/* Copyright 2002-2017 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.attitudes;
import org.hipparchus.geometry.euclidean.threed.Rotation;
import org.hipparchus.geometry.euclidean.threed.RotationConvention;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.errors.OrekitException;
import org.orekit.frames.Frame;
import org.orekit.frames.Transform;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.PVCoordinates;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.TimeStampedAngularCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/**
* This class handles yaw compensation attitude provider.
* <p>
* Yaw compensation is mainly used for Earth observation satellites. As a
* satellites moves along its track, the image of ground points move on
* the focal point of the optical sensor. This motion is a combination of
* the satellite motion, but also on the Earth rotation and on the current
* attitude (in particular if the pointing includes Roll or Pitch offset).
* In order to reduce geometrical distortion, the yaw angle is changed a
* little from the simple ground pointing attitude such that the apparent
* motion of ground points is along a prescribed axis (orthogonal to the
* optical sensors rows), taking into account all effects.
* </p>
* <p>
* This attitude is implemented as a wrapper on top of an underlying ground
* pointing law that defines the roll and pitch angles.
* </p>
* <p>
* Instances of this class are guaranteed to be immutable.
* </p>
* @see GroundPointing
* @author Véronique Pommier-Maurussane
*/
public class YawCompensation extends GroundPointing implements AttitudeProviderModifier {
/** Serializable UID. */
private static final long serialVersionUID = 20150529L;
/** J axis. */
private static final PVCoordinates PLUS_J =
new PVCoordinates(Vector3D.PLUS_J, Vector3D.ZERO, Vector3D.ZERO);
/** K axis. */
private static final PVCoordinates PLUS_K =
new PVCoordinates(Vector3D.PLUS_K, Vector3D.ZERO, Vector3D.ZERO);
/** Underlying ground pointing attitude provider. */
private final GroundPointing groundPointingLaw;
/** Creates a new instance.
* @param inertialFrame frame in which orbital velocities are computed
* @param groundPointingLaw ground pointing attitude provider without yaw compensation
* @exception OrekitException if the frame specified is not a pseudo-inertial frame
* @since 7.1
*/
public YawCompensation(final Frame inertialFrame, final GroundPointing groundPointingLaw)
throws OrekitException {
super(inertialFrame, groundPointingLaw.getBodyFrame());
this.groundPointingLaw = groundPointingLaw;
}
/** Get the underlying (ground pointing) attitude provider.
* @return underlying attitude provider, which in this case is a {@link GroundPointing} instance
*/
public AttitudeProvider getUnderlyingAttitudeProvider() {
return groundPointingLaw;
}
/** {@inheritDoc} */
protected TimeStampedPVCoordinates getTargetPV(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame)
throws OrekitException {
return groundPointingLaw.getTargetPV(pvProv, date, frame);
}
/** Compute the base system state at given date, without compensation.
* @param pvProv provider for PV coordinates
* @param date date at which state is requested
* @param frame reference frame from which attitude is computed
* @return satellite base attitude state, i.e without compensation.
* @throws OrekitException if some specific error occurs
*/
public Attitude getBaseState(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame)
throws OrekitException {
return groundPointingLaw.getAttitude(pvProv, date, frame);
}
/** {@inheritDoc} */
@Override
public Attitude getAttitude(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame)
throws OrekitException {
final Transform bodyToRef = getBodyFrame().getTransformTo(frame, date);
// compute sliding target ground point
final PVCoordinates slidingRef = getTargetPV(pvProv, date, frame);
final PVCoordinates slidingBody = bodyToRef.getInverse().transformPVCoordinates(slidingRef);
// compute relative position of sliding ground point with respect to satellite
final PVCoordinates relativePosition =
new PVCoordinates(pvProv.getPVCoordinates(date, frame), slidingRef);
// compute relative velocity of fixed ground point with respect to sliding ground point
// the velocity part of the transformPVCoordinates for the sliding point ps
// from body frame to reference frame is:
// d(ps_ref)/dt = r(d(ps_body)/dt + dq/dt) - Ω ⨯ ps_ref
// where r is the rotation part of the transform, Ω is the corresponding
// angular rate, and dq/dt is the derivative of the translation part of the
// transform (the translation itself, without derivative, is hidden in the
// ps_ref part in the cross product).
// The sliding point ps is co-located to a fixed ground point pf (i.e. they have the
// same position at time of computation), but this fixed point as zero velocity
// with respect to the ground. So the velocity part of the transformPVCoordinates
// for this fixed point can be computed using the same formula as above:
// from body frame to reference frame is:
// d(pf_ref)/dt = r(0 + dq/dt) - Ω ⨯ pf_ref
// so remembering that the two points are at the same location at computation time,
// i.e. that at t=0 pf_ref=ps_ref, the relative velocity between the fixed point
// and the sliding point is given by the simple expression:
// d(ps_ref)/dt - d(pf_ref)/dt = r(d(ps_body)/dt)
// the acceleration is computed by differentiating the expression, which gives:
// d²(ps_ref)/dt² - d²(pf_ref)/dt² = r(d²(ps_body)/dt²) - Ω ⨯ [d(ps_ref)/dt - d(pf_ref)/dt]
final Vector3D v = bodyToRef.getRotation().applyTo(slidingBody.getVelocity());
final Vector3D a = new Vector3D(+1, bodyToRef.getRotation().applyTo(slidingBody.getAcceleration()),
-1, Vector3D.crossProduct(bodyToRef.getRotationRate(), v));
final PVCoordinates relativeVelocity = new PVCoordinates(v, a, Vector3D.ZERO);
final PVCoordinates relativeNormal =
PVCoordinates.crossProduct(relativePosition, relativeVelocity).normalize();
// attitude definition :
// . Z satellite axis points to sliding target
// . target relative velocity is in (Z,X) plane, in the -X half plane part
return new Attitude(frame,
new TimeStampedAngularCoordinates(date,
relativePosition.normalize(),
relativeNormal.normalize(),
PLUS_K, PLUS_J,
1.0e-9));
}
/** Compute the yaw compensation angle at date.
* @param pvProv provider for PV coordinates
* @param date date at which compensation is requested
* @param frame reference frame from which attitude is computed
* @return yaw compensation angle for orbit.
* @throws OrekitException if some specific error occurs
*/
public double getYawAngle(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame)
throws OrekitException {
final Rotation rBase = getBaseState(pvProv, date, frame).getRotation();
final Rotation rCompensated = getAttitude(pvProv, date, frame).getRotation();
final Rotation compensation = rCompensated.compose(rBase.revert(), RotationConvention.VECTOR_OPERATOR);
return -compensation.applyTo(Vector3D.PLUS_I).getAlpha();
}
}