/* Copyright 2002-2021 CS GROUP
    * Licensed to CS GROUP (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.models.earth.ionosphere;
  
  import org.hipparchus.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.FieldSinCos;
  import org.hipparchus.util.MathArrays;
  import org.hipparchus.util.SinCos;
  import org.orekit.time.DateComponents;
  import org.orekit.time.DateTimeComponents;
  import org.orekit.time.TimeComponents;
  
  /**
   * This class perfoms the computation of the parameters used by the NeQuick model.
   *
   * @author Bryan Cazabonne
   *
   * @see "European Union (2016). European GNSS (Galileo) Open Service-Ionospheric Correction
   *       Algorithm for Galileo Single Frequency Users. 1.2."
   *
   * @since 10.1
   */
  class FieldNeQuickParameters <T extends CalculusFieldElement<T>> {
  
      /** Solar zenith angle at day night transition, degrees. */
      private static final double X0 = 86.23292796211615;
  
      /** F2 layer maximum density. */
      private final T nmF2;
  
      /** F2 layer maximum density height [km]. */
      private final T hmF2;
  
      /** F1 layer maximum density height [km]. */
      private final T hmF1;
  
      /** E layer maximum density height [km]. */
      private final T hmE;
  
      /** F2 layer bottom thickness parameter [km]. */
      private final T b2Bot;
  
      /** F1 layer top thickness parameter [km]. */
      private final T b1Top;
  
      /** F1 layer bottom thickness parameter [km]. */
      private final T b1Bot;
  
      /** E layer top thickness parameter [km]. */
      private final T beTop;
  
      /** E layer bottom thickness parameter [km]. */
      private final T beBot;
  
      /** topside thickness parameter [km]. */
      private final T h0;
  
      /** Layer amplitudes. */
      private final T[] amplitudes;
  
      /**
       * Build a new instance.
       * @param field field of the elements
       * @param dateTime current date time components
       * @param f2 F2 coefficients used by the F2 layer
       * @param fm3 Fm3 coefficients used by the F2 layer
       * @param latitude latitude of a point along the integration path, in radians
       * @param longitude longitude of a point along the integration path, in radians
       * @param alpha effective ionisation level coefficients
       * @param modipGrip modip grid
       */
      FieldNeQuickParameters(final Field<T> field, final DateTimeComponents dateTime, final double[][][] f2,
                             final double[][][] fm3, final T latitude, final T longitude,
                             final double[] alpha, final double[][] modipGrip) {
  
          // Zero
          final T zero = field.getZero();
  
          // MODIP in degrees
          final T modip = computeMODIP(latitude, longitude, modipGrip);
          // Effective ionisation level Az
          final T az = computeAz(modip, alpha);
          // Effective sunspot number (Eq. 19)
         final T azr = FastMath.sqrt(az.subtract(63.7).multiply(1123.6).add(167273.0)).subtract(408.99);
         // Date and Time components
         final DateComponents date = dateTime.getDate();
         final TimeComponents time = dateTime.getTime();
         // Hours
         final double hours  = time.getSecondsInUTCDay() / 3600.0;
         // Effective solar zenith angle in radians
         final T xeff = computeEffectiveSolarAngle(date.getMonth(), hours, latitude, longitude);
 
         // Coefficients for F2 layer parameters
         // Compute the array of interpolated coefficients for foF2 (Eq. 44)
         final T[][] af2 = MathArrays.buildArray(field, 76, 13);
         for (int j = 0; j < 76; j++) {
             for (int k = 0; k < 13; k++ ) {
                 af2[j][k] = azr.multiply(0.01).negate().add(1.0).multiply(f2[0][j][k]).add(azr.multiply(0.01).multiply(f2[1][j][k]));
             }
         }
 
         // Compute the array of interpolated coefficients for M(3000)F2 (Eq. 46)
         final T[][] am3 = MathArrays.buildArray(field, 49, 9);
         for (int j = 0; j < 49; j++) {
             for (int k = 0; k < 9; k++ ) {
                 am3[j][k] = azr.multiply(0.01).negate().add(1.0).multiply(fm3[0][j][k]).add(azr.multiply(0.01).multiply(fm3[1][j][k]));
             }
         }
 
         // E layer maximum density height in km (Eq. 78)
         this.hmE = field.getZero().add(120.0);
         // E layer critical frequency in MHz
         final T foE = computefoE(date.getMonth(), az, xeff, latitude);
         // E layer maximum density in 10^11 m-3 (Eq. 36)
         final T nmE = foE.multiply(foE).multiply(0.124);
 
