BistaticRange.java
/* Copyright 2002-2023 Mark Rutten
* 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.
* Mark Rutten 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 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.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.Constants;
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 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.
* <p>
* The motion of the stations and the spacecraft during the signal flight time are taken into account.
* </p>
*
* @author Mark Rutten
* @since 11.2
*/
public class BistaticRange extends GroundReceiverMeasurement<BistaticRange> {
/** Type of the measurement. */
public static final String MEASUREMENT_TYPE = "BistaticRange";
/**
* Ground station from which transmission is made.
*/
private final GroundStation emitter;
/**
* Simple constructor.
*
* @param emitter ground station from which transmission is performed
* @param receiver ground station from which measurement is performed
* @param date date of the measurement
* @param range observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param satellite satellite related to this measurement
* @since 11.2
*/
public BistaticRange(final GroundStation emitter, final GroundStation receiver, final AbsoluteDate date,
final double range, final double sigma, final double baseWeight,
final ObservableSatellite satellite) {
super(receiver, true, date, range, sigma, baseWeight, satellite);
addParameterDriver(emitter.getClockOffsetDriver());
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<BistaticRange> theoreticalEvaluationWithoutDerivatives(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
final SpacecraftState state = states[0];
// Coordinates of the spacecraft
final TimeStampedPVCoordinates pva = state.getPVCoordinates();
// transform between station and inertial frame, expressed as a gradient
// The components of station's position in offset frame are the 3 last derivative parameters
final Transform offsetToInertialRx = getReceiverStation().getOffsetToInertial(state.getFrame(), getDate(), false);
final AbsoluteDate downlinkDate = offsetToInertialRx.getDate();
// Station position in inertial frame at end of the downlink leg
final TimeStampedPVCoordinates stationReceiver =
offsetToInertialRx.transformPVCoordinates(new TimeStampedPVCoordinates(downlinkDate,
Vector3D.ZERO, Vector3D.ZERO, Vector3D.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 double tauD = signalTimeOfFlight(pva, stationReceiver.getPosition(), downlinkDate);
// Transit state & Transit state (re)computed with gradients
final double delta = downlinkDate.durationFrom(state.getDate());
final double deltaMTauD = delta - tauD;
final SpacecraftState transitState = state.shiftedBy(deltaMTauD);
final TimeStampedPVCoordinates transitStateDS = pva.shiftedBy(deltaMTauD);
// transform between secondary station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
// The components of secondary station's position in offset frame are the 3 last derivative parameters
final AbsoluteDate transitDate = downlinkDate.shiftedBy(-tauD);
final Transform offsetToInertialTxApprox = getEmitterStation().getOffsetToInertial(state.getFrame(), transitDate, true);
// Secondary station PV in inertial frame at transit time
final TimeStampedPVCoordinates transmitApprox =
offsetToInertialTxApprox.transformPVCoordinates(new TimeStampedPVCoordinates(transitDate,
Vector3D.ZERO, Vector3D.ZERO, Vector3D.ZERO));
// Uplink time of flight from secondary station to transit state of leg2
final double tauU = signalTimeOfFlight(transmitApprox, transitStateDS.getPosition(), transitStateDS.getDate());
// Total time of flight
final double tauTotal = tauU - deltaMTauD;
// Absolute date of transmission
final AbsoluteDate transmitDate = downlinkDate.shiftedBy(tauTotal);
final Transform transmitToInert = emitter.getOffsetToInertial(state.getFrame(), transmitDate, true);
// Secondary station PV in inertial frame at rebound date on secondary station
final TimeStampedPVCoordinates stationTransmitter =
transmitToInert.transformPVCoordinates(new TimeStampedPVCoordinates(transmitDate,
Vector3D.ZERO, Vector3D.ZERO, Vector3D.ZERO));
// Prepare the evaluation
final EstimatedMeasurementBase<BistaticRange> estimated =
new EstimatedMeasurementBase<>(this,
iteration, evaluation,
new SpacecraftState[] {
transitState
},
new TimeStampedPVCoordinates[] {
stationReceiver,
transitStateDS,
stationTransmitter
});
// Range value
final double tau = tauD + tauU;
final double range = tau * Constants.SPEED_OF_LIGHT;
estimated.setEstimatedValue(range);
return estimated;
}
/**
* {@inheritDoc}
*/
@Override
protected EstimatedMeasurement<BistaticRange> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
final SpacecraftState state = states[0];
// Range 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...)
