TimeSpanDragForce.java

  1. /* Copyright 2002-2025 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.forces.drag;

  19. import java.util.ArrayList;
  20. import java.util.List;
  21. import java.util.stream.Stream;

  23. import org.hipparchus.CalculusFieldElement;
  24. import org.hipparchus.Field;
  25. import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  26. import org.hipparchus.geometry.euclidean.threed.Vector3D;
  27. import org.hipparchus.util.MathArrays;
  28. import org.orekit.annotation.DefaultDataContext;
  29. import org.orekit.frames.Frame;
  30. import org.orekit.models.earth.atmosphere.Atmosphere;
  31. import org.orekit.propagation.FieldSpacecraftState;
  32. import org.orekit.propagation.SpacecraftState;
  33. import org.orekit.propagation.events.DateDetector;
  34. import org.orekit.propagation.events.EventDetector;
  35. import org.orekit.propagation.events.FieldDateDetector;
  36. import org.orekit.propagation.events.FieldEventDetector;
  37. import org.orekit.time.AbsoluteDate;
  38. import org.orekit.time.FieldAbsoluteDate;
  39. import org.orekit.time.TimeScale;
  40. import org.orekit.time.TimeScalesFactory;
  41. import org.orekit.utils.ParameterDriver;
  42. import org.orekit.utils.ParameterDriversProvider;
  43. import org.orekit.utils.TimeSpanMap;
  44. import org.orekit.utils.TimeSpanMap.Span;
  45. import org.orekit.utils.TimeSpanMap.Transition;


