BistaticRangeRate.java
/* Copyright 2002-2024 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
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*
* 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 org.hipparchus.analysis.differentiation.Gradient;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.frames.FieldTransform;
import org.orekit.frames.Transform;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeSpanMap.Span;
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 GroundReceiverMeasurement<BistaticRangeRate> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "BistaticRangeRate";
/** Emitter ground station. */
private final GroundStation emitter;
/** 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(receiver, true, date, rangeRate, sigma, baseWeight, 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());
this.emitter = emitter;
}
/** 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 getStation();
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurementBase<BistaticRangeRate> theoreticalEvaluationWithoutDerivatives(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
final GroundReceiverCommonParametersWithoutDerivatives common = computeCommonParametersWithout(states[0]);
final TimeStampedPVCoordinates transitPV = common.getTransitPV();
final AbsoluteDate transitDate = transitPV.getDate();
// Approximate emitter location at transit time
final Transform emitterToInertial =
getEmitterStation().getOffsetToInertial(common.getState().getFrame(), transitDate, true);
final TimeStampedPVCoordinates emitterApprox =
emitterToInertial.transformPVCoordinates(new TimeStampedPVCoordinates(transitDate,
Vector3D.ZERO, Vector3D.ZERO, Vector3D.ZERO));
// Uplink time of flight from emitter station to transit state
final double tauU = signalTimeOfFlight(emitterApprox, transitPV.getPosition(), transitDate);
// Secondary station PV in inertial frame at rebound date on secondary station
final TimeStampedPVCoordinates emitterPV = emitterApprox.shiftedBy(-tauU);
// Prepare the evaluation
final EstimatedMeasurementBase<BistaticRangeRate> estimated =
new EstimatedMeasurementBase<>(this,
iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
common.getStationDownlink(),
transitPV,
emitterPV
});
// Range-rate components
final Vector3D receiverDirection = common.getStationDownlink().getPosition()
.subtract(transitPV.getPosition()).normalize();
final Vector3D emitterDirection = emitterPV.getPosition()
.subtract(transitPV.getPosition()).normalize();
final Vector3D receiverVelocity = common.getStationDownlink().getVelocity()
.subtract(transitPV.getVelocity());
final Vector3D emitterVelocity = emitterPV.getVelocity()
.subtract(transitPV.getVelocity());
// range rate
final double rangeRate = Vector3D.dotProduct(receiverDirection, receiverVelocity) +
Vector3D.dotProduct(emitterDirection, emitterVelocity);
estimated.setEstimatedValue(rangeRate);
return estimated;
}
/** {@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
// and station parameters
// ----------------------
//
// Parameters:
// - 0..2 - Position of the spacecraft in inertial frame
// - 3..5 - Velocity of the spacecraft in inertial frame
// - 6..n - measurements parameters (clock offset, station offsets, pole, prime meridian, sat clock offset...)
final GroundReceiverCommonParametersWithDerivatives common = computeCommonParametersWithDerivatives(state);
final int nbParams = common.getTauD().getFreeParameters();
final TimeStampedFieldPVCoordinates<Gradient> transitPV = common.getTransitPV();
final FieldAbsoluteDate<Gradient> transitDate = transitPV.getDate();
// Approximate emitter location (at transit time)
final FieldVector3D<Gradient> zero = FieldVector3D.getZero(common.getTauD().getField());
final FieldTransform<Gradient> emitterToInertial =
getEmitterStation().getOffsetToInertial(state.getFrame(), transitDate, nbParams, common.getIndices());
final TimeStampedFieldPVCoordinates<Gradient> emitterApprox =
emitterToInertial.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(transitDate,
zero, zero, zero));
// Uplink time of flight from emiiter to transit state
final Gradient tauU = signalTimeOfFlight(emitterApprox, transitPV.getPosition(), transitPV.getDate());
// Emitter coordinates at transmit time
final TimeStampedFieldPVCoordinates<Gradient> emitterPV = emitterApprox.shiftedBy(tauU.negate());
// Prepare the evaluation
final EstimatedMeasurement<BistaticRangeRate> estimated = new EstimatedMeasurement<>(this,
iteration, evaluation,
new SpacecraftState[] {
common.getTransitState()
},
new TimeStampedPVCoordinates[] {
common.getStationDownlink().toTimeStampedPVCoordinates(),
common.getTransitPV().toTimeStampedPVCoordinates(),
emitterPV.toTimeStampedPVCoordinates()
});
// Range-rate components
final FieldVector3D<Gradient> receiverDirection = common.getStationDownlink().getPosition()
.subtract(transitPV.getPosition()).normalize();
final FieldVector3D<Gradient> emitterDirection = emitterPV.getPosition()
.subtract(transitPV.getPosition()).normalize();
final FieldVector3D<Gradient> receiverVelocity = common.getStationDownlink().getVelocity()
.subtract(transitPV.getVelocity());
final FieldVector3D<Gradient> emitterVelocity = emitterPV.getVelocity()
.subtract(transitPV.getVelocity());
// range rate
final Gradient rangeRate = FieldVector3D.dotProduct(receiverDirection, receiverVelocity)
.add(FieldVector3D.dotProduct(emitterDirection, emitterVelocity));
estimated.setEstimatedValue(rangeRate.getValue());
// Range partial derivatives with respect to state
final double[] derivatives = rangeRate.getGradient();
estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
// set partial derivatives with respect to parameters
// (beware element at index 0 is the value, not a derivative)
for (final ParameterDriver driver : getParametersDrivers()) {
for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
final Integer index = common.getIndices().get(span.getData());
if (index != null) {
estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
}
}
}
return estimated;
}
}