InterSatellitesPhase.java
/* Copyright 2002-2021 CS GROUP
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* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.estimation.measurements.gnss;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.orekit.estimation.measurements.AbstractMeasurement;
import org.orekit.estimation.measurements.EstimatedMeasurement;
import org.orekit.estimation.measurements.ObservableSatellite;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Phase measurement between two satellites.
* <p>
* The measurement is considered to be a signal emitted from
* a remote satellite and received by a local satellite.
* Its value is the number of cycles between emission and reception.
* The motion of both spacecrafts during the signal flight time
* are taken into account. The date of the measurement corresponds to the
* reception on ground of the emitted signal.
* </p>
* @author Bryan Cazabonne
* @since 10.3
*/
public class InterSatellitesPhase extends AbstractMeasurement<InterSatellitesPhase> {
/** Name for ambiguity driver. */
public static final String AMBIGUITY_NAME = "ambiguity";
/** Driver for ambiguity. */
private final ParameterDriver ambiguityDriver;
/** Wavelength of the phase observed value [m]. */
private final double wavelength;
/** Constructor.
* @param local satellite which receives the signal and performs the measurement
* @param remote emote satellite which simply emits the signal
* @param date date of the measurement
* @param phase observed value (cycles)
* @param wavelength phase observed value wavelength (m)
* @param sigma theoretical standard deviation
* @param baseWeight base weight
*/
public InterSatellitesPhase(final ObservableSatellite local,
final ObservableSatellite remote,
final AbsoluteDate date, final double phase,
final double wavelength, final double sigma,
final double baseWeight) {
// Call to super constructor
super(date, phase, sigma, baseWeight, Arrays.asList(local, remote));
// Initialize phase ambiguity driver
ambiguityDriver = new ParameterDriver(AMBIGUITY_NAME, 0.0, 1.0,
Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY);
// Add parameter drivers
addParameterDriver(ambiguityDriver);
addParameterDriver(local.getClockOffsetDriver());
addParameterDriver(remote.getClockOffsetDriver());
// Initialize fields
this.wavelength = wavelength;
}
/** Get the wavelength.
* @return wavelength (m)
*/
public double getWavelength() {
return wavelength;
}
/** Get the driver for phase ambiguity.
* @return the driver for phase ambiguity
*/
public ParameterDriver getAmbiguityDriver() {
return ambiguityDriver;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<InterSatellitesPhase> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// Phase derivatives are computed with respect to spacecrafts states in inertial frame
// ----------------------
//
// Parameters:
// - 0..2 - Position of the receiver satellite in inertial frame
// - 3..5 - Velocity of the receiver satellite in inertial frame
// - 6..8 - Position of the remote satellite in inertial frame
// - 9..11 - Velocity of the remote satellite in inertial frame
// - 12.. - Measurement parameters: ambiguity, local clock offset, remote clock offset...
int nbParams = 12;
final Map<String, Integer> indices = new HashMap<>();
for (ParameterDriver phaseMeasurementDriver : getParametersDrivers()) {
if (phaseMeasurementDriver.isSelected()) {
indices.put(phaseMeasurementDriver.getName(), nbParams++);
}
}
// Coordinates of both satellites
final SpacecraftState local = states[0];
final TimeStampedFieldPVCoordinates<Gradient> pvaL = getCoordinates(local, 0, nbParams);
final SpacecraftState remote = states[1];
final TimeStampedFieldPVCoordinates<Gradient> pvaR = getCoordinates(remote, 6, nbParams);
// Compute propagation times
// Downlink delay
final Gradient dtl = getSatellites().get(0).getClockOffsetDriver().getValue(nbParams, indices);
final FieldAbsoluteDate<Gradient> arrivalDate = new FieldAbsoluteDate<>(getDate(), dtl.negate());
final TimeStampedFieldPVCoordinates<Gradient> s1Downlink =
pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
final Gradient tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);
// Transit state
final double delta = getDate().durationFrom(remote.getDate());
final Gradient deltaMTauD = tauD.negate().add(delta);
// prepare the evaluation
final EstimatedMeasurement<InterSatellitesPhase> estimatedPhase =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
local.shiftedBy(deltaMTauD.getValue()),
remote.shiftedBy(deltaMTauD.getValue())
}, new TimeStampedPVCoordinates[] {
remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
local.shiftedBy(delta).getPVCoordinates()
});
// Clock offsets
final Gradient dtr = getSatellites().get(1).getClockOffsetDriver().getValue(nbParams, indices);
// Phase value
final double cOverLambda = Constants.SPEED_OF_LIGHT / wavelength;
final Gradient ambiguity = ambiguityDriver.getValue(nbParams, indices);
final Gradient phase = tauD.add(dtl).subtract(dtr).multiply(cOverLambda).add(ambiguity);
estimatedPhase.setEstimatedValue(phase.getValue());
// Range partial derivatives with respect to states
final double[] derivatives = phase.getGradient();
estimatedPhase.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
estimatedPhase.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 6, 12));
// Set partial derivatives with respect to parameters
for (final ParameterDriver driver : getParametersDrivers()) {
final Integer index = indices.get(driver.getName());
if (index != null) {
estimatedPhase.setParameterDerivatives(driver, derivatives[index]);
}
}
// Return the estimated measurement
return estimatedPhase;
}
}