AngularAzEl.java

  1. /* Copyright 2002-2022 CS GROUP
  2.  * Licensed to CS GROUP (CS) under one or more
  3.  * contributor license agreements.  See the NOTICE file distributed with
  4.  * this work for additional information regarding copyright ownership.
  5.  * CS licenses this file to You under the Apache License, Version 2.0
  6.  * (the "License"); you may not use this file except in compliance with
  7.  * the License.  You may obtain a copy of the License at
  8.  *
  9.  *   http://www.apache.org/licenses/LICENSE-2.0
  10.  *
  11.  * Unless required by applicable law or agreed to in writing, software
  12.  * distributed under the License is distributed on an "AS IS" BASIS,
  13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14.  * See the License for the specific language governing permissions and
  15.  * limitations under the License.
  16.  */
  17. package org.orekit.estimation.measurements;

  19. import java.util.Arrays;
  20. import java.util.Collections;
  21. import java.util.HashMap;
  22. import java.util.Map;

  24. import org.hipparchus.Field;
  25. import org.hipparchus.analysis.differentiation.Gradient;
  26. import org.hipparchus.analysis.differentiation.GradientField;
  27. import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  28. import org.hipparchus.util.MathUtils;
  29. import org.orekit.frames.FieldTransform;
  30. import org.orekit.propagation.SpacecraftState;
  31. import org.orekit.time.AbsoluteDate;
  32. import org.orekit.time.FieldAbsoluteDate;
  33. import org.orekit.utils.ParameterDriver;
  34. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  35. import org.orekit.utils.TimeStampedPVCoordinates;

  37. /** Class modeling an Azimuth-Elevation measurement from a ground station.
  38.  * The motion of the spacecraft during the signal flight time is taken into
  39.  * account. The date of the measurement corresponds to the reception on
  40.  * ground of the reflected signal.
  41.  *
  42.  * @author Thierry Ceolin
  43.  * @since 8.0
  44.  */
  45. public class AngularAzEl extends AbstractMeasurement<AngularAzEl> {

  47.     /** Ground station from which measurement is performed. */
  48.     private final GroundStation station;

  50.     /** Simple constructor.
  51.      * @param station ground station from which measurement is performed
  52.      * @param date date of the measurement
  53.      * @param angular observed value
  54.      * @param sigma theoretical standard deviation
  55.      * @param baseWeight base weight
  56.      * @param satellite satellite related to this measurement
  57.      * @since 9.3
  58.      */
  59.     public AngularAzEl(final GroundStation station, final AbsoluteDate date,
  60.                        final double[] angular, final double[] sigma, final double[] baseWeight,
  61.                        final ObservableSatellite satellite) {
  62.         super(date, angular, sigma, baseWeight, Collections.singletonList(satellite));
  63.         addParameterDriver(station.getClockOffsetDriver());
  64.         addParameterDriver(station.getEastOffsetDriver());
  65.         addParameterDriver(station.getNorthOffsetDriver());
  66.         addParameterDriver(station.getZenithOffsetDriver());
  67.         addParameterDriver(station.getPrimeMeridianOffsetDriver());
  68.         addParameterDriver(station.getPrimeMeridianDriftDriver());
  69.         addParameterDriver(station.getPolarOffsetXDriver());
  70.         addParameterDriver(station.getPolarDriftXDriver());
  71.         addParameterDriver(station.getPolarOffsetYDriver());
  72.         addParameterDriver(station.getPolarDriftYDriver());
  73.         this.station = station;
  74.     }

  76.     /** Get the ground station from which measurement is performed.
  77.      * @return ground station from which measurement is performed
  78.      */
  79.     public GroundStation getStation() {
  80.         return station;
  81.     }

  83.     /** {@inheritDoc} */
  84.     @Override
  85.     protected EstimatedMeasurement<AngularAzEl> theoreticalEvaluation(final int iteration, final int evaluation,
  86.                                                                       final SpacecraftState[] states) {

  88.         final SpacecraftState state = states[0];

  90.         // Azimuth/elevation derivatives are computed with respect to spacecraft state in inertial frame
  91.         // and station parameters
  92.         // ----------------------
  93.         //
  94.         // Parameters:
  95.         //  - 0..2 - Position of the spacecraft in inertial frame
  96.         //  - 3..5 - Velocity of the spacecraft in inertial frame
  97.         //  - 6..n - station parameters (clock offset, station offsets, pole, prime meridian...)

