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    * 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
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    *
    *   http://www.apache.org/licenses/LICENSE-2.0
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   * Unless required by applicable law or agreed to in writing, software
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  package org.orekit.gnss.attitude;
  
  import org.hipparchus.analysis.UnivariateFunction;
  import org.hipparchus.analysis.differentiation.DSFactory;
  import org.hipparchus.analysis.differentiation.DerivativeStructure;
  import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
  import org.hipparchus.analysis.solvers.UnivariateSolverUtils;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.FieldSinCos;
  import org.hipparchus.util.SinCos;
  import org.orekit.frames.Frame;
  import org.orekit.frames.LOFType;
  import org.orekit.frames.Transform;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.time.TimeStamped;
  import org.orekit.utils.ExtendedPVCoordinatesProvider;
  import org.orekit.utils.FieldPVCoordinates;
  import org.orekit.utils.PVCoordinates;
  import org.orekit.utils.PVCoordinatesProvider;
  import org.orekit.utils.TimeStampedAngularCoordinates;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /**
   * Boilerplate computations for GNSS attitude.
   *
   * <p>
   * This class is intended to hold throw-away data pertaining to <em>one</em> call
   * to {@link GNSSAttitudeProvider#getAttitude(org.orekit.utils.PVCoordinatesProvider,
   * org.orekit.time.AbsoluteDate, org.orekit.frames.Frame) getAttitude}.
   * </p>
   *
   * @author Luc Maisonobe
   * @since 9.2
   */
  class GNSSAttitudeContext implements TimeStamped {
  
      /** Derivation order. */
      private static final int ORDER = 2;
  
      /** Time derivation factory. */
      private static final DSFactory FACTORY = new DSFactory(1, ORDER);
  
      /** Constant Y axis. */
      private static final PVCoordinates PLUS_Y_PV =
              new PVCoordinates(Vector3D.PLUS_J, Vector3D.ZERO, Vector3D.ZERO);
  
      /** Constant Z axis. */
      private static final PVCoordinates MINUS_Z_PV =
              new PVCoordinates(Vector3D.MINUS_K, Vector3D.ZERO, Vector3D.ZERO);
  
      /** Limit value below which we shoud use replace beta by betaIni. */
      private static final double BETA_SIGN_CHANGE_PROTECTION = FastMath.toRadians(0.07);
  
      /** Current date. */
      private final AbsoluteDate date;
  
      /** Current date. */
      private final FieldAbsoluteDate<DerivativeStructure> dateDS;
  
      /** Provider for Sun position. */
      private final ExtendedPVCoordinatesProvider sun;
  
      /** Provider for spacecraft position. */
      private final PVCoordinatesProvider pvProv;
  
      /** Inertial frame where velocity are computed. */
      private final Frame inertialFrame;
  
      /** Cosine of the angle between spacecraft and Sun direction. */
      private final DerivativeStructure svbCos;
  
      /** Morning/Evening half orbit indicator. */
      private final boolean morning;
  
      /** Relative orbit angle to turn center. */
      private final DerivativeStructure delta;
  
      /** Spacecraft angular velocity. */
      private double muRate;
  
     /** Limit cosine for the midnight turn. */
     private double cNight;
 
     /** Limit cosine for the noon turn. */
     private double cNoon;
 
     /** Turn time data. */
     private TurnSpan turnSpan;
 
     /** Simple constructor.
      * @param date current date
      * @param sun provider for Sun position
      * @param pvProv provider for spacecraft position
      * @param inertialFrame inertial frame where velocity are computed
      * @param turnSpan turn time data, if a turn has already been identified in the date neighborhood,
      * null otherwise
      */
     GNSSAttitudeContext(final AbsoluteDate date,
                         final ExtendedPVCoordinatesProvider sun, final PVCoordinatesProvider pvProv,
                         final Frame inertialFrame, final TurnSpan turnSpan) {
 
         this.date          = date;
         this.dateDS        = addDerivatives(date);
         this.sun           = sun;
         this.pvProv        = pvProv;
         this.inertialFrame = inertialFrame;
         final FieldPVCoordinates<DerivativeStructure> sunPVDS = sun.getPVCoordinates(dateDS, inertialFrame);
 
