path: root/ast/erfa/dat.c

#include "erfa.h"

int eraDat(int iy, int im, int id, double fd, double *deltat )
/*
**  - - - - - - -
**   e r a D a t
**  - - - - - - -
**
**  For a given UTC date, calculate delta(AT) = TAI-UTC.
**
**     :------------------------------------------:
**     :                                          :
**     :                 IMPORTANT                :
**     :                                          :
**     :  A new version of this function must be  :
**     :  produced whenever a new leap second is  :
**     :  announced.  There are four items to     :
**     :  change on each such occasion:           :
**     :                                          :
**     :  1) A new line must be added to the set  :
**     :     of statements that initialize the    :
**     :     array "changes".                     :
**     :                                          :
**     :  2) The constant IYV must be set to the  :
**     :     current year.                        :
**     :                                          :
**     :  3) The "Latest leap second" comment     :
**     :     below must be set to the new leap    :
**     :     second date.                         :
**     :                                          :
**     :  4) The "This revision" comment, later,  :
**     :     must be set to the current date.     :
**     :                                          :
**     :  Change (2) must also be carried out     :
**     :  whenever the function is re-issued,     :
**     :  even if no leap seconds have been       :
**     :  added.                                  :
**     :                                          :
**     :  Latest leap second:  2016 December 31   :
**     :                                          :
**     :__________________________________________:
**
**  Given:
**     iy     int      UTC:  year (Notes 1 and 2)
**     im     int            month (Note 2)
**     id     int            day (Notes 2 and 3)
**     fd     double         fraction of day (Note 4)
**
**  Returned:
**     deltat double   TAI minus UTC, seconds
**
**  Returned (function value):
**            int      status (Note 5):
**                       1 = dubious year (Note 1)
**                       0 = OK
**                      -1 = bad year
**                      -2 = bad month
**                      -3 = bad day (Note 3)
**                      -4 = bad fraction (Note 4)
**                      -5 = internal error (Note 5)
**
**  Notes:
**
**  1) UTC began at 1960 January 1.0 (JD 2436934.5) and it is improper
**     to call the function with an earlier date.  If this is attempted,
**     zero is returned together with a warning status.
**
**     Because leap seconds cannot, in principle, be predicted in
**     advance, a reliable check for dates beyond the valid range is
**     impossible.  To guard against gross errors, a year five or more
**     after the release year of the present function (see the constant
**     IYV) is considered dubious.  In this case a warning status is
**     returned but the result is computed in the normal way.
**
**     For both too-early and too-late years, the warning status is +1.
**     This is distinct from the error status -1, which signifies a year
**     so early that JD could not be computed.
**
**  2) If the specified date is for a day which ends with a leap second,
**     the UTC-TAI value returned is for the period leading up to the
**     leap second.  If the date is for a day which begins as a leap
**     second ends, the UTC-TAI returned is for the period following the
**     leap second.
**
**  3) The day number must be in the normal calendar range, for example
**     1 through 30 for April.  The "almanac" convention of allowing
**     such dates as January 0 and December 32 is not supported in this
**     function, in order to avoid confusion near leap seconds.
**
**  4) The fraction of day is used only for dates before the
**     introduction of leap seconds, the first of which occurred at the
**     end of 1971.  It is tested for validity (0 to 1 is the valid
**     range) even if not used;  if invalid, zero is used and status -4
**     is returned.  For many applications, setting fd to zero is
**     acceptable;  the resulting error is always less than 3 ms (and
**     occurs only pre-1972).
**
**  5) The status value returned in the case where there are multiple
**     errors refers to the first error detected.  For example, if the
**     month and day are 13 and 32 respectively, status -2 (bad month)
**     will be returned.  The "internal error" status refers to a
**     case that is impossible but causes some compilers to issue a
**     warning.
**
**  6) In cases where a valid result is not available, zero is returned.
**
**  References:
**
**  1) For dates from 1961 January 1 onwards, the expressions from the
**     file ftp://maia.usno.navy.mil/ser7/tai-utc.dat are used.
**
**  2) The 5ms timestep at 1961 January 1 is taken from 2.58.1 (p87) of
**     the 1992 Explanatory Supplement.
**
**  Called:
**     eraCal2jd    Gregorian calendar to JD
**
**  Copyright (C) 2013-2016, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
/* Release year for this version of eraDat */
   enum { IYV = 2016};

