TDOA.java
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package org.orekit.estimation.measurements;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.orekit.frames.Frame;
import org.orekit.propagation.SpacecraftState;
import org.orekit.signal.DifferencesOfSignalArrival;
import org.orekit.signal.SignalReceptionCondition;
import org.orekit.signal.SignalTravelTimeModel;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a Time Difference of Arrival measurement with a satellite as emitter
* and two observers 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 sensors and the satellite during the signal flight time are taken into account.
* </p>
* @author Pascal Parraud
* @since 11.2
*/
public class TDOA extends DualReceiverMeasurement<TDOA> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "TDOA";
/** Constructor.
* @param primeObserver observer that gives the measurement date
* @param secondObserver observer that gives the measurement value
* @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 Observer primeObserver, final Observer secondObserver,
final AbsoluteDate date, final double tdoa, final double sigma, final double baseWeight,
final ObservableSatellite satellite) {
this(primeObserver, secondObserver, date, tdoa, new MeasurementQuality(sigma, baseWeight), new SignalTravelTimeModel(), satellite);
}
/** Constructor.
* @param primeObserver observer that gives the measurement date
* @param secondObserver observer that gives the measurement value
* @param date date of the measurement
* @param tdoa observed value (s)
* @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 TDOA(final Observer primeObserver, final Observer secondObserver,
final AbsoluteDate date, final double tdoa, final MeasurementQuality measurementQuality,
final SignalTravelTimeModel signalTravelTimeModel, final ObservableSatellite satellite) {
super(primeObserver, secondObserver, date, new double[] {tdoa}, measurementQuality,
signalTravelTimeModel, satellite);
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<TDOA> theoreticalEvaluationWithoutDerivatives(final int iteration, final int evaluation,
final SpacecraftState[] states) {
final SpacecraftState state = states[0];
final Frame frame = state.getFrame();
// Compute emission and reception dates
final AbsoluteDate firstReceptionDate = getPrimeObserver().getCorrectedReceptionDate(getDate());
final PVCoordinatesProvider emitter = AbstractParticipant.extractPVCoordinatesProvider(state, state.getPVCoordinates());
final DifferencesOfSignalArrival differencesOfSignalArrival = new DifferencesOfSignalArrival(getSignalTravelTimeModel());
final TimeStampedPVCoordinates primePV = getPrimeObserver().getPVCoordinatesProvider().getPVCoordinates(firstReceptionDate, frame);
final SignalReceptionCondition receptionCondition = new SignalReceptionCondition(firstReceptionDate,
primePV.getPosition(), frame);
final double[] delays = differencesOfSignalArrival.computeDelays(receptionCondition,
getSecondObserver().getPVCoordinatesProvider(), emitter);
final AbsoluteDate emissionDate = firstReceptionDate.shiftedBy(-delays[0]);
final AbsoluteDate secondReceptionDate = emissionDate.shiftedBy(delays[1]);
// The measured TDOA is (tau1 + clockOffset1) - (tau2 + clockOffset2)
final double offset1 = getPrimeObserver().getOffsetValue(firstReceptionDate);
final double offset2 = getSecondObserver().getOffsetValue(secondReceptionDate);
final double tdoa = (firstReceptionDate.durationFrom(emissionDate) + offset1) - (secondReceptionDate.durationFrom(emissionDate) + offset2);
// Prepare the evaluation
final TimeStampedPVCoordinates emitterPV = emitter.getPVCoordinates(emissionDate, frame);
final TimeStampedPVCoordinates secondPV = getSecondObserver().getPVCoordinatesProvider().getPVCoordinates(secondReceptionDate, frame);
final EstimatedMeasurement<TDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] { state.shiftedBy(emissionDate.durationFrom(state.getDate())) },
new TimeStampedPVCoordinates[] { emitterPV, tdoa > 0.0 ? secondPV : primePV, tdoa > 0.0 ? primePV : secondPV });
// set TDOA value
estimated.setEstimatedValue(tdoa);
return estimated;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<TDOA> theoreticalEvaluation(final int iteration, final int evaluation,
final SpacecraftState[] states) {
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);
// The measured TDOA is (tau1 + clockOffset1) - (tau2 + clockOffset2)
final Gradient offset1 = getPrimeObserver().getFieldOffsetValue(nbParams, emissionDate.toAbsoluteDate(), paramIndices);
final Gradient offset2 = getSecondObserver().getFieldOffsetValue(nbParams, emissionDate.toAbsoluteDate(), paramIndices);
final Gradient tdoaG = firstDelay.add(offset1).subtract(secondDelay.add(offset2));
final double tdoa = tdoaG.getValue();
// Evaluate the TDOA value and derivatives
final SpacecraftState state = states[0];
final Frame frame = state.getFrame();
final TimeStampedPVCoordinates primePV = getPrimeObserver().getPVCoordinatesProvider().getPVCoordinates(firstReceptionDate.toAbsoluteDate(), frame);
final TimeStampedPVCoordinates secondPV = getSecondObserver().getPVCoordinatesProvider().getPVCoordinates(secondReceptionDate.toAbsoluteDate(), frame);
final SpacecraftState emitterState = state.shiftedBy(emissionDate.toAbsoluteDate().durationFrom(state.getDate()));
final EstimatedMeasurement<TDOA> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] { emitterState },
new TimeStampedPVCoordinates[] { emitterState.getPVCoordinates(), tdoa > 0.0 ? secondPV : primePV, tdoa > 0.0 ? primePV : secondPV });
// set TDOA value
fillEstimation(tdoaG, paramIndices, estimated);
return estimated;
}
}