TopocentricFrame.java

  1. /* Copyright 2002-2013 CS Systèmes d'Information
  2.  * Licensed to CS Systèmes d'Information (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.frames;

  19. import org.apache.commons.math3.analysis.UnivariateFunction;
  20. import org.apache.commons.math3.analysis.solvers.BracketingNthOrderBrentSolver;
  21. import org.apache.commons.math3.analysis.solvers.UnivariateSolver;
  22. import org.apache.commons.math3.exception.TooManyEvaluationsException;
  23. import org.apache.commons.math3.geometry.euclidean.threed.Rotation;
  24. import org.apache.commons.math3.geometry.euclidean.threed.Vector3D;
  25. import org.apache.commons.math3.util.FastMath;
  26. import org.apache.commons.math3.util.MathUtils;
  27. import org.orekit.bodies.BodyShape;
  28. import org.orekit.bodies.GeodeticPoint;
  29. import org.orekit.errors.OrekitException;
  30. import org.orekit.errors.OrekitExceptionWrapper;
  31. import org.orekit.time.AbsoluteDate;
  32. import org.orekit.utils.Constants;
  33. import org.orekit.utils.PVCoordinates;
  34. import org.orekit.utils.PVCoordinatesProvider;


  37. /** Topocentric frame.
  38.  * <p>Frame associated to a position near the surface of a body shape.</p>
  39.  * <p>
  40.  * The origin of the frame is at the defining {@link GeodeticPoint geodetic point}
  41.  * location, and the right-handed canonical trihedra is:
  42.  * </p>
  43.  * <ul>
  44.  *   <li>X axis in the local horizontal plane (normal to zenith direction) and
  45.  *   following the local parallel towards East</li>
  46.  *   <li>Y axis in the horizontal plane (normal to zenith direction) and
  47.  *   following the local meridian towards North</li>
  48.  *   <li>Z axis towards Zenith direction</li>
  49.  * </ul>
  50.  * @author V&eacute;ronique Pommier-Maurussane
  51.  */
  52. public class TopocentricFrame extends Frame implements PVCoordinatesProvider {

  54.     /** Serializable UID. */
  55.     private static final long serialVersionUID = -5997915708080966466L;

  57.     /** Body shape on which the local point is defined. */
  58.     private final BodyShape parentShape;

  60.     /** Point where the topocentric frame is defined. */
  61.     private final GeodeticPoint point;

  63.     /** Simple constructor.
  64.      * @param parentShape body shape on which the local point is defined
  65.      * @param point local surface point where topocentric frame is defined
  66.      * @param name the string representation
  67.      */
  68.     public TopocentricFrame(final BodyShape parentShape, final GeodeticPoint point,
  69.                             final String name) {

  71.         super(parentShape.getBodyFrame(),
  72.               new Transform(AbsoluteDate.J2000_EPOCH,
  73.                             new Transform(AbsoluteDate.J2000_EPOCH,
  74.                                           parentShape.transform(point).negate()),
  75.                             new Transform(AbsoluteDate.J2000_EPOCH,
  76.                                           new Rotation(point.getEast(), point.getZenith(),
  77.                                                        Vector3D.PLUS_I, Vector3D.PLUS_K),
  78.                                           Vector3D.ZERO)),
  79.               name, false);
  80.         this.parentShape = parentShape;
  81.         this.point = point;
  82.     }

  84.     /** Get the body shape on which the local point is defined.
  85.      * @return body shape on which the local point is defined
  86.      */
  87.     public BodyShape getParentShape() {
  88.         return parentShape;
  89.     }

  91.     /** Get the surface point defining the origin of the frame.
  92.      * @return surface point defining the origin of the frame
  93.      */
  94.     public GeodeticPoint getPoint() {
  95.         return point;
  96.     }

  98.     /** Get the zenith direction of topocentric frame, expressed in parent shape frame.
  99.      * <p>The zenith direction is defined as the normal to local horizontal plane.</p>
  100.      * @return unit vector in the zenith direction
  101.      * @see #getNadir()
  102.      */
  103.     public Vector3D getZenith() {
  104.         return point.getZenith();
  105.     }

  107.     /** Get the nadir direction of topocentric frame, expressed in parent shape frame.
  108.      * <p>The nadir direction is the opposite of zenith direction.</p>
  109.      * @return unit vector in the nadir direction
  110.      * @see #getZenith()
  111.      */
  112.     public Vector3D getNadir() {
  113.         return point.getNadir();
  114.     }

