DualReceiverMeasurement.java
/* Copyright 2025-2026 Hawkeye 360 (HE360)
* 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
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
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package org.orekit.estimation.measurements;
import java.util.Arrays;
import java.util.Collections;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.orekit.frames.Frame;
import org.orekit.propagation.SpacecraftState;
import org.orekit.signal.DifferencesOfSignalArrival;
import org.orekit.signal.FieldSignalReceptionCondition;
import org.orekit.signal.SignalTravelTimeModel;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.FieldPVCoordinatesProvider;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeSpanMap;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
/**
* Class modeling a twice-received measurement using a primary and secondary observer.
* <p>
* The measurement is considered to be a signal:
* <ul>
* <li>Emitted by the observed spacecraft</li>
* <li>Received by the first observer</li>
* <li>Received by the second observer</li>
* </ul>
* The date of the measurement corresponds to the reception on of the signal by the first observer.
* <p>
* The motion of the observers and the spacecraft during the signal flight time are taken into account.
* </p>
*
* @author Brianna Aubin
* @since 14.0
*/
abstract class DualReceiverMeasurement<T extends AbstractMeasurement<T>> extends SignalBasedMeasurement<T> {
/**
* First observer to receive signal. Determines measurement date.
*/
private final Observer primeObserver;
/**
* Second Observer to receive signal. Determines measurement value.
*/
private final Observer secondObserver;
/**
* Simple constructor.
*
* @param primeObserver observer from which transmission is performed
* @param secondObserver observer from which measurement is performed
* @param date date of the measurement
* @param value observed value
* @param measurementQuality measurement quality data as used in orbit determination
* @param signalTravelTimeModel signal travel time model
* @param satellite satellite related to this measurement
*/
protected DualReceiverMeasurement(final Observer primeObserver, final Observer secondObserver,
final AbsoluteDate date, final double[] value,
final MeasurementQuality measurementQuality,
final SignalTravelTimeModel signalTravelTimeModel,
final ObservableSatellite satellite) {
super(date, false, value, measurementQuality, signalTravelTimeModel,
Collections.singletonList(satellite));
// Add the parameters for the receiver
addParametersDrivers(primeObserver.getParametersDrivers());
addParametersDrivers(secondObserver.getParametersDrivers());
// Set emitter
this.primeObserver = primeObserver;
this.secondObserver = secondObserver;
}
/** Get the prime ground station, the one that receives the signal first.
* @return prime ground station
* @deprecated as of 14.0, replaced by {@link #getPrimeObserver()}
*/
@Deprecated
public GroundStation getPrimeStation() {
if (!(primeObserver instanceof GroundStation)) {
return null;
}
return (GroundStation) primeObserver;
}
/** Get the prime observer, the one that receives the signal first.
* @return prime observer
*/
public Observer getPrimeObserver() {
return primeObserver;
}
/** Get the second ground station, the one that receives the signal first.
* @return second ground station
* @deprecated as of 14.0, replaced by {@link #getSecondObserver()}
*/
@Deprecated
public GroundStation getSecondStation() {
if (!(secondObserver instanceof GroundStation)) {
return null;
}
return (GroundStation) secondObserver;
}
/** Get the second observer, the one that gives the measurement.
* @return second observer
*/
public Observer getSecondObserver() {
return secondObserver;
}
/**
* Compute signal delays (always positive).
* @param states observed states
* @return delays (to prime and second sensors)
*/
protected Gradient[] computeDelays(final SpacecraftState[] states) {
// Derivatives are computed with respect to spacecraft state in inertial frame and measurement model 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);
final FieldPVCoordinatesProvider<Gradient> emitter = AbstractParticipant.extractFieldPVCoordinatesProvider(state, pva);
final FieldAbsoluteDate<Gradient> firstReceptionDate = getPrimeObserver().getCorrectedReceptionDateField(getDate(), nbParams, paramIndices);
// Compute delays
final DifferencesOfSignalArrival differencesOfSignalArrival = new DifferencesOfSignalArrival(getSignalTravelTimeModel());
final FieldVector3D<Gradient> primePosition = getPrimeObserver().getFieldPVCoordinatesProvider(nbParams, paramIndices)
.getPosition(firstReceptionDate, frame);
final FieldSignalReceptionCondition<Gradient> receptionCondition = new FieldSignalReceptionCondition<>(firstReceptionDate, primePosition,
frame);
return differencesOfSignalArrival.computeDelays(receptionCondition,
getSecondObserver().getFieldPVCoordinatesProvider(nbParams, paramIndices), emitter);
}
/**
* 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 (TimeSpanMap.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]);
}
}
}
}
}