BistaticRange.java

/* Copyright 2002-2025 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 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.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 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 = signalTimeOfFlightAdjustableEmitter(emitterApprox, transitPV.getPosition(), transitDate,
                                                                common.getState().getFrame());

        // Secondary station PV in inertial frame at rebound date on secondary station
        final TimeStampedPVCoordinates emitterPV = emitterApprox.shiftedBy(-tauU);

        // Prepare the evaluation
        final EstimatedMeasurementBase<BistaticRange> estimated =
                        new EstimatedMeasurementBase<>(this,
                                                       iteration, evaluation,
                                                       new SpacecraftState[] {
                                                           common.getTransitState()
                                                       },
                                                       new TimeStampedPVCoordinates[] {
                                                           common.getStationDownlink(),
                                                           transitPV,
                                                           emitterPV
                                                       });

        // Clock offsets
        final double dte = getEmitterStation().getClockOffsetDriver().getValue(common.getState().getDate());
        final double dtr = getReceiverStation().getClockOffsetDriver().getValue(common.getState().getDate());

        // Range value
        final double tau = common.getTauD() + tauU + dtr - dte;
        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];

        // Bistatic 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...)
        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 = signalTimeOfFlightAdjustableEmitter(emitterApprox, transitPV.getPosition(),
                                                                  transitPV.getDate(), state.getFrame());

        // Emitter coordinates at transmit time
        final TimeStampedFieldPVCoordinates<Gradient> emitterPV = emitterApprox.shiftedBy(tauU.negate());

        // Prepare the evaluation
        final EstimatedMeasurement<BistaticRange> estimated = new EstimatedMeasurement<>(this,
                iteration, evaluation,
                new SpacecraftState[] {
                    common.getTransitState()
                },
                new TimeStampedPVCoordinates[] {
                    common.getStationDownlink().toTimeStampedPVCoordinates(),
                    common.getTransitPV().toTimeStampedPVCoordinates(),
                    emitterPV.toTimeStampedPVCoordinates()
                });

        // Clock offsets
        final Gradient dte = getEmitterStation().getClockOffsetDriver().getValue(nbParams, common.getIndices(), state.getDate());
        final Gradient dtr = getReceiverStation().getClockOffsetDriver().getValue(nbParams, common.getIndices(), state.getDate());

        // Range value
        final Gradient tau = common.getTauD().add(tauU).add(dtr).subtract(dte);
        final Gradient range = tau.multiply(Constants.SPEED_OF_LIGHT);

        estimated.setEstimatedValue(range.getValue());

        // Range first order derivatives with respect to state
        final double[] derivatives = range.getGradient();
        estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));

        // Set first order derivatives with respect to parameters
        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;
    }

}