         // Time argument (Eq. 49)
         final double t = FastMath.toRadians(15 * hours) - FastMath.PI;
         // Compute Fourier time series for foF2 and M(3000)F2
         final T[] cf2 = computeCF2(field, af2, t);
         final T[] cm3 = computeCm3(field, am3, t);
         // F2 layer critical frequency in MHz
         final T foF2 = computefoF2(field, modip, cf2, latitude, longitude);
         // Maximum Usable Frequency factor
         final T mF2  = computeMF2(field, modip, cm3, latitude, longitude);
         // F2 layer maximum density in 10^11 m-3
         this.nmF2 = foF2.multiply(foF2).multiply(0.124);
         // F2 layer maximum density height in km
         this.hmF2 = computehmF2(field, foE, foF2, mF2);
 
         // F1 layer critical frequency in MHz
         final T foF1 = computefoF1(field, foE, foF2);
         // F1 layer maximum density in 10^11 m-3
         final T nmF1;
         if (foF1.getReal() <= 0.0 && foE.getReal() > 2.0) {
             final T foEpopf = foE.add(0.5);
             nmF1 = foEpopf.multiply(foEpopf).multiply(0.124);
         } else {
             nmF1 = foF1.multiply(foF1).multiply(0.124);
         }
         // F1 layer maximum density height in km
         this.hmF1 = hmF2.add(hmE).multiply(0.5);
 
         // Thickness parameters (Eq. 85 to 89)
         final T a = clipExp(FastMath.log(foF2.multiply(foF2)).multiply(0.857).add(FastMath.log(mF2).multiply(2.02)).add(-3.467)).multiply(0.01);
         this.b2Bot = nmF2.divide(a).multiply(0.385);
         this.b1Top = hmF2.subtract(hmF1).multiply(0.3);
         this.b1Bot = hmF1.subtract(hmE).multiply(0.5);
         this.beTop = FastMath.max(b1Bot, zero.add(7.0));
         this.beBot = zero.add(5.0);
 
         // Layer amplitude coefficients
         this.amplitudes = computeLayerAmplitudes(field, nmE, nmF1, foF1);
 
         // Topside thickness parameter
         this.h0 = computeH0(field, date.getMonth(), azr);
     }
 
     /**
      * Get the F2 layer maximum density.
      * @return nmF2
      */
     public T getNmF2() {
         return nmF2;
     }
 
     /**
      * Get the F2 layer maximum density height.
      * @return hmF2 in km
      */
     public T getHmF2() {
         return hmF2;
     }
 
     /**
      * Get the F1 layer maximum density height.
      * @return hmF1 in km
      */
     public T getHmF1() {
         return hmF1;
     }
 
     /**
      * Get the E layer maximum density height.
      * @return hmE in km
      */
     public T getHmE() {
         return hmE;
     }
 
     /**
      * Get the F2 layer thickness parameter (bottom).
      * @return B2Bot in km
      */
     public T getB2Bot() {
         return b2Bot;
     }
 
     /**
      * Get the F1 layer thickness parameter (top).
      * @return B1Top in km
      */
     public T getB1Top() {
         return b1Top;
     }
 
     /**
      * Get the F1 layer thickness parameter (bottom).
      * @return B1Bot in km
      */
     public T getB1Bot() {
         return b1Bot;
     }
 
     /**
      * Get the E layer thickness parameter (bottom).
      * @return BeBot in km
      */
     public T getBEBot() {
         return beBot;
     }
 
     /**
      * Get the E layer thickness parameter (top).
      * @return BeTop in km
      */
     public T getBETop() {
         return beTop;
     }
 
     /**
      * Get the F2, F1 and E layer amplitudes.
      * <p>
      * The resulting element is an array having the following form:
      * <ul>
      * <li>double[0] = A1 → F2 layer amplitude
      * <li>double[1] = A2 → F1 layer amplitude
      * <li>double[2] = A3 → E  layer amplitude
      * </ul>
      * @return layer amplitudes
      */
     public T[] getLayerAmplitudes() {
         return amplitudes.clone();
     }
 
     /**
      * Get the topside thickness parameter H0.
      * @return H0 in km
      */
     public T getH0() {
         return h0;
     }
 
