tttcg.c

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#include "erfa.h"

int eraTttcg(double tt1, double tt2, double *tcg1, double *tcg2)
/*
**  - - - - - - - - -
**   e r a T t t c g
**  - - - - - - - - -
**
**  Time scale transformation:  Terrestrial Time, TT, to Geocentric
**  Coordinate Time, TCG.
**
**  Given:
**     tt1,tt2    double    TT as a 2-part Julian Date
**
**  Returned:
**     tcg1,tcg2  double    TCG as a 2-part Julian Date
**
**  Returned (function value):
**                int       status:  0 = OK
**
**  Note:
**
**     tt1+tt2 is Julian Date, apportioned in any convenient way between
**     the two arguments, for example where tt1 is the Julian Day Number
**     and tt2 is the fraction of a day.  The returned tcg1,tcg2 follow
**     suit.
**
**  References:
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**     IAU 2000 Resolution B1.9
**
**  Copyright (C) 2013-2015, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{

/* 1977 Jan 1 00:00:32.184 TT, as MJD */
   static const double t77t = ERFA_DJM77 + ERFA_TTMTAI/ERFA_DAYSEC;

/* TT to TCG rate */
   static const double elgg = ERFA_ELG/(1.0-ERFA_ELG);

/* Result, safeguarding precision. */
   if ( tt1 > tt2 ) {
      *tcg1 = tt1;
      *tcg2 = tt2 + ( ( tt1 - ERFA_DJM0 ) + ( tt2 - t77t ) ) * elgg;
   } else {
      *tcg1 = tt1 + ( ( tt2 - ERFA_DJM0 ) + ( tt1 - t77t ) ) * elgg;
      *tcg2 = tt2;
   }

/* Status (always OK). */
   return 0;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2015, NumFOCUS Foundation.
**  All rights reserved.
**  
**  This library is derived, with permission, from the International
**  Astronomical Union's "Standards of Fundamental Astronomy" library,
**  available from http://www.iausofa.org.
**  
**  The ERFA version is intended to retain identical functionality to
**  the SOFA library, but made distinct through different function and
**  file names, as set out in the SOFA license conditions.  The SOFA
**  original has a role as a reference standard for the IAU and IERS,
**  and consequently redistribution is permitted only in its unaltered
**  state.  The ERFA version is not subject to this restriction and
**  therefore can be included in distributions which do not support the
**  concept of "read only" software.
**  
**  Although the intent is to replicate the SOFA API (other than
**  replacement of prefix names) and results (with the exception of
**  bugs;  any that are discovered will be fixed), SOFA is not
**  responsible for any errors found in this version of the library.
**  
**  If you wish to acknowledge the SOFA heritage, please acknowledge
**  that you are using a library derived from SOFA, rather than SOFA
**  itself.
**  
**  
**  TERMS AND CONDITIONS
**  
**  Redistribution and use in source and binary forms, with or without
**  modification, are permitted provided that the following conditions
**  are met:
**  
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**    notice, this list of conditions and the following disclaimer.
**  
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