  116.    /** Get the north direction of topocentric frame, expressed in parent shape frame.
  117.      * <p>The north direction is defined in the horizontal plane
  118.      * (normal to zenith direction) and following the local meridian.</p>
  119.      * @return unit vector in the north direction
  120.      * @see #getSouth()
  121.      */
  122.     public Vector3D getNorth() {
  123.         return point.getNorth();
  124.     }

  126.     /** Get the south direction of topocentric frame, expressed in parent shape frame.
  127.      * <p>The south direction is the opposite of north direction.</p>
  128.      * @return unit vector in the south direction
  129.      * @see #getNorth()
  130.      */
  131.     public Vector3D getSouth() {
  132.         return point.getSouth();
  133.     }

  135.     /** Get the east direction of topocentric frame, expressed in parent shape frame.
  136.      * <p>The east direction is defined in the horizontal plane
  137.      * in order to complete direct triangle (east, north, zenith).</p>
  138.      * @return unit vector in the east direction
  139.      * @see #getWest()
  140.      */
  141.     public Vector3D getEast() {
  142.         return point.getEast();
  143.     }

  145.     /** Get the west direction of topocentric frame, expressed in parent shape frame.
  146.      * <p>The west direction is the opposite of east direction.</p>
  147.      * @return unit vector in the west direction
  148.      * @see #getEast()
  149.      */
  150.     public Vector3D getWest() {
  151.         return point.getWest();
  152.     }

  154.     /** Get the elevation of a point with regards to the local point.
  155.      * <p>The elevation is the angle between the local horizontal and
  156.      * the direction from local point to given point.</p>
  157.      * @param extPoint point for which elevation shall be computed
  158.      * @param frame frame in which the point is defined
  159.      * @param date computation date
  160.      * @return elevation of the point
  161.      * @exception OrekitException if frames transformations cannot be computed
  162.      */
  163.     public double getElevation(final Vector3D extPoint, final Frame frame,
  164.                                final AbsoluteDate date)
  165.         throws OrekitException {

  167.         // Transform given point from given frame to topocentric frame
  168.         final Transform t = frame.getTransformTo(this, date);
  169.         final Vector3D extPointTopo = t.transformPosition(extPoint);

  171.         // Elevation angle is PI/2 - angle between zenith and given point direction
  172.         return extPointTopo.getDelta();
  173.     }

  175.     /** Get the azimuth of a point with regards to the topocentric frame center point.
  176.      * <p>The azimuth is the angle between the North direction at local point and
  177.      * the projection in local horizontal plane of the direction from local point
  178.      * to given point. Azimuth angles are counted clockwise, i.e positive towards the East.</p>
  179.      * @param extPoint point for which elevation shall be computed
  180.      * @param frame frame in which the point is defined
  181.      * @param date computation date
  182.      * @return azimuth of the point
  183.      * @exception OrekitException if frames transformations cannot be computed
  184.      */
  185.     public double getAzimuth(final Vector3D extPoint, final Frame frame,
  186.                              final AbsoluteDate date)
  187.         throws OrekitException {

  189.         // Transform given point from given frame to topocentric frame
  190.         final Transform t = getTransformTo(frame, date).getInverse();
  191.         final Vector3D extPointTopo = t.transformPosition(extPoint);

  193.         // Compute azimuth
  194.         double azimuth = FastMath.atan2(extPointTopo.getX(), extPointTopo.getY());
  195.         if (azimuth < 0.) {
  196.             azimuth += MathUtils.TWO_PI;
  197.         }
  198.         return azimuth;

  200.     }

  202.     /** Get the range of a point with regards to the topocentric frame center point.
  203.      * @param extPoint point for which range shall be computed
  204.      * @param frame frame in which the point is defined
  205.      * @param date computation date
  206.      * @return range (distance) of the point
  207.      * @exception OrekitException if frames transformations cannot be computed
  208.      */
  209.     public double getRange(final Vector3D extPoint, final Frame frame,
  210.                            final AbsoluteDate date)
  211.         throws OrekitException {

  213.         // Transform given point from given frame to topocentric frame
  214.         final Transform t = frame.getTransformTo(this, date);
  215.         final Vector3D extPointTopo = t.transformPosition(extPoint);

  217.         // Compute range
  218.         return extPointTopo.getNorm();

