TorqueFree.java
/* Copyright 2023 Luc Maisonobe
* 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.attitudes;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.CalculusFieldElement;
import org.hipparchus.Field;
import org.hipparchus.geometry.euclidean.threed.FieldRotation;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Rotation;
import org.hipparchus.geometry.euclidean.threed.RotationConvention;
import org.hipparchus.geometry.euclidean.threed.RotationOrder;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.ode.DenseOutputModel;
import org.hipparchus.ode.FieldDenseOutputModel;
import org.hipparchus.ode.FieldExpandableODE;
import org.hipparchus.ode.FieldODEState;
import org.hipparchus.ode.FieldOrdinaryDifferentialEquation;
import org.hipparchus.ode.ODEState;
import org.hipparchus.ode.OrdinaryDifferentialEquation;
import org.hipparchus.ode.nonstiff.DormandPrince853FieldIntegrator;
import org.hipparchus.ode.nonstiff.DormandPrince853Integrator;
import org.hipparchus.special.elliptic.jacobi.CopolarN;
import org.hipparchus.special.elliptic.jacobi.FieldCopolarN;
import org.hipparchus.special.elliptic.jacobi.FieldJacobiElliptic;
import org.hipparchus.special.elliptic.jacobi.JacobiElliptic;
import org.hipparchus.special.elliptic.jacobi.JacobiEllipticBuilder;
import org.hipparchus.special.elliptic.legendre.LegendreEllipticIntegral;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.MathArrays;
import org.orekit.frames.Frame;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.FieldPVCoordinatesProvider;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.TimeStampedAngularCoordinates;
import org.orekit.utils.TimeStampedFieldAngularCoordinates;
/** This class handles torque-free motion of a general (non-symmetrical) body.
* <p>
* This attitude model is analytical, it can be called at any arbitrary date
* before or after the date of the initial attitude. Despite being an analytical
* model, it is <em>not</em> an approximation. It provides the attitude exactly
* in O(1) time.
* </p>
* <p>
* The equations are based on Landau and Lifchitz Course of Theoretical Physics,
* Mechanics vol 1, chapter 37. Some adaptations have been made to Landau and
* Lifchitz equations:
* </p>
* <ul>
* <li>inertia can be in any order</li>
* <li>initial conditions can be arbitrary</li>
* <li>signs of several equations have been fixed to work for all initial conditions</li>
* <li>equations have been rewritten to work in all octants</li>
* <li>the φ angle model is based on a precomputed quadrature over one period computed
* at construction (the Landau and Lifchitz equations 37.17 to 37.20 seem to be wrong)</li>
* </ul>
* <p>
* The precomputed quadrature is performed numerically, but as it is performed only once at
* construction and the full integrated model over one period is saved, it can be applied
* analytically later on for any number of periods, hence we consider this attitude mode
* to be analytical.
* </p>
* @author Luc Maisonobe
* @author Lucas Girodet
* @since 12.0
*/
public class TorqueFree implements AttitudeProvider {
/** Initial attitude. */
private final Attitude initialAttitude;
/** Spacecraft inertia. */
private final Inertia inertia;
/** Regular model for primitive double arguments. */
private final DoubleModel doubleModel;
/** Cached field-based models. */
private final transient Map<Field<? extends CalculusFieldElement<?>>, FieldModel<? extends CalculusFieldElement<?>>> cachedModels;
/** Simple constructor.
* @param initialAttitude initial attitude
* @param inertia spacecraft inertia
*/
public TorqueFree(final Attitude initialAttitude, final Inertia inertia) {
this.initialAttitude = initialAttitude;
this.inertia = inertia;
// prepare the regular model
this.doubleModel = new DoubleModel();
// set an empty cache for field-based models that will be lazily build
this.cachedModels = new HashMap<>();
}
/** Get the initial attitude.
* @return initial attitude
*/
public Attitude getInitialAttitude() {
return initialAttitude;
}
/** Get the spacecraft inertia.