         final TimeStampedPVCoordinates svPV = pvProv.getPVCoordinates(date, inertialFrame);
         final FieldPVCoordinates<DerivativeStructure> svPVDS  = svPV.toDerivativeStructurePV(FACTORY.getCompiler().getOrder());
         this.svbCos  = FieldVector3D.dotProduct(sunPVDS.getPosition(), svPVDS.getPosition()).
                        divide(sunPVDS.getPosition().getNorm().multiply(svPVDS.getPosition().getNorm()));
         this.morning = Vector3D.dotProduct(svPV.getVelocity(), sunPVDS.getPosition().toVector3D()) >= 0.0;
 
         this.muRate = svPV.getAngularVelocity().getNorm();
 
         this.turnSpan = turnSpan;
 
         final DerivativeStructure absDelta;
         if (svbCos.getValue() <= 0) {
             // night side
             absDelta = inOrbitPlaneAbsoluteAngle(svbCos.acos().negate().add(FastMath.PI));
         } else {
             // Sun side
             absDelta = inOrbitPlaneAbsoluteAngle(svbCos.acos());
         }
         delta = FastMath.copySign(absDelta, -absDelta.getPartialDerivative(1));
 
     }
 
     /** Convert a date, removing derivatives.
      * @param d date to convert
      * @return date without derivatives
      */
     private AbsoluteDate removeDerivatives(final FieldAbsoluteDate<DerivativeStructure> d) {
         return d.toAbsoluteDate();
     }
 
     /** Convert a date, adding derivatives.
      * @param d date to convert
      * @return date without derivatives
      */
     private FieldAbsoluteDate<DerivativeStructure> addDerivatives(final AbsoluteDate d) {
         return new FieldAbsoluteDate<>(FACTORY.getDerivativeField(), d).
                shiftedBy(FACTORY.variable(0, 0.0));
     }
 
     /** Compute nominal yaw steering.
      * @param d computation date
      * @return nominal yaw steering
      */
     public TimeStampedAngularCoordinates nominalYaw(final AbsoluteDate d) {
         final PVCoordinates svPV = pvProv.getPVCoordinates(d, inertialFrame);
         return new TimeStampedAngularCoordinates(d,
                                                  svPV.normalize(),
                                                  PVCoordinates.crossProduct(sun.getPVCoordinates(d, inertialFrame), svPV).normalize(),
                                                  MINUS_Z_PV,
                                                  PLUS_Y_PV,
                                                  1.0e-9);
     }
 
     /** Compute Sun elevation.
      * @param d computation date
      * @return Sun elevation
      */
     public double beta(final AbsoluteDate d) {
         final TimeStampedPVCoordinates svPV = pvProv.getPVCoordinates(d, inertialFrame);
         return 0.5 * FastMath.PI - Vector3D.angle(sun.getPVCoordinates(d, inertialFrame).getPosition(), svPV.getMomentum());
     }
 
     /** Compute Sun elevation.
      * @param d computation date
      * @return Sun elevation
      */
     private DerivativeStructure betaDS(final FieldAbsoluteDate<DerivativeStructure> d) {
         final TimeStampedPVCoordinates svPV = pvProv.getPVCoordinates(removeDerivatives(d), inertialFrame);
         final FieldPVCoordinates<DerivativeStructure> svPVDS  = svPV.toDerivativeStructurePV(FACTORY.getCompiler().getOrder());
         return FieldVector3D.angle(sun.getPVCoordinates(d, inertialFrame).getPosition(), svPVDS.getMomentum()).
                negate().
                add(0.5 * FastMath.PI);
     }
 
     /** Compute Sun elevation.
      * @return Sun elevation
      */
     public DerivativeStructure betaDS() {
         return betaDS(dateDS);
     }
 
     /** {@inheritDoc} */
     @Override
     public AbsoluteDate getDate() {
         return date;
     }
 
     /** Get the turn span.
      * @return turn span, may be null if context is outside of turn
      */
     public TurnSpan getTurnSpan() {
         return turnSpan;
     }
 