     /**
      * Computes the value of the modified dip latitude (MODIP) for the
      * given latitude and longitude.
      *
      * @param lat receiver latitude, radians
      * @param lon receiver longitude, radians
      * @param stModip modip grid
      * @return the MODIP in degrees
      */
     private T computeMODIP(final T lat, final T lon, final double[][] stModip) {
 
         // Zero
         final T zero = lat.getField().getZero();
 
         // For the MODIP computation, the latitude and longitude have to be converted in degrees
         final T latitude  = FastMath.toDegrees(lat);
         final T longitude = FastMath.toDegrees(lon);
 
         // Extreme cases
         if (latitude.getReal() == 90.0 || latitude.getReal() == -90.0) {
             return latitude;
         }
 
         // Auxiliary parameter l (Eq. 6 to 8)
         final int lF = (int) ((longitude.getReal() + 180) * 0.1);
         int l = lF - 2;
         if (l < 0) {
             l += 36;
         } else if (l > 33) {
             l -= 36;
         }
 
         // Auxiliary parameter a (Eq. 9 to 11)
         final T a  = latitude.add(90).multiply(0.2).add(1.0);
         final T aF = FastMath.floor(a);
         // Eq. 10
         final T x = a.subtract(aF);
         // Eq. 11
         final int i = (int) aF.getReal() - 2;
 
         // zi coefficients (Eq. 12 and 13)
         final T z1 = interpolate(zero.add(stModip[i + 1][l + 2]), zero.add(stModip[i + 2][l + 2]),
                                       zero.add(stModip[i + 3][l + 2]), zero.add(stModip[i + 4][l + 2]), x);
         final T z2 = interpolate(zero.add(stModip[i + 1][l + 3]), zero.add(stModip[i + 2][l + 3]),
                                       zero.add(stModip[i + 3][l + 3]), zero.add(stModip[i + 4][l + 3]), x);
         final T z3 = interpolate(zero.add(stModip[i + 1][l + 4]), zero.add(stModip[i + 2][l + 4]),
                                       zero.add(stModip[i + 3][l + 4]), zero.add(stModip[i + 4][l + 4]), x);
         final T z4 = interpolate(zero.add(stModip[i + 1][l + 5]), zero.add(stModip[i + 2][l + 5]),
                                       zero.add(stModip[i + 3][l + 5]), zero.add(stModip[i + 4][l + 5]), x);
 
         // Auxiliary parameter b (Eq. 14 and 15)
         final T b  = longitude.add(180).multiply(0.1);
         final T bF = FastMath.floor(b);
         final T y  = b.subtract(bF);
 
         // MODIP (Ref Eq. 16)
         final T modip = interpolate(z1, z2, z3, z4, y);
 
         return modip;
     }
 
     /**
      * This method computes the effective ionisation level Az.
      * <p>
      * This parameter is used for the computation of the Total Electron Content (TEC).
      * </p>
      * @param modip modified dip latitude (MODIP) in degrees
      * @param alpha effective ionisation level coefficients
      * @return the ionisation level Az
      */
     private T computeAz(final T modip, final double[] alpha) {
         // Field
         final Field<T> field = modip.getField();
         // Zero
         final T zero = field.getZero();
         // Particular condition (Eq. 17)
         if (alpha[0] == 0.0 && alpha[1] == 0.0 && alpha[2] == 0.0) {
             return zero.add(63.7);
         }
         // Az = a0 + modip * a1 + modip^2 * a2 (Eq. 18)
         T az = modip.multiply(alpha[2]).add(alpha[1]).multiply(modip).add(alpha[0]);
         // If Az < 0 -> Az = 0
         az = FastMath.max(zero, az);
         // If Az > 400 -> Az = 400
         az = FastMath.min(zero.add(400.0), az);
         return az;
     }
 
