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.Arrays;
import java.util.Collections;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.FieldPVCoordinatesProvider;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeSpanMap.Span;
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 AbstractMeasurement<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;
/** First ground station to receiver the measurement. */
private final GroundStation primeStation;
/** Second ground station, the one that gives the measurement, i.e. the delay. */
private final GroundStation secondStation;
/** Simple constructor.
* @param primeStation ground station that gives the date of the measurement
* @param secondStation ground station 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 GroundStation primeStation, final GroundStation secondStation,
final double centreFrequency,
final AbsoluteDate date, final double fdoa, final double sigma,
final double baseWeight, final ObservableSatellite satellite) {
super(date, false, fdoa, sigma, baseWeight, Collections.singletonList(satellite));
// add parameter drivers for the secondary station
addParametersDrivers(primeStation.getParametersDrivers());
addParametersDrivers(secondStation.getParametersDrivers());
// Set values
this.primeStation = primeStation;
this.secondStation = secondStation;
this.centreFrequency = centreFrequency;
}
/** Get centre frequency of carrier wave.
* @return frequency value (Hz)
*/
public double getCentreFrequency() {
return centreFrequency;
}
/** Get the prime ground station, the one that receives the signal first.
* @return prime ground station
*/
public GroundStation getPrimeStation() {
return primeStation;
}
/** Get the second ground station, the one that gives the measurement.
* @return second ground station
*/
public GroundStation getSecondStation() {
return secondStation;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<FDOA> theoreticalEvaluationWithoutDerivatives(final int iteration, final int evaluation,
final SpacecraftState[] states) {
final CommonParametersWithoutDerivatives common =
getPrimeStation().computeRemoteParametersWithout(states, getSatellites().get(0), getDate(), false);
final TimeStampedPVCoordinates emitterPV = common.getTransitPV();
final AbsoluteDate emitterDate = emitterPV.getDate();
// Time of flight from emitter to second station
final PVCoordinatesProvider secondPVCoordinatesProvider = getSecondStation().getPVCoordinatesProvider();
final SignalTravelTimeAdjustableReceiver signalTimeOfFlight = new SignalTravelTimeAdjustableReceiver(secondPVCoordinatesProvider);
final double tau2 = signalTimeOfFlight.compute(emitterPV.getPosition(), emitterDate, states[0].getFrame());
// Secondary station PV in inertial frame at receive at second station
final TimeStampedPVCoordinates secondPV = secondPVCoordinatesProvider.getPVCoordinates(emitterDate.shiftedBy(tau2), states[0].getFrame());
// The measured TDOA is (tau1 + clockOffset1) - (tau2 + clockOffset2)
final double offset1 = getPrimeStation().getClockOffsetDriver().getValue(emitterDate);
final double offset2 = getSecondStation().getClockOffsetDriver().getValue(emitterDate);
final double tdoa = (common.getTauD() + offset1) - (tau2 + offset2);
// Evaluate the FDOA value
// -------------------------------------------
final EstimatedMeasurement<FDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
emitterPV,
tdoa > 0.0 ? secondPV : common.getRemotePV(),
tdoa > 0.0 ? common.getRemotePV() : secondPV
});
// Range-rate components
final Vector3D primeDirection = common.getRemotePV().getPosition()
.subtract(emitterPV.getPosition()).normalize();
final Vector3D secondDirection = secondPV.getPosition()
.subtract(emitterPV.getPosition()).normalize();
final Vector3D primeVelocity = common.getRemotePV().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) {
// FDOA derivatives are computed with respect to spacecraft state in inertial frame
// and station parameters
// ----------------------
//
// Parameters:
// - 0..2 - Position of the spacecraft in inertial frame
// - 3..5 - Velocity of the spacecraft in inertial frame
// - 6..n - measurements parameters (clock offset, station offsets, pole, prime meridian, sat clock offset...)
final CommonParametersWithDerivatives common =
getPrimeStation().computeRemoteParametersWith(states, getSatellites().get(0), getDate(), false, getParametersDrivers());
final int nbParams = common.getTauD().getFreeParameters();
final TimeStampedFieldPVCoordinates<Gradient> emitterPV = common.getTransitPV();
final FieldAbsoluteDate<Gradient> emitterDate = emitterPV.getDate();
// Obtain time at which signal arrives at second station from emitter
final FieldPVCoordinatesProvider<Gradient> fieldPvCoordinatesProvider = getSecondStation().getFieldPVCoordinatesProvider(nbParams, common.getIndices());
final FieldSignalTravelTimeAdjustableReceiver<Gradient> fieldComputer = new FieldSignalTravelTimeAdjustableReceiver<Gradient>(fieldPvCoordinatesProvider);
final Gradient tau2 = fieldComputer.compute(emitterPV.getPosition(), emitterDate, emitterDate, states[0].getFrame());
// Second station coordinates at receive time
final TimeStampedFieldPVCoordinates<Gradient> secondPV =
fieldPvCoordinatesProvider.getPVCoordinates(emitterDate.shiftedBy(tau2), states[0].getFrame());
// The measured TDOA is (tau1 + clockOffset1) - (tau2 + clockOffset2)
final Gradient offset1 = getPrimeStation().getClockOffsetDriver()
.getValue(nbParams, common.getIndices(), emitterDate.toAbsoluteDate());
final Gradient offset2 = getSecondStation().getClockOffsetDriver()
.getValue(nbParams, common.getIndices(), emitterDate.toAbsoluteDate());
final Gradient tdoaG = common.getTauD().add(offset1).subtract(tau2.add(offset2));
final double tdoa = tdoaG.getValue();
// Evaluate the TDOA value and derivatives
// -------------------------------------------
final TimeStampedPVCoordinates pv1 = common.getRemotePV().toTimeStampedPVCoordinates();
final TimeStampedPVCoordinates pv2 = secondPV.toTimeStampedPVCoordinates();
final EstimatedMeasurement<FDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
emitterPV.toTimeStampedPVCoordinates(),
tdoa > 0 ? pv2 : pv1,
tdoa > 0 ? pv1 : pv2
});
// Range-rate components
final FieldVector3D<Gradient> primeDirection = common.getRemotePV().getPosition()
.subtract(emitterPV.getPosition()).normalize();
final FieldVector3D<Gradient> secondDirection = secondPV.getPosition()
.subtract(emitterPV.getPosition()).normalize();
final FieldVector3D<Gradient> primeVelocity = common.getRemotePV().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);
estimated.setEstimatedValue(fdoa.getValue());
// Range first order derivatives with respect to state
final double[] derivatives = fdoa.getGradient();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
// set first order derivatives with respect to parameters
for (final ParameterDriver driver : getParametersDrivers()) {
for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
final Integer index = common.getIndices().get(span.getData());
if (index != null) {
estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
}
}
}
return estimated;
}
}