     /** Get the cosine of the angle between spacecraft and Sun direction.
      * @return cosine of the angle between spacecraft and Sun direction.
      */
     public double getSVBcos() {
         return svbCos.getValue();
     }
 
     /** Get a Sun elevation angle that does not change sign within the turn.
      * <p>
      * This method either returns the current beta or replaces it with the
      * value at turn start, so the sign remains constant throughout the
      * turn. As of 9.2, it is used for GPS, Glonass and Galileo.
      * </p>
      * @return secured Sun elevation angle
      * @see #beta(AbsoluteDate)
      * @see #betaDS(FieldAbsoluteDate)
      */
     public double getSecuredBeta() {
         final double beta = beta(getDate());
         return FastMath.abs(beta) < BETA_SIGN_CHANGE_PROTECTION ?
                beta(turnSpan.getTurnStartDate()) :
                beta;
     }
 
     /** Check if a linear yaw model is still active or if we already reached target yaw.
      * @param linearPhi value of the linear yaw model
      * @param phiDot slope of the linear yaw model
      * @return true if linear model is still active
      */
     public boolean linearModelStillActive(final double linearPhi, final double phiDot) {
         final double dt0 = turnSpan.getTurnEndDate().durationFrom(date);
         final UnivariateFunction yawReached = dt -> {
             final AbsoluteDate  t       = date.shiftedBy(dt);
             final Vector3D      pSun    = sun.getPVCoordinates(t, inertialFrame).getPosition();
             final PVCoordinates pv      = pvProv.getPVCoordinates(t, inertialFrame);
             final Vector3D      pSat    = pv.getPosition();
             final Vector3D      targetX = Vector3D.crossProduct(pSat, Vector3D.crossProduct(pSun, pSat)).normalize();
 
             final double        phi         = linearPhi + dt * phiDot;
             final SinCos        sc          = FastMath.sinCos(phi);
             final Vector3D      pU          = pv.getPosition().normalize();
             final Vector3D      mU          = pv.getMomentum().normalize();
             final Vector3D      omega       = new Vector3D(-phiDot, pU);
             final Vector3D      currentX    = new Vector3D(-sc.sin(), mU, -sc.cos(), Vector3D.crossProduct(pU, mU));
             final Vector3D      currentXDot = Vector3D.crossProduct(omega, currentX);
 
             return Vector3D.dotProduct(targetX, currentXDot);
         };
         final double fullTurn = 2 * FastMath.PI / FastMath.abs(phiDot);
         final double dtMin    = FastMath.min(turnSpan.getTurnStartDate().durationFrom(date), dt0 - 60.0);
         final double dtMax    = FastMath.max(dtMin + fullTurn, dt0 + 60.0);
         double[] bracket = UnivariateSolverUtils.bracket(yawReached, dt0,
                                                          dtMin, dtMax, fullTurn / 100, 1.0, 100);
         if (yawReached.value(bracket[0]) <= 0.0) {
             // we have bracketed the wrong crossing
             bracket = UnivariateSolverUtils.bracket(yawReached, 0.5 * (bracket[0] + bracket[1] + fullTurn),
                                                     bracket[1], bracket[1] + fullTurn, fullTurn / 100, 1.0, 100);
         }
         final double dt = new BracketingNthOrderBrentSolver(1.0e-3, 5).
                           solve(100, yawReached, bracket[0], bracket[1]);
         turnSpan.updateEnd(date.shiftedBy(dt), date);
 
         return dt > 0.0;
 
     }
 
     /** Set up the midnight/noon turn region.
      * @param cosNight limit cosine for the midnight turn
      * @param cosNoon limit cosine for the noon turn
      * @return true if spacecraft is in the midnight/noon turn region
      */
     public boolean setUpTurnRegion(final double cosNight, final double cosNoon) {
         this.cNight = cosNight;
         this.cNoon  = cosNoon;
 
         if (svbCos.getValue() < cNight || svbCos.getValue() > cNoon) {
             // we are within turn triggering zone
             return true;
         } else {
             // we are outside of turn triggering zone,
             // but we may still be trying to recover nominal attitude at the end of a turn
             return inTurnTimeRange();
         }
 