  48. /** Time span atmospheric drag force model.
  49.  *  <p>
  50.  *  This class is closely related to {@link org.orekit.forces.drag.DragForce DragForce} class.<br>
  51.  *  The difference is that it has a {@link TimeSpanMap} of {@link DragSensitive} objects as attribute
  52.  *  instead of a single {@link DragSensitive} object. <br>
  53.  *  The idea behind this model is to allow the user to design a drag force model that can see its physical parameters
  54.  *  (drag coefficient and lift ratio) change with time, at dates chosen by the user. <br>
  55.  *  </p>
  56.  *  <p>
  57.  *  This is a behavior that can be sought in operational orbit determination.<br>
  58.  *  Indeed the solar activity has a strong influence on the local atmospheric density, and thus on the drag force effect.<br>
  59.  *  Solar activity is a physical phenomenon that is difficult to model and predict. <br>
  60.  *  The errors induced by this incomplete modeling can be estimated through the drag coefficients.<br>
  61.  *  Being able to define and estimate drag coefficients depending on user-chosen dates in a piecewise fashion allows for
  62.  *  a better  modeling of solar activity uncertainties.
  63.  *  </p>
  64.  *  <p>
  65.  *  A typical operational use case is to have a daily solar activity with three-hourly magnetic indexes provided by an
  66.  *  international organization (NOAA for example).<br>
  67.  *  Given this input, a user can define a piecewise drag force model with daily or three-hourly drag coefficients.<br>
  68.  *  Each timed coefficient will absorb a part of the uncertainties in the solar activity and will allow for a more accurate
  69.  *  orbit determination
  70.  *  </p>
  71.  *  <b>Usage</b>:<ul>
  72.  *  <li><u>Construction</u>: constructor takes an atmospheric model and a DragSensitive model.<br>
  73.  *  This last model will be your initial DragSensitive model and it will be initially valid for the whole time line.<br>
  74.  *  The real validity of this first entry will be truncated as other DragSensitive models are added.
  75.  *  <li><u>Time spans</u>: DragSensitive models are added using methods {@link #addDragSensitiveValidAfter(DragSensitive, AbsoluteDate)}
  76.  *   or {@link #addDragSensitiveValidBefore(DragSensitive, AbsoluteDate)}.<br>
  77.  *   Recommendations are the same than the ones in {@link TimeSpanMap}, meaning: <ul>
  78.  *   <li>As an entry is added, it truncates the validity of the neighboring entries already present in the map;
  79.  *   <li><b>The transition dates should be entered only once</b>. Repeating a transition date will lead to unexpected result and is not supported;
  80.  *   <li>It is advised to order your DragSensitive models chronologically when adding them to avoid any confusion.
  81.  *   </ul>
  82.  *   <li><u>Naming the parameter drivers</u>: It is strongly advised to give a custom name to the {@link ParameterDriver}(s)
  83.  *   of each DragSensitive model that is added to the object. This will allow you keeping track of the evolution of your models.<br>
  84.  *   Different names are mandatory to differentiate the different drivers.<br>
  85.  *   If you do not specify a name, a default name will be chosen. Example for the drag coefficient:<ul>
  86.  *   <li>Initial DragSensitive model: the driver's default name is "{@link DragSensitive#DRAG_COEFFICIENT}";
  87.  *   <li>Using {@link #addDragSensitiveValidAfter(DragSensitive, AbsoluteDate)}: the driver's default name is
  88.  *   "{@link DragSensitive#DRAG_COEFFICIENT} + {@link #DATE_AFTER} + date.toString()"
  89.  *   <li>Using {@link #addDragSensitiveValidBefore(DragSensitive, AbsoluteDate)}: the driver's default name is
  90.  *   "{@link DragSensitive#DRAG_COEFFICIENT} + {@link #DATE_BEFORE} + date.toString()"
  91.  *   </ul>
  92.  *   </ul>
  93.  *  <b>Example following previous recommendations</b>:<ul>
  94.  *  <li>Given:
  95.  *  <ul>
  96.  *  <li><code>atmosphere</code>: an {@link Atmosphere atmospheric model};
  97.  *  <li><code>isotropicDrag0, 1 and 2</code>: three {@link org.orekit.forces.drag.IsotropicDrag IsotropicDrag} models;
  98.  *  <li><code>date</code>: an {@link AbsoluteDate}.
  99.  *  </ul>
  100.  *  <li>Name the drivers:<br>
  101.  *  <code>isotropicDrag0.getDragParametersDrivers()[0].setName = "Cd0";</code><br>
  102.  *  <code>isotropicDrag1.getDragParametersDrivers()[0].setName = "Cd1";</code><br>
  103.  *  <code>isotropicDrag2.getDragParametersDrivers()[0].setName = "Cd2";</code><br>
  104.  *  <li>Initialize the model: <br>
  105.  *  <code>TimeSpanDragForce force = new TimeSpanDragForce(atmosphere, isotropicDrag0);</code>
  106.  *  <li>Set the second and third model one Julian day apart each:<br>
  107.  *  <code>force.addDragSensitiveValidAfter(isotropicDrag1, date.shiftedBy(Constants.JULIAN_DAY));</code><br>
  108.  *  <code>force.addDragSensitiveValidAfter(isotropicDrag2, date.shiftedBy(2 * Constants.JULIAN_DAY));</code><br>
  109.  *  <li>With this, your model will have the following properties:
  110.  *  <ul>
  111.  *  <li>t in ]-∞, date + 1 day [ / Cd = Cd0
  112.  *  <li>t in [date + 1 day, date + 2days [ / Cd = Cd1
  113.  *  <li>t in [date + 2 days, +∞ [ / Cd = Cd2
  114.  *  </ul>
  115.  *  </ul>
  116.  *  <p>
  117.  *  <b>Warning</b>:<br> The TimeSpanDragForce model is versatile and you could end up with non-physical modeling.<br>
  118.  *  For example you could add 2 {@link org.orekit.forces.drag.IsotropicDrag IsotropicDrag} models with different areas,
  119.  *  or one {@link org.orekit.forces.drag.IsotropicDrag IsotropicDrag} model and then one
  120.  *  {@link org.orekit.forces.BoxAndSolarArraySpacecraft BoxAndSolarArraySpacecraft} model.<br>
  121.  *  It is up to you to ensure that your models are consistent with each other, Orekit will not perform any check for that.
  122.  *  </p>
  123.  * @author Maxime Journot
  124.  * @since 10.2
  125.  */
  126. public class TimeSpanDragForce extends AbstractDragForceModel {

