InterSatellitesRange.java
/* Copyright 2002-2018 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (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;
import java.util.Arrays;
import org.hipparchus.Field;
import org.hipparchus.analysis.differentiation.DSFactory;
import org.hipparchus.analysis.differentiation.DerivativeStructure;
import org.orekit.errors.OrekitException;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** One-way or two-way range measurements between two satellites.
* <p>
* Satellite 1 is always considered to be the satellite that receives the
* signal and computes the measurement.
* </p>
* <p>
* For one-way measurements, a signal is emitted by satellite 2 and received
* by satellite 1. The measurement value is the elapsed time between emission
* and reception divided by c were c is the speed of light.
* </p>
* <p>
* For two-way measurements, a signal is emitted by satellite 1, reflected on
* satellite 2, and received back by satellite 1 again. The measurement value
* is the elapsed time between emission and reception divided by 2c were c is
* the speed of light.
* </p>
* <p>
* The motion of both satellites during the signal flight time is
* taken into account. The date of the measurement corresponds to
* the reception of the signal by satellite 1.
* </p>
* @author Luc Maisonobe
* @since 9.0
*/
public class InterSatellitesRange extends AbstractMeasurement<InterSatellitesRange> {
/** Flag indicating whether it is a two-way measurement. */
private final boolean twoway;
/** Simple constructor.
* @param satellite1Index index of satellite 1 propagator
* (i.e. the satellite which receives the signal and performs
* the measurement)
* @param satellite2Index index of satellite 2 propagator
* (i.e. the satellite which simply emits the signal in the one-way
* case, or reflects the signal in the two-way case)
* @param twoWay flag indicating whether it is a two-way measurement
* @param date date of the measurement
* @param range observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @exception OrekitException if a {@link org.orekit.utils.ParameterDriver}
* name conflict occurs
*/
public InterSatellitesRange(final int satellite1Index, final int satellite2Index,
final boolean twoWay,
final AbsoluteDate date, final double range,
final double sigma, final double baseWeight)
throws OrekitException {
super(date, range, sigma, baseWeight, Arrays.asList(satellite1Index, satellite2Index));
this.twoway = twoWay;
}
/** Check if the instance represents a two-way measurement.
* @return true if the instance represents a two-way measurement
*/
public boolean isTwoWay() {
return twoway;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<InterSatellitesRange> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states)
throws OrekitException {
// Range derivatives are computed with respect to spacecrafts states in inertial frame
// ----------------------
//
// Parameters:
// - 0..2 - Position of the satellite 1 in inertial frame
// - 3..5 - Velocity of the satellite 1 in inertial frame
// - 6..8 - Position of the satellite 2 in inertial frame
// - 9..11 - Velocity of the satellite 2 in inertial frame
final int nbParams = 12;
final DSFactory factory = new DSFactory(nbParams, 1);
final Field<DerivativeStructure> field = factory.getDerivativeField();
// coordinates of both satellites
final SpacecraftState state1 = states[getPropagatorsIndices().get(0)];
final TimeStampedFieldPVCoordinates<DerivativeStructure> pva1 = getCoordinates(state1, 0, factory);
final SpacecraftState state2 = states[getPropagatorsIndices().get(1)];
final TimeStampedFieldPVCoordinates<DerivativeStructure> pva2 = getCoordinates(state2, 6, factory);
// compute propagation times
// (if state has already been set up to pre-compensate propagation delay,
// we will have delta == tauD and transitState will be the same as state)
// downlink delay
final FieldAbsoluteDate<DerivativeStructure> arrivalDate = new FieldAbsoluteDate<>(field, getDate());
final TimeStampedFieldPVCoordinates<DerivativeStructure> s1Downlink =
pva1.shiftedBy(arrivalDate.durationFrom(pva1.getDate()));
final DerivativeStructure tauD = signalTimeOfFlight(pva2, s1Downlink.getPosition(), arrivalDate);
// Transit state
final double delta = getDate().durationFrom(state2.getDate());
final DerivativeStructure deltaMTauD = tauD.negate().add(delta);
// prepare the evaluation
final EstimatedMeasurement<InterSatellitesRange> estimated;
final DerivativeStructure range;
if (twoway) {
// Transit state (re)computed with derivative structures
final TimeStampedFieldPVCoordinates<DerivativeStructure> transitStateDS = pva2.shiftedBy(deltaMTauD);
// uplink delay
final DerivativeStructure tauU = signalTimeOfFlight(pva1,
transitStateDS.getPosition(),
transitStateDS.getDate());
estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
state1.shiftedBy(deltaMTauD.getValue()),
state2.shiftedBy(deltaMTauD.getValue())
}, new TimeStampedPVCoordinates[] {
state1.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(),
state2.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
state1.shiftedBy(delta).getPVCoordinates()
});
// Range value
range = tauD.add(tauU).multiply(0.5 * Constants.SPEED_OF_LIGHT);
} else {
estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
state1.shiftedBy(deltaMTauD.getValue()),
state2.shiftedBy(deltaMTauD.getValue())
}, new TimeStampedPVCoordinates[] {
state2.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
state1.shiftedBy(delta).getPVCoordinates()
});
// Range value
range = tauD.multiply(Constants.SPEED_OF_LIGHT);
}
estimated.setEstimatedValue(range.getValue());
// Range partial derivatives with respect to states
final double[] derivatives = range.getAllDerivatives();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 1, 7));
estimated.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 7, 13));
return estimated;
}
}