/* Reference dates (MJD) and drift rates (s/day), pre leap seconds */
   static const double drift[][2] = {
      { 37300.0, 0.0012960 },
      { 37300.0, 0.0012960 },
      { 37300.0, 0.0012960 },
      { 37665.0, 0.0011232 },
      { 37665.0, 0.0011232 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 38761.0, 0.0012960 },
      { 39126.0, 0.0025920 },
      { 39126.0, 0.0025920 }
   };

/* Number of Delta(AT) expressions before leap seconds were introduced */
   enum { NERA1 = (int) (sizeof drift / sizeof (double) / 2) };

/* Dates and Delta(AT)s */
   static const struct {
      int iyear, month;
      double delat;
   } changes[] = {
      { 1960,  1,  1.4178180 },
      { 1961,  1,  1.4228180 },
      { 1961,  8,  1.3728180 },
      { 1962,  1,  1.8458580 },
      { 1963, 11,  1.9458580 },
      { 1964,  1,  3.2401300 },
      { 1964,  4,  3.3401300 },
      { 1964,  9,  3.4401300 },
      { 1965,  1,  3.5401300 },
      { 1965,  3,  3.6401300 },
      { 1965,  7,  3.7401300 },
      { 1965,  9,  3.8401300 },
      { 1966,  1,  4.3131700 },
      { 1968,  2,  4.2131700 },
      { 1972,  1, 10.0       },
      { 1972,  7, 11.0       },
      { 1973,  1, 12.0       },
      { 1974,  1, 13.0       },
      { 1975,  1, 14.0       },
      { 1976,  1, 15.0       },
      { 1977,  1, 16.0       },
      { 1978,  1, 17.0       },
      { 1979,  1, 18.0       },
      { 1980,  1, 19.0       },
      { 1981,  7, 20.0       },
      { 1982,  7, 21.0       },
      { 1983,  7, 22.0       },
      { 1985,  7, 23.0       },
      { 1988,  1, 24.0       },
      { 1990,  1, 25.0       },
      { 1991,  1, 26.0       },
      { 1992,  7, 27.0       },
      { 1993,  7, 28.0       },
      { 1994,  7, 29.0       },
      { 1996,  1, 30.0       },
      { 1997,  7, 31.0       },
      { 1999,  1, 32.0       },
      { 2006,  1, 33.0       },
      { 2009,  1, 34.0       },
      { 2012,  7, 35.0       },
      { 2015,  7, 36.0       },
      { 2017,  1, 37.0       }
   };

/* Number of Delta(AT) changes */
   enum { NDAT = (int) (sizeof changes / sizeof changes[0]) };

/* Miscellaneous local variables */
   int j, i, m;
   double da, djm0, djm;


/* Initialize the result to zero. */
   *deltat = da = 0.0;

/* If invalid fraction of a day, set error status and give up. */
   if (fd < 0.0 || fd > 1.0) return -4;

/* Convert the date into an MJD. */
   j = eraCal2jd(iy, im, id, &djm0, &djm);

/* If invalid year, month, or day, give up. */
   if (j < 0) return j;

/* If pre-UTC year, set warning status and give up. */
   if (iy < changes[0].iyear) return 1;

/* If suspiciously late year, set warning status but proceed. */
   if (iy > IYV + 5) j = 1;

/* Combine year and month to form a date-ordered integer... */
   m = 12*iy + im;

/* ...and use it to find the preceding table entry. */
   for (i = NDAT-1; i >=0; i--) {
      if (m >= (12 * changes[i].iyear + changes[i].month)) break;
   }

/* Prevent underflow warnings. */
   if (i < 0) return -5;

/* Get the Delta(AT). */
   da = changes[i].delat;

/* If pre-1972, adjust for drift. */
   if (i < NERA1) da += (djm + fd - drift[i][0]) * drift[i][1];

/* Return the Delta(AT) value. */
   *deltat = da;

/* Return the status. */
   return j;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2016, 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:
**  
**  1 Redistributions of source code must retain the above copyright
**    notice, this list of conditions and the following disclaimer.
**  
**  2 Redistributions in binary form must reproduce the above copyright
**    notice, this list of conditions and the following disclaimer in
**    the documentation and/or other materials provided with the
**    distribution.
**  
**  3 Neither the name of the Standards Of Fundamental Astronomy Board,
**    the International Astronomical Union nor the names of its
**    contributors may be used to endorse or promote products derived
**    from this software without specific prior written permission.
**  
**  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
**  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
**  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
**  FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
**  COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
**  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
**  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
**  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
**  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
**  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
**  ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
**  POSSIBILITY OF SUCH DAMAGE.
**  
*/