  128.     /** Prefix for dates before in the parameter drivers' name. */
  129.     public static final String DATE_BEFORE = " - Before ";

  131.     /** Prefix for dates after in the parameter drivers' name. */
  132.     public static final String DATE_AFTER = " - After ";

  134.     /** TimeSpanMap of DragSensitive objects. */
  135.     private final TimeSpanMap<DragSensitive> dragSensitiveTimeSpanMap;

  137.     /** Time scale used for the default names of the drag parameter drivers. */
  138.     private final TimeScale timeScale;

  140.     /** Constructor with default UTC time scale for the default names of the drag parameter drivers.
  141.      * @param atmosphere atmospheric model
  142.      * @param spacecraft Time scale used for the default names of the drag parameter drivers
  143.      */
  144.     @DefaultDataContext
  145.     public TimeSpanDragForce(final Atmosphere atmosphere,
  146.                              final DragSensitive spacecraft) {
  147.         super(atmosphere);
  148.         this.dragSensitiveTimeSpanMap = new TimeSpanMap<>(spacecraft);
  149.         this.timeScale = TimeScalesFactory.getUTC();
  150.     }

  152.     /** Constructor.
  153.      * @param atmosphere atmospheric model
  154.      * @param spacecraft the initial object physical and geometric information
  155.      * @param timeScale Time scale used for the default names of the drag parameter drivers
  156.      */
  157.     public TimeSpanDragForce(final Atmosphere atmosphere,
  158.                              final DragSensitive spacecraft,
  159.                              final TimeScale timeScale) {
  160.         super(atmosphere);
  161.         this.dragSensitiveTimeSpanMap = new TimeSpanMap<>(spacecraft);
  162.         this.timeScale = timeScale;
  163.     }

  165.     /** Add a DragSensitive entry valid before a limit date.<br>
  166.      * Using <code>addDragSensitiveValidBefore(entry, t)</code> will make <code>entry</code>
  167.      * valid in ]-∞, t[ (note the open bracket).
  168.      * @param dragSensitive DragSensitive entry
  169.      * @param latestValidityDate date before which the entry is valid
  170.      * (must be different from <b>all</b> dates already used for transitions)
  171.      */
  172.     public void addDragSensitiveValidBefore(final DragSensitive dragSensitive, final AbsoluteDate latestValidityDate) {
  173.         dragSensitiveTimeSpanMap.addValidBefore(changeDragParameterDriversNames(dragSensitive,
  174.                                                                                 latestValidityDate,
  175.                                                                                 DATE_BEFORE),
  176.                                                 latestValidityDate, false);
  177.     }

  179.     /** Add a DragSensitive entry valid after a limit date.<br>
  180.      * Using <code>addDragSensitiveValidAfter(entry, t)</code> will make <code>entry</code>
  181.      * valid in [t, +∞[ (note the closed bracket).
  182.      * @param dragSensitive DragSensitive entry
  183.      * @param earliestValidityDate date after which the entry is valid
  184.      * (must be different from <b>all</b> dates already used for transitions)
  185.      */
  186.     public void addDragSensitiveValidAfter(final DragSensitive dragSensitive, final AbsoluteDate earliestValidityDate) {
  187.         dragSensitiveTimeSpanMap.addValidAfter(changeDragParameterDriversNames(dragSensitive,
  188.                                                                                earliestValidityDate,
  189.                                                                                DATE_AFTER),
  190.                                                earliestValidityDate, false);
  191.     }

