameters must be 6 (PV), 7 (PV + drag coefficient) or 8 (PV + drag coefficient + lift ratio)
if (p != 6 && p != 7 && p != 8) {
return false;
}
// Check that the first 6 parameters are position and velocity
@SuppressWarnings("unchecked")
final FieldPVCoordinates<Gradient> pv = (FieldPVCoordinates<Gradient>) state.getPVCoordinates();
return isVariable(pv.getPosition().getX(), 0) &&
isVariable(pv.getPosition().getY(), 1) &&
isVariable(pv.getPosition().getZ(), 2) &&
isVariable(pv.getVelocity().getX(), 3) &&
isVariable(pv.getVelocity().getY(), 4) &&
isVariable(pv.getVelocity().getZ(), 5);
} catch (ClassCastException cce) {
return false;
}
}
/** Check if a derivative represents a specified variable.
* @param ds derivative to check
* @param index index of the variable
* @return true if the derivative represents a specified variable
*/
protected boolean isVariable(final DerivativeStructure ds, final int index) {
final double[] derivatives = ds.getAllDerivatives();
boolean check = true;
for (int i = 1; i < derivatives.length; ++i) {
check &= derivatives[i] == ((index + 1 == i) ? 1.0 : 0.0);
}
return check;
}
/** Check if a derivative represents a specified variable.
* @param g derivative to check
* @param index index of the variable
* @return true if the derivative represents a specified variable
*/
protected boolean isVariable(final Gradient g, final int index) {
final double[] derivatives = g.getGradient();
boolean check = true;
for (int i = 0; i < derivatives.length; ++i) {
check &= derivatives[i] == ((index == i) ? 1.0 : 0.0);
}
return check;
}
/** Compute density and its derivatives.
* Using finite differences for the derivatives.
* And doing the actual computation only for the derivatives with respect to position (others are set to 0.).
* <p>
* From a theoretical point of view, this method computes the same values
* as {@link Atmosphere#getDensity(FieldAbsoluteDate, FieldVector3D, Frame)} in the
* specific case of {@link DerivativeStructure} with respect to state, so
* it is less general. However, it is *much* faster in this important case.
* </p>
* <p>
* The derivatives should be computed with respect to position. The input
* parameters already take into account the free parameters (6, 7 or 8 depending
* on derivation with respect to drag coefficient and lift ratio being considered or not)
* and order (always 1). Free parameters at indices 0, 1 and 2 correspond to derivatives
* with respect to position. Free parameters at indices 3, 4 and 5 correspond
* to derivatives with respect to velocity (these derivatives will remain zero
* as the atmospheric density does not depend on velocity). Free parameter
* at indexes 6 and 7 (if present) corresponds to derivatives with respect to drag coefficient
* and/or lift ratio (one of these or both).
* This 2 last derivatives will remain zero as atmospheric density does not depend on them.
* </p>
* @param date current date
* @param frame inertial reference frame for state (both orbit and attitude)
* @param position position of spacecraft in inertial frame
* @return the density and its derivatives
*/
protected DerivativeStructure getDSDensityWrtStateUsingFiniteDifferences(final AbsoluteDate date,
final Frame frame,
final FieldVector3D<DerivativeStructure> position) {
// Retrieve derivation properties for parameter T
// It is implied here that T is a DerivativeStructure
// With order 1 and 6, 7 or 8 free parameters
// This is all checked before in method isStateDerivatives
final DSFactory factory = position.getX().getFactory();
// Build a DerivativeStructure using only derivatives with respect to position
final DSFactory factory3 = new DSFactory(3, 1);
final FieldVector3D<DerivativeStructure> position3 =
new FieldVector3D<>(factory3.variable(0, position.getX().getReal()),
factory3.variable(1, position.getY().getReal()),
factory3.variable(2, position.getZ().getReal()));
// Get atmosphere properties in atmosphere own frame
final Frame atmFrame = atmosphere.getFrame();
final Transform toBody = frame.getTransformTo(atmFrame, date);
final FieldVector3D<DerivativeStructure> posBodyDS = toBody.transformPosition(position3);
final Vector3D posBody = posBodyDS.toVector3D();
// Estimate density model by finite differences and composition
// Using a delta of 1m
final double delta = 1.0;
final double x = posBody.getX();
final double y = posBody.getY();
final double z = posBody.getZ();
final double rho0 = atmosphere.getDensity(date, posBody, atmFrame);
final double dRhodX = (atmosphere.getDensity(date, new Vector3D(x + delta, y, z), atmFrame) - rho0) / delta;
final double dRhodY = (atmosphere.getDensity(date, new Vector3D(x, y + delta, z), atmFrame) - rho0) / delta;
final double dRhodZ = (atmosphere.getDensity(date, new Vector3D(x, y, z + delta), atmFrame) - rho0) / delta;
final double[] dXdQ = posBodyDS.getX().getAllDerivatives();
final double[] dYdQ = posBodyDS.getY().getAllDerivatives();
final double[] dZdQ = posBodyDS.getZ().getAllDerivatives();
// Density with derivatives:
// - The value and only the 3 first derivatives (those with respect to spacecraft position) are computed
// - Others are set to 0.
