AbstractInterSatellitesMeasurement.java
/* Copyright 2022-2026 Luc Maisonobe
* 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.
* 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
* 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.gnss;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.analysis.differentiation.GradientField;
import org.orekit.estimation.measurements.AbstractMeasurement;
import org.orekit.estimation.measurements.CommonParametersWithDerivatives;
import org.orekit.estimation.measurements.CommonParametersWithoutDerivatives;
import org.orekit.estimation.measurements.FieldSignalTravelTimeAdjustableEmitter;
import org.orekit.estimation.measurements.ObservableSatellite;
import org.orekit.estimation.measurements.ObservedMeasurement;
import org.orekit.estimation.measurements.SignalTravelTimeAdjustableEmitter;
import org.orekit.frames.Frame;
import org.orekit.time.clocks.ClockOffset;
import org.orekit.time.clocks.FieldClockOffset;
import org.orekit.time.clocks.QuadraticClockModel;
import org.orekit.time.clocks.QuadraticFieldClockModel;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.AbsolutePVCoordinates;
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;
/** Base class for measurement between two satellites that are both estimated.
* <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 spacecraft 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>
* @param <T> type of the measurement
* @author Luc Maisonobe
* @since 12.1
*/
public abstract class AbstractInterSatellitesMeasurement<T extends ObservedMeasurement<T>> extends AbstractMeasurement<T> {
/** Constructor.
* @param date date of the measurement
* @param observed observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param local satellite which receives the signal and performs the measurement
* @param remote remote satellite which simply emits the signal
*/
protected AbstractInterSatellitesMeasurement(final AbsoluteDate date, final double observed,
final double sigma, final double baseWeight,
final ObservableSatellite local,
final ObservableSatellite remote) {
// Call to super constructor
super(date, false, observed, sigma, baseWeight, Arrays.asList(local, remote));
}
/** Retrieves the clock of the satellite being treated as "remote"
* in this function (i.e. sat number 2).
* @return ObservableSatellite clock
*/
protected QuadraticClockModel getRemoteClock() {
return getSatellites().get(1).getQuadraticClockModel();
}
/** Get emitting satellite clock provider.
* @param freeParameters total number of free parameters in the gradient
* @param indices indices of the differentiation parameters in derivatives computations,
* must be span name and not driver name
* @return emitting satellite clock provider
*/
protected QuadraticFieldClockModel<Gradient> getRemoteClock(final int freeParameters,
final Map<String, Integer> indices) {
return getRemoteClock().toGradientModel(freeParameters, indices, getDate());
}
/** Return the FieldPVCoordinatesProvider.
* @param state ObservableSatellite spacecraft state
* @return pos/vel coordinates provider for values with Gradient field
* @since 14.0
*/
protected PVCoordinatesProvider getRemotePV(final SpacecraftState state) {
return new AbsolutePVCoordinates(state.getFrame(), state.getPVCoordinates());
}
/** Return the FieldPVCoordinatesProvider.
* @param state ObservableSatellite spacecraft state
* @param freeParameters number of free parameters
* @return pos/vel coordinates provider for values with Gradient field
* @since 14.0
*/
protected FieldPVCoordinatesProvider<Gradient> getRemotePV(final SpacecraftState state,
final int freeParameters) {
// convert the SpacecraftState to a FieldPVCoordinatesProvider<Gradient>
return (date, frame) -> {
// set up the derivatives with respect to remote state at its date
final TimeStampedFieldPVCoordinates<Gradient> pv0 = getCoordinates(state, 6, freeParameters);
// shift to desired date
final TimeStampedFieldPVCoordinates<Gradient> shifted = pv0.shiftedBy(date.durationFrom(state.getDate()));
// transform to desired frame
return state.getFrame().getTransformTo(frame, state.getDate()).transformPVCoordinates(shifted);
};
}
/** Compute common estimation parameters.
