BistaticRangeRate.java
/* Copyright 2002-2022 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
* 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.Collections;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.analysis.differentiation.GradientField;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.orekit.frames.FieldTransform;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** Class modeling a bistatic range rate measurement using
* an emitter ground station and a receiver ground station.
* <p>
* The measurement is considered to be a signal:
* <ul>
* <li>Emitted from the emitter ground station</li>
* <li>Reflected on the spacecraft</li>
* <li>Received on the receiver ground station</li>
* </ul>
* The date of the measurement corresponds to the reception on ground of the reflected signal.
* The quantity measured at the receiver is the bistatic radial velocity as the sum of the radial
* velocities with respect to the two stations.
* <p>
* The motion of the stations and the spacecraft during the signal flight time are taken into account.
* </p><p>
* The Doppler measurement can be obtained by multiplying the velocity by (fe/c), where
* fe is the emission frequency.
* </p>
*
* @author Pascal Parraud
* @since 11.2
*/
public class BistaticRangeRate extends AbstractMeasurement<BistaticRangeRate> {
/** Emitter ground station. */
private final GroundStation emitter;
/** Receiver ground station. */
private final GroundStation receiver;
/** Simple constructor.
* @param emitter emitter ground station
* @param receiver receiver ground station
* @param date date of the measurement
* @param rangeRate observed value, m/s
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
*/
public BistaticRangeRate(final GroundStation emitter, final GroundStation receiver,
final AbsoluteDate date, final double rangeRate, final double sigma,
final double baseWeight, final ObservableSatellite satellite) {
super(date, rangeRate, sigma, baseWeight, Collections.singletonList(satellite));
// add parameter drivers for the emitter, clock offset is not used
addParameterDriver(emitter.getEastOffsetDriver());
addParameterDriver(emitter.getNorthOffsetDriver());
addParameterDriver(emitter.getZenithOffsetDriver());
addParameterDriver(emitter.getPrimeMeridianOffsetDriver());
addParameterDriver(emitter.getPrimeMeridianDriftDriver());
addParameterDriver(emitter.getPolarOffsetXDriver());
addParameterDriver(emitter.getPolarDriftXDriver());
addParameterDriver(emitter.getPolarOffsetYDriver());
addParameterDriver(emitter.getPolarDriftYDriver());
// add parameter drivers for the receiver
addParameterDriver(receiver.getClockOffsetDriver());
addParameterDriver(receiver.getEastOffsetDriver());
addParameterDriver(receiver.getNorthOffsetDriver());
addParameterDriver(receiver.getZenithOffsetDriver());
addParameterDriver(receiver.getPrimeMeridianOffsetDriver());
addParameterDriver(receiver.getPrimeMeridianDriftDriver());
addParameterDriver(receiver.getPolarOffsetXDriver());
addParameterDriver(receiver.getPolarDriftXDriver());
addParameterDriver(receiver.getPolarOffsetYDriver());
addParameterDriver(receiver.getPolarDriftYDriver());
this.emitter = emitter;
this.receiver = receiver;
}
/** Get the emitter ground station.
* @return emitter ground station
*/
public GroundStation getEmitterStation() {
return emitter;
}
/** Get the receiver ground station.
* @return receiver ground station
*/
public GroundStation getReceiverStation() {
return receiver;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<BistaticRangeRate> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
final SpacecraftState state = states[0];
// Bistatic range rate derivatives are computed with respect to:
// - Spacecraft state in inertial frame
// - Emitter station parameters
// - Receiver station parameters
// --------------------------
// - 0..2 - Position of the spacecraft in inertial frame
// - 3..5 - Velocity of the spacecraft in inertial frame
// - 6..n - stations' parameters (stations' offsets, pole, prime meridian...)