     /**
      * This method computes the effective solar zenith angle.
      * <p>
      * The effective solar zenith angle is compute as a function of the
      * solar zenith angle and the solar zenith angle at day night transition.
      * </p>
      * @param month current month of the year
      * @param hours universal time (hours)
      * @param latitude in radians
      * @param longitude in radians
      * @return the effective solar zenith angle, radians
      */
     private T computeEffectiveSolarAngle(final int month,
                                          final double hours,
                                          final T latitude,
                                          final T longitude) {
         // Zero
         final T zero = latitude.getField().getZero();
         // Local time (Eq.4)
         final T lt = longitude.divide(FastMath.toRadians(15.0)).add(hours);
         // Day of year at the middle of the month (Eq. 20)
         final double dy = 30.5 * month - 15.0;
         // Time (Eq. 21)
         final double t = dy + (18 - hours) / 24;
         // Arguments am and al (Eq. 22 and 23)
         final double am = FastMath.toRadians(0.9856 * t - 3.289);
         final double al = am + FastMath.toRadians(1.916 * FastMath.sin(am) + 0.020 * FastMath.sin(2.0 * am) + 282.634);
         // Sine and cosine of solar declination (Eq. 24 and 25)
         final double sDec = 0.39782 * FastMath.sin(al);
         final double cDec = FastMath.sqrt(1. - sDec * sDec);
         // Solar zenith angle, deg (Eq. 26 and 27)
         final FieldSinCos<T> scLat   = FastMath.sinCos(latitude);
         final T coef    = lt.negate().add(12.0).multiply(FastMath.PI / 12);
         final T cZenith = scLat.sin().multiply(sDec).add(scLat.cos().multiply(cDec).multiply(FastMath.cos(coef)));
         final T angle   = FastMath.atan2(FastMath.sqrt(cZenith.multiply(cZenith).negate().add(1.0)), cZenith);
         final T x       = FastMath.toDegrees(angle);
         // Effective solar zenith angle (Eq. 28)
         final T xeff = join(clipExp(x.multiply(0.2).negate().add(20.0)).multiply(0.24).negate().add(90.0), x, zero.add(12.0), x.subtract(X0));
         return FastMath.toRadians(xeff);
     }
 
     /**
      * This method computes the E layer critical frequency at a given location.
      * @param month current month
      * @param az ffective ionisation level
      * @param xeff effective solar zenith angle in radians
      * @param latitude latitude in radians
      * @return the E layer critical frequency at a given location in MHz
      */
     private T computefoE(final int month, final T az,
                          final T xeff, final T latitude) {
         // The latitude has to be converted in degrees
         final T lat = FastMath.toDegrees(latitude);
         // Square root of the effective ionisation level
         final T sqAz = FastMath.sqrt(az);
         // seas parameter (Eq. 30 to 32)
         final int seas;
         if (month == 1 || month == 2 || month == 11 || month == 12) {
             seas = -1;
         } else if (month == 3 || month == 4 || month == 9 || month == 10) {
             seas = 0;
         } else {
             seas = 1;
         }
         // Latitudinal dependence (Eq. 33 and 34)
         final T ee = clipExp(lat.multiply(0.3));
         final T seasp = ee.subtract(1.0).divide(ee.add(1.0)).multiply(seas);
         // Critical frequency (Eq. 35)
         final T coef = seasp.multiply(0.019).negate().add(1.112);
         final T foE = FastMath.sqrt(coef .multiply(coef).multiply(sqAz).multiply(FastMath.cos(xeff).pow(0.6)).add(0.49));
         return foE;
     }
 
     /**
      * Computes the F2 layer height of maximum electron density.
      * @param field field of the elements
      * @param foE E layer layer critical frequency in MHz
      * @param foF2 F2 layer layer critical frequency in MHz
      * @param mF2 maximum usable frequency factor
      * @return hmF2 in km
      */
     private T computehmF2(final Field<T> field, final T foE, final T foF2, final T mF2) {
         // Zero
         final T zero = field.getZero();
         // Ratio
         final T fo = foF2.divide(foE);
         final T ratio = join(fo, zero.add(1.75), zero.add(20.0), fo.subtract(1.75));
 
         // deltaM parameter
         T deltaM = zero.subtract(0.012);
         if (foE.getReal() >= 1e-30) {
             deltaM = deltaM.add(ratio.subtract(1.215).divide(0.253).reciprocal());
         }
 
         // hmF2 Eq. 80
         final T mF2Sq = mF2.multiply(mF2);
         final T temp  = FastMath.sqrt(mF2Sq.multiply(0.0196).add(1.0).divide(mF2Sq.multiply(1.2967).subtract(1.0)));
         final T height  = mF2.multiply(1490.0).multiply(temp).divide(mF2.add(deltaM)).subtract(176.0);
         return height;
     }
 
