InterSatellitesRange.java
/* Copyright 2002-2020 CS GROUP
* 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,
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*/
package org.orekit.estimation.measurements;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
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;
/** One-way or two-way range measurements between two satellites.
* <p>
* For one-way measurements, a signal is emitted by a remote satellite and received
* by local satellite. The measurement value is the elapsed time between emission
* and reception multiplied by c where c is the speed of light.
* </p>
* <p>
* For two-way measurements, a signal is emitted by local satellite, reflected on
* remote satellite, and received back by local satellite. The measurement value
* is the elapsed time between emission and reception multiplied by c/2 where c
* is the speed of light.
* </p>
* <p>
* Since 9.3, this class also uses the clock offsets of both satellites,
* which manage the value that must be added to each satellite reading of time to
* compute the real physical date. In this measurement, these offsets have two effects:
* </p>
* <ul>
* <li>as measurement date is evaluated at reception time, the real physical date
* of the measurement is the observed date to which the local satellite clock
* offset is subtracted</li>
* <li>as range is evaluated using the total signal time of flight, for one-way
* measurements the observed range is the real physical signal time of flight to
* which (Δtl - Δtr) ⨉ c is added, where Δtl (resp. Δtr) is the clock offset for the
* local satellite (resp. remote satellite). A similar effect exists in
* two-way measurements but it is computed as (Δtl - Δtl) ⨉ c / 2 as the local satellite
* clock is used for both initial emission and final reception and therefore it evaluates
* to zero.</li>
* </ul>
* <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 local satellite which receives the signal and performs the measurement
* @param remote 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
* @since 9.3
*/
public InterSatellitesRange(final ObservableSatellite local,
final ObservableSatellite remote,
final boolean twoWay,
final AbsoluteDate date, final double range,
final double sigma, final double baseWeight) {
super(date, range, sigma, baseWeight, Arrays.asList(local, remote));
// for one way and two ways measurements, the local satellite clock offsets affects the measurement
addParameterDriver(local.getClockOffsetDriver());
if (!twoWay) {
// for one way measurements, the remote satellite clock offsets also affects the measurement
addParameterDriver(remote.getClockOffsetDriver());
}
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) {
// Range 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: local clock offset, remote clock offset...
int nbParams = 12;
final Map<String, Integer> indices = new HashMap<>();
for (ParameterDriver driver : getParametersDrivers()) {
if (driver.isSelected()) {
indices.put(driver.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
// (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 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<InterSatellitesRange> estimated;
final Gradient range;
if (twoway) {
// Transit state (re)computed with derivative structures
final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaR.shiftedBy(deltaMTauD);
// uplink delay
final Gradient tauU = signalTimeOfFlight(pvaL,
transitStateDS.getPosition(),
transitStateDS.getDate());
estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
local.shiftedBy(deltaMTauD.getValue()),
remote.shiftedBy(deltaMTauD.getValue())
}, new TimeStampedPVCoordinates[] {
local.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(),
remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
local.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[] {
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);
// Range value
range = tauD.add(dtl).subtract(dtr).multiply(Constants.SPEED_OF_LIGHT);
}
estimated.setEstimatedValue(range.getValue());
// Range partial derivatives with respect to states
final double[] derivatives = range.getGradient();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
estimated.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) {
estimated.setParameterDerivatives(driver, derivatives[index]);
}
}
return estimated;
}
}