int nbParams = 6;
final Map<String, Integer> indices = new HashMap<String, Integer>();
for (ParameterDriver driver : getParametersDrivers()) {
// we have to check for duplicate keys because emitter and receiver stations share
// pole and prime meridian parameters names that must be considered
// as one set only (they are combined together by the estimation engine)
if (driver.isSelected() && !indices.containsKey(driver.getName())) {
indices.put(driver.getName(), nbParams++);
}
}
final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));
// coordinates of the spacecraft as a gradient
final TimeStampedFieldPVCoordinates<Gradient> pvaG = getCoordinates(state, 0, nbParams);
// transform between receiver station frame and inertial frame
// at the real date of measurement, i.e. taking station clock offset into account
final FieldTransform<Gradient> receiverToInertial =
receiver.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
final FieldAbsoluteDate<Gradient> measurementDateG = receiverToInertial.getFieldDate();
// Receiver PV in inertial frame at the end of the downlink leg
final TimeStampedFieldPVCoordinates<Gradient> receiverPV =
receiverToInertial.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(measurementDateG,
zero, zero, zero));
// 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 tauD = signalTimeOfFlight(pvaG, receiverPV.getPosition(), measurementDateG);
final Gradient delta = measurementDateG.durationFrom(state.getDate());
final Gradient deltaMTauD = delta.subtract(tauD);
// Transit state
final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());
// Transit PV
final TimeStampedFieldPVCoordinates<Gradient> transitPV = pvaG.shiftedBy(deltaMTauD);
// Downlink range-rate
final EstimatedMeasurement<BistaticRangeRate> evalDownlink =
oneWayTheoreticalEvaluation(iteration, evaluation, true,
receiverPV, transitPV, transitState, indices);
// transform between emitter station frame and inertial frame at the transit date
// clock offset from receiver is already compensated
final FieldTransform<Gradient> emitterToInertial =
emitter.getOffsetToInertial(state.getFrame(), transitPV.getDate(), nbParams, indices);
// emitter PV in inertial frame at the end of the uplink leg
final TimeStampedFieldPVCoordinates<Gradient> emitterPV =
emitterToInertial.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(transitPV.getDate(),
zero, zero, zero));
// Uplink delay
final Gradient tauU = signalTimeOfFlight(emitterPV, transitPV.getPosition(), transitPV.getDate());
// emitter position in inertial frame at the end of the uplink leg
final TimeStampedFieldPVCoordinates<Gradient> emitterUplink = emitterPV.shiftedBy(tauU.negate());
// Uplink range-rate
final EstimatedMeasurement<BistaticRangeRate> evalUplink =
oneWayTheoreticalEvaluation(iteration, evaluation, false,
emitterUplink, transitPV, transitState, indices);
// combine uplink and downlink values
final EstimatedMeasurement<BistaticRangeRate> estimated =
new EstimatedMeasurement<>(this, iteration, evaluation,
evalDownlink.getStates(),
new TimeStampedPVCoordinates[] {
evalUplink.getParticipants()[0],
evalDownlink.getParticipants()[0],
evalDownlink.getParticipants()[1]
});
estimated.setEstimatedValue(evalDownlink.getEstimatedValue()[0] + evalUplink.getEstimatedValue()[0]);
// combine uplink and downlink partial derivatives with respect to state
final double[][] sd1 = evalDownlink.getStateDerivatives(0);
final double[][] sd2 = evalUplink.getStateDerivatives(0);
final double[][] sd = new double[sd1.length][sd1[0].length];
for (int i = 0; i < sd.length; ++i) {
for (int j = 0; j < sd[0].length; ++j) {
sd[i][j] = sd1[i][j] + sd2[i][j];
}
}
estimated.setStateDerivatives(0, sd);
// combine uplink and downlink partial derivatives with respect to parameters
evalDownlink.getDerivativesDrivers().forEach(driver -> {
final double[] pd1 = evalDownlink.getParameterDerivatives(driver);
final double[] pd2 = evalUplink.getParameterDerivatives(driver);
final double[] pd = new double[pd1.length];
for (int i = 0; i < pd.length; ++i) {
pd[i] = pd1[i] + pd2[i];
}
estimated.setParameterDerivatives(driver, pd);
});
return estimated;
}
/** Evaluate range rate measurement in one-way.
* @param iteration iteration number
* @param evaluation evaluations counter
* @param downlink indicator for downlink leg
* @param stationPV station coordinates when signal is at station
* @param transitPV spacecraft coordinates at onboard signal transit
* @param transitState orbital state at onboard signal transit
* @param indices indices of the estimated parameters in derivatives computations
* @return theoretical value for the current leg
*/
private EstimatedMeasurement<BistaticRangeRate> oneWayTheoreticalEvaluation(final int iteration, final int evaluation, final boolean downlink,
final TimeStampedFieldPVCoordinates<Gradient> stationPV,
final TimeStampedFieldPVCoordinates<Gradient> transitPV,
final SpacecraftState transitState,
final Map<String, Integer> indices) {
// prepare the evaluation
final EstimatedMeasurement<BistaticRangeRate> estimated =
new EstimatedMeasurement<BistaticRangeRate>(this, iteration, evaluation,
new SpacecraftState[] {
transitState
}, new TimeStampedPVCoordinates[] {
(downlink ? transitPV : stationPV).toTimeStampedPVCoordinates(),
(downlink ? stationPV : transitPV).toTimeStampedPVCoordinates()
});
// range rate value
final FieldVector3D<Gradient> stationPosition = stationPV.getPosition();
final FieldVector3D<Gradient> relativePosition = stationPosition.subtract(transitPV.getPosition());
final FieldVector3D<Gradient> stationVelocity = stationPV.getVelocity();
final FieldVector3D<Gradient> relativeVelocity = stationVelocity.subtract(transitPV.getVelocity());
// radial direction
final FieldVector3D<Gradient> lineOfSight = relativePosition.normalize();
// range rate
final Gradient rangeRate = FieldVector3D.dotProduct(relativeVelocity, lineOfSight);
estimated.setEstimatedValue(rangeRate.getValue());
// compute partial derivatives of (rr) with respect to spacecraft state Cartesian coordinates
final double[] derivatives = rangeRate.getGradient();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
// 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;
}
}