MelbourneWubbenaCombination.java
/* Copyright 2002-2024 CS GROUP
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* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
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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.ArrayList;
import java.util.List;
import org.hipparchus.util.FastMath;
import org.orekit.files.rinex.observation.ObservationData;
import org.orekit.files.rinex.observation.ObservationDataSet;
import org.orekit.gnss.MeasurementType;
import org.orekit.gnss.SatelliteSystem;
/**
* Melbourne-Wübbena combination.
* <p>
* This combination allows, thanks to the wide-lane combination, a larger wavelength
* than each signal individually. Moreover, the measurement noise is reduced by the
* narrow-lane combination of code measurements.
* </p>
* <pre>
* mMW = ΦWL- RNL
* mMW = λWL * NWL+ b + ε
* </pre>
* With:
* <ul>
* <li>mMW : Melbourne-Wübbena measurement.</li>
* <li>ΦWL : Wide-Lane phase measurement.</li>
* <li>RNL : Narrow-Lane code measurement.</li>
* <li>λWL : Wide-Lane wavelength.</li>
* <li>NWL : Wide-Lane ambiguity (Nf1 - Nf2).</li>
* <li>b : Satellite and receiver instrumental delays.</li>
* <li>ε : Measurement noise.</li>
* </ul>
* <p>
* {@link NarrowLaneCombination Narrow-Lane} and {@link WideLaneCombination Wide-Lane}
* combinations shall be performed with the same pair of frequencies.
* </p>
*
* @see "Detector based in code and carrier phase data: The Melbourne-Wübbena combination,
* J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, 2011"
*
* @author Bryan Cazabonne
* @since 10.1
*/
public class MelbourneWubbenaCombination implements MeasurementCombination {
/** Threshold for frequency comparison. */
private static final double THRESHOLD = 1.0e-4;
/** Satellite system used for the combination. */
private final SatelliteSystem system;
/**
* Package private constructor for the factory.
* @param system satellite system for which the combination is applied
*/
MelbourneWubbenaCombination(final SatelliteSystem system) {
this.system = system;
}
/** {@inheritDoc} */
@Override
public CombinedObservationDataSet combine(final ObservationDataSet observations) {
// Wide-Lane combination
final WideLaneCombination wideLane = MeasurementCombinationFactory.getWideLaneCombination(system);
final CombinedObservationDataSet combinedWL = wideLane.combine(observations);
// Narrow-Lane combination
final NarrowLaneCombination narrowLane = MeasurementCombinationFactory.getNarrowLaneCombination(system);
final CombinedObservationDataSet combinedNL = narrowLane.combine(observations);
// Initialize list of combined observation data
final List<CombinedObservationData> combined = new ArrayList<>();
// Loop on Wide-Lane measurements
for (CombinedObservationData odWL : combinedWL.getObservationData()) {
// Only consider combined phase measurements
if (odWL.getMeasurementType() == MeasurementType.CARRIER_PHASE) {
// Loop on Narrow-Lane measurements
for (CombinedObservationData odNL : combinedNL.getObservationData()) {
// Only consider combined range measurements
if (odNL.getMeasurementType() == MeasurementType.PSEUDO_RANGE) {
// Verify if the combinations have used the same frequencies
final boolean isCombinationPossible = isCombinationPossible(odWL, odNL);
if (isCombinationPossible) {
// Combined value and frequency
final double combinedValue = odWL.getValue() - odNL.getValue();
final double combinedFrequency = odWL.getCombinedFrequency();
// Used observation data to build the Melbourn-Wübbena measurement
final List<ObservationData> usedData = new ArrayList<>(4);
usedData.add(0, odWL.getUsedObservationData().get(0));
usedData.add(1, odWL.getUsedObservationData().get(1));
usedData.add(2, odNL.getUsedObservationData().get(0));
usedData.add(3, odNL.getUsedObservationData().get(1));
// Update the combined observation data list
combined.add(new CombinedObservationData(combinedValue, combinedFrequency,
CombinationType.MELBOURNE_WUBBENA,
MeasurementType.COMBINED_RANGE_PHASE,
usedData));
}
}
}
}
}
return new CombinedObservationDataSet(observations.getSatellite().getSystem(),
observations.getSatellite().getPRN(),
observations.getDate(),
observations.getRcvrClkOffset(), combined);
}
/**
* Verifies if the Melbourne-Wübbena combination is possible between both combined observation data.
* <p>
* This method compares the frequencies of the combined measurement to decide
* if the combination of measurements is possible.
* The combination is possible if :
* <pre>
* abs(f1<sub>WL</sub> - f2<sub>WL</sub>) = abs(f1<sub>NL</sub> - f2<sub>NL</sub>)
* </pre>
* </p>
* @param odWL Wide-Lane measurement
* @param odNL Narrow-Lane measurement
* @return true if the Melbourne-Wübbena combination is possible
*/
private boolean isCombinationPossible(final CombinedObservationData odWL, final CombinedObservationData odNL) {
// Frequencies
final double[] frequency = new double[4];
int j = 0;
for (int i = 0; i < odWL.getUsedObservationData().size(); i++) {
frequency[j++] = odWL.getUsedObservationData().get(i).getObservationType().getFrequency(system).getFrequency();
frequency[j++] = odNL.getUsedObservationData().get(i).getObservationType().getFrequency(system).getFrequency();
}
// Verify if used frequencies are the same.
// Possible numerical error is taken into account by using a threshold of acceptance
return (FastMath.abs(frequency[0] - frequency[2]) - FastMath.abs(frequency[1] - frequency[3])) < THRESHOLD;
}
/** {@inheritDoc} */
@Override
public String getName() {
return CombinationType.MELBOURNE_WUBBENA.getName();
}
}