  193.     /** Get the {@link DragSensitive} model valid at a date.
  194.      * @param date the date of validity
  195.      * @return the DragSensitive model valid at date
  196.      */
  197.     public DragSensitive getDragSensitive(final AbsoluteDate date) {
  198.         return dragSensitiveTimeSpanMap.get(date);
  199.     }

  201.     /** Get the {@link DragSensitive} {@link Span} containing a specified date.
  202.      * @param date date belonging to the desired time span
  203.      * @return the DragSensitive time span containing the specified date
  204.      */
  205.     public Span<DragSensitive> getDragSensitiveSpan(final AbsoluteDate date) {
  206.         return dragSensitiveTimeSpanMap.getSpan(date);
  207.     }

  209.     /** Extract a range of the {@link DragSensitive} map.
  210.      * <p>
  211.      * The object returned will be a new independent instance that will contain
  212.      * only the transitions that lie in the specified range.
  213.      * </p>
  214.      * See the {@link TimeSpanMap#extractRange TimeSpanMap.extractRange method} for more.
  215.      * @param start earliest date at which a transition is included in the range
  216.      * (may be set to {@link AbsoluteDate#PAST_INFINITY} to keep all early transitions)
  217.      * @param end latest date at which a transition is included in the r
  218.      * (may be set to {@link AbsoluteDate#FUTURE_INFINITY} to keep all late transitions)
  219.      * @return a new TimeSpanMap instance of DragSensitive with all transitions restricted to the specified range
  220.      */
  221.     public TimeSpanMap<DragSensitive> extractDragSensitiveRange(final AbsoluteDate start, final AbsoluteDate end) {
  222.         return dragSensitiveTimeSpanMap.extractRange(start, end);
  223.     }

  225.     /** Get the first {@link Span time span} of the drag sensitive time span map.
  226.      * @return the first {@link Span time span} of the drag sensitive time span map
  227.      * @since 11.1
  228.      */
  229.     public Span<DragSensitive> getFirstSpan() {
  230.         return dragSensitiveTimeSpanMap.getFirstSpan();
  231.     }

  233.     /** {@inheritDoc} */
  234.     @Override
  235.     public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {

  237.         // Local atmospheric density
  238.         final AbsoluteDate date     = s.getDate();
  239.         final Frame        frame    = s.getFrame();
  240.         final Vector3D     position = s.getPosition();
  241.         final double rho    = getAtmosphere().getDensity(date, position, frame);

  243.         // Spacecraft relative velocity with respect to the atmosphere
  244.         final Vector3D vAtm = getAtmosphere().getVelocity(date, position, frame);
  245.         final Vector3D relativeVelocity = vAtm.subtract(s.getPVCoordinates().getVelocity());

  247.         // Extract the proper parameters valid at date from the input array
  248.         final double[] extractedParameters = extractParameters(parameters, date);

  250.         // Compute and return drag acceleration
  251.         return getDragSensitive(date).dragAcceleration(s, rho, relativeVelocity, extractedParameters);

  253.     }

  255.     /** {@inheritDoc} */
  256.     @Override
  257.     public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
  258.                                                                              final T[] parameters) {
  259.         // Density and its derivatives
  260.         final T rho = getFieldDensity(s);

  262.         // Spacecraft relative velocity with respect to the atmosphere
  263.         final FieldAbsoluteDate<T> date     = s.getDate();
  264.         final Frame                frame    = s.getFrame();
  265.         final FieldVector3D<T>     position = s.getPosition();
  266.         final FieldVector3D<T> vAtm = getAtmosphere().getVelocity(date, position, frame);
  267.         final FieldVector3D<T> relativeVelocity = vAtm.subtract(s.getPVCoordinates().getVelocity());

  269.         // Extract the proper parameters valid at date from the input array
  270.         final T[] extractedParameters = extractParameters(parameters, date);

  272.         // Compute and return drag acceleration
  273.         return getDragSensitive(date.toAbsoluteDate()).dragAcceleration(s, rho, relativeVelocity, extractedParameters);
  274.     }

