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.forces.gravity.potential;
18  
19  import org.orekit.time.AbsoluteDate;
20  import org.orekit.time.TimeStamped;
21  
22  /** Interface used to provide normalized spherical harmonics coefficients.
23   * @see GravityFields
24   * @author Luc Maisonobe
25   * @since 6.0
26   */
27  public interface NormalizedSphericalHarmonicsProvider extends SphericalHarmonicsProvider {
28  
29      /**
30       * The normalized geopotential coefficients at a specific instance in time.
31       * @see NormalizedSphericalHarmonicsProvider
32       * @see NormalizedSphericalHarmonicsProvider#onDate(AbsoluteDate)
33       * @since 6.1
34       */
35      interface NormalizedSphericalHarmonics extends TimeStamped {
36  
37          /** Get a spherical harmonic cosine coefficient.
38           * @param n degree of the coefficient
39           * @param m order of the coefficient
40           * @return normalized coefficient Cnm
41           */
42          double getNormalizedCnm(int n, int m);
43  
44          /** Get a spherical harmonic sine coefficient.
45           * @param n degree of the coefficient
46           * @param m order of the coefficient
47           * @return normalized coefficient Snm
48           */
49          double getNormalizedSnm(int n, int m);
50  
51      }
52  
53      /**
54       * Get the normalized spherical harmonic coefficients at a specific instance in time.
55       *
56       * @param date of evaluation
57       * @return normalized coefficients on {@code date}.
58       * @since 6.1
59       */
60      NormalizedSphericalHarmonics onDate(AbsoluteDate date);
61  
62      /**
63       * Get the normalized coefficient of degree 2 and order 0 at a specific instance in time.
64       *
65       * @param date of evaluation (may be null if model is not time-dependent)
66       * @return normalized C20 on {@code date}.
67       * @since 12.1
68       */
69      default double getNormalizedC20(final AbsoluteDate date) {
70          return onDate(date).getNormalizedCnm(2, 0);
71      }
72  
73  }