* @return spacecraft inertia
*/
public Inertia getInertia() {
return inertia;
}
/** {@inheritDoc} */
public Attitude getAttitude(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame) {
// attitude from model
final Attitude attitude =
new Attitude(initialAttitude.getReferenceFrame(), doubleModel.evaluate(date));
// fix frame
return attitude.withReferenceFrame(frame);
}
/** {@inheritDoc} */
public <T extends CalculusFieldElement<T>> FieldAttitude<T> getAttitude(final FieldPVCoordinatesProvider<T> pvProv,
final FieldAbsoluteDate<T> date,
final Frame frame) {
// get the model for specified field
@SuppressWarnings("unchecked")
FieldModel<T> fm = (FieldModel<T>) cachedModels.get(date.getField());
if (fm == null) {
// create a model for this field
fm = new FieldModel<>(date.getField());
cachedModels.put(date.getField(), fm);
}
// attitude from model
final FieldAttitude<T> attitude =
new FieldAttitude<>(initialAttitude.getReferenceFrame(), fm.evaluate(date));
// fix frame
return attitude.withReferenceFrame(frame);
}
/** Torque-free model. */
private class DoubleModel {
/** Inertia sorted to get a motion about axis 3. */
private final Inertia sortedInertia;
/** State scaling factor. */
private final double o1Scale;
/** State scaling factor. */
private final double o2Scale;
/** State scaling factor. */
private final double o3Scale;
/** Jacobi elliptic function. */
private final JacobiElliptic jacobi;
/** Time scaling factor. */
private final double tScale;
/** Time reference for rotation rate. */
private final AbsoluteDate tRef;
/** Offset rotation between initial inertial frame and the frame with moment vector and Z axis aligned. */
private final Rotation inertToAligned;
/** Rotation to switch to the converted axes frame. */
private final Rotation sortedToBody;
/** Period of rotation rate. */
private final double period;
/** Slope of the linear part of the phi model. */
private final double phiSlope;
/** DenseOutputModel of phi. */
private final DenseOutputModel phiQuadratureModel;
/** Integral part of quadrature model over one period. */
private final double integOnePeriod;
/** Simple constructor.
*/
DoubleModel() {
// build inertia tensor
final double i1 = inertia.getInertiaAxis1().getI();
final Vector3D a1 = inertia.getInertiaAxis1().getA();
final double i2 = inertia.getInertiaAxis2().getI();
final Vector3D a2 = inertia.getInertiaAxis2().getA();
final double i3 = inertia.getInertiaAxis3().getI();
final Vector3D a3 = inertia.getInertiaAxis3().getA();
final Vector3D n1 = a1.normalize();
final Vector3D n2 = a2.normalize();
final Vector3D n3 = Vector3D.dotProduct(Vector3D.crossProduct(a1, a2), a3) > 0 ?
a3.normalize() : a3.normalize().negate();
final Vector3D omega0 = initialAttitude.getSpin();
final Vector3D m0 = new Vector3D(i1 * Vector3D.dotProduct(omega0, n1), n1,
i2 * Vector3D.dotProduct(omega0, n2), n2,
i3 * Vector3D.dotProduct(omega0, n3), n3);
// sort axes in increasing moments of inertia order
Inertia tmpInertia = new Inertia(new InertiaAxis(i1, n1), new InertiaAxis(i2, n2), new InertiaAxis(i3, n3));
if (tmpInertia.getInertiaAxis1().getI() > tmpInertia.getInertiaAxis2().getI()) {
tmpInertia = tmpInertia.swap12();
}
if (tmpInertia.getInertiaAxis2().getI() > tmpInertia.getInertiaAxis3().getI()) {
tmpInertia = tmpInertia.swap23();
}
if (tmpInertia.getInertiaAxis1().getI() > tmpInertia.getInertiaAxis2().getI()) {
tmpInertia = tmpInertia.swap12();
}
// in order to simplify implementation, we want the motion to be about axis 3
// which is either the minimum or the maximum inertia axis
final double o1 = Vector3D.dotProduct(omega0, n1);
final double o2 = Vector3D.dotProduct(omega0, n2);
final double o3 = Vector3D.dotProduct(omega0, n3);
