1   /* Copyright 2002-2024 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;
18  
19  import org.hipparchus.CalculusFieldElement;
20  
21  /** Geocentric Coordinate Time.
22   * <p>Coordinate time at the center of mass of the Earth.
23   * This time scale depends linearly from {@link TTScale Terrestrial Time}.</p>
24   * <p>This is intended to be accessed thanks to {@link TimeScales},
25   * so there is no public constructor.</p>
26   * @author Luc Maisonobe
27   * @see AbsoluteDate
28   */
29  public class TCGScale implements TimeScale {
30  
31      /** Serializable UID. */
32      private static final long serialVersionUID = 20240720L;
33  
34      /** LG rate. */
35      private static final double LG_RATE = 6.969290134e-10;
36  
37      /** Reference date for TCG.
38       * <p>The reference date is such that the four following instants are equal:</p>
39       * <ul>
40       *   <li>1977-01-01T00:00:32.184 TT</li>
41       *   <li>1977-01-01T00:00:32.184 TCG</li>
42       *   <li>1977-01-01T00:00:32.184 TCB</li>
43       *   <li>1977-01-01T00:00:00.000 TAI</li>
44       * </ul>
45       */
46      private final AbsoluteDate referenceDate;
47  
48      /** Offset between TT and TAI scales. */
49      private final TimeOffset ttOffset;
50  
51      /**
52       * Package private constructor for the factory.
53       *
54       * @param tt  TT time scale.
55       * @param tai TAI time scale.
56       */
57      TCGScale(final TimeScale tt, final TimeScale tai) {
58          referenceDate = new AbsoluteDate(1977, 1, 1, tai);
59          ttOffset = tt.offsetFromTAI(referenceDate);
60      }
61  
62      /** {@inheritDoc} */
63      @Override
64      public TimeOffset offsetFromTAI(final AbsoluteDate date) {
65          return ttOffset.add(new TimeOffset(LG_RATE * date.durationFrom(referenceDate)));
66      }
67  
68      /** {@inheritDoc} */
69      @Override
70      public <T extends CalculusFieldElement<T>> T offsetFromTAI(final FieldAbsoluteDate<T> date) {
71          return date.durationFrom(referenceDate).multiply(LG_RATE).add(ttOffset.toDouble());
72      }
73  
74      /** {@inheritDoc} */
75      public String getName() {
76          return "TCG";
77      }
78  
79      /** {@inheritDoc} */
80      public String toString() {
81          return getName();
82      }
83  
84  }