1   /* Copyright 2002-2026 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.propagation;
18  
19  import org.hipparchus.analysis.polynomials.SmoothStepFactory;
20  import org.orekit.errors.OrekitException;
21  import org.orekit.frames.Frame;
22  import org.orekit.orbits.AbstractOrbitInterpolator;
23  import org.orekit.orbits.CartesianOrbit;
24  import org.orekit.orbits.Orbit;
25  import org.orekit.propagation.analytical.AbstractAnalyticalPropagator;
26  import org.orekit.time.AbsoluteDate;
27  import org.orekit.utils.PVCoordinates;
28  
29  import java.util.List;
30  
31  /**
32   * Orbit blender.
33   * <p>
34   * Its purpose is to interpolate orbit state between tabulated orbit states using the concept of blending, exposed in :
35   * "Efficient Covariance Interpolation using Blending of Approximate State Error Transitions" by Sergei Tanygin, and applying
36   * it to orbit states instead of covariances.
37   * <p>
38   * It propagates tabulated values to the interpolating time using given propagator and then blend each propagated
39   * states using a smoothstep function. It gives especially good results as explained
40   * <a href="https://orekit.org/doc/technical-notes/Implementation_of_covariance_interpolation_in_Orekit.pdf">here</a>
41   * compared to Hermite interpolation when time steps between tabulated values get significant (In LEO, &gt; 10 mn for
42   * example).
43   * <p>
44   * <b>In most cases, an analytical propagator would be used to quickly fill the gap between tabulated values and recreate
45   * a dense ephemeris</b>.
46   * <p>
47   * However, a fully configured and accurate numerical propagator can be used to recreate an even
48   * more precise ephemeris in case the initial tabulated values were obtained from an external source.
49   * <p>
50   * Note that in the current implementation, the returned blended orbit is necessarily Cartesian.
51   *
52   * @author Vincent Cucchietti
53   * @see org.hipparchus.analysis.polynomials.SmoothStepFactory
54   * @see org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction
55   * @see Propagator
56   * @see AbstractAnalyticalPropagator
57   *
58   * @since 12.0
59   */
60  public class OrbitBlender extends AbstractOrbitInterpolator {
61  
62      /** Propagator used to propagate tabulated orbits to interpolating time. */
63      private final Propagator blendingPropagator;
64  
65      /** Blending function. */
66      private final SmoothStepFactory.SmoothStepFunction blendingFunction;
67  
68      /**
69       * Default constructor.
70       * <p>
71       * <b>In most cases, an analytical propagator would be used to quickly fill the gap between tabulated values and recreate
72       * a dense ephemeris</b>.
73       * <p>
74       * However, a fully configured and accurate numerical propagator can be used to recreate an even
75       * more precise ephemeris in case the initial tabulated values were obtained from an external source.
76       *
77       * @param blendingFunction
78       * {@link org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction smoothstep function} used for
79       * blending
80       * @param blendingPropagator propagator used to propagate tabulated orbits to interpolating time
81       * @param outputInertialFrame output inertial frame
82       *
83       * @throws OrekitException if output frame is not inertial
84       * @see org.hipparchus.analysis.polynomials.SmoothStepFactory.SmoothStepFunction
85       */
86      public OrbitBlender(final SmoothStepFactory.SmoothStepFunction blendingFunction,
87                          final Propagator blendingPropagator,
88                          final Frame outputInertialFrame) {
89          super(DEFAULT_INTERPOLATION_POINTS, 0., outputInertialFrame);
90          this.blendingFunction   = blendingFunction;
91          this.blendingPropagator = blendingPropagator;
92      }
93  
94      /** {@inheritDoc} */
95      @Override
96      protected Orbit interpolate(final InterpolationData interpolationData) {
97  
98          // Get first and last entry
99          final List<Orbit> neighborList  = interpolationData.getNeighborList();
100         final Orbit       previousOrbit = neighborList.getFirst();
101         final Orbit       nextOrbit     = neighborList.get(1);
102 
103         // Propagate orbits
104         final AbsoluteDate interpolationDate = interpolationData.getInterpolationDate();
105         final Orbit forwardedOrbit  = propagateOrbit(previousOrbit, interpolationDate);
106         final Orbit backwardedOrbit = propagateOrbit(nextOrbit, interpolationDate);
107 
108         // Extract position-velocity-acceleration coordinates
109         final PVCoordinates forwardedPV  = forwardedOrbit.getPVCoordinates(getOutputInertialFrame());
110         final PVCoordinates backwardedPV = backwardedOrbit.getPVCoordinates(getOutputInertialFrame());
111 
112         // Blend PV coordinates
113         final double timeParameter = getTimeParameter(interpolationDate, previousOrbit.getDate(), nextOrbit.getDate());
114         final double blendingValue = blendingFunction.value(timeParameter);
115 
116         final PVCoordinates blendedPV = forwardedPV.blendArithmeticallyWith(backwardedPV, blendingValue);
117 
118         // Output new blended instance
119         return new CartesianOrbit(blendedPV, getOutputInertialFrame(), interpolationDate, previousOrbit.getMu());
120     }
121 
122     /**
123      * Propagate orbit using predefined {@link Propagator propagator}.
124      *
125      * @param tabulatedOrbit tabulated orbit to propagate
126      * @param propagationDate propagation date
127      *
128      * @return orbit propagated to propagation date
129      */
130     private Orbit propagateOrbit(final Orbit tabulatedOrbit,
131                                  final AbsoluteDate propagationDate) {
132         blendingPropagator.resetInitialState(new SpacecraftState(tabulatedOrbit));
133         return blendingPropagator.propagate(propagationDate).getOrbit();
134     }
135 }