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erfasrc/src/taiutc.c  view on Meta::CPAN

#include "erfa.h"

int eraTaiutc(double tai1, double tai2, double *utc1, double *utc2)
/*
**  - - - - - - - - - -
**   e r a T a i u t c
**  - - - - - - - - - -
**
**  Time scale transformation:  International Atomic Time, TAI, to
**  Coordinated Universal Time, UTC.
**
**  Given:
**     tai1,tai2  double   TAI as a 2-part Julian Date (Note 1)
**
**  Returned:
**     utc1,utc2  double   UTC as a 2-part quasi Julian Date (Notes 1-3)
**
**  Returned (function value):
**                int      status: +1 = dubious year (Note 4)
**                                  0 = OK
**                                 -1 = unacceptable date
**
**  Notes:
**
**  1) tai1+tai2 is Julian Date, apportioned in any convenient way
**     between the two arguments, for example where tai1 is the Julian
**     Day Number and tai2 is the fraction of a day.  The returned utc1
**     and utc2 form an analogous pair, except that a special convention
**     is used, to deal with the problem of leap seconds - see the next
**     note.
**
**  2) JD cannot unambiguously represent UTC during a leap second unless
**     special measures are taken.  The convention in the present
**     function is that the JD day represents UTC days whether the
**     length is 86399, 86400 or 86401 SI seconds.  In the 1960-1972 era
**     there were smaller jumps (in either direction) each time the
**     linear UTC(TAI) expression was changed, and these "mini-leaps"
**     are also included in the ERFA convention.
**
**  3) The function eraD2dtf can be used to transform the UTC quasi-JD
**     into calendar date and clock time, including UTC leap second
**     handling.
**
**  4) The warning status "dubious year" flags UTCs that predate the
**     introduction of the time scale or that are too far in the future
**     to be trusted.  See eraDat for further details.
**
**  Called:
**     eraUtctai    UTC to TAI
**
**  References:
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**     Explanatory Supplement to the Astronomical Almanac,
**     P. Kenneth Seidelmann (ed), University Science Books (1992)
**
**  Copyright (C) 2013-2020, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   int big1;
   int i, j;
   double a1, a2, u1, u2, g1, g2;


/* Put the two parts of the TAI into big-first order. */
   big1 = ( fabs(tai1) >= fabs(tai2) );
   if ( big1 ) {
      a1 = tai1;
      a2 = tai2;
   } else {
      a1 = tai2;
      a2 = tai1;
   }

/* Initial guess for UTC. */
   u1 = a1;
   u2 = a2;

/* Iterate (though in most cases just once is enough). */
   for ( i = 0; i < 3; i++ ) {

   /* Guessed UTC to TAI. */
      j = eraUtctai(u1, u2, &g1, &g2);
      if ( j < 0 ) return j;

   /* Adjust guessed UTC. */
      u2 += a1 - g1;
      u2 += a2 - g2;
   }

/* Return the UTC result, preserving the TAI order. */
   if ( big1 ) {
      *utc1 = u1;
      *utc2 = u2;
   } else {
      *utc1 = u2;
      *utc2 = u1;
   }