int nbParams = 6;
final Map<String, Integer> indices = new HashMap<>();
for (ParameterDriver driver : getParametersDrivers()) {
if (driver.isSelected()) {
for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
if (!indices.containsKey(span.getData())) {
indices.put(span.getData(), nbParams++);
}
}
}
}
final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));
// Coordinates of the spacecraft expressed as a gradient
final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);
// transform between station and inertial frame, expressed as a gradient
// The components of station's position in offset frame are the 3 last derivative parameters
final FieldTransform<Gradient> offsetToInertialRx =
getReceiverStation().getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
final FieldAbsoluteDate<Gradient> downlinkDateDS = offsetToInertialRx.getFieldDate();
// Station position in inertial frame at end of the downlink leg
final TimeStampedFieldPVCoordinates<Gradient> stationReceiver =
offsetToInertialRx.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
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(pvaDS, stationReceiver.getPosition(), downlinkDateDS);
// Transit state & Transit state (re)computed with gradients
final Gradient delta = downlinkDateDS.durationFrom(state.getDate());
final Gradient deltaMTauD = tauD.negate().add(delta);
final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());
final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaDS.shiftedBy(deltaMTauD);
// transform between secondary station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
// The components of secondary station's position in offset frame are the 3 last derivative parameters
final FieldAbsoluteDate<Gradient> transitDate = downlinkDateDS.shiftedBy(tauD.negate());
final FieldTransform<Gradient> offsetToInertialTxApprox =
getEmitterStation().getOffsetToInertial(state.getFrame(), transitDate, nbParams, indices);
// Secondary station PV in inertial frame at transit time
final TimeStampedFieldPVCoordinates<Gradient> transmitApprox =
offsetToInertialTxApprox.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(transitDate,
zero, zero, zero));
// Uplink time of flight from secondary station to transit state of leg2
final Gradient tauU = signalTimeOfFlight(transmitApprox, transitStateDS.getPosition(), transitStateDS.getDate());
// Total time of flight
final Gradient tauTotal = deltaMTauD.negate().add(tauU);
// Absolute date of transmission
final FieldAbsoluteDate<Gradient> transmitDateDS = downlinkDateDS.shiftedBy(tauTotal);
final FieldTransform<Gradient> transmitToInert =
emitter.getOffsetToInertial(state.getFrame(), transmitDateDS, nbParams, indices);
// Secondary station PV in inertial frame at rebound date on secondary station
final TimeStampedFieldPVCoordinates<Gradient> stationTransmitter =
transmitToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(transmitDateDS,
zero, zero, zero));
// Prepare the evaluation
final EstimatedMeasurement<BistaticRange> estimated = new EstimatedMeasurement<>(this,
iteration, evaluation,
new SpacecraftState[] {
transitState
},
new TimeStampedPVCoordinates[] {
stationReceiver.toTimeStampedPVCoordinates(),
transitStateDS.toTimeStampedPVCoordinates(),
stationTransmitter.toTimeStampedPVCoordinates()
});
// Range value
final Gradient tau = tauD.add(tauU);
final Gradient range = tau.multiply(Constants.SPEED_OF_LIGHT);
estimated.setEstimatedValue(range.getValue());
// Range partial derivatives with respect to state
final double[] derivatives = range.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 = indices.get(span.getData());
if (index != null) {
estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
}
}
}
return estimated;
}
}