BistaticRangeRate.java
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package org.orekit.estimation.measurements;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.analysis.differentiation.GradientField;
import org.orekit.estimation.measurements.model.TwoLegsRangeRateModel;
import org.orekit.estimation.measurements.signal.SignalTravelTimeModel;
import org.orekit.frames.Frame;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.FieldPVCoordinatesProvider;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a bistatic range rate measurement using
* an emitter ground station and a receiver ground station.
* <p>
* The measurement is considered to be a signal:
* <ul>
* <li>Emitted from the emitter ground station</li>
* <li>Reflected on the spacecraft</li>
* <li>Received on the receiver ground station</li>
* </ul>
* The date of the measurement corresponds to the reception on ground of the reflected signal.
* The quantity measured at the receiver is the bistatic radial velocity as the sum of the radial
* velocities with respect to the two stations.
* <p>
* The motion of the stations and the spacecraft during the signal flight time are taken into account.
* </p><p>
* The Doppler measurement can be obtained by multiplying the velocity by (fe/c), where
* fe is the emission frequency.
* </p>
*
* @author Pascal Parraud
* @since 11.2
*/
public class BistaticRangeRate extends BistaticRangeRelatedMeasurement<BistaticRangeRate> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "BistaticRangeRate";
/** Range-rate measurement model. */
private final TwoLegsRangeRateModel twoLegsRangeRateModel;
/** Simple constructor.
* @param emitter emitter ground station
* @param receiver receiver ground station
* @param date date of the measurement
* @param rangeRate observed value, m/s
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
*/
public BistaticRangeRate(final GroundStation emitter, final GroundStation receiver, final AbsoluteDate date,
final double rangeRate, final double sigma, final double baseWeight,
final ObservableSatellite satellite) {
this(emitter, receiver, date, rangeRate, sigma, baseWeight, new SignalTravelTimeModel(), satellite);
}
/** Simple constructor.
* @param emitter emitter ground station
* @param receiver receiver ground station
* @param date date of the measurement
* @param rangeRate observed value, m/s
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param signalTravelTimeModel signal travel time model
* @param satellite satellite related to this measurement
*/
public BistaticRangeRate(final GroundStation emitter, final GroundStation receiver, final AbsoluteDate date,
final double rangeRate, final double sigma, final double baseWeight,
final SignalTravelTimeModel signalTravelTimeModel, final ObservableSatellite satellite) {
super(emitter, receiver, true, date, new double[] {rangeRate}, new double[] {sigma}, new double[] {baseWeight},
signalTravelTimeModel, satellite);
// Add class member values
this.twoLegsRangeRateModel = new TwoLegsRangeRateModel(getTwoLegsSignalTimer());
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<BistaticRangeRate> theoreticalEvaluationWithoutDerivatives(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// Compute participants (position-velocities at signal transmissions)
final SpacecraftState state = states[0];
final Frame frame = state.getFrame();
final TimeStampedPVCoordinates[] participants = getParticipants(state);
// Extract dates
final AbsoluteDate emissionDate = participants[0].getDate();
final AbsoluteDate transitDate = participants[1].getDate();
final AbsoluteDate receptionDate = participants[2].getDate();
// Prepare the evaluation
final double shift = transitDate.durationFrom(state);
final SpacecraftState transitState = state.shiftedBy(shift);
final EstimatedMeasurementBase<BistaticRangeRate> estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
new SpacecraftState[] { transitState }, participants);
// Compute range rate
final PVCoordinatesProvider observablePVCoordinates = AbstractMeasurementObject.extractPVCoordinatesProvider(state,
state.getPVCoordinates());
final double rangeRate = twoLegsRangeRateModel.value(frame, participants[2], receptionDate,
observablePVCoordinates, transitDate, getEmitterStation().getPVCoordinatesProvider(), emissionDate);
estimated.setEstimatedValue(rangeRate);
return estimated;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<BistaticRangeRate> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// Bistatic range-rate 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...)
// Compute time shifts w.r.t. recorded observation date
final Gradient[] shifts = getFieldShifts(states);
final GradientField field = shifts[0].getField();
final FieldAbsoluteDate<Gradient> receptionDate = new FieldAbsoluteDate<>(field, getDate()).shiftedBy(shifts[0]);
final FieldAbsoluteDate<Gradient> transitDate = receptionDate.shiftedBy(shifts[1]);
final FieldAbsoluteDate<Gradient> emissionDate = transitDate.shiftedBy(shifts[2]);
// Prepare the evaluation
final SpacecraftState state = states[0];
final Frame frame = state.getFrame();
final double shift = transitDate.toAbsoluteDate().durationFrom(state);
final SpacecraftState transitState = state.shiftedBy(shift);
final int nbParams = field.getZero().getFreeParameters();
final Map<String, Integer> paramIndices = getParameterIndices(states);
final FieldPVCoordinatesProvider<Gradient> receiver = getReceiverStation().getFieldPVCoordinatesProvider(nbParams,
paramIndices);
final TimeStampedFieldPVCoordinates<Gradient> receiverPV = receiver.getPVCoordinates(receptionDate, frame);
final EstimatedMeasurement<BistaticRangeRate> estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] { transitState },
new TimeStampedPVCoordinates[] {
getEmitterStation().getPVCoordinatesProvider().getPVCoordinates(emissionDate.toAbsoluteDate(), frame),
transitState.getPVCoordinates(),
receiverPV.toTimeStampedPVCoordinates()});
// Compute range rate
final FieldPVCoordinatesProvider<Gradient> emitter = getEmitterStation().getFieldPVCoordinatesProvider(nbParams,
paramIndices);
final FieldPVCoordinatesProvider<Gradient> observable = AbstractMeasurementObject.extractFieldPVCoordinatesProvider(state,
AbstractMeasurement.getCoordinates(state, 0, nbParams));
final Gradient rangeRate = twoLegsRangeRateModel.value(frame, receiverPV, receptionDate, observable,
transitDate, emitter, emissionDate);
fillEstimation(rangeRate, getParameterIndices(states), estimated);
return estimated;
}
}