1   /* Copyright 2002-2025 CS GROUP
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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  
21  /**
22   * Interface used to provide spherical harmonics coefficients.
23   * <p>
24   * Two interfaces are provided to distinguish between normalized and un-normalized
25   * coefficients: {@link NormalizedSphericalHarmonicsProvider} and {@link
26   * UnnormalizedSphericalHarmonicsProvider}. To account for gravity perturbations all
27   * providers are capable of providing the coefficients on specific dates, using the {@link
28   * NormalizedSphericalHarmonicsProvider#onDate(AbsoluteDate) onDate(AbsoluteDate)}
29   * methods.
30   * <p>
31   * Typical usage when evaluating the geopotential:
32   * <pre><code>
33   *     NormalizedSphericalHarmonicsProvider provider = ...;
34   *     NormalizedSphericalHarmonics coeffs = provider.onDate(date);
35   *     double c20 = coeffs.getNormalizedCnm(2, 0);
36   * </code></pre>
37   *
38   * @author Luc Maisonobe
39   * @see GravityFields
40   * @since 6.0
41   */
42  public interface SphericalHarmonicsProvider extends TideSystemProvider {
43  
44      /** Get the maximal supported degree.
45       * @return maximal supported degree
46       */
47      int getMaxDegree();
48  
49      /** Get the maximal supported order.
50       * @return maximal supported order
51       */
52      int getMaxOrder();
53  
54      /** Get the central body attraction coefficient.
55       * @return mu (m³/s²)
56       */
57      double getMu();
58  
59      /** Get the value of the central body reference radius.
60       * @return ae (m)
61       */
62      double getAe();
63  
64      /** Get the reference date for the harmonics.
65       * <p>
66       * For piecewise models, the latest reference date is returned.
67       * </p>
68       * @return reference date for the harmonics (may be null if no reference date is defined)
69       */
70      AbsoluteDate getReferenceDate();
71  
72  }