     }
 
     /** Get the relative orbit angle to turn center.
      * @return relative orbit angle to turn center
      */
     public DerivativeStructure getDeltaDS() {
         return delta;
     }
 
     /** Get the orbit angle since solar midnight.
      * @return orbit angle since solar midnight
      */
     public double getOrbitAngleSinceMidnight() {
         final double absAngle = inOrbitPlaneAbsoluteAngle(FastMath.PI - FastMath.acos(svbCos.getValue()));
         return morning ? absAngle : -absAngle;
     }
 
     /** Check if spacecraft is in the half orbit closest to Sun.
      * @return true if spacecraft is in the half orbit closest to Sun
      */
     public boolean inSunSide() {
         return svbCos.getValue() > 0;
     }
 
     /** Get yaw at turn start.
      * @param sunBeta Sun elevation above orbital plane
      * (it <em>may</em> be different from {@link #getBeta()} in
      * some special cases)
      * @return yaw at turn start
      */
     public double getYawStart(final double sunBeta) {
         final double halfSpan = 0.5 * turnSpan.getTurnDuration() * muRate;
         return computePhi(sunBeta, FastMath.copySign(halfSpan, svbCos.getValue()));
     }
 
     /** Get yaw at turn end.
      * @param sunBeta Sun elevation above orbital plane
      * (it <em>may</em> be different from {@link #getBeta()} in
      * some special cases)
      * @return yaw at turn end
      */
     public double getYawEnd(final double sunBeta) {
         final double halfSpan = 0.5 * turnSpan.getTurnDuration() * muRate;
         return computePhi(sunBeta, FastMath.copySign(halfSpan, -svbCos.getValue()));
     }
 
     /** Compute yaw rate.
      * @param sunBeta Sun elevation above orbital plane
      * (it <em>may</em> be different from {@link #getBeta()} in
      * some special cases)
      * @return yaw rate
      */
     public double yawRate(final double sunBeta) {
         return (getYawEnd(sunBeta) - getYawStart(sunBeta)) / turnSpan.getTurnDuration();
     }
 
     /** Get the spacecraft angular velocity.
      * @return spacecraft angular velocity
      */
     public double getMuRate() {
         return muRate;
     }
 
     /** Project a spacecraft/Sun angle into orbital plane.
      * <p>
      * This method is intended to find the limits of the noon and midnight
      * turns in orbital plane. The return angle is signed, depending on the
      * spacecraft being before or after turn middle point.
      * </p>
      * @param angle spacecraft/Sun angle (or spacecraft/opposite-of-Sun)
      * @return angle projected into orbital plane, always positive
      */
     private DerivativeStructure inOrbitPlaneAbsoluteAngle(final DerivativeStructure angle) {
         return FastMath.acos(FastMath.cos(angle).divide(FastMath.cos(beta(getDate()))));
     }
 
     /** Project a spacecraft/Sun angle into orbital plane.
      * <p>
      * This method is intended to find the limits of the noon and midnight
      * turns in orbital plane. The return angle is always positive. The
      * correct sign to apply depends on the spacecraft being before or
      * after turn middle point.
      * </p>
      * @param angle spacecraft/Sun angle (or spacecraft/opposite-of-Sun)
      * @return angle projected into orbital plane, always positive
      */
     public double inOrbitPlaneAbsoluteAngle(final double angle) {
         return FastMath.acos(FastMath.cos(angle) / FastMath.cos(beta(getDate())));
     }
 
     /** Compute yaw.
      * @param sunBeta Sun elevation above orbital plane
      * (it <em>may</em> be different from {@link #getBeta()} in
      * some special cases)
      * @param inOrbitPlaneAngle in orbit angle between spacecraft
      * and Sun (or opposite of Sun) projection
      * @return yaw angle
      */
     public double computePhi(final double sunBeta, final double inOrbitPlaneAngle) {
         return FastMath.atan2(-FastMath.tan(sunBeta), FastMath.sin(inOrbitPlaneAngle));
     }
 