  276.     /**{@inheritDoc}
  277.      * <p>
  278.      * A date detector is used to cleanly stop the propagator and reset
  279.      * the state derivatives at transition dates.
  280.      * </p>
  281.      */
  282.     @Override
  283.     public Stream<EventDetector> getEventDetectors() {

  285.         // Get the transitions' dates from the TimeSpanMap
  286.         final AbsoluteDate[] transitionDates = getTransitionDates();

  288.         // create detector and return it in Stream
  289.         final DateDetector detector = getDateDetector(transitionDates);
  290.         return Stream.of(detector);
  291.     }

  293.     /** {@inheritDoc}
  294.      * <p>
  295.      * A date detector is used to cleanly stop the propagator and reset
  296.      * the state derivatives at transition dates.
  297.      * </p>
  298.      */
  299.     @Override
  300.     public <T extends CalculusFieldElement<T>> Stream<FieldEventDetector<T>> getFieldEventDetectors(final Field<T> field) {

  302.         // Get the transitions' dates from the TimeSpanMap
  303.         final AbsoluteDate[] transitionDates = getTransitionDates();

  305.         // create detector and return it in Stream
  306.         final FieldDateDetector<T> detector = getFieldDateDetector(field, transitionDates);
  307.         return Stream.of(detector);
  308.     }

  310.     /** {@inheritDoc}
  311.      * <p>
  312.      * All the parameter drivers of all DragSensitive models are returned in an array.
  313.      * Models are ordered chronologically.
  314.      * </p>
  315.      */
  316.     @Override
  317.     public List<ParameterDriver> getParametersDrivers() {

  319.         // Get all transitions from the TimeSpanMap
  320.         final List<ParameterDriver> listParameterDrivers = new ArrayList<>();

  322.         // Loop on the spans
  323.         for (Span<DragSensitive> span = getFirstSpan(); span != null; span = span.next()) {
  324.             // Add all the parameter drivers of the span
  325.             for (ParameterDriver driver : span.getData().getDragParametersDrivers()) {
  326.                 // Add the driver only if the name does not exist already
  327.                 if (!findByName(listParameterDrivers, driver.getName())) {
  328.                     listParameterDrivers.add(driver);
  329.                 }
  330.             }
  331.         }

  333.         // Return an array of parameter drivers with no duplicated name
  334.         return listParameterDrivers;

  336.     }

  338.     /** Extract the proper parameter drivers' values from the array in input of the
  339.      * {@link #acceleration(SpacecraftState, double[]) acceleration} method.
  340.      *  Parameters are filtered given an input date.
  341.      * @param parameters the input parameters array
  342.      * @param date the date
  343.      * @return the parameters given the date
  344.      */
  345.     public double[] extractParameters(final double[] parameters, final AbsoluteDate date) {

  347.         // Get the drag parameter drivers of the date
  348.         final List<ParameterDriver> dragParameterDriver = getDragSensitive(date).getDragParametersDrivers();

  350.         // Find out the indexes of the parameters in the whole array of parameters
  351.         final List<ParameterDriver> allParameters = getParametersDrivers();
  352.         final double[] outParameters = new double[dragParameterDriver.size()];
  353.         int index = 0;
  354.         for (int i = 0; i < allParameters.size(); i++) {
  355.             final String driverName = allParameters.get(i).getName();
  356.             for (ParameterDriver dragDriver : dragParameterDriver) {
  357.                 if (dragDriver.getName().equals(driverName)) {
  358.                     outParameters[index++] = parameters[i];
  359.                 }
  360.             }
  361.         }
  362.         return outParameters;
  363.     }