     /**
      * Computes cf2 coefficients.
      * @param field field of the elements
      * @param af2 interpolated coefficients for foF2
      * @param t time argument
      * @return the cf2 coefficients array
      */
     private T[] computeCF2(final Field<T> field, final T[][] af2, final double t) {
         // Eq. 50
         final T[] cf2 = MathArrays.buildArray(field, 76);
         for (int i = 0; i < cf2.length; i++) {
             T sum = field.getZero();
             for (int k = 0; k < 6; k++) {
                 final SinCos sc = FastMath.sinCos((k + 1) * t);
                 sum = sum.add(af2[i][2 * k + 1].multiply(sc.sin()).add(af2[i][2 * (k + 1)].multiply(sc.cos())));
             }
             cf2[i] = af2[i][0].add(sum);
         }
         return cf2;
     }
 
     /**
      * Computes Cm3 coefficients.
      * @param field field of the elements
      * @param am3 interpolated coefficients for foF2
      * @param t time argument
      * @return the Cm3 coefficients array
      */
     private T[] computeCm3(final Field<T> field, final T[][] am3, final double t) {
         // Eq. 51
         final T[] cm3 = MathArrays.buildArray(field, 49);
         for (int i = 0; i < cm3.length; i++) {
             T sum = field.getZero();
             for (int k = 0; k < 4; k++) {
                 final SinCos sc = FastMath.sinCos((k + 1) * t);
                 sum = sum.add(am3[i][2 * k + 1].multiply(sc.sin()).add(am3[i][2 * (k + 1)].multiply(sc.cos())));
             }
             cm3[i] = am3[i][0].add(sum);
         }
         return cm3;
     }
 
     /**
      * This method computes the F2 layer critical frequency.
      * @param field field of the elements
      * @param modip modified DIP latitude, in degrees
      * @param cf2 Fourier time series for foF2
      * @param latitude latitude in radians
      * @param longitude longitude in radians
      * @return the F2 layer critical frequency, MHz
      */
     private T computefoF2(final Field<T> field, final T modip, final T[] cf2,
                           final T latitude, final T longitude) {
 
         // One
         final T one = field.getOne();
 
         // Legendre grades (Eq. 63)
         final int[] q = new int[] {
             12, 12, 9, 5, 2, 1, 1, 1, 1
         };
 
         // Array for geographic terms
         final T[] g = MathArrays.buildArray(field, cf2.length);
         g[0] = one;
 
         // MODIP coefficients Eq. 57
         final T sinMODIP = FastMath.sin(FastMath.toRadians(modip));
         final T[] m = MathArrays.buildArray(field, 12);
         m[0] = one;
         for (int i = 1; i < q[0]; i++) {
             m[i] = sinMODIP.multiply(m[i - 1]);
             g[i] = m[i];
         }
 
         // Latitude coefficients (Eq. 58)
         final T cosLat = FastMath.cos(latitude);
         final T[] p = MathArrays.buildArray(field, 8);
         p[0] = cosLat;
         for (int n = 2; n < 9; n++) {
             p[n - 1] = cosLat.multiply(p[n - 2]);
         }
 
         // latitude and longitude terms
         int index = 12;
         for (int i = 1; i < q.length; i++) {
             final FieldSinCos<T> sc = FastMath.sinCos(longitude.multiply(i));
             for (int j = 0; j < q[i]; j++) {
                 g[index++] = m[j].multiply(p[i - 1]).multiply(sc.cos());
                 g[index++] = m[j].multiply(p[i - 1]).multiply(sc.sin());
             }
         }
 
         // Compute foF2 by linear combination
         final T frequency = one.linearCombination(g, cf2);
         return frequency;
     }
 
     /**
      * This method computes the Maximum Usable Frequency factor.
      * @param field field of the elements
      * @param modip modified DIP latitude, in degrees
      * @param cm3 Fourier time series for M(3000)F2
      * @param latitude latitude in radians
      * @param longitude longitude in radians
      * @return the Maximum Usable Frequency factor
      */
     private T computeMF2(final Field<T> field, final T modip, final T[] cm3,
                          final T latitude, final T longitude) {
 
         // One
         final T one = field.getOne();
         // Legendre grades (Eq. 71)
         final int[] r = new int[] {
             7, 8, 6, 3, 2, 1, 1
         };
 
         // Array for geographic terms
         final T[] g = MathArrays.buildArray(field, cm3.length);
         g[0] = one;
 
