GLONASSDate.java

  1. /* Copyright 2002-2025 CS GROUP
  2.  * Licensed to CS GROUP (CS) under one or more
  3.  * contributor license agreements.  See the NOTICE file distributed with
  4.  * this work for additional information regarding copyright ownership.
  5.  * CS licenses this file to You under the Apache License, Version 2.0
  6.  * (the "License"); you may not use this file except in compliance with
  7.  * the License.  You may obtain a copy of the License at
  8.  *
  9.  *   http://www.apache.org/licenses/LICENSE-2.0
  10.  *
  11.  * Unless required by applicable law or agreed to in writing, software
  12.  * distributed under the License is distributed on an "AS IS" BASIS,
  13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14.  * See the License for the specific language governing permissions and
  15.  * limitations under the License.
  16.  */
  17. package org.orekit.time;

  19. import org.hipparchus.util.FastMath;
  20. import org.orekit.annotation.DefaultDataContext;
  21. import org.orekit.data.DataContext;
  22. import org.orekit.propagation.analytical.gnss.data.GNSSConstants;
  23. import org.orekit.utils.Constants;

  25. /**
  26.  * Container for date in GLONASS form.
  27.  * @author Bryan Cazabonne
  28.  * @see AbsoluteDate
  29.  * @see "GLONASS Interface Control Document v1.0, 2016"
  30.  * @since 10.0
  31.  */
  32. public class GLONASSDate implements TimeStamped {

  34.     /** Constant for date computation. */
  35.     private static final int C1 = 44195;

  37.     /** Constant for date computation. */
  38.     private static final int C2 = 45290;

  40.     /** The number of the current day in a four year interval N<sub>a</sub>. */
  41.     private final int na;

  43.     /** The number of the current four year interval N<sub>4</sub>. */
  44.     private final int n4;

  46.     /** Number of seconds since N<sub>a</sub>. */
  47.     private final double secInNa;

  49.     /** Current Julian date JD0. */
  50.     private double jd0;

  52.     /** Greenwich Mean Sidereal Time (rad). */
  53.     private double gmst;

  55.     /** Corresponding date. */
  56.     private final AbsoluteDate date;

  58.     /** Build an instance corresponding to a GLONASS date.
  59.      *
  60.      * <p>This method uses the {@link DataContext#getDefault() default data context}.
  61.      *
  62.      * @param na the number of the current day in a four year interval
  63.      * @param n4 the number of the current four year interval
  64.      * @param secInNa the number of seconds since na start
  65.      * @see #GLONASSDate(int, int, double, TimeScale)
  66.      */
  67.     @DefaultDataContext
  68.     public GLONASSDate(final int na, final int n4, final double secInNa) {
  69.         this(na, n4, secInNa, DataContext.getDefault().getTimeScales().getGLONASS());
  70.     }

  72.     /**
  73.      * Build an instance corresponding to a GLONASS date.
  74.      *
  75.      * @param na      the number of the current day in a four year interval
  76.      * @param n4      the number of the current four year interval
  77.      * @param secInNa the number of seconds since na start
  78.      * @param glonass time scale.
  79.      * @since 10.1
  80.      */
  81.     public GLONASSDate(final int na,
  82.                        final int n4,
  83.                        final double secInNa,
  84.                        final TimeScale glonass) {
  85.         this.na      = na;
  86.         this.n4      = n4;
  87.         this.secInNa = secInNa;
  88.         // Compute JD0
  89.         final int ratio = FastMath.round((float) (na - 3) / (25 + C1 + C2));
  90.         this.jd0  = 1461 * (n4 - 1) + na + 2450082.5 - ratio;
  91.         // GMST
  92.         this.gmst = computeGMST();
  93.         this.date = computeDate(glonass);
  94.     }

  96.     /** Build an instance from an absolute date.
  97.      *
  98.      * <p>This method uses the {@link DataContext#getDefault() default data context}.
  99.      *
  100.      * @param date absolute date to consider
  101.      * @see #GLONASSDate(AbsoluteDate, TimeScale)
  102.      */
  103.     @DefaultDataContext
  104.     public GLONASSDate(final AbsoluteDate date) {
  105.         this(date, DataContext.getDefault().getTimeScales().getGLONASS());
  106.     }

