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.Arrays;
import java.util.Collections;
import org.hipparchus.analysis.differentiation.Gradient;
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 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 AbstractMeasurement<BistaticRange> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "BistaticRange";
/**
* Ground station from which transmission is made.
*/
private final GroundStation emitter;
/**
* Ground station that performs measurement.
*/
private final GroundStation receiver;
/**
* 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) {
super(date, true, range, sigma, baseWeight, Collections.singletonList(satellite));
// Add the parameters for the emitter and receiver
addParametersDrivers(receiver.getParametersDrivers());
addParametersDrivers(emitter.getParametersDrivers());
// Set emitter and receiver values
this.receiver = receiver;
this.emitter = emitter;
}
/** Get the emitter ground station.
* @return emitter ground station
*/
public GroundStation getEmitterStation() {
return emitter;
}
/** Get the receiver ground station.
* @return receiver ground station
*/
public GroundStation getReceiverStation() {
return receiver;
}
/**
* {@inheritDoc}
*/
@Override
protected EstimatedMeasurementBase<BistaticRange> theoreticalEvaluationWithoutDerivatives(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
final CommonParametersWithoutDerivatives common = getReceiverStation().
computeRemoteParametersWithout(states, getSatellites().get(0), getDate(), false);
final TimeStampedPVCoordinates transitPV = common.getTransitPV();
final AbsoluteDate transitDate = transitPV.getDate();
// Uplink time of flight from emitter station to transit state
final PVCoordinatesProvider emitterPVCoordinatesProvider = getEmitterStation().getPVCoordinatesProvider();
final SignalTravelTimeAdjustableEmitter signalTimeOfFlight = new SignalTravelTimeAdjustableEmitter(emitterPVCoordinatesProvider);
final double tauU = signalTimeOfFlight.compute(transitPV.getPosition(), transitDate, states[0].getFrame());
// Secondary station PV in inertial frame at rebound date on secondary station
final TimeStampedPVCoordinates emitterPV = emitterPVCoordinatesProvider.getPVCoordinates(transitDate.shiftedBy(-tauU), states[0].getFrame());
// Prepare the evaluation
final EstimatedMeasurementBase<BistaticRange> estimated =
new EstimatedMeasurementBase<>(this,
iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
common.getRemotePV(),
transitPV,
emitterPV
});
// Clock offsets
final double dte = getEmitterStation().getClockOffsetDriver().getValue(common.getState().getDate());
final double dtr = getReceiverStation().getClockOffsetDriver().getValue(common.getState().getDate());
// Range value
//final double tau = commonR.getTauD() + tauU + dtr - dte;
final double tau = common.getTauD() + tauU + 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) {
final SpacecraftState state = states[0];
// Bistatic range 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 = getReceiverStation().
computeRemoteParametersWith(states, getSatellites().get(0), getDate(), false, getParametersDrivers());
final int nbParams = common.getTauD().getFreeParameters();
final TimeStampedFieldPVCoordinates<Gradient> transitPV = common.getTransitPV();
final FieldAbsoluteDate<Gradient> transitDate = transitPV.getDate();
// Uplink time of flight from emitter to transit state
// states[0].getPVCoordinates(states[0].getFrame()).shiftedBy(transitDate.durationFrom(states[0].getDate()).getValue());
// does not QUITE equal transitPV
// transitPV gradients do NOT equal pvaDownlink gradient once the movement to the proper time is made
final FieldPVCoordinatesProvider<Gradient> emitterPVCoordsProvider = getEmitterStation().getFieldPVCoordinatesProvider(nbParams, common.getIndices());
final FieldSignalTravelTimeAdjustableEmitter<Gradient> fieldComputer = new FieldSignalTravelTimeAdjustableEmitter<>(emitterPVCoordsProvider);
final Gradient tauU = fieldComputer.compute(transitPV.getPosition(), transitPV.getDate(), state.getFrame());
// Emitter coordinates at transmit time
final TimeStampedFieldPVCoordinates<Gradient> emitterPV
= emitterPVCoordsProvider.getPVCoordinates(transitDate, states[0].getFrame()).shiftedBy(tauU.negate());
// Prepare the evaluation
final EstimatedMeasurement<BistaticRange> estimated = new EstimatedMeasurement<>(this,
iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
common.getRemotePV().toTimeStampedPVCoordinates(),
common.getTransitPV().toTimeStampedPVCoordinates(),
emitterPV.toTimeStampedPVCoordinates()
});
// Clock offsets
final Gradient dte = getEmitterStation().getClockOffsetDriver().getValue(nbParams, common.getIndices(), state.getDate());
final Gradient dtr = getReceiverStation().getClockOffsetDriver().getValue(nbParams, common.getIndices(), state.getDate());
// Range value
final Gradient tau = common.getTauD().add(tauU).add(dtr).subtract(dte);
final Gradient range = tau.multiply(Constants.SPEED_OF_LIGHT);
estimated.setEstimatedValue(range.getValue());
// Range first order derivatives with respect to state
final double[] derivatives = range.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;
}
}