BistaticRange.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.
* Mark Rutten 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.analysis.differentiation.GradientField;
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.Constants;
import org.orekit.utils.TimeStampedPVCoordinates;
/**
* Class modeling a bistatic range 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.
* <p>
* The motion of the stations and the spacecraft during the signal flight time are taken into account.
* </p>
*
* @author Mark Rutten
* @since 11.2
*/
public class BistaticRange extends BistaticRangeRelatedMeasurement<BistaticRange> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "BistaticRange";
/**
* Simple constructor.
*
* @param emitter ground station from which transmission is performed
* @param receiver ground station from which measurement is performed
* @param date date of the measurement
* @param range observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
* @since 11.2
*/
public BistaticRange(final GroundStation emitter, final GroundStation receiver, final AbsoluteDate date,
final double range, final double sigma, final double baseWeight,
final ObservableSatellite satellite) {
this(emitter, receiver, date, range, sigma, baseWeight, new SignalTravelTimeModel(), satellite);
}
/**
* Simple constructor.
*
* @param emitter ground station from which transmission is performed
* @param receiver ground station from which measurement is performed
* @param date date of the measurement
* @param range observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param signalTravelTimeModel signal travel time model
* @param satellite satellite related to this measurement
* @since 14.0
*/
public BistaticRange(final GroundStation emitter, final GroundStation receiver, final AbsoluteDate date,
final double range, final double sigma, final double baseWeight,
final SignalTravelTimeModel signalTravelTimeModel,
final ObservableSatellite satellite) {
super(emitter, receiver, true, date, new double[] { range }, new double[] { sigma }, new double[] { baseWeight },
signalTravelTimeModel, satellite);
}
/**
* {@inheritDoc}
*/
@Override
protected EstimatedMeasurementBase<BistaticRange> theoreticalEvaluationWithoutDerivatives(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// Compute participants (position-velocities at signal transmissions)
final SpacecraftState state = states[0];
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<BistaticRange> estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
new SpacecraftState[] { transitState }, participants);
// Clock offsets
final double dte = getEmitterStation().getClockOffsetDriver().getValue(emissionDate);
final double dtr = getReceiverStation().getClockOffsetDriver().getValue(receptionDate);
// Range value
final double firstLegDelay = transitDate.durationFrom(emissionDate);
final double secondLegDelay = receptionDate.durationFrom(transitDate);
final double tau = firstLegDelay + secondLegDelay + dtr - dte;
final double range = tau * Constants.SPEED_OF_LIGHT;
estimated.setEstimatedValue(range);
return estimated;
}
/**
* {@inheritDoc}
*/
@Override
protected EstimatedMeasurement<BistaticRange> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// 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 EstimatedMeasurement<BistaticRange> estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] { transitState },
new TimeStampedPVCoordinates[] {
getEmitterStation().getPVCoordinatesProvider().getPVCoordinates(emissionDate.toAbsoluteDate(), frame),
transitState.getPVCoordinates(),
getReceiverStation().getPVCoordinatesProvider().getPVCoordinates(receptionDate.toAbsoluteDate(), frame)});
// Clock offsets
final int nbParams = field.getZero().getFreeParameters();
final Map<String, Integer> paramIndices = getParameterIndices(states);
final Gradient dte = getEmitterStation().getClockOffsetDriver().getValue(nbParams, paramIndices, emissionDate.toAbsoluteDate());
final Gradient dtr = getReceiverStation().getClockOffsetDriver().getValue(nbParams, paramIndices, receptionDate.toAbsoluteDate());
// Range value
final Gradient tau = (shifts[1].add(shifts[2])).negate().add(dtr).subtract(dte);
final Gradient range = tau.multiply(Constants.SPEED_OF_LIGHT);
fillEstimation(range, paramIndices, estimated);
return estimated;
}
}