* @param states states of all spacecraft involved in the measurement
* @param clockOffsetAlreadyApplied if true, the specified {@code date} is as read
* by the receiver clock (i.e. clock offset <em>not</em> compensated), if false,
* the specified {@code date} was already compensated and is a physical absolute date
* @return common parameters
*/
protected CommonParametersWithoutDerivatives computeCommonParametersWithout(final SpacecraftState[] states,
final boolean clockOffsetAlreadyApplied) {
// local and remote satellites
final Frame frame = states[0].getFrame();
final TimeStampedPVCoordinates pvaLocal = states[0].getPVCoordinates(frame);
final ClockOffset localClock = getSatellites().get(0).
getQuadraticClockModel().getOffset(getDate());
final double localClockOffset = localClock.getOffset();
final PVCoordinatesProvider remotePV = getRemotePV(states[1]);
// take clock offset into account
final AbsoluteDate arrivalDate = clockOffsetAlreadyApplied ? getDate() : getDate().shiftedBy(-localClockOffset);
// Downlink delay
final double deltaT = arrivalDate.durationFrom(states[0]);
final TimeStampedPVCoordinates pvaDownlink = pvaLocal.shiftedBy(deltaT);
final SignalTravelTimeAdjustableEmitter signalTimeOfFlight = new SignalTravelTimeAdjustableEmitter(remotePV);
final double tauD = signalTimeOfFlight.compute(arrivalDate, pvaDownlink.getPosition(), arrivalDate, frame);
// Remote satellite at signal emission
final AbsoluteDate emissionDate = arrivalDate.shiftedBy(-tauD);
final ClockOffset remoteClock = getRemoteClock().getOffset(emissionDate);
return new CommonParametersWithoutDerivatives(states[0], tauD,
localClock, remoteClock,
states[0],
pvaDownlink,
remotePV.getPVCoordinates(emissionDate, frame));
}
/** Compute common estimation parameters.
* @param states states of all spacecraft involved in the measurement
* @param clockOffsetAlreadyApplied if true, the specified {@code date} is as read
* by the receiver clock (i.e. clock offset <em>not</em> compensated), if false,
* the specified {@code date} was already compensated and is a physical absolute date
* @return common parameters
*/
protected CommonParametersWithDerivatives computeCommonParametersWith(final SpacecraftState[] states,
final boolean clockOffsetAlreadyApplied) {
final Frame frame = states[0].getFrame();
// measurement derivatives are computed with respect to spacecraft state in inertial frame
// Parameters:
// - 6k..6k+2 - Position of spacecraft k (counting k from 0 to nbSat-1) in inertial frame
// - 6k+3..6k+5 - Velocity of spacecraft k (counting k from 0 to nbSat-1) in inertial frame
// - 6nbSat..n - measurements parameters (clock offset, etc)
int nbParams = 6 * states.length;
final Map<String, Integer> paramIndices = new HashMap<>();
for (ParameterDriver measurementDriver : getParametersDrivers()) {
if (measurementDriver.isSelected()) {
for (Span<String> span = measurementDriver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
paramIndices.put(span.getData(), nbParams++);
}
}
}
final FieldAbsoluteDate<Gradient> gDate = new FieldAbsoluteDate<>(GradientField.getField(nbParams),
getDate());
// local and remote satellites
final TimeStampedFieldPVCoordinates<Gradient> pvaLocal = getCoordinates(states[0], 0, nbParams);
final QuadraticFieldClockModel<Gradient> localClock = getSatellites().get(0).getQuadraticClockModel().
toGradientModel(nbParams, paramIndices, getDate());
final FieldClockOffset<Gradient> localClockOffset = localClock.getOffset(gDate);
final FieldPVCoordinatesProvider<Gradient> remotePV = getRemotePV(states[1], nbParams);
// take clock offset into account
final FieldAbsoluteDate<Gradient> arrivalDate = clockOffsetAlreadyApplied ?
gDate : gDate.shiftedBy(localClockOffset.getOffset().negate());
// Downlink delay
final Gradient deltaT = arrivalDate.durationFrom(states[0].getDate());
final TimeStampedFieldPVCoordinates<Gradient> pvaDownlink = pvaLocal.shiftedBy(deltaT);
final FieldSignalTravelTimeAdjustableEmitter<Gradient> fieldComputer = new FieldSignalTravelTimeAdjustableEmitter<>(remotePV);
final Gradient tauD = fieldComputer.compute(arrivalDate, pvaDownlink.getPosition(), arrivalDate, frame);
// Remote satellite at signal emission
final FieldAbsoluteDate<Gradient> emissionDate = arrivalDate.shiftedBy(tauD.negate());
final QuadraticFieldClockModel<Gradient> remoteClock = getRemoteClock(nbParams, paramIndices);
final FieldClockOffset<Gradient> remoteClockOffset = remoteClock.getOffset(emissionDate);
return new CommonParametersWithDerivatives(states[0], paramIndices, tauD,
localClockOffset, remoteClockOffset,
states[0], pvaDownlink,
remotePV.getPVCoordinates(emissionDate, frame));
}
}