UpdatableFrame.java

/* Copyright 2002-2013 CS Systèmes d'Information
 * Licensed to CS Systèmes d'Information (CS) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * CS 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.frames;

import java.util.concurrent.atomic.AtomicReference;

import org.orekit.errors.FrameAncestorException;
import org.orekit.errors.OrekitException;
import org.orekit.errors.OrekitMessages;
import org.orekit.time.AbsoluteDate;


/** Frame whose transform from its parent can be updated.
 * <p>This class allows to control the relative position of two parts
 * of the global frames tree using any two frames in each part as
 * control handles. Consider the following simplified frames tree as an
 * example:</p>
 * <pre>
 *              GCRF
 *                |
 *  --------------------------------
 *  |             |                |
 * Sun        satellite          Earth
 *                |                |
 *        on-board antenna   ground station
 *                                 |
 *                          tracking antenna
 * </pre>
 * <p>Tracking measurements really correspond to the link between the ground
 * and on-board antennas. This is tightly linked to the transform between
 * these two frames, however neither frame is the direct parent frame of the
 * other one: the path involves four intermediate frames. When we process a
 * measurement, what we really want to update is the transform that defines
 * the satellite frame with respect to its parent GCRF frame.</p>
 * <p>In order to implement the above case, the satellite frame is defined
 * as an instance of this class and its {@link #updateTransform(Frame, Frame,
 * Transform, AbsoluteDate) updateTransform} would be called each time we want
 * to adjust the frame, i.e. each time we get a new measurement between the
 * two antennas.</p>
 * @author Luc Maisonobe
 */
public class UpdatableFrame extends Frame {

    /** Serializable UID. */
    private static final long serialVersionUID = -2075893064211339303L;

    /** Build a non-inertial frame from its transform with respect to its parent.
     * <p>calling this constructor is equivalent to call
     * {@link #UpdatableFrame(Frame, Transform, String, boolean)
     * UpdatableFrame(parent, transform, name, false)}.</p>
     * @param parent parent frame (must be non-null)
     * @param transform transform from parent frame to instance
     * @param name name of the frame
     * @exception IllegalArgumentException if the parent frame is null
     */
    public UpdatableFrame(final Frame parent, final Transform transform, final String name)
        throws IllegalArgumentException {
        this(parent, transform, name, false);
    }

    /** Build a frame from its transform with respect to its parent.
     * <p>The convention for the transform is that it is from parent
     * frame to instance. This means that the two following frames
     * are similar:</p>
     * <pre>
     * Frame frame1 = new Frame(FramesFactory.getGCRF(), new Transform(t1, t2));
     * Frame frame2 = new Frame(new Frame(FramesFactory.getGCRF(), t1), t2);
     * </pre>
     * @param parent parent frame (must be non-null)
     * @param transform transform from parent frame to instance
     * @param name name of the frame
     * @param pseudoInertial true if frame is considered pseudo-inertial
     * (i.e. suitable for propagating orbit)
     * @exception IllegalArgumentException if the parent frame is null
     */
    public UpdatableFrame(final Frame parent, final Transform transform, final String name,
                          final boolean pseudoInertial)
        throws IllegalArgumentException {
        super(parent, new UpdatableProvider(transform), name, pseudoInertial);
    }

    /** Update the transform from parent frame implicitly according to two other
     * frames.

     * <p>This method allows to control the relative position of two parts
     * of the global frames tree using any two frames in each part as
     * control handles. Consider the following simplified frames tree as an
     * example:</p>
     * <pre>
     *              GCRF
     *                |
     *  --------------------------------
     *  |             |                |
     * Sun        satellite          Earth
     *                |                |
     *        on-board antenna   ground station
     *                                 |
     *                          tracking antenna
     * </pre>
     * <p>Tracking measurements really correspond to the link between the ground
     * and on-board antennas. This is tightly linked to the transform between
     * these two frames, however neither frame is the direct parent frame of the
     * other one: the path involves four intermediate frames. When we process a
     * measurement, what we really want to update is the transform that defines
     * the satellite frame with respect to its parent GCRF frame. This
     * is the purpose of this method. This update is done by the following call,
     * where <code>measurementTransform</code> represents the measurement as a
     * simple translation transform between the two antenna frames:</p>
     * <pre><code>
     * satellite.updateTransform(onBoardAntenna, trackingAntenna,
     *                           measurementTransform, date);
     * </code></pre>
     * <p>One way to represent the behavior of the method is to consider the
     * sub-tree rooted at the instance on one hand (satellite and on-board antenna
     * in the example above) and the tree containing all the other frames on the
     * other hand (GCRF, Sun, Earth, ground station, tracking antenna).
     * Both tree are considered as two solid sets linked together by a flexible
     * spring, which is the transform we want to update. The method stretches the
     * spring to make sure the transform between the two specified frames (one in
     * each tree part) matches the specified transform.</p>
     * @param f1 first control frame (may be the instance itself)
     * @param f2 second control frame (may be the instance itself)
     * @param f1Tof2 desired transform from first to second control frame
     * @param date date of the transform
     * @exception OrekitException if the path between the two control frames does
     * not cross the link between instance and its parent frame or if some
     * intermediate transform fails
     */
    public void updateTransform(final Frame f1, final Frame f2, final Transform f1Tof2,
                                final AbsoluteDate date) throws OrekitException {

        Frame fA = f1;
        Frame fB = f2;
        Transform fAtoB = f1Tof2;

        // make sure f1 is not a child of the instance
        if (fA.isChildOf(this) || (fA == this)) {

            if (fB.isChildOf(this) || (fB == this)) {
                throw new FrameAncestorException(OrekitMessages.FRAME_ANCESTOR_OF_BOTH_FRAMES,
                                                 getName(), fA.getName(), fB.getName());
            }

            // swap f1 and f2 to make sure the child is f2
            final Frame tmp = fA;
            fA = fB;
            fB = tmp;
            fAtoB = fAtoB.getInverse();

        } else  if (!(fB.isChildOf(this) || (fB == this))) {
            throw new FrameAncestorException(OrekitMessages.FRAME_ANCESTOR_OF_NEITHER_FRAME,
                                             getName(), fA.getName(), fB.getName());
        }

        // rebuild the transform by traveling from parent to self
        // WITHOUT using the existing provider from parent to self that will be updated
        final Transform parentTofA   = getParent().getTransformTo(fA, date);
        final Transform fBtoSelf     = fB.getTransformTo(this, date);
        final Transform fAtoSelf     = new Transform(date, fAtoB, fBtoSelf);
        final Transform parentToSelf = new Transform(date, parentTofA, fAtoSelf);

        // update the existing provider from parent to self
        ((UpdatableProvider) getTransformProvider()).setTransform(parentToSelf);

    }

    /** Local provider for transforms. */
    private static class UpdatableProvider implements TransformProvider {

        /** Serializable UID. */
        private static final long serialVersionUID = 4436954500689776331L;

        /** Current transform. */
        private AtomicReference<Transform> transform;

        /** Simple constructor.
         * @param transform initial value of the transform
         */
        public UpdatableProvider(final Transform transform) {
            this.transform = new AtomicReference<Transform>(transform);
        }

        /** Update the transform from the parent frame to the instance.
         * @param transform new transform from parent frame to instance
         */
        public void setTransform(final Transform transform) {
            this.transform.set(transform);
        }

        /** {@inheritDoc} */
        public Transform getTransform(final AbsoluteDate date) {
            return transform.get();
        }

    }

}