BistaticRangeRelatedMeasurement.java
/* Copyright 2022-2026 Romain Serra
* 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.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.orekit.estimation.measurements.signal.SignalTravelTimeModel;
import org.orekit.estimation.measurements.signal.TwoLegsSignalTravelTimer;
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.ParameterDriver;
import org.orekit.utils.TimeSpanMap.Span;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/**
* Class modeling a bistatic 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 Romain Serra
* @since 14.0
*/
abstract class BistaticRangeRelatedMeasurement<T extends GroundReceiverMeasurement<T>> extends GroundReceiverMeasurement<T> {
/**
* Ground station from which transmission is made.
*/
private final GroundStation emitter;
/** Two-way signal model .*/
private final TwoLegsSignalTravelTimer twoLegsSignalTimer;
/**
* Simple constructor.
*
* @param emitter ground station from which transmission is performed
* @param receiver ground station from which measurement is performed
* @param twoWay flag on two-way type
* @param date date of the measurement
* @param value 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
*/
protected BistaticRangeRelatedMeasurement(final GroundStation emitter, final GroundStation receiver,
final boolean twoWay, final AbsoluteDate date,
final double[] value, final double[] sigma, final double[] baseWeight,
final SignalTravelTimeModel signalTravelTimeModel,
final ObservableSatellite satellite) {
super(receiver, twoWay, date, value, sigma, baseWeight, signalTravelTimeModel, satellite);
// Add the parameters for the receiver
addParametersDrivers(emitter.getParametersDrivers());
// Set emitter
this.emitter = emitter;
this.twoLegsSignalTimer = new TwoLegsSignalTravelTimer(signalTravelTimeModel);
}
/**
* Getter for the two legs timer.
* @return model
*/
public TwoLegsSignalTravelTimer getTwoLegsSignalTimer() {
return twoLegsSignalTimer;
}
/** Get the emitter ground station.
* @return emitter ground station
*/
public GroundStation getEmitterStation() {
return emitter;
}
/**
* Method returning the full kinematic coordinates of signal participants at transmission dates.
* @param state observable state
* @return signal participants
*/
protected TimeStampedPVCoordinates[] getParticipants(final SpacecraftState state) {
// Compute actual reception date
final AbsoluteDate receptionDate = getCorrectedReceptionDate();
// Compute light time delays
final PVCoordinatesProvider receiverPVProvider = getReceiverStation().getPVCoordinatesProvider();
final Frame frame = state.getFrame();
final TimeStampedPVCoordinates receiverPV = receiverPVProvider.getPVCoordinates(receptionDate, frame);
final PVCoordinatesProvider satellitePVProvider = AbstractMeasurementObject.extractPVCoordinatesProvider(state,
state.getPVCoordinates());
final double[] delays = getTwoLegsSignalTimer().computeDelays(frame, receiverPV.getPosition(), receptionDate,
satellitePVProvider, getEmitterStation().getPVCoordinatesProvider());
// Form dates
final AbsoluteDate transitDate = receptionDate.shiftedBy(-delays[1]);
final AbsoluteDate emissionDate = transitDate.shiftedBy(-delays[0]);
final double shift = transitDate.durationFrom(state);
final SpacecraftState transitState = state.shiftedBy(shift);
return new TimeStampedPVCoordinates[] { emitter.getPVCoordinatesProvider().getPVCoordinates(emissionDate, frame),
transitState.getPVCoordinates(), receiverPV };
}
/**
* Method computing consecutive Field time shifts of participants, starting from observation date.
* @param states observables
* @return time shifts
*/
protected Gradient[] getFieldShifts(final SpacecraftState[] states) {
// 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 SpacecraftState state = states[0];
final Frame frame = state.getFrame();
final Map<String, Integer> paramIndices = getParameterIndices(states);
final int nbParams = 6 * states.length + paramIndices.size();
final TimeStampedFieldPVCoordinates<Gradient> pva = AbstractMeasurement.getCoordinates(state, 0, nbParams);
// Compute actual reception date
final FieldAbsoluteDate<Gradient> receptionDate = getCorrectedReceptionDateField(nbParams, paramIndices);
// Compute light time delays
final FieldPVCoordinatesProvider<Gradient> receiverPVProvider = getReceiverStation().getFieldPVCoordinatesProvider(nbParams, paramIndices);
final TimeStampedFieldPVCoordinates<Gradient> receiverPV = receiverPVProvider.getPVCoordinates(receptionDate, frame);
final FieldPVCoordinatesProvider<Gradient> satellitePVProvider = AbstractMeasurementObject.extractFieldPVCoordinatesProvider(state, pva);
final FieldPVCoordinatesProvider<Gradient> emitterPVProvider = getEmitterStation().getFieldPVCoordinatesProvider(nbParams, paramIndices);
final Gradient[] delays = getTwoLegsSignalTimer().computeDelays(frame, receiverPV.getPosition(), receptionDate,
satellitePVProvider, emitterPVProvider);
return new Gradient[] { receptionDate.durationFrom(getDate()), delays[1].negate(), delays[0].negate() };
}
/**
* Fill estimated measurements with value and derivatives.
* @param quantity estimated quantity
* @param paramIndices indices mapping parameter names to derivative indices
* @param estimated theoretical measurement class
*/
protected void fillEstimation(final Gradient quantity, final Map<String, Integer> paramIndices,
final EstimatedMeasurement<T> estimated) {
estimated.setEstimatedValue(quantity.getValue());
// First order derivatives with respect to state
final double[] derivatives = quantity.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 = paramIndices.get(span.getData());
if (index != null) {
estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
}
}
}
}
}