  220.     }

  222.     /** Get the range rate of a point with regards to the topocentric frame center point.
  223.      * @param extPV point/velocity for which range rate shall be computed
  224.      * @param frame frame in which the point is defined
  225.      * @param date computation date
  226.      * @return range rate of the point (positive if point departs from frame)
  227.      * @exception OrekitException if frames transformations cannot be computed
  228.      */
  229.     public double getRangeRate(final PVCoordinates extPV, final Frame frame,
  230.                                final AbsoluteDate date)
  231.         throws OrekitException {

  233.         // Transform given point from given frame to topocentric frame
  234.         final Transform t = frame.getTransformTo(this, date);
  235.         final PVCoordinates extPVTopo = t.transformPVCoordinates(extPV);

  237.         // Compute range rate (doppler) : relative rate along the line of sight
  238.         return Vector3D.dotProduct(extPVTopo.getPosition(), extPVTopo.getVelocity()) /
  239.                extPVTopo.getPosition().getNorm();

  241.     }

  243.     /**
  244.      * Compute the limit visibility point for a satellite in a given direction.
  245.      * <p>
  246.      * This method can be used to compute visibility circles around ground stations
  247.      * for example, using a simple loop on azimuth, with either a fixed elevation
  248.      * or an elevation that depends on azimuth to take ground masks into account.
  249.      * </p>
  250.      * @param radius satellite distance to Earth center
  251.      * @param azimuth pointing azimuth from station
  252.      * @param elevation pointing elevation from station
  253.      * @return limit visibility point for the satellite
  254.      * @throws OrekitException if point cannot be found
  255.      */
  256.     public GeodeticPoint computeLimitVisibilityPoint(final double radius,
  257.                                                      final double azimuth, final double elevation)
  258.         throws OrekitException {
  259.         try {
  260.             // convergence threshold on point position: 1mm
  261.             final double deltaP = 0.001;
  262.             final UnivariateSolver solver =
  263.                     new BracketingNthOrderBrentSolver(deltaP / Constants.WGS84_EARTH_EQUATORIAL_RADIUS,
  264.                                                       deltaP, deltaP, 5);

  266.             // find the distance such that a point in the specified direction and at the solved-for
  267.             // distance is exactly at the specified radius
  268.             final double distance = solver.solve(1000, new UnivariateFunction() {
  269.                 /** {@inheritDoc} */
  270.                 public double value(final double x) {
  271.                     try {
  272.                         final GeodeticPoint gp = pointAtDistance(azimuth, elevation, x);
  273.                         return parentShape.transform(gp).getNorm() - radius;
  274.                     } catch (OrekitException oe) {
  275.                         throw new OrekitExceptionWrapper(oe);
  276.                     }
  277.                 }
  278.             }, 0, 2 * radius);

  280.             // return the limit point
  281.             return pointAtDistance(azimuth, elevation, distance);

  283.         } catch (TooManyEvaluationsException tmee) {
  284.             throw new OrekitException(tmee);
  285.         } catch (OrekitExceptionWrapper lwe) {
  286.             throw lwe.getException();
  287.         }
  288.     }

  290.     /** Compute the point observed from the station at some specified distance.
  291.      * @param azimuth pointing azimuth from station
  292.      * @param elevation pointing elevation from station
  293.      * @param distance distance to station
  294.      * @return observed point
  295.      * @exception OrekitException if point cannot be computed
  296.      */
  297.     public GeodeticPoint pointAtDistance(final double azimuth, final double elevation,
  298.                                          final double distance)
  299.         throws OrekitException {
  300.         final double cosAz = FastMath.cos(azimuth);
  301.         final double sinAz = FastMath.sin(azimuth);
  302.         final double cosEl = FastMath.cos(elevation);
  303.         final double sinEl = FastMath.sin(elevation);
  304.         final Vector3D  observed = new Vector3D(distance * cosEl * sinAz,
  305.                                                 distance * cosEl * cosAz,
  306.                                                 distance * sinEl);
  307.         return parentShape.transform(observed, this, AbsoluteDate.J2000_EPOCH);
  308.     }

  310.     /** Get the {@link PVCoordinates} of the topocentric frame origin in the selected frame.
  311.      * @param date current date
  312.      * @param frame the frame where to define the position
  313.      * @return position/velocity of the topocentric frame origin (m and m/s)
  314.      * @exception OrekitException if position cannot be computed in given frame
  315.      */
  316.     public PVCoordinates getPVCoordinates(final AbsoluteDate date, final Frame frame)
  317.         throws OrekitException {
  318.         return getTransformTo(frame, date).transformPVCoordinates(PVCoordinates.ZERO);
  319.     }

  321. }
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