final double o12 = o1 * o1;
final double o22 = o2 * o2;
final double o32 = o3 * o3;
final double twoE = i1 * o12 + i2 * o22 + i3 * o32;
final double m2 = i1 * i1 * o12 + i2 * i2 * o22 + i3 * i3 * o32;
final double separatrixInertia = (twoE == 0) ? 0.0 : m2 / twoE;
final boolean clockwise;
if (separatrixInertia < tmpInertia.getInertiaAxis2().getI()) {
// motion is about minimum inertia axis
// we swap axes to put them in decreasing moments order
// motion will be clockwise about axis 3
clockwise = true;
tmpInertia = tmpInertia.swap13();
} else {
// motion is about maximum inertia axis
// we keep axes in increasing moments order
// motion will be counter-clockwise about axis 3
clockwise = false;
}
sortedInertia = tmpInertia;
final double i1C = tmpInertia.getInertiaAxis1().getI();
final double i2C = tmpInertia.getInertiaAxis2().getI();
final double i3C = tmpInertia.getInertiaAxis3().getI();
final double i32 = i3C - i2C;
final double i31 = i3C - i1C;
final double i21 = i2C - i1C;
// convert initial conditions to Euler angles such the M is aligned with Z in sorted computation frame
sortedToBody = new Rotation(Vector3D.PLUS_I, Vector3D.PLUS_J,
tmpInertia.getInertiaAxis1().getA(), tmpInertia.getInertiaAxis2().getA());
final Vector3D omega0Sorted = sortedToBody.applyInverseTo(omega0);
final Vector3D m0Sorted = sortedToBody.applyInverseTo(m0);
final double phi0 = 0; // this angle can be set arbitrarily, so 0 is a fair value (Eq. 37.13 - 37.14)
final double theta0 = FastMath.acos(m0Sorted.getZ() / m0Sorted.getNorm());
final double psi0 = FastMath.atan2(m0Sorted.getX(), m0Sorted.getY()); // it is really atan2(x, y), not atan2(y, x) as usual!
// compute offset rotation between inertial frame aligned with momentum and regular inertial frame
final Rotation alignedToSorted0 = new Rotation(RotationOrder.ZXZ, RotationConvention.FRAME_TRANSFORM,
phi0, theta0, psi0);
inertToAligned = alignedToSorted0.
applyInverseTo(sortedToBody.applyInverseTo(initialAttitude.getRotation()));
// Ω is always o1Scale * cn((t-tref) * tScale), o2Scale * sn((t-tref) * tScale), o3Scale * dn((t-tref) * tScale)
tScale = FastMath.copySign(FastMath.sqrt(i32 * (m2 - twoE * i1C) / (i1C * i2C * i3C)),
clockwise ? -omega0Sorted.getZ() : omega0Sorted.getZ());
o1Scale = FastMath.sqrt((twoE * i3C - m2) / (i1C * i31));
o2Scale = FastMath.sqrt((twoE * i3C - m2) / (i2C * i32));
o3Scale = FastMath.copySign(FastMath.sqrt((m2 - twoE * i1C) / (i3C * i31)), omega0Sorted.getZ());
final double k2 = (twoE == 0) ? 0.0 : i21 * (twoE * i3C - m2) / (i32 * (m2 - twoE * i1C));
jacobi = JacobiEllipticBuilder.build(k2);
period = 4 * LegendreEllipticIntegral.bigK(k2) / tScale;
final double dtRef;
if (o1Scale == 0) {
// special case where twoE * i3C = m2, then o2Scale is also zero
// motion is exactly along one axis
dtRef = 0;
} else {
if (FastMath.abs(omega0Sorted.getX()) >= FastMath.abs(omega0Sorted.getY())) {
if (omega0Sorted.getX() >= 0) {
// omega is roughly towards +I
dtRef = -jacobi.arcsn(omega0Sorted.getY() / o2Scale) / tScale;
} else {
// omega is roughly towards -I
dtRef = jacobi.arcsn(omega0Sorted.getY() / o2Scale) / tScale - 0.5 * period;
}
} else {
if (omega0Sorted.getY() >= 0) {
// omega is roughly towards +J
dtRef = -jacobi.arccn(omega0Sorted.getX() / o1Scale) / tScale;
} else {
// omega is roughly towards -J
dtRef = jacobi.arccn(omega0Sorted.getX() / o1Scale) / tScale;
}
}
}
tRef = initialAttitude.getDate().shiftedBy(dtRef);
phiSlope = FastMath.sqrt(m2) / i3C;
phiQuadratureModel = computePhiQuadratureModel(dtRef);
integOnePeriod = phiQuadratureModel.getInterpolatedState(phiQuadratureModel.getFinalTime()).getPrimaryState()[0];
}
/** Compute the model for φ angle.
