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    * CS licenses this file to You under the Apache License, Version 2.0
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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.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.util.FastMath;
  import org.orekit.frames.Frame;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.ExtendedPositionProvider;
  import org.orekit.utils.TimeStampedAngularCoordinates;
  import org.orekit.utils.TimeStampedFieldAngularCoordinates;
  
  /**
   * Attitude providers for GPS block IIF navigation satellites.
   * <p>
   * This class is based on the May 2017 version of J. Kouba eclips.f
   * subroutine available at <a href="http://acc.igs.org/orbits">IGS Analysis
   * Center Coordinator site</a>. The eclips.f code itself is not used ; its
   * hard-coded data are used and its low level models are used, but the
   * structure of the code and the API have been completely rewritten.
   * </p>
   * @author J. Kouba original fortran routine
   * @author Luc Maisonobe Java translation
   * @since 9.2
   */
  public class GPSBlockIIF extends AbstractGNSSAttitudeProvider {
  
      /** Default yaw rates for all spacecrafts in radians per seconds. */
      public static final double DEFAULT_YAW_RATE = FastMath.toRadians(0.11);
  
      /** Default yaw bias (rad). */
      public static final double DEFAULT_YAW_BIAS = FastMath.toRadians(-0.7);
  
      /** Satellite-Sun angle limit for a midnight turn maneuver. */
      private static final double NIGHT_TURN_LIMIT = FastMath.toRadians(180.0 - 13.25);
  
      /** Margin on turn end. */
      private static final double END_MARGIN = 1800.0;
  
      /** Yaw rate. */
      private final double yawRate;
  
      /** Yaw bias. */
      private final double yawBias;
  
      /** Simple constructor.
       * @param yawRate yaw rate to use in radians per seconds (typically {@link #DEFAULT_YAW_RATE})
       * @param yawBias yaw bias to use (rad) (typicall {@link #DEFAULT_YAW_BIAS})
       * @param validityStart start of validity for this provider
       * @param validityEnd end of validity for this provider
       * @param sun provider for Sun position
       * @param inertialFrame inertial frame where velocity are computed
       */
      public GPSBlockIIF(final double yawRate, final double yawBias,
                         final AbsoluteDate validityStart, final AbsoluteDate validityEnd,
                         final ExtendedPositionProvider sun, final Frame inertialFrame) {
          super(validityStart, validityEnd, sun, inertialFrame);
          this.yawRate = yawRate;
          this.yawBias = yawBias;
      }
  
      /** {@inheritDoc} */
      @Override
      protected TimeStampedAngularCoordinates correctedYaw(final GNSSAttitudeContext context) {
  
          // noon beta angle limit from yaw rate
          final double aNoon  = FastMath.atan(context.getMuRate() / yawRate);
          final double aNight = NIGHT_TURN_LIMIT;
          final double cNoon  = FastMath.cos(aNoon);
          final double cNight = FastMath.cos(aNight);
  
          if (context.setUpTurnRegion(cNight, cNoon)) {
  
              final double absBeta = FastMath.abs(context.beta(context.getDate()));
              context.setHalfSpan(context.inSunSide() ?
                                  absBeta * FastMath.sqrt(aNoon / absBeta - 1.0) :
                                  context.inOrbitPlaneAbsoluteAngle(aNight - FastMath.PI),
                                  END_MARGIN);
              if (context.inTurnTimeRange()) {
  
                  // we need to ensure beta sign does not change during the turn
                  final double beta     = context.getSecuredBeta();
                  final double phiStart = context.getYawStart(beta);
                  final double dtStart  = context.timeSinceTurnStart();
                 final double phiDot;
                 final double linearPhi;
                 if (context.inSunSide()) {
                     // noon turn
                     if (beta > yawBias && beta < 0) {
                         // noon turn problem for small negative beta in block IIF
                         // rotation is in the wrong direction for these spacecrafts
                         phiDot    = FastMath.copySign(yawRate, beta);
                         linearPhi = phiStart + phiDot * dtStart;
                     } else {
                         // regular noon turn
                         phiDot    = -FastMath.copySign(yawRate, beta);
                         linearPhi = phiStart + phiDot * dtStart;
                     }
                 } else {
                     // midnight turn
                     phiDot    = context.yawRate(beta);
                     linearPhi = phiStart + phiDot * dtStart;
                 }
 
                 if (context.linearModelStillActive(linearPhi, phiDot)) {
                     // we are still in the linear model phase
                     return context.turnCorrectedAttitude(linearPhi, phiDot);
                 }
 
 
             }
 
         }
 
         // in nominal yaw mode
         return context.nominalYaw(context.getDate());
 
     }
 
     /** {@inheritDoc} */
     @Override
     protected <T extends CalculusFieldElement<T>> TimeStampedFieldAngularCoordinates<T> correctedYaw(final GNSSFieldAttitudeContext<T> context) {
 
         final Field<T> field = context.getDate().getField();
 
         // noon beta angle limit from yaw rate
         final T      aNoon  = FastMath.atan(context.getMuRate().divide(yawRate));
         final T      aNight = field.getZero().newInstance(NIGHT_TURN_LIMIT);
         final double cNoon  = FastMath.cos(aNoon.getReal());
         final double cNight = FastMath.cos(aNight.getReal());
 
         if (context.setUpTurnRegion(cNight, cNoon)) {
 
             final T absBeta = FastMath.abs(context.beta(context.getDate()));
             context.setHalfSpan(context.inSunSide() ?
                                 absBeta.multiply(FastMath.sqrt(aNoon.divide(absBeta).subtract(1.0))) :
                                 context.inOrbitPlaneAbsoluteAngle(aNight.subtract(aNoon.getPi())),
                                 END_MARGIN);
             if (context.inTurnTimeRange()) {
 
                 // we need to ensure beta sign does not change during the turn
                 final T beta     = context.getSecuredBeta();
                 final T phiStart = context.getYawStart(beta);
                 final T dtStart  = context.timeSinceTurnStart();
                 final T phiDot;
                 final T linearPhi;
                 if (context.inSunSide()) {
                     // noon turn
                     if (beta.getReal() > yawBias && beta.getReal() < 0) {
                         // noon turn problem for small negative beta in block IIF
                         // rotation is in the wrong direction for these spacecrafts
                         phiDot    = field.getZero().newInstance(FastMath.copySign(yawRate, beta.getReal()));
                         linearPhi = phiStart.add(phiDot.multiply(dtStart));
                     } else {
                         // regular noon turn
                         phiDot    = field.getZero().newInstance(-FastMath.copySign(yawRate, beta.getReal()));
                         linearPhi = phiStart.add(phiDot.multiply(dtStart));
                     }
                 } else {
                     // midnight turn
                     phiDot    = context.yawRate(beta);
                     linearPhi = phiStart.add(phiDot.multiply(dtStart));
                 }
 
                 if (context.linearModelStillActive(linearPhi, phiDot)) {
                     // we are still in the linear model phase
                     return context.turnCorrectedAttitude(linearPhi, phiDot);
                 }
 
             }
 
         }
 
         // in nominal yaw mode
         return context.nominalYaw(context.getDate());
 
     }
 
 }