FDOA.java
/* Copyright 2002-2026 Mark Rutten
* 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.estimation.measurements;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.frames.Frame;
import org.orekit.propagation.SpacecraftState;
import org.orekit.signal.SignalTravelTimeModel;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.PVCoordinates;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a Frequency Difference of Arrival measurement with a satellite as emitter
* and two ground stations as receivers.
* <p>
* FDOA measures the difference in signal arrival frequency between the emitter and receivers,
* corresponding to a difference in range-rate from the two receivers to the emitter.
* </p><p>
* The date of the measurement corresponds to the reception of the signal by the prime station.
* The measurement corresponds to the frequency of the signal received at the prime station at
* the date of the measurement minus the frequency of the signal received at the second station:
* <code>fdoa = f<sub>1</sub> - f<sub>2</sub></code>
* </p><p>
* The motion of the stations and the satellite during the signal flight time are taken into account.
* </p>
* @author Mark Rutten
* @since 12.0
*/
public class FDOA extends DualReceiverMeasurement<FDOA> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "FDOA";
/** Centre frequency of the signal emitted from the satellite. */
private final double centreFrequency;
/** Constructor with default signal travel time model.
* @param primeObserver observer that gives the date of the measurement
* @param secondObserver observer that gives the measurement
* @param centreFrequency satellite emitter frequency (Hz)
* @param date date of the measurement
* @param fdoa observed value (Hz)
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
*/
public FDOA(final Observer primeObserver, final Observer secondObserver,
final double centreFrequency, final AbsoluteDate date, final double fdoa, final double sigma,
final double baseWeight, final ObservableSatellite satellite) {
this(primeObserver, secondObserver, centreFrequency, date, fdoa, new MeasurementQuality(sigma, baseWeight), new SignalTravelTimeModel(),
satellite);
}
/** Constructor.
* @param primeObserver observer that gives the date of the measurement
* @param secondObserver observer that gives the measurement
* @param centreFrequency satellite emitter frequency (Hz)
* @param date date of the measurement
* @param fdoa observed value (Hz)
* @param measurementQuality measurement quality data as used in orbit determination
* @param signalTravelTimeModel signal travel time model
* @param satellite satellite related to this measurement
* @since 14.0
*/
public FDOA(final Observer primeObserver, final Observer secondObserver,
final double centreFrequency, final AbsoluteDate date, final double fdoa,
final MeasurementQuality measurementQuality, final SignalTravelTimeModel signalTravelTimeModel,
final ObservableSatellite satellite) {
super(primeObserver, secondObserver, date, new double[] {fdoa}, measurementQuality,
signalTravelTimeModel, satellite);
this.centreFrequency = centreFrequency;
}
/** Get centre frequency of carrier wave.
* @return frequency value (Hz)
*/
public double getCentreFrequency() {
return centreFrequency;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<FDOA> theoreticalEvaluationWithoutDerivatives(final int iteration, final int evaluation,
final SpacecraftState[] states) {
// Evaluate the TDOA value
final TDOA tdoa = new TDOA(getPrimeObserver(), getSecondObserver(), getDate(), 0., new MeasurementQuality(1),
getSignalTravelTimeModel(), getSatellites().get(0));
final EstimatedMeasurementBase<TDOA> estimatedTdoa = tdoa.theoreticalEvaluationWithoutDerivatives(iteration,
evaluation, states);
// Prepare the FDOA model
final EstimatedMeasurement<FDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation, estimatedTdoa.getStates(),
estimatedTdoa.getParticipants());