         // MODIP coefficients Eq. 57
         final T sinMODIP = FastMath.sin(FastMath.toRadians(modip));
         final T[] m = MathArrays.buildArray(field, 12);
         m[0] = one;
         for (int i = 1; i < 12; i++) {
             m[i] = sinMODIP.multiply(m[i - 1]);
             if (i < 7) {
                 g[i] = m[i];
             }
         }
 
         // Latitude coefficients (Eq. 58)
         final T cosLat = FastMath.cos(latitude);
         final T[] p = MathArrays.buildArray(field, 8);
         p[0] = cosLat;
         for (int n = 2; n < 9; n++) {
             p[n - 1] = cosLat.multiply(p[n - 2]);
         }
 
         // latitude and longitude terms
         int index = 7;
         for (int i = 1; i < r.length; i++) {
             final FieldSinCos<T> sc = FastMath.sinCos(longitude.multiply(i));
             for (int j = 0; j < r[i]; j++) {
                 g[index++] = m[j].multiply(p[i - 1]).multiply(sc.cos());
                 g[index++] = m[j].multiply(p[i - 1]).multiply(sc.sin());
             }
         }
 
         // Compute m3000 by linear combination
         final T m3000 = one.linearCombination(g, cm3);
         return m3000;
     }
 
     /**
      * This method computes the F1 layer critical frequency.
      * <p>
      * This computation performs the algorithm exposed in Annex F
      * of the reference document.
      * </p>
      * @param field field of the elements
      * @param foE the E layer critical frequency, MHz
      * @return the F1 layer critical frequency, MHz
      * @param foF2 the F2 layer critical frequency, MHz
      */
     private T computefoF1(final Field<T> field, final T foE, final T foF2) {
         final T zero = field.getZero();
         final T temp  = join(foE.multiply(1.4), zero, zero.add(1000.0), foE.subtract(2.0));
         final T temp2 = join(zero, temp, zero.add(1000.0), foE.subtract(temp));
         final T value = join(temp2, temp2.multiply(0.85), zero.add(60.0), foF2.multiply(0.85).subtract(temp2));
         if (value.getReal() < 1.0E-6) {
             return zero;
         } else {
             return value;
         }
     }
 
     /**
      * This method allows the computation of the F2, F1 and E layer amplitudes.
      * <p>
      * The resulting element is an array having the following form:
      * <ul>
      * <li>double[0] = A1 → F2 layer amplitude
      * <li>double[1] = A2 → F1 layer amplitude
      * <li>double[2] = A3 → E  layer amplitude
      * </ul>
      * </p>
      * @param field field of the elements
      * @param nmE E layer maximum density in 10^11 m-3
      * @param nmF1 F1 layer maximum density in 10^11 m-3
      * @param foF1 F1 layer critical frequency in MHz
      * @return a three components array containing the layer amplitudes
      */
     private T[] computeLayerAmplitudes(final Field<T> field, final T nmE, final T nmF1, final T foF1) {
         // Zero
         final T zero = field.getZero();
 
         // Initialize array
         final T[] amplitude = MathArrays.buildArray(field, 3);
 
         // F2 layer amplitude (Eq. 90)
         final T a1 = nmF2.multiply(4.0);
         amplitude[0] = a1;
 
         // F1 and E layer amplitudes (Eq. 91 to 98)
         if (foF1.getReal() < 0.5) {
             amplitude[1] = zero;
             amplitude[2] = nmE.subtract(epst(a1, hmF2, b2Bot, hmE)).multiply(4.0);
         } else {
             T a2a = zero;
             T a3a = nmE.multiply(4.0);
             for (int i = 0; i < 5; i++) {
                 a2a = nmF1.subtract(epst(a1, hmF2, b2Bot, hmF1)).subtract(epst(a3a, hmE, beTop, hmF1)).multiply(4.0);
                 a2a = join(a2a, nmF1.multiply(0.8), field.getOne(), a2a.subtract(nmF1.multiply(0.8)));
                 a3a = nmE.subtract(epst(a2a, hmF1, b1Bot, hmE)).subtract(epst(a1, hmF2, b2Bot, hmE)).multiply(4.0);
             }
             amplitude[1] = a2a;
             amplitude[2] = join(a3a, zero.add(0.05), zero.add(60.0), a3a.subtract(0.005));
         }
 
         return amplitude;
     }
 
     /**
      * This method computes the topside thickness parameter H0.
      *
      * @param field field of the elements
      * @param month current month
      * @param azr effective sunspot number
      * @return H0 in km
      */
     private T computeH0(final Field<T> field, final int month, final T azr) {
 
