InterSatDirectViewDetector.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.propagation.events;

  19. import org.hipparchus.geometry.euclidean.threed.Vector3D;
  20. import org.hipparchus.util.FastMath;
  21. import org.orekit.bodies.OneAxisEllipsoid;
  22. import org.orekit.frames.Frame;
  23. import org.orekit.propagation.PropagatorsParallelizer;
  24. import org.orekit.propagation.SpacecraftState;
  25. import org.orekit.propagation.events.handlers.ContinueOnEvent;
  26. import org.orekit.propagation.events.handlers.EventHandler;
  27. import org.orekit.time.AbsoluteDate;
  28. import org.orekit.utils.PVCoordinatesProvider;


  31. /** Detector for inter-satellites direct view (i.e. no masking by central body limb).
  32.  * <p>
  33.  * As this detector needs two satellites, it embeds one {@link
  34.  * PVCoordinatesProvider coordinates provider} for the secondary satellite
  35.  * and is registered as an event detector in the propagator of the primary
  36.  * satellite. The secondary satellite provider will therefore be driven by this
  37.  * detector (and hence by the propagator in which this detector is registered).
  38.  * </p>
  39.  * <p>
  40.  * In order to avoid infinite recursion, care must be taken to have the secondary
  41.  * satellite provider being <em>completely independent</em> from anything else.
  42.  * In particular, if the provider is a propagator, it should <em>not</em> be run
  43.  * together in a {@link PropagatorsParallelizer propagators parallelizer} with
  44.  * the propagator this detector is registered in. It is fine however to configure
  45.  * two separate propagators PsA and PsB with similar settings for the secondary satellite
  46.  * and one propagator Pm for the primary satellite and then use Psa in this detector
  47.  * registered within Pm while Pm and Psb are run in the context of a {@link
  48.  * PropagatorsParallelizer propagators parallelizer}.
  49.  * </p>
  50.  * <p>
  51.  * For efficiency reason during the event search loop, it is recommended to have
  52.  * the secondary provider be an analytical propagator or an ephemeris. A numerical propagator
  53.  * as a secondary propagator works but is expected to be computationally costly.
  54.  * </p>
  55.  * <p>
  56.  * The {@code g} function of this detector is positive when satellites can see
  57.  * each other directly and negative when the central body limb is in between and
  58.  * blocks the direct view.
  59.  * </p>
  60.  * <p>
  61.  * This detector only checks masking by central body limb, it does not take into
  62.  * account satellites antenna patterns. If these patterns must be considered, then
  63.  * this detector can be {@link BooleanDetector#andCombine(EventDetector...) and combined}
  64.  * with  the {@link BooleanDetector#notCombine(EventDetector) logical not} of
  65.  * {@link FieldOfViewDetector field of view detectors}.
  66.  * </p>
  67.  * @author Luc Maisonobe
  68.  * @since 9.3
  69.  */
  70. public class InterSatDirectViewDetector extends AbstractDetector<InterSatDirectViewDetector> {

  72.     /** Central body. */
  73.     private final OneAxisEllipsoid body;

  75.     /** Equatorial radius squared. */
  76.     private final double ae2;

  78.     /** 1 minus flatness squared. */
  79.     private final double g2;

  81.     /** Coordinates provider for the secondary satellite. */
  82.     private final PVCoordinatesProvider secondary;

  84.     /** simple constructor.
  85.      *
  86.      * @param body central body
  87.      * @param secondary provider for the secondary satellite
  88.      */
  89.     public InterSatDirectViewDetector(final OneAxisEllipsoid body, final PVCoordinatesProvider secondary) {
  90.         this(body, secondary, DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, DEFAULT_MAX_ITER,
  91.              new ContinueOnEvent<>());
  92.     }

