Position.java
/* Copyright 2002-2024 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,
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* See the License for the specific language governing permissions and
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*/
package org.orekit.estimation.measurements;
import java.util.Collections;
import org.hipparchus.exception.LocalizedCoreFormats;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.orekit.errors.OrekitException;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a position only measurement.
* <p>
* For position-velocity measurement see {@link PV}.
* </p>
* @see PV
* @author Luc Maisonobe
* @since 9.3
*/
public class Position extends AbstractMeasurement<Position> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "Position";
/** Identity matrix, for states derivatives. */
private static final double[][] IDENTITY = new double[][] {
{
1, 0, 0, 0, 0, 0
}, {
0, 1, 0, 0, 0, 0
}, {
0, 0, 1, 0, 0, 0
}
};
/** Covariance matrix of the position only measurement (size 3x3). */
private final double[][] covarianceMatrix;
/** Constructor with one double for the standard deviation.
* <p>The double is the position's standard deviation, common to the 3 position's components.</p>
* <p>
* The measurement must be in the orbit propagation frame.
* </p>
* @param date date of the measurement
* @param position position
* @param sigmaPosition theoretical standard deviation on position components
* @param baseWeight base weight
* @param satellite satellite related to this measurement
* @since 9.3
*/
public Position(final AbsoluteDate date, final Vector3D position,
final double sigmaPosition, final double baseWeight,
final ObservableSatellite satellite) {
this(date, position,
new double[] {
sigmaPosition,
sigmaPosition,
sigmaPosition
}, baseWeight, satellite);
}
/** Constructor with one vector for the standard deviation.
* <p>The 3-sized vector represents the square root of the diagonal elements of the covariance matrix.</p>
* <p>The measurement must be in the orbit propagation frame.</p>
* @param date date of the measurement
* @param position position
* @param sigmaPosition 3-sized vector of the standard deviations of the position
* @param baseWeight base weight
* @param satellite satellite related to this measurement
* @since 9.3
*/
public Position(final AbsoluteDate date, final Vector3D position,
final double[] sigmaPosition, final double baseWeight, final ObservableSatellite satellite) {
this(date, position, buildPvCovarianceMatrix(sigmaPosition), baseWeight, satellite);
}
/** Constructor with full covariance matrix and all inputs.
* <p>The fact that the covariance matrix is symmetric and positive definite is not checked.</p>
* <p>The measurement must be in the orbit propagation frame.</p>
* @param date date of the measurement
* @param position position
* @param covarianceMatrix 3x3 covariance matrix of the position only measurement
* @param baseWeight base weight
* @param satellite satellite related to this measurement
* @since 9.3
*/
public Position(final AbsoluteDate date, final Vector3D position,
final double[][] covarianceMatrix, final double baseWeight,
final ObservableSatellite satellite) {
super(date,
new double[] {
position.getX(), position.getY(), position.getZ()
}, extractSigmas(covarianceMatrix),
new double[] {
baseWeight, baseWeight, baseWeight
}, Collections.singletonList(satellite));
this.covarianceMatrix = covarianceMatrix.clone();
}
/** Get the position.
* @return position
*/
public Vector3D getPosition() {
final double[] pv = getObservedValue();
return new Vector3D(pv[0], pv[1], pv[2]);
}
/** Get the covariance matrix.
* @return the covariance matrix
*/
public double[][] getCovarianceMatrix() {
return covarianceMatrix.clone();
}
/** Get the correlation coefficients matrix.
* <p>This is the 3x3 matrix M such that:
* <p>Mij = Pij/(σi.σj)
* <p>Where:
* <ul>
* <li>P is the covariance matrix
* <li>σi is the i-th standard deviation (σi² = Pii)
* </ul>
* @return the correlation coefficient matrix (3x3)
*/
public double[][] getCorrelationCoefficientsMatrix() {
// Get the standard deviations
final double[] sigmas = getTheoreticalStandardDeviation();
// Initialize the correlation coefficients matric to the covariance matrix
final double[][] corrCoefMatrix = new double[sigmas.length][sigmas.length];
// Divide by the standard deviations
for (int i = 0; i < sigmas.length; i++) {
for (int j = 0; j < sigmas.length; j++) {
corrCoefMatrix[i][j] = covarianceMatrix[i][j] / (sigmas[i] * sigmas[j]);
}
}
return corrCoefMatrix;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<Position> theoreticalEvaluationWithoutDerivatives(final int iteration, final int evaluation,
final SpacecraftState[] states) {
// PV value
final TimeStampedPVCoordinates pv = states[0].getPVCoordinates();
// prepare the evaluation
final EstimatedMeasurementBase<Position> estimated =
new EstimatedMeasurementBase<>(this, iteration, evaluation, states,
new TimeStampedPVCoordinates[] {
pv
});
estimated.setEstimatedValue(new double[] {
pv.getPosition().getX(), pv.getPosition().getY(), pv.getPosition().getZ()
});
return estimated;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<Position> theoreticalEvaluation(final int iteration, final int evaluation,
final SpacecraftState[] states) {
// PV value
final TimeStampedPVCoordinates pv = states[0].getPVCoordinates();
// prepare the evaluation
final EstimatedMeasurement<Position> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation, states,
new TimeStampedPVCoordinates[] {
pv
});
estimated.setEstimatedValue(new double[] {
pv.getPosition().getX(), pv.getPosition().getY(), pv.getPosition().getZ()
});
// partial derivatives with respect to state
estimated.setStateDerivatives(0, IDENTITY);
return estimated;
}
/** Extract standard deviations from a 3x3 position covariance matrix.
* Check the size of the position covariance matrix first.
* @param pCovarianceMatrix the 3x" position covariance matrix
* @return the standard deviations (3-sized vector), they are
* the square roots of the diagonal elements of the covariance matrix in input.
*/
private static double[] extractSigmas(final double[][] pCovarianceMatrix) {
// Check the size of the covariance matrix, should be 3x3
if (pCovarianceMatrix.length != 3 || pCovarianceMatrix[0].length != 3) {
throw new OrekitException(LocalizedCoreFormats.DIMENSIONS_MISMATCH_2x2,
pCovarianceMatrix.length, pCovarianceMatrix[0],
3, 3);
}
// Extract the standard deviations (square roots of the diagonal elements)
final double[] sigmas = new double[3];
for (int i = 0; i < sigmas.length; i++) {
sigmas[i] = FastMath.sqrt(pCovarianceMatrix[i][i]);
}
return sigmas;
}
/** Build a 3x3 position covariance matrix from a 3-sized vector (position standard deviations).
* Check the size of the vector first.
* @param sigmaP 3-sized vector with position standard deviations
* @return the 3x3 position covariance matrix
*/
private static double[][] buildPvCovarianceMatrix(final double[] sigmaP) {
// Check the size of the vector first
if (sigmaP.length != 3) {
throw new OrekitException(LocalizedCoreFormats.DIMENSIONS_MISMATCH, sigmaP.length, 3);
}
// Build the 3x3 position covariance matrix
final double[][] pvCovarianceMatrix = new double[3][3];
for (int i = 0; i < sigmaP.length; i++) {
pvCovarianceMatrix[i][i] = sigmaP[i] * sigmaP[i];
}
return pvCovarianceMatrix;
}
}