* @param dtRef start time
* @return model for φ angle
*/
private DenseOutputModel computePhiQuadratureModel(final double dtRef) {
final double i1C = sortedInertia.getInertiaAxis1().getI();
final double i2C = sortedInertia.getInertiaAxis2().getI();
final double i3C = sortedInertia.getInertiaAxis3().getI();
final double i32 = i3C - i2C;
final double i31 = i3C - i1C;
final double i21 = i2C - i1C;
// coefficients for φ model
final double b = phiSlope * i32 * i31;
final double c = i1C * i32;
final double d = i3C * i21;
// integrate the quadrature phi term over one period
final DormandPrince853Integrator integ = new DormandPrince853Integrator(1.0e-6 * period, 1.0e-2 * period,
phiSlope * period * 1.0e-13, 1.0e-13);
final DenseOutputModel model = new DenseOutputModel();
integ.addStepHandler(model);
integ.integrate(new OrdinaryDifferentialEquation() {
/** {@inheritDoc} */
@Override
public int getDimension() {
return 1;
}
/** {@inheritDoc} */
@Override
public double[] computeDerivatives(final double t, final double[] y) {
final double sn = jacobi.valuesN((t - dtRef) * tScale).sn();
return new double[] {
b / (c + d * sn * sn)
};
}
}, new ODEState(0, new double[1]), period);
return model;
}
/** Evaluate torque-free motion model.
* @param date evaluation date
* @return body orientation at date
*/
public TimeStampedAngularCoordinates evaluate(final AbsoluteDate date) {
// angular velocity
final CopolarN valuesN = jacobi.valuesN(date.durationFrom(tRef) * tScale);
final Vector3D omegaSorted = new Vector3D(o1Scale * valuesN.cn(), o2Scale * valuesN.sn(), o3Scale * valuesN.dn());
final Vector3D omegaBody = sortedToBody.applyTo(omegaSorted);
// acceleration
final Vector3D accelerationSorted = new Vector3D(o1Scale * tScale * valuesN.cn() * valuesN.dn(),
o2Scale * tScale * -valuesN.sn() * valuesN.dn(),
o3Scale * tScale * -jacobi.getM() * valuesN.sn() * valuesN.cn());
final Vector3D accelerationBody = sortedToBody.applyTo(accelerationSorted);
// first Euler angles are directly linked to angular velocity
final double dt = date.durationFrom(initialAttitude.getDate());
final double psi = FastMath.atan2(sortedInertia.getInertiaAxis1().getI() * omegaSorted.getX(),
sortedInertia.getInertiaAxis2().getI() * omegaSorted.getY());
final double theta = FastMath.acos(omegaSorted.getZ() / phiSlope);
final double phiLinear = phiSlope * dt;
// third Euler angle results from a quadrature
final int nbPeriods = (int) FastMath.floor(dt / period);
final double tStartInteg = nbPeriods * period;
final double integPartial = phiQuadratureModel.getInterpolatedState(dt - tStartInteg).getPrimaryState()[0];
final double phiQuadrature = nbPeriods * integOnePeriod + integPartial;
final double phi = phiLinear + phiQuadrature;
// rotation between computation frame (aligned with momentum) and sorted computation frame
// (it is simply the angles equations provided by Landau & Lifchitz)
final Rotation alignedToSorted = new Rotation(RotationOrder.ZXZ, RotationConvention.FRAME_TRANSFORM,
phi, theta, psi);
// combine with offset rotation to get back from regular inertial frame to body frame
final Rotation inertToBody = sortedToBody.applyTo(alignedToSorted.applyTo(inertToAligned));
return new TimeStampedAngularCoordinates(date, inertToBody, omegaBody, accelerationBody);
}
}
/** Torque-free model.
* @param <T> type of the field elements
*/
private class FieldModel <T extends CalculusFieldElement<T>> {
/** Inertia sorted to get a motion about axis 3. */
private final FieldInertia<T> sortedInertia;
/** State scaling factor. */
private final T o1Scale;
/** State scaling factor. */
private final T o2Scale;
/** State scaling factor. */
private final T o3Scale;
/** Jacobi elliptic function. */
private final FieldJacobiElliptic<T> jacobi;
/** Time scaling factor. */
private final T tScale;
/** Time reference for rotation rate. */
private final FieldAbsoluteDate<T> tRef;
/** Offset rotation between initial inertial frame and the frame with moment vector and Z axis aligned. */
private final FieldRotation<T> inertToAligned;
/** Rotation to switch to the converted axes frame. */
private final FieldRotation<T> sortedToBody;
/** Period of rotation rate. */
private final T period;
/** Slope of the linear part of the phi model. */
private final T phiSlope;
/** DenseOutputModel of phi. */
private final FieldDenseOutputModel<T> phiQuadratureModel;
/** Integral part of quadrature model over one period. */
private final T integOnePeriod;
/** Simple constructor.