  94.     /** Private constructor.
  95.      * @param body central body
  96.      * @param secondary provider for the secondary satellite
  97.      * @param maxCheck  maximum checking interval (s)
  98.      * @param threshold convergence threshold (s)
  99.      * @param maxIter   maximum number of iterations in the event time search
  100.      * @param handler   event handler to call at event occurrences
  101.      */
  102.     private InterSatDirectViewDetector(final OneAxisEllipsoid body,
  103.                                        final PVCoordinatesProvider secondary,
  104.                                        final double maxCheck,
  105.                                        final double threshold,
  106.                                        final int maxIter,
  107.                                        final EventHandler<? super InterSatDirectViewDetector> handler) {
  108.         super(maxCheck, threshold, maxIter, handler);
  109.         this.body  = body;
  110.         this.ae2   = body.getEquatorialRadius() * body.getEquatorialRadius();
  111.         this.g2    = (1.0 - body.getFlattening()) * (1.0 - body.getFlattening());
  112.         this.secondary = secondary;
  113.     }

  115.     /** Get the central body.
  116.      * @return central body
  117.      */
  118.     public OneAxisEllipsoid getCentralBody() {
  119.         return body;
  120.     }

  122.     /** Get the provider for the secondary satellite.
  123.      * @return provider for the secondary satellite
  124.      */
  125.     public PVCoordinatesProvider getSecondary() {
  126.         return secondary;
  127.     }

  129.     /** {@inheritDoc} */
  130.     @Override
  131.     protected InterSatDirectViewDetector create(final double newMaxCheck,
  132.                                                 final double newThreshold,
  133.                                                 final int newMaxIter,
  134.                                                 final EventHandler<? super InterSatDirectViewDetector> newHandler) {
  135.         return new InterSatDirectViewDetector(body, secondary, newMaxCheck, newThreshold, newMaxIter, newHandler);
  136.     }

  138.     /** {@inheritDoc}
  139.      * <p>
  140.      * The {@code g} function of this detector is positive when satellites can see
  141.      * each other directly and negative when the central body limb is in between and
  142.      * blocks the direct view.
  143.      * </p>
  144.      */
  145.     @Override
  146.     public double g(final SpacecraftState state) {

  148.         // get the line between primary and secondary in body frame
  149.         final AbsoluteDate date    = state.getDate();
  150.         final Frame        frame   = body.getBodyFrame();
  151.         final Vector3D     pPrimary = state.getPVCoordinates(frame).getPosition();
  152.         final Vector3D     pSecondary  = secondary.getPVCoordinates(date, frame).getPosition();

  154.         // points along the primary/secondary lines are defined as
  155.         // xk = x + k * dx, yk = y + k * dy, zk = z + k * dz
  156.         // so k is 0 at primary and 1 at secondary
  157.         final double x  = pPrimary.getX();
  158.         final double y  = pPrimary.getY();
  159.         final double z  = pPrimary.getZ();
  160.         final double dx = pSecondary.getX() - x;
  161.         final double dy = pSecondary.getY() - y;
  162.         final double dz = pSecondary.getZ() - z;

  164.         // intersection between line and central body surface
  165.         // is a root of a 2nd degree polynomial :
  166.         // a k^2 - 2 b k + c = 0
  167.         final double a =   g2 * (dx * dx + dy * dy) + dz * dz;
  168.         final double b = -(g2 * (x * dx + y * dy) + z * dz);
  169.         final double c =   g2 * (x * x + y * y - ae2) + z * z;
  170.         final double s = b * b - a * c;
  171.         if (s < 0) {
  172.             // the quadratic has no solution, the line between primary and secondary
  173.             // doesn't crosses central body limb, direct view is possible
  174.             // return a positive value, preserving continuity across zero crossing
  175.             return -s;
  176.         }

  178.         // the quadratic has two solutions (degenerated to one if s = 0)
  179.         // direct view is blocked when one of these solutions is between 0 and 1
  180.         final double k1 = (b < 0) ? (b - FastMath.sqrt(s)) / a : c / (b + FastMath.sqrt(s));
  181.         final double k2 = c / (a * k1);
  182.         if (FastMath.max(k1, k2) < 0.0 || FastMath.min(k1, k2) > 1.0) {
  183.             // the intersections are either behind primary or farther away than secondary
  184.             // along the line, direct view is possible
  185.             // return a positive value, preserving continuity across zero crossing
  186.             return s;
  187.         } else {
  188.             // part of the central body is between primary and secondary
  189.             // this includes unrealistic cases where primary, secondary or both are inside the central body ;-)
  190.             // in all these cases, direct view is blocked
  191.             // return a negative value, preserving continuity across zero crossing
  192.             return -s;
  193.         }

  195.     }

  197. }
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