  108.     /**
  109.      * Build an instance from an absolute date.
  110.      *
  111.      * @param date    absolute date to consider
  112.      * @param glonass time scale.
  113.      * @since 10.1
  114.      */
  115.     public GLONASSDate(final AbsoluteDate date, final TimeScale glonass) {
  116.         final DateTimeComponents dateTime = date.getComponents(glonass);
  117.         // N4
  118.         final int year = dateTime.getDate().getYear();
  119.         this.n4 = ((int) (year - 1996) / 4) + 1;
  120.         // Na
  121.         final int start = 1996 + 4 * (n4 - 1);
  122.         final double duration = date.durationFrom(new AbsoluteDate(start, 1, 1, glonass));
  123.         this.na = (int) (duration / 86400) + 1;
  124.         this.secInNa = dateTime.getTime().getSecondsInLocalDay();
  125.         // Compute JD0
  126.         final int ratio = FastMath.round((float) (na - 3) / (25 + C1 + C2));
  127.         this.jd0 = 1461 * (n4 - 1) + na + 2450082.5 - ratio;
  128.         // GMST
  129.         this.gmst = computeGMST();
  130.         this.date = date;
  131.     }

  133.     @Override
  134.     public AbsoluteDate getDate() {
  135.         return date;
  136.     }

  138.     /** Get the number of seconds since N<sub>a</sub> start.
  139.      * @return number of seconds since N<sub>a</sub> start
  140.      */
  141.     public double getSecInDay() {
  142.         return secInNa;
  143.     }

  145.     /** Get the number of the current day in a four year interval.
  146.      * @return the number of the current day in a four year interval
  147.      */
  148.     public int getDayNumber() {
  149.         return na;
  150.     }

  152.     /** Get the number of the current four year interval.
  153.      * @return the number of the current four year interval
  154.      */
  155.     public int getIntervalNumber() {
  156.         return n4;
  157.     }

  159.     /** Get the current Julian date JD0.
  160.      * @return the current date JD0
  161.      */
  162.     public double getJD0() {
  163.         return jd0;
  164.     }

  166.     /** Get the Greenwich Mean Sidereal Time.
  167.      * @return the Greenwich Mean Sidereal Time (rad)
  168.      */
  169.     public double getGMST() {
  170.         return gmst;
  171.     }

  173.     /** Compute the Greenwich Mean Sidereal Time using the current Julian date JD0.
  174.      * @return the Greenwich Mean Sidereal Time (rad)
  175.      */
  176.     private double computeGMST() {
  177.         final double ref = 2451545.0;
  178.         // Earth's rotation angle in radians
  179.         final double era = 2. * GNSSConstants.GLONASS_PI *
  180.                         (0.7790572732640 + 1.00273781191135448 * (jd0 - ref));
  181.         // Time from Epoch 2000 (1st January, 00:00 UTC) till current Epoch in Julian centuries
  182.         final double time = (jd0 - ref) / Constants.JULIAN_CENTURY;
  183.         // Time to the power n
  184.         final double time2 = time * time;
  185.         final double time3 = time2 * time;
  186.         final double time4 = time2 * time2;
  187.         final double time5 = time2 * time3;
  188.         // GMST computation
  189.         final double gTime = era + 7.03270726e-8 + time * 2.23603658710194e-2 +
  190.                         time2 * 6.7465784654e-6 - time3 * 2.1332e-12 - time4 * 1.452308e-10 - time5 * 1.784e-13;
  191.         return gTime;
  192.     }

  194.     /** Compute the GLONASS date.
  195.      * @return the date
  196.      * @param glonass time scale.
  197.      */
  198.     private AbsoluteDate computeDate(final TimeScale glonass) {
  199.         // Compute the number of Julian day for the current date
  200.         final double jdn = jd0 + 0.5;
  201.         // Coefficients
  202.         final int a = (int) (jdn + 32044);
  203.         final int b = (4 * a + 3) / 146097;
  204.         final int c = a - (146097 * b) / 4;
  205.         final int d = (4 * c + 3) / 1461;
  206.         final int e = c - (1461 * d) / 4;
  207.         final int m = (5 * e + 2) / 153;
  208.         // Year, month and day
  209.         final int day   = e - (153 * m + 2) / 5 + 1;
  210.         final int month = m + 3 - 12 * (m / 10);
  211.         final int year  = 100 * b + d - 4800 + m / 10;
  212.         return new AbsoluteDate(new DateComponents(year, month, day),
  213.                                 new TimeComponents(secInNa),
  214.                                 glonass);
  215.     }

  217. }
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