* @param field field to which elements belong
*/
FieldModel(final Field<T> field) {
final double i1 = inertia.getInertiaAxis1().getI();
final Vector3D a1 = inertia.getInertiaAxis1().getA();
final double i2 = inertia.getInertiaAxis2().getI();
final Vector3D a2 = inertia.getInertiaAxis2().getA();
final double i3 = inertia.getInertiaAxis3().getI();
final Vector3D a3 = inertia.getInertiaAxis3().getA();
final T zero = field.getZero();
final T fI1 = zero.newInstance(i1);
final FieldVector3D<T> fA1 = new FieldVector3D<>(field, a1);
final T fI2 = zero.newInstance(i2);
final FieldVector3D<T> fA2 = new FieldVector3D<>(field, a2);
final T fI3 = zero.newInstance(i3);
final FieldVector3D<T> fA3 = new FieldVector3D<>(field, a3);
// build inertia tensor
final FieldVector3D<T> n1 = fA1.normalize();
final FieldVector3D<T> n2 = fA2.normalize();
final FieldVector3D<T> n3 = Vector3D.dotProduct(Vector3D.crossProduct(a1, a2), a3) > 0 ?
fA3.normalize() : fA3.normalize().negate();
final FieldVector3D<T> omega0 = new FieldVector3D<>(field, initialAttitude.getSpin());
final FieldVector3D<T> m0 = new FieldVector3D<>(fI1.multiply(FieldVector3D.dotProduct(omega0, n1)), n1,
fI2.multiply(FieldVector3D.dotProduct(omega0, n2)), n2,
fI3.multiply(FieldVector3D.dotProduct(omega0, n3)), n3);
// sort axes in increasing moments of inertia order
FieldInertia<T> tmpInertia = new FieldInertia<>(new FieldInertiaAxis<>(fI1, n1),
new FieldInertiaAxis<>(fI2, n2),
new FieldInertiaAxis<>(fI3, n3));
if (tmpInertia.getInertiaAxis1().getI().subtract(tmpInertia.getInertiaAxis2().getI()).getReal() > 0) {
tmpInertia = tmpInertia.swap12();
}
if (tmpInertia.getInertiaAxis2().getI().subtract(tmpInertia.getInertiaAxis3().getI()).getReal() > 0) {
tmpInertia = tmpInertia.swap23();
}
if (tmpInertia.getInertiaAxis1().getI().subtract(tmpInertia.getInertiaAxis2().getI()).getReal() > 0) {
tmpInertia = tmpInertia.swap12();
}
// in order to simplify implementation, we want the motion to be about axis 3
// which is either the minimum or the maximum inertia axis
final T o1 = FieldVector3D.dotProduct(omega0, n1);
final T o2 = FieldVector3D.dotProduct(omega0, n2);
final T o3 = FieldVector3D.dotProduct(omega0, n3);
final T o12 = o1.multiply(o1);
final T o22 = o2.multiply(o2);
final T o32 = o3.multiply(o3);
final T twoE = fI1.multiply(o12).add(fI2.multiply(o22)).add(fI3.multiply(o32));
final T m2 = fI1.multiply(fI1).multiply(o12).add(fI2.multiply(fI2).multiply(o22)).add(fI3.multiply(fI3).multiply(o32));
final T separatrixInertia = (twoE.isZero()) ? zero : m2.divide(twoE);
final boolean clockwise;
if (separatrixInertia.subtract(tmpInertia.getInertiaAxis2().getI()).getReal() < 0) {
// motion is about minimum inertia axis
// we swap axes to put them in decreasing moments order
// motion will be clockwise about axis 3
clockwise = true;
tmpInertia = tmpInertia.swap13();
} else {
// motion is about maximum inertia axis
// we keep axes in increasing moments order
// motion will be counter-clockwise about axis 3
clockwise = false;
}
sortedInertia = tmpInertia;
final T i1C = tmpInertia.getInertiaAxis1().getI();
final T i2C = tmpInertia.getInertiaAxis2().getI();
final T i3C = tmpInertia.getInertiaAxis3().getI();
final T i32 = i3C.subtract(i2C);
final T i31 = i3C.subtract(i1C);
final T i21 = i2C.subtract(i1C);
// convert initial conditions to Euler angles such the M is aligned with Z in sorted computation frame
sortedToBody = new FieldRotation<>(FieldVector3D.getPlusI(field),
FieldVector3D.getPlusJ(field),
tmpInertia.getInertiaAxis1().getA(),
tmpInertia.getInertiaAxis2().getA());
final FieldVector3D<T> omega0Sorted = sortedToBody.applyInverseTo(omega0);
final FieldVector3D<T> m0Sorted = sortedToBody.applyInverseTo(m0);
final T phi0 = zero; // this angle can be set arbitrarily, so 0 is a fair value (Eq. 37.13 - 37.14)
final T theta0 = FastMath.acos(m0Sorted.getZ().divide(m0Sorted.getNorm()));
final T psi0 = FastMath.atan2(m0Sorted.getX(), m0Sorted.getY()); // it is really atan2(x, y), not atan2(y, x) as usual!
