TDOA.java
/* Copyright 2002-2022 CS GROUP
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* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.estimation.measurements;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.analysis.differentiation.GradientField;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.util.FastMath;
import org.orekit.frames.FieldTransform;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a Time Difference of Arrival measurement with a satellite as emitter
* and two ground stations as receivers.
* <p>
* TDOA measures the difference in signal arrival time between the emitter and receivers,
* corresponding to a difference in ranges 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 date of the measurement minus
* the date of reception of the signal by the second station:
* <code>tdoa = tr<sub>1</sub> - tr<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 Pascal Parraud
* @since 11.2
*/
public class TDOA extends AbstractMeasurement<TDOA> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "TDOA";
/** Prime ground station, the one that gives the date of 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 date date of the measurement
* @param tdoa observed value (s)
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
*/
public TDOA(final GroundStation primeStation, final GroundStation secondStation,
final AbsoluteDate date, final double tdoa, final double sigma,
final double baseWeight, final ObservableSatellite satellite) {
super(date, tdoa, sigma, baseWeight, Collections.singletonList(satellite));
// add parameter drivers for the primary station
addParameterDriver(primeStation.getClockOffsetDriver());
addParameterDriver(primeStation.getEastOffsetDriver());
addParameterDriver(primeStation.getNorthOffsetDriver());
addParameterDriver(primeStation.getZenithOffsetDriver());
addParameterDriver(primeStation.getPrimeMeridianOffsetDriver());
addParameterDriver(primeStation.getPrimeMeridianDriftDriver());
addParameterDriver(primeStation.getPolarOffsetXDriver());
addParameterDriver(primeStation.getPolarDriftXDriver());
addParameterDriver(primeStation.getPolarOffsetYDriver());
addParameterDriver(primeStation.getPolarDriftYDriver());
// add parameter drivers for the secondary station
addParameterDriver(secondStation.getClockOffsetDriver());
addParameterDriver(secondStation.getEastOffsetDriver());
addParameterDriver(secondStation.getNorthOffsetDriver());
addParameterDriver(secondStation.getZenithOffsetDriver());
addParameterDriver(secondStation.getPrimeMeridianOffsetDriver());
addParameterDriver(secondStation.getPrimeMeridianDriftDriver());
addParameterDriver(secondStation.getPolarOffsetXDriver());
addParameterDriver(secondStation.getPolarDriftXDriver());
addParameterDriver(secondStation.getPolarOffsetYDriver());
addParameterDriver(secondStation.getPolarDriftYDriver());
this.primeStation = primeStation;
this.secondStation = secondStation;
}
/** Get the prime ground station, the one that gives the date of the measurement.
* @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 EstimatedMeasurement<TDOA> theoreticalEvaluation(final int iteration, final int evaluation,
final SpacecraftState[] states) {
final SpacecraftState state = states[0];
// TDOA derivatives are computed with respect to:
// - Spacecraft state in inertial frame
// - Prime station parameters
// - Second station parameters
// --------------------------
// - 0..2 - Position of the spacecraft in inertial frame
// - 3..5 - Velocity of the spacecraft in inertial frame
// - 6..n - stations' parameters (clock offset, station offsets, pole, prime meridian...)
int nbParams = 6;
final Map<String, Integer> indices = new HashMap<>();
for (ParameterDriver driver : getParametersDrivers()) {
// we have to check for duplicate keys because primary and secondary station share
// pole and prime meridian parameters names that must be considered
// as one set only (they are combined together by the estimation engine)
if (driver.isSelected() && !indices.containsKey(driver.getName())) {
indices.put(driver.getName(), nbParams++);
}
}
final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));
// coordinates of the spacecraft as a gradient
final TimeStampedFieldPVCoordinates<Gradient> pvaG = getCoordinates(state, 0, nbParams);
// transform between prime station frame and inertial frame
// at the real date of measurement, i.e. taking station clock offset into account
final FieldTransform<Gradient> primeToInert =
primeStation.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
final FieldAbsoluteDate<Gradient> measurementDateG = primeToInert.getFieldDate();
// prime station PV in inertial frame at the real date of the measurement
final TimeStampedFieldPVCoordinates<Gradient> primePV =
primeToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(measurementDateG,
zero, zero, zero));
// compute downlink delay from emitter to prime receiver
final Gradient tau1 = signalTimeOfFlight(pvaG, primePV.getPosition(), measurementDateG);
// elapsed time between state date and signal arrival to the prime receiver
final Gradient dtMtau1 = measurementDateG.durationFrom(state.getDate()).subtract(tau1);
// satellite state at signal emission
final SpacecraftState emitterState = state.shiftedBy(dtMtau1.getValue());
// satellite pv at signal emission (re)computed with gradient
final TimeStampedFieldPVCoordinates<Gradient> emitterPV = pvaG.shiftedBy(dtMtau1);
// second station PV in inertial frame at real date of signal reception
TimeStampedFieldPVCoordinates<Gradient> secondPV;
// initialize search loop of the reception date by second station
Gradient tau2 = tau1;
double delta;
int count = 0;
do {
final double previous = tau2.getValue();
// date of signal arrival on second receiver
final AbsoluteDate dateAt2 = emitterState.getDate().shiftedBy(previous);
// transform between second station frame and inertial frame
// at the date of signal arrival, taking clock offset into account
final FieldTransform<Gradient> secondToInert =
secondStation.getOffsetToInertial(state.getFrame(), dateAt2,
nbParams, indices);
// second receiver position in inertial frame at the real date of signal reception
secondPV = secondToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(secondToInert.getFieldDate(),
zero, zero, zero));
// downlink delay from emitter to second receiver
tau2 = linkDelay(emitterPV.getPosition(), secondPV.getPosition());
// Change in the computed downlink delay
delta = FastMath.abs(tau2.getValue() - previous);
} while (count++ < 10 && delta >= 2 * FastMath.ulp(tau2.getValue()));
// The measured TDOA is (tau1 + clockOffset1) - (tau2 + clockOffset2)
final Gradient offset1 = primeStation.getClockOffsetDriver().getValue(nbParams, indices);
final Gradient offset2 = secondStation.getClockOffsetDriver().getValue(nbParams, indices);
final Gradient tdoaG = tau1.add(offset1).subtract(tau2.add(offset2));
final double tdoa = tdoaG.getValue();
// Evaluate the TDOA value and derivatives
// -------------------------------------------
final TimeStampedPVCoordinates pv1 = primePV.toTimeStampedPVCoordinates();
final TimeStampedPVCoordinates pv2 = secondPV.toTimeStampedPVCoordinates();
final EstimatedMeasurement<TDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
emitterState
},
new TimeStampedPVCoordinates[] {
emitterPV.toTimeStampedPVCoordinates(),
tdoa > 0 ? pv2 : pv1,
tdoa > 0 ? pv1 : pv2
});
// set TDOA value
estimated.setEstimatedValue(tdoa);
// set partial derivatives with respect to state
final double[] derivatives = tdoaG.getGradient();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
// set partial derivatives with respect to parameters
for (final ParameterDriver driver : getParametersDrivers()) {
final Integer index = indices.get(driver.getName());
if (index != null) {
estimated.setParameterDerivatives(driver, derivatives[index]);
}
}
return estimated;
}
/** Compute propagation delay on a link.
* @param emitter the position of the emitter
* @param receiver the position of the receiver (same frame as emitter)
* @return the propagation delay
*/
private Gradient linkDelay(final FieldVector3D<Gradient> emitter,
final FieldVector3D<Gradient> receiver) {
return receiver.distance(emitter).divide(Constants.SPEED_OF_LIGHT);
}
}