final int p = factory.getCompiler().getFreeParameters();
final double[] rhoAll = new double[p + 1];
rhoAll[0] = rho0;
for (int i = 1; i < 4; ++i) {
rhoAll[i] = dRhodX * dXdQ[i] + dRhodY * dYdQ[i] + dRhodZ * dZdQ[i];
}
return factory.build(rhoAll);
}
/** Compute density and its derivatives.
* Using finite differences for the derivatives.
* And doing the actual computation only for the derivatives with respect to position (others are set to 0.).
* <p>
* From a theoretical point of view, this method computes the same values
* as {@link Atmosphere#getDensity(FieldAbsoluteDate, FieldVector3D, Frame)} in the
* specific case of {@link Gradient} with respect to state, so
* it is less general. However, it is *much* faster in this important case.
* </p>
* <p>
* The derivatives should be computed with respect to position. The input
* parameters already take into account the free parameters (6, 7 or 8 depending
* on derivation with respect to drag coefficient and lift ratio being considered or not)
* and order (always 1). Free parameters at indices 0, 1 and 2 correspond to derivatives
* with respect to position. Free parameters at indices 3, 4 and 5 correspond
* to derivatives with respect to velocity (these derivatives will remain zero
* as the atmospheric density does not depend on velocity). Free parameter
* at indexes 6 and 7 (if present) corresponds to derivatives with respect to drag coefficient
* and/or lift ratio (one of these or both).
* This 2 last derivatives will remain zero as atmospheric density does not depend on them.
* </p>
* @param date current date
* @param frame inertial reference frame for state (both orbit and attitude)
* @param position position of spacecraft in inertial frame
* @return the density and its derivatives
*/
protected Gradient getGradientDensityWrtStateUsingFiniteDifferences(final AbsoluteDate date,
final Frame frame,
final FieldVector3D<Gradient> position) {
// Build a Gradient using only derivatives with respect to position
final FieldVector3D<Gradient> position3 =
new FieldVector3D<>(Gradient.variable(3, 0, position.getX().getReal()),
Gradient.variable(3, 1, position.getY().getReal()),
Gradient.variable(3, 2, position.getZ().getReal()));
// Get atmosphere properties in atmosphere own frame
final Frame atmFrame = atmosphere.getFrame();
final Transform toBody = frame.getTransformTo(atmFrame, date);
final FieldVector3D<Gradient> posBodyDS = toBody.transformPosition(position3);
final Vector3D posBody = posBodyDS.toVector3D();
// Estimate density model by finite differences and composition
// Using a delta of 1m
final double delta = 1.0;
final double x = posBody.getX();
final double y = posBody.getY();
final double z = posBody.getZ();
final double rho0 = atmosphere.getDensity(date, posBody, atmFrame);
final double dRhodX = (atmosphere.getDensity(date, new Vector3D(x + delta, y, z), atmFrame) - rho0) / delta;
final double dRhodY = (atmosphere.getDensity(date, new Vector3D(x, y + delta, z), atmFrame) - rho0) / delta;
final double dRhodZ = (atmosphere.getDensity(date, new Vector3D(x, y, z + delta), atmFrame) - rho0) / delta;
final double[] dXdQ = posBodyDS.getX().getGradient();
final double[] dYdQ = posBodyDS.getY().getGradient();
final double[] dZdQ = posBodyDS.getZ().getGradient();
// Density with derivatives:
// - The value and only the 3 first derivatives (those with respect to spacecraft position) are computed
// - Others are set to 0.
final int p = position.getX().getFreeParameters();
final double[] rhoAll = new double[p];
for (int i = 0; i < 3; ++i) {
rhoAll[i] = dRhodX * dXdQ[i] + dRhodY * dYdQ[i] + dRhodZ * dZdQ[i];
}
return new Gradient(rho0, rhoAll);
}
}