     /** Set turn half span and compute corresponding turn time range.
      * @param halfSpan half span of the turn region, as an angle in orbit plane
      * @param endMargin margin in seconds after turn end
      */
     public void setHalfSpan(final double halfSpan, final double endMargin) {
 
         final AbsoluteDate start = date.shiftedBy((delta.getValue() - halfSpan) / muRate);
         final AbsoluteDate end   = date.shiftedBy((delta.getValue() + halfSpan) / muRate);
         final AbsoluteDate estimationDate = getDate();
 
         if (turnSpan == null) {
             turnSpan = new TurnSpan(start, end, estimationDate, endMargin);
         } else {
             turnSpan.updateStart(start, estimationDate);
             turnSpan.updateEnd(end, estimationDate);
         }
     }
 
     /** Check if context is within turn range.
      * @return true if context is within range extended by end margin
      */
     public boolean inTurnTimeRange() {
         return turnSpan != null && turnSpan.inTurnTimeRange(getDate());
     }
 
     /** Get elapsed time since turn start.
      * @return elapsed time from turn start to current date
      */
     public double timeSinceTurnStart() {
         return getDate().durationFrom(turnSpan.getTurnStartDate());
     }
 
     /** Generate an attitude with turn-corrected yaw.
      * @param yaw yaw value to apply
      * @param yawDot yaw first time derivative
      * @return attitude with specified yaw
      */
     public TimeStampedAngularCoordinates turnCorrectedAttitude(final double yaw, final double yawDot) {
         return turnCorrectedAttitude(FACTORY.build(yaw, yawDot, 0.0));
     }
 
     /** Generate an attitude with turn-corrected yaw.
      * @param yaw yaw value to apply
      * @return attitude with specified yaw
      */
     public TimeStampedAngularCoordinates turnCorrectedAttitude(final DerivativeStructure yaw) {
 
         // Earth pointing (Z aligned with position) with linear yaw (momentum with known cos/sin in the X/Y plane)
         final PVCoordinates svPV          = pvProv.getPVCoordinates(date, inertialFrame);
         final Vector3D      p             = svPV.getPosition();
         final Vector3D      v             = svPV.getVelocity();
         final Vector3D      a             = svPV.getAcceleration();
         final double        r2            = p.getNormSq();
         final double        r             = FastMath.sqrt(r2);
         final Vector3D      keplerianJerk = new Vector3D(-3 * Vector3D.dotProduct(p, v) / r2, a, -a.getNorm() / r, v);
         final PVCoordinates velocity      = new PVCoordinates(v, a, keplerianJerk);
         final PVCoordinates momentum      = PVCoordinates.crossProduct(svPV, velocity);
 
         final FieldSinCos<DerivativeStructure> sc = FastMath.sinCos(yaw);
         final DerivativeStructure c = sc.cos().negate();
         final DerivativeStructure s = sc.sin().negate();
         final Vector3D m0 = new Vector3D(s.getValue(),              c.getValue(),              0.0);
         final Vector3D m1 = new Vector3D(s.getPartialDerivative(1), c.getPartialDerivative(1), 0.0);
         final Vector3D m2 = new Vector3D(s.getPartialDerivative(2), c.getPartialDerivative(2), 0.0);
         return new TimeStampedAngularCoordinates(date,
                                                  svPV.normalize(), momentum.normalize(),
                                                  MINUS_Z_PV, new PVCoordinates(m0, m1, m2),
                                                  1.0e-9);
 
     }
 
     /** Compute Orbit Normal (ON) yaw.
      * @return Orbit Normal yaw, using inertial frame as reference
      */
     public TimeStampedAngularCoordinates orbitNormalYaw() {
         final Transform t = LOFType.LVLH_CCSDS.transformFromInertial(date, pvProv.getPVCoordinates(date, inertialFrame));
         return new TimeStampedAngularCoordinates(date,
                                                  t.getRotation(),
                                                  t.getRotationRate(),
                                                  t.getRotationAcceleration());
     }
 
 }