// Range-rate components
final PVCoordinates emitterPV = estimated.getParticipants()[0];
final PVCoordinates primePV = estimated.getParticipants()[1];
final PVCoordinates secondPV = estimated.getParticipants()[2];
final Vector3D primeDirection = primePV.getPosition().subtract(emitterPV.getPosition()).normalize();
final Vector3D secondDirection = secondPV.getPosition().subtract(emitterPV.getPosition()).normalize();
final Vector3D primeVelocity = primePV.getVelocity().subtract(emitterPV.getVelocity());
final Vector3D secondVelocity = secondPV.getVelocity().subtract(emitterPV.getVelocity());
// range rate difference
final double rangeRateDifference = Vector3D.dotProduct(primeDirection, primeVelocity) -
Vector3D.dotProduct(secondDirection, secondVelocity);
// set FDOA value
final double rangeRateToHz = -centreFrequency / Constants.SPEED_OF_LIGHT;
estimated.setEstimatedValue(rangeRateDifference * rangeRateToHz);
return estimated;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<FDOA> theoreticalEvaluation(final int iteration, final int evaluation,
final SpacecraftState[] states) {
// Compute emission date and reception ones
final Map<String, Integer> paramIndices = getParameterIndices(states);
final int nbParams = 6 * states.length + paramIndices.size();
final Gradient[] delays = computeDelays(states);
final Gradient firstDelay = delays[0];
final Gradient secondDelay = delays[1];
final FieldAbsoluteDate<Gradient> firstReceptionDate = getPrimeObserver().getCorrectedReceptionDateField(getDate(), nbParams, paramIndices);
final FieldAbsoluteDate<Gradient> emissionDate = firstReceptionDate.shiftedBy(firstDelay.negate());
final FieldAbsoluteDate<Gradient> secondReceptionDate = emissionDate.shiftedBy(secondDelay);
// Prepare the FDOA estimation
final SpacecraftState state = states[0];
final Frame frame = state.getFrame();
final TimeStampedFieldPVCoordinates<Gradient> primePV = getPrimeObserver().getFieldPVCoordinatesProvider(nbParams, paramIndices)
.getPVCoordinates(firstReceptionDate, frame);
final TimeStampedFieldPVCoordinates<Gradient> secondPV = getSecondObserver().getFieldPVCoordinatesProvider(nbParams, paramIndices)
.getPVCoordinates(secondReceptionDate, frame);
final SpacecraftState emitterState = state.shiftedBy(emissionDate.toAbsoluteDate().durationFrom(state.getDate()));
final double tdoa = delays[0].getReal() + delays[1].getReal();
final EstimatedMeasurement<FDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] { emitterState },
new TimeStampedPVCoordinates[] { emitterState.getPVCoordinates(),
tdoa > 0.0 ? secondPV.toTimeStampedPVCoordinates() : primePV.toTimeStampedPVCoordinates(),
tdoa > 0.0 ? primePV.toTimeStampedPVCoordinates() : secondPV.toTimeStampedPVCoordinates() });
// Range-rate components
final TimeStampedFieldPVCoordinates<Gradient> pva = AbstractMeasurement.getCoordinates(state, 0, nbParams);
final TimeStampedFieldPVCoordinates<Gradient> emitterPV = AbstractParticipant.extractFieldPVCoordinatesProvider(state, pva)
.getPVCoordinates(emissionDate, frame);
final FieldVector3D<Gradient> primeDirection = primePV.getPosition().subtract(emitterPV.getPosition()).normalize();
final FieldVector3D<Gradient> secondDirection = secondPV.getPosition().subtract(emitterPV.getPosition()).normalize();
final FieldVector3D<Gradient> primeVelocity = primePV.getVelocity().subtract(emitterPV.getVelocity());
final FieldVector3D<Gradient> secondVelocity = secondPV.getVelocity().subtract(emitterPV.getVelocity());
// range rate difference
final Gradient rangeRateDifference = FieldVector3D.dotProduct(primeDirection, primeVelocity)
.subtract(FieldVector3D.dotProduct(secondDirection, secondVelocity));
// set FDOA value
final double rangeRateToHz = -centreFrequency / Constants.SPEED_OF_LIGHT;
final Gradient fdoa = rangeRateDifference.multiply(rangeRateToHz);
fillEstimation(fdoa, getParameterIndices(states), estimated);
return estimated;
}
}