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erfasrc/src/ut1utc.c  view on Meta::CPAN

#include "erfa.h"

int eraUt1utc(double ut11, double ut12, double dut1,
              double *utc1, double *utc2)
/*
**  - - - - - - - - - -
**   e r a U t 1 u t c
**  - - - - - - - - - -
**
**  Time scale transformation:  Universal Time, UT1, to Coordinated
**  Universal Time, UTC.
**
**  Given:
**     ut11,ut12  double   UT1 as a 2-part Julian Date (Note 1)
**     dut1       double   Delta UT1: UT1-UTC in seconds (Note 2)
**
**  Returned:
**     utc1,utc2  double   UTC as a 2-part quasi Julian Date (Notes 3,4)
**
**  Returned (function value):
**                int      status: +1 = dubious year (Note 5)
**                                  0 = OK
**                                 -1 = unacceptable date
**
**  Notes:
**
**  1) ut11+ut12 is Julian Date, apportioned in any convenient way
**     between the two arguments, for example where ut11 is the Julian
**     Day Number and ut12 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 Note 3.
**
**  2) Delta UT1 can be obtained from tabulations provided by the
**     International Earth Rotation and Reference Systems Service.  The
**     value changes abruptly by 1s at a leap second;  however, close to
**     a leap second the algorithm used here is tolerant of the "wrong"
**     choice of value being made.
**
**  3) JD cannot unambiguously represent UTC during a leap second unless
**     special measures are taken.  The convention in the present
**     function is that the returned quasi JD day UTC1+UTC2 represents
**     UTC days whether the length is 86399, 86400 or 86401 SI seconds.
**
**  4) The function eraD2dtf can be used to transform the UTC quasi-JD
**     into calendar date and clock time, including UTC leap second
**     handling.
**
**  5) 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:
**     eraJd2cal    JD to Gregorian calendar
**     eraDat       delta(AT) = TAI-UTC
**     eraCal2jd    Gregorian calendar to JD
**
**  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, iy, im, id, js;
   double duts, u1, u2, d1, dats1, d2, fd, dats2, ddats, us1, us2, du;


/* UT1-UTC in seconds. */
   duts = dut1;

/* Put the two parts of the UT1 into big-first order. */
   big1 = ( fabs(ut11) >= fabs(ut12) );
   if ( big1 ) {
      u1 = ut11;
      u2 = ut12;
   } else {
      u1 = ut12;
      u2 = ut11;
   }

/* See if the UT1 can possibly be in a leap-second day. */
   d1 = u1;
   dats1 = 0;
   for ( i = -1; i <= 3; i++ ) {
      d2 = u2 + (double) i;
      if ( eraJd2cal(d1, d2, &iy, &im, &id, &fd) ) return -1;
      js = eraDat(iy, im, id, 0.0, &dats2);
      if ( js < 0 ) return -1;
      if ( i == - 1 ) dats1 = dats2;
      ddats = dats2 - dats1;
      if ( fabs(ddats) >= 0.5 ) {

      /* Yes, leap second nearby: ensure UT1-UTC is "before" value. */
         if ( ddats * duts >= 0 ) duts -= ddats;

      /* UT1 for the start of the UTC day that ends in a leap. */
         if ( eraCal2jd(iy, im, id, &d1, &d2) ) return -1;
         us1 = d1;
         us2 = d2 - 1.0 + duts/ERFA_DAYSEC;

      /* Is the UT1 after this point? */
         du = u1 - us1;
         du += u2 - us2;
         if ( du > 0 ) {

         /* Yes:  fraction of the current UTC day that has elapsed. */
            fd = du * ERFA_DAYSEC / ( ERFA_DAYSEC + ddats );

         /* Ramp UT1-UTC to bring about ERFA's JD(UTC) convention. */