// compute offset rotation between inertial frame aligned with momentum and regular inertial frame
final FieldRotation<T> alignedToSorted0 = new FieldRotation<>(RotationOrder.ZXZ, RotationConvention.FRAME_TRANSFORM,
phi0, theta0, psi0);
inertToAligned = alignedToSorted0.
applyInverseTo(sortedToBody.applyInverseTo(new FieldRotation<>(field, initialAttitude.getRotation())));
// Ω is always o1Scale * cn((t-tref) * tScale), o2Scale * sn((t-tref) * tScale), o3Scale * dn((t-tref) * tScale)
tScale = FastMath.copySign(FastMath.sqrt(i32.multiply(m2.subtract(twoE.multiply(i1C))).divide(i1C.multiply(i2C).multiply(i3C))),
clockwise ? omega0Sorted.getZ().negate() : omega0Sorted.getZ());
o1Scale = FastMath.sqrt(twoE.multiply(i3C).subtract(m2).divide(i1C.multiply(i31)));
o2Scale = FastMath.sqrt(twoE.multiply(i3C).subtract(m2).divide(i2C.multiply(i32)));
o3Scale = FastMath.copySign(FastMath.sqrt(m2.subtract(twoE.multiply(i1C)).divide(i3C.multiply(i31))),
omega0Sorted.getZ());
final T k2 = (twoE.isZero()) ?
zero :
i21.multiply(twoE.multiply(i3C).subtract(m2)).
divide(i32.multiply(m2.subtract(twoE.multiply(i1C))));
jacobi = JacobiEllipticBuilder.build(k2);
period = LegendreEllipticIntegral.bigK(k2).multiply(4).divide(tScale);
final T dtRef;
if (o1Scale.isZero()) {
// special case where twoE * i3C = m2, then o2Scale is also zero
// motion is exactly along one axis
dtRef = zero;
} else {
if (FastMath.abs(omega0Sorted.getX()).subtract(FastMath.abs(omega0Sorted.getY())).getReal() >= 0) {
if (omega0Sorted.getX().getReal() >= 0) {
// omega is roughly towards +I
dtRef = jacobi.arcsn(omega0Sorted.getY().divide(o2Scale)).divide(tScale).negate();
} else {
// omega is roughly towards -I
dtRef = jacobi.arcsn(omega0Sorted.getY().divide(o2Scale)).divide(tScale).subtract(period.multiply(0.5));
}
} else {
if (omega0Sorted.getY().getReal() >= 0) {
// omega is roughly towards +J
dtRef = jacobi.arccn(omega0Sorted.getX().divide(o1Scale)).divide(tScale).negate();
} else {
// omega is roughly towards -J
dtRef = jacobi.arccn(omega0Sorted.getX().divide(o1Scale)).divide(tScale);
}
}
}
tRef = new FieldAbsoluteDate<>(field, initialAttitude.getDate()).shiftedBy(dtRef);
phiSlope = FastMath.sqrt(m2).divide(i3C);
phiQuadratureModel = computePhiQuadratureModel(dtRef);
integOnePeriod = phiQuadratureModel.getInterpolatedState(phiQuadratureModel.getFinalTime()).getPrimaryState()[0];
}
/** Compute the model for φ angle.