         // One
         final T one = field.getOne();
 
         // Auxiliary parameter ka (Eq. 99 and 100)
         final T ka;
         if (month > 3 && month < 10) {
             // month = 4,5,6,7,8,9
             ka = azr.multiply(0.014).add(hmF2.multiply(0.008)).negate().add(6.705);
         } else {
             // month = 1,2,3,10,11,12
             final T ratio = hmF2.divide(b2Bot);
             ka = ratio.multiply(ratio).multiply(0.097).add(nmF2.multiply(0.153)).add(-7.77);
         }
 
         // Auxiliary parameter kb (Eq. 101 and 102)
         T kb = join(ka, one.multiply(2.0), one, ka.subtract(2.0));
         kb = join(one.multiply(8.0), kb, one, kb.subtract(8.0));
 
         // Auxiliary parameter Ha (Eq. 103)
         final T hA = kb.multiply(b2Bot);
 
         // Auxiliary parameters x and v (Eq. 104 and 105)
         final T x = hA.subtract(150.0).multiply(0.01);
         final T v = x.multiply(0.041163).subtract(0.183981).multiply(x).add(1.424472);
 
         // Topside thickness parameter (Eq. 106)
         final T h = hA.divide(v);
         return h;
     }
 
     /**
      * A clipped exponential function.
      * <p>
      * This function, describe in section F.2.12.2 of the reference document, is
      * recommanded for the computation of exponential values.
      * </p>
      * @param power power for exponential function
      * @return clipped exponential value
      */
     private T clipExp(final T power) {
         final T zero = power.getField().getZero();
         if (power.getReal() > 80.0) {
             return zero.add(5.5406E34);
         } else if (power.getReal() < -80) {
             return zero.add(1.8049E-35);
         } else {
             return FastMath.exp(power);
         }
     }
 
     /**
      * This method provides a third order interpolation function
      * as recommended in the reference document (Ref Eq. 128 to Eq. 138)
      *
      * @param z1 z1 coefficient
      * @param z2 z2 coefficient
      * @param z3 z3 coefficient
      * @param z4 z4 coefficient
      * @param x position
      * @return a third order interpolation
      */
     private T interpolate(final T z1, final T z2,
                           final T z3, final T z4,
                           final T x) {
 
         if (FastMath.abs(2.0 * x.getReal()) < 1e-10) {
             return z2;
         }
 
         final T delta = x.multiply(2.0).subtract(1.0);
         final T g1 = z3.add(z2);
         final T g2 = z3.subtract(z2);
         final T g3 = z4.add(z1);
         final T g4 = z4.subtract(z1).divide(3.0);
         final T a0 = g1.multiply(9.0).subtract(g3);
         final T a1 = g2.multiply(9.0).subtract(g4);
         final T a2 = g3.subtract(g1);
         final T a3 = g4.subtract(g2);
         final T zx = delta.multiply(a3).add(a2).multiply(delta).add(a1).multiply(delta).add(a0).multiply(0.0625);
 
         return zx;
     }
 
     /**
      * Allows smooth joining of functions f1 and f2
      * (i.e. continuous first derivatives) at origin.
      * <p>
      * This function, describe in section F.2.12.1 of the reference document, is
      * recommanded for computational efficiency.
      * </p>
      * @param dF1 first function
      * @param dF2 second function
      * @param dA width of transition region
      * @param dX x value
      * @return the computed value
      */
     private T join(final T dF1, final T dF2,
                    final T dA, final T dX) {
         final T ee = clipExp(dA.multiply(dX));
         return dF1.multiply(ee).add(dF2).divide(ee.add(1.0));
     }
 
     /**
      * The Epstein function.
      * <p>
      * This function, describe in section 2.5.1 of the reference document, is used
      * as a basis analytical function in NeQuick for the construction of the ionospheric layers.
      * </p>
      * @param x x parameter
      * @param y y parameter
      * @param z z parameter
      * @param w w parameter
      * @return value of the epstein function
      */
     private T epst(final T x, final T y,
                    final T z, final T w) {
         final T ex  = clipExp(w.subtract(y).divide(z));
         final T opex = ex.add(1.0);
         final T epst = x.multiply(ex).divide(opex.multiply(opex));
         return epst;
     }
 
 }