  99.         // Get the number of parameters used for derivation
  100.         // Place the selected drivers into a map
  101.         int nbParams = 6;
  102.         final Map<String, Integer> indices = new HashMap<>();
  103.         for (ParameterDriver driver : getParametersDrivers()) {
  104.             if (driver.isSelected()) {
  105.                 indices.put(driver.getName(), nbParams++);
  106.             }
  107.         }
  108.         final Field<Gradient>         field   = GradientField.getField(nbParams);
  109.         final FieldVector3D<Gradient> zero    = FieldVector3D.getZero(field);

  111.         // Coordinates of the spacecraft expressed as a gradient
  112.         final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);

  114.         // Transform between station and inertial frame, expressed as a gradient
  115.         // The components of station's position in offset frame are the 3 last derivative parameters
  116.         final FieldTransform<Gradient> offsetToInertialDownlink =
  117.                         station.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
  118.         final FieldAbsoluteDate<Gradient> downlinkDateDS =
  119.                         offsetToInertialDownlink.getFieldDate();

  121.         // Station position/velocity in inertial frame at end of the downlink leg
  122.         final TimeStampedFieldPVCoordinates<Gradient> stationDownlink =
  123.                         offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
  124.                                                                                                             zero, zero, zero));
  125.         // Station topocentric frame (east-north-zenith) in inertial frame expressed as Gradient
  126.         final FieldVector3D<Gradient> east   = offsetToInertialDownlink.transformVector(FieldVector3D.getPlusI(field));
  127.         final FieldVector3D<Gradient> north  = offsetToInertialDownlink.transformVector(FieldVector3D.getPlusJ(field));
  128.         final FieldVector3D<Gradient> zenith = offsetToInertialDownlink.transformVector(FieldVector3D.getPlusK(field));

  130.         // Compute propagation times
  131.         // (if state has already been set up to pre-compensate propagation delay,
  132.         //  we will have delta == tauD and transitState will be the same as state)

  134.         // Downlink delay
  135.         final Gradient tauD = signalTimeOfFlight(pvaDS, stationDownlink.getPosition(), downlinkDateDS);

  137.         // Transit state
  138.         final Gradient        delta        = downlinkDateDS.durationFrom(state.getDate());
  139.         final Gradient        deltaMTauD   = tauD.negate().add(delta);
  140.         final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());

  142.         // Transit state (re)computed with derivative structures
  143.         final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaDS.shiftedBy(deltaMTauD);

  145.         // Station-satellite vector expressed in inertial frame
  146.         final FieldVector3D<Gradient> staSat = transitStateDS.getPosition().subtract(stationDownlink.getPosition());

  148.         // Compute azimuth/elevation
  149.         final Gradient baseAzimuth = staSat.dotProduct(east).atan2(staSat.dotProduct(north));
  150.         final double   twoPiWrap   = MathUtils.normalizeAngle(baseAzimuth.getReal(), getObservedValue()[0]) -
  151.                                                 baseAzimuth.getReal();
  152.         final Gradient azimuth     = baseAzimuth.add(twoPiWrap);
  153.         final Gradient elevation   = staSat.dotProduct(zenith).divide(staSat.getNorm()).asin();

  155.         // Prepare the estimation
  156.         final EstimatedMeasurement<AngularAzEl> estimated =
  157.                         new EstimatedMeasurement<>(this, iteration, evaluation,
  158.                                                    new SpacecraftState[] {
  159.                                                        transitState
  160.                                                    }, new TimeStampedPVCoordinates[] {
  161.                                                        transitStateDS.toTimeStampedPVCoordinates(),
  162.                                                        stationDownlink.toTimeStampedPVCoordinates()
  163.                                                    });

  165.         // azimuth - elevation values
  166.         estimated.setEstimatedValue(azimuth.getValue(), elevation.getValue());

  168.         // Partial derivatives of azimuth/elevation with respect to state
  169.         // (beware element at index 0 is the value, not a derivative)
  170.         final double[] azDerivatives = azimuth.getGradient();
  171.         final double[] elDerivatives = elevation.getGradient();
  172.         estimated.setStateDerivatives(0,
  173.                                       Arrays.copyOfRange(azDerivatives, 0, 6), Arrays.copyOfRange(elDerivatives, 0, 6));

  175.         // Set partial derivatives with respect to parameters
  176.         // (beware element at index 0 is the value, not a derivative)
  177.         for (final ParameterDriver driver : getParametersDrivers()) {
  178.             final Integer index = indices.get(driver.getName());
  179.             if (index != null) {
  180.                 estimated.setParameterDerivatives(driver, azDerivatives[index], elDerivatives[index]);
  181.             }
  182.         }

  184.         return estimated;
  185.     }
  186. }
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