* @param dtRef start time
* @return model for φ angle
*/
private FieldDenseOutputModel<T> computePhiQuadratureModel(final T dtRef) {
final T zero = dtRef.getField().getZero();
final T i1C = sortedInertia.getInertiaAxis1().getI();
final T i2C = sortedInertia.getInertiaAxis2().getI();
final T i3C = sortedInertia.getInertiaAxis3().getI();
final T i32 = i3C.subtract(i2C);
final T i31 = i3C.subtract(i1C);
final T i21 = i2C.subtract(i1C);
// coefficients for φ model
final T b = phiSlope.multiply(i32).multiply(i31);
final T c = i1C.multiply(i32);
final T d = i3C.multiply(i21);
// integrate the quadrature phi term on one period
final DormandPrince853FieldIntegrator<T> integ = new DormandPrince853FieldIntegrator<>(dtRef.getField(),
1.0e-6 * period.getReal(),
1.0e-2 * period.getReal(),
phiSlope.getReal() * period.getReal() * 1.0e-13,
1.0e-13);
final FieldDenseOutputModel<T> model = new FieldDenseOutputModel<>();
integ.addStepHandler(model);
integ.integrate(new FieldExpandableODE<T>(new FieldOrdinaryDifferentialEquation<T>() {
/** {@inheritDoc} */
@Override
public int getDimension() {
return 1;
}
/** {@inheritDoc} */
@Override
public T[] computeDerivatives(final T t, final T[] y) {
final T sn = jacobi.valuesN(t.subtract(dtRef).multiply(tScale)).sn();
final T[] yDot = MathArrays.buildArray(dtRef.getField(), 1);
yDot[0] = b.divide(c.add(d.multiply(sn).multiply(sn)));
return yDot;
}
}), new FieldODEState<T>(zero, MathArrays.buildArray(dtRef.getField(), 1)), period);
return model;
}
/** Evaluate torque-free motion model.
* @param date evaluation date
* @return body orientation at date
*/
public TimeStampedFieldAngularCoordinates<T> evaluate(final FieldAbsoluteDate<T> date) {
// angular velocity
final FieldCopolarN<T> valuesN = jacobi.valuesN(date.durationFrom(tRef).multiply(tScale));
final FieldVector3D<T> omegaSorted = new FieldVector3D<>(valuesN.cn().multiply(o1Scale),
valuesN.sn().multiply(o2Scale),
valuesN.dn().multiply(o3Scale));
final FieldVector3D<T> omegaBody = sortedToBody.applyTo(omegaSorted);
// acceleration
final FieldVector3D<T> accelerationSorted =
new FieldVector3D<>(o1Scale.multiply(tScale).multiply(valuesN.cn()).multiply(valuesN.dn()),
o2Scale.multiply(tScale).multiply(valuesN.sn().negate()).multiply(valuesN.dn()),
o3Scale.multiply(tScale).multiply(jacobi.getM().negate()).multiply(valuesN.sn()).multiply(valuesN.cn()));
final FieldVector3D<T> accelerationBody = sortedToBody.applyTo(accelerationSorted);
// first Euler angles are directly linked to angular velocity
final T dt = date.durationFrom(initialAttitude.getDate());
final T psi = FastMath.atan2(sortedInertia.getInertiaAxis1().getI().multiply(omegaSorted.getX()),
sortedInertia.getInertiaAxis2().getI().multiply(omegaSorted.getY()));
final T theta = FastMath.acos(omegaSorted.getZ().divide(phiSlope));
final T phiLinear = dt.multiply(phiSlope);
// third Euler angle results from a quadrature
final int nbPeriods = (int) FastMath.floor(dt.divide(period)).getReal();
final T tStartInteg = period.multiply(nbPeriods);
final T integPartial = phiQuadratureModel.getInterpolatedState(dt.subtract(tStartInteg)).getPrimaryState()[0];
final T phiQuadrature = integOnePeriod.multiply(nbPeriods).add(integPartial);
final T phi = phiLinear.add(phiQuadrature);
// rotation between computation frame (aligned with momentum) and sorted computation frame
// (it is simply the angles equations provided by Landau & Lifchitz)
final FieldRotation<T> alignedToSorted = new FieldRotation<>(RotationOrder.ZXZ, RotationConvention.FRAME_TRANSFORM,
phi, theta, psi);
// combine with offset rotation to get back from regular inertial frame to body frame
final FieldRotation<T> inertToBody = sortedToBody.applyTo(alignedToSorted.applyTo(inertToAligned));
return new TimeStampedFieldAngularCoordinates<>(date, inertToBody, omegaBody, accelerationBody);
}
}
}