  365.     /** Extract the proper parameter drivers' values from the array in input of the
  366.      * {@link #acceleration(FieldSpacecraftState, CalculusFieldElement[]) acceleration} method.
  367.      *  Parameters are filtered given an input date.
  368.      * @param parameters the input parameters array
  369.      * @param date the date
  370.      * @param <T> extends CalculusFieldElement
  371.      * @return the parameters given the date
  372.      */
  373.     public <T extends CalculusFieldElement<T>> T[] extractParameters(final T[] parameters,
  374.                                                                  final FieldAbsoluteDate<T> date) {

  376.         // Get the drag parameter drivers of the date
  377.         final List<ParameterDriver> dragPD = getDragSensitive(date.toAbsoluteDate()).getDragParametersDrivers();

  379.         // Find out the indexes of the parameters in the whole array of parameters
  380.         final List<ParameterDriver> allParameters = getParametersDrivers();
  381.         final T[] outParameters = MathArrays.buildArray(date.getField(), dragPD.size());
  382.         int index = 0;
  383.         for (int i = 0; i < allParameters.size(); i++) {
  384.             final String driverName = allParameters.get(i).getName();
  385.             for (ParameterDriver dragDriver : dragPD) {
  386.                 if (dragDriver.getName().equals(driverName)) {
  387.                     outParameters[index++] = parameters[i];
  388.                 }
  389.             }
  390.         }
  391.         return outParameters;
  392.     }

  394.     /** Find if a parameter driver with a given name already exists in a list of parameter drivers.
  395.      * @param driversList the list of parameter drivers
  396.      * @param name the parameter driver's name to filter with
  397.      * @return true if the name was found, false otherwise
  398.      */
  399.     private boolean findByName(final List<ParameterDriver> driversList, final String name) {
  400.         return ParameterDriversProvider.findByName(driversList, name);
  401.     }

  403.     /** Get the dates of the transitions for the drag sensitive models {@link TimeSpanMap}.
  404.      * @return dates of the transitions for the drag sensitive models {@link TimeSpanMap}
  405.      */
  406.     private AbsoluteDate[] getTransitionDates() {

  408.         // Get all transitions
  409.         final List<AbsoluteDate> listDates = new ArrayList<>();

  411.         // Extract all the transitions' dates
  412.         for (Transition<DragSensitive> transition = getFirstSpan().getEndTransition(); transition != null; transition = transition.next()) {
  413.             listDates.add(transition.getDate());
  414.         }
  415.         // Return the array of transition dates
  416.         return listDates.toArray(new AbsoluteDate[0]);
  417.     }

  419.     /** Change the parameter drivers names of a {@link DragSensitive} model, if needed.
  420.      * <p>
  421.      * This is done to avoid that several parameter drivers have the same name.<br>
  422.      * It is done only if the user hasn't modify the DragSensitive parameter drivers default names.
  423.      * </p>
  424.      * @param dragSensitive the DragSensitive model
  425.      * @param date the date used in the parameter driver's name
  426.      * @param datePrefix the date prefix used in the parameter driver's name
  427.      * @return the DragSensitive with its drivers' names changed
  428.      */
  429.     private DragSensitive changeDragParameterDriversNames(final DragSensitive dragSensitive,
  430.                                                           final AbsoluteDate date,
  431.                                                           final String datePrefix) {
  432.         // Loop on the parameter drivers of the DragSensitive model
  433.         for (ParameterDriver driver: dragSensitive.getDragParametersDrivers()) {
  434.             final String driverName = driver.getName();

  436.             // If the name is the default name for DragSensitive parameter drivers
  437.             // Modify the name to add the prefix and the date
  438.             if (driverName.equals(DragSensitive.GLOBAL_DRAG_FACTOR) ||
  439.                 driverName.equals(DragSensitive.DRAG_COEFFICIENT) ||
  440.                 driverName.equals(DragSensitive.LIFT_RATIO)) {
  441.                 driver.setName(driverName + datePrefix + date.toString(timeScale));
  442.             }
  443.         }
  444.         return dragSensitive;
  445.     }

  447. }
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