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17  package org.orekit.propagation.analytical.tle;
18  
19  import org.hipparchus.util.FastMath;
20  import org.hipparchus.util.SinCos;
21  import org.orekit.annotation.DefaultDataContext;
22  import org.orekit.attitudes.AttitudeProvider;
23  import org.orekit.data.DataContext;
24  import org.orekit.frames.Frame;
25  
26  /** This class contains methods to compute propagated coordinates with the SGP4 model.
27   * <p>
28   * The user should not bother in this class since it is handled internaly by the
29   * {@link TLEPropagator}.
30   * </p>
31   * <p>This implementation is largely inspired from the paper and source code <a
32   * href="https://www.celestrak.com/publications/AIAA/2006-6753/">Revisiting Spacetrack
33   * Report #3</a> and is fully compliant with its results and tests cases.</p>
34   * @author Felix R. Hoots, Ronald L. Roehrich, December 1980 (original fortran)
35   * @author David A. Vallado, Paul Crawford, Richard Hujsak, T.S. Kelso (C++ translation and improvements)
36   * @author Fabien Maussion (java translation)
37   */
38  public class SGP4 extends TLEPropagator {
39  
40      /** If perige is less than 220 km, some calculus are avoided. */
41      private boolean lessThan220;
42  
43      /** (1 + eta * cos(M0))³. */
44      private double delM0;
45  
46      // CHECKSTYLE: stop JavadocVariable check
47      private double d2;
48      private double d3;
49      private double d4;
50      private double t3cof;
51      private double t4cof;
52      private double t5cof;
53      private double sinM0;
54      private double omgcof;
55      private double xmcof;
56      private double c5;
57      // CHECKSTYLE: resume JavadocVariable check
58  
59      /** Constructor for a unique initial TLE.
60       *
61       * <p>This constructor uses the {@link DataContext#getDefault() default data context}.
62       *
63       * @param initialTLE the TLE to propagate.
64       * @param attitudeProvider provider for attitude computation
65       * @param mass spacecraft mass (kg)
66       * @see #SGP4(TLE, AttitudeProvider, double, Frame)
67       */
68      @DefaultDataContext
69      public SGP4(final TLE initialTLE, final AttitudeProvider attitudeProvider,
70                  final double mass) {
71          this(initialTLE, attitudeProvider, mass,
72                  DataContext.getDefault().getFrames().getTEME());
73      }
74  
75      /** Constructor for a unique initial TLE.
76       * @param initialTLE the TLE to propagate.
77       * @param attitudeProvider provider for attitude computation
78       * @param mass spacecraft mass (kg)
79       * @param teme the TEME frame to use for propagation.
80       * @since 10.1
81       */
82      public SGP4(final TLE initialTLE,
83                  final AttitudeProvider attitudeProvider,
84                  final double mass,
85                  final Frame teme) {
86          super(initialTLE, attitudeProvider, mass, teme);
87      }
88  
89      /** Initialization proper to each propagator (SGP or SDP).
90       */
91      protected void sxpInitialize() {
92  
93          // For perigee less than 220 kilometers, the equations are truncated to
94          // linear variation in sqrt a and quadratic variation in mean anomaly.
95          // Also, the c3 term, the delta omega term, and the delta m term are dropped.
96          lessThan220 = perige < 220;
97          if (!lessThan220) {
98              final SinCos scM0 = FastMath.sinCos(tle.getMeanAnomaly());
99              final double c1sq = c1 * c1;
100             delM0 = 1.0 + eta * scM0.cos();
101             delM0 *= delM0 * delM0;
102             d2 = 4 * a0dp * tsi * c1sq;
103             final double temp = d2 * tsi * c1 / 3.0;
104             d3 = (17 * a0dp + s4) * temp;
105             d4 = 0.5 * temp * a0dp * tsi * (221 * a0dp + 31 * s4) * c1;
106             t3cof = d2 + 2 * c1sq;
107             t4cof = 0.25 * (3 * d3 + c1 * (12 * d2 + 10 * c1sq));
108             t5cof = 0.2 * (3 * d4 + 12 * c1 * d3 + 6 * d2 * d2 + 15 * c1sq * (2 * d2 + c1sq));
109             sinM0 = scM0.sin();
110             if (tle.getE() < 1e-4) {
111                 omgcof = 0.;
112                 xmcof = 0.;
113             } else  {
114                 final double c3 = coef * tsi * TLEConstants.A3OVK2 * xn0dp *
115                                   TLEConstants.NORMALIZED_EQUATORIAL_RADIUS * sini0 / tle.getE();
116                 xmcof = -TLEConstants.TWO_THIRD * coef * tle.getBStar() *
117                         TLEConstants.NORMALIZED_EQUATORIAL_RADIUS / eeta;
118                 omgcof = tle.getBStar() * c3 * FastMath.cos(tle.getPerigeeArgument());
119             }
120         }
121 
122         c5 = 2 * coef1 * a0dp * beta02 * (1 + 2.75 * (etasq + eeta) + eeta * etasq);
123         // initialized
124     }
125 
126     /** Propagation proper to each propagator (SGP or SDP).
127      * @param tSince the offset from initial epoch (min)
128      */
129     protected void sxpPropagate(final double tSince) {
130 
131         // Update for secular gravity and atmospheric drag.
132         final double xmdf = tle.getMeanAnomaly() + xmdot * tSince;
133         final double omgadf = tle.getPerigeeArgument() + omgdot * tSince;
134         final double xn0ddf = tle.getRaan() + xnodot * tSince;
135         omega = omgadf;
136         double xmp = xmdf;
137         final double tsq = tSince * tSince;
138         xnode = xn0ddf + xnodcf * tsq;
139         double tempa = 1 - c1 * tSince;
140         double tempe = tle.getBStar(tle.getDate().shiftedBy(tSince)) * c4 * tSince;
141         double templ = t2cof * tsq;
142 
143         if (!lessThan220) {
144             final double delomg = omgcof * tSince;
145             double delm = 1. + eta * FastMath.cos(xmdf);
146             delm = xmcof * (delm * delm * delm - delM0);
147             final double temp = delomg + delm;
148             xmp = xmdf + temp;
149             omega = omgadf - temp;
150             final double tcube = tsq * tSince;
151             final double tfour = tSince * tcube;
152             tempa = tempa - d2 * tsq - d3 * tcube - d4 * tfour;
153             tempe = tempe + tle.getBStar(tle.getDate().shiftedBy(tSince)) * c5 * (FastMath.sin(xmp) - sinM0);
154             templ = templ + t3cof * tcube + tfour * (t4cof + tSince * t5cof);
155         }
156 
157         a = a0dp * tempa * tempa;
158         e = tle.getE() - tempe;
159 
160         // A highly arbitrary lower limit on e,  of 1e-6:
161         if (e < 1e-6) {
162             e = 1e-6;
163         }
164 
165         xl = xmp + omega + xnode + xn0dp * templ;
166 
167         i = tle.getI();
168 
169     }
170 
171 }