Procedure

     DELTET ( Delta ET, ET - UTC )

     SUBROUTINE DELTET ( EPOCH, EPTYPE, DELTA )

Abstract

     Return the value of Delta ET (ET-UTC) for an input epoch.

Required_Reading

     TIME
     KERNEL

Keywords

     TIME

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION EPOCH
     CHARACTER*(*)    EPTYPE
     DOUBLE PRECISION DELTA

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     EPOCH      I   Input epoch (seconds past J2000).
     EPTYPE     I   Type of input epoch ('UTC' or 'ET').
     DELTA      O   Delta ET (ET-UTC) at input epoch.

Detailed_Input

     EPOCH    is the epoch at which "delta ET" is to be computed.
              EPOCH may be either UTC or ephemeris seconds past
              J2000, as specified by EPTYPE.

     EPTYPE   is the type of input epoch. It may be either
              of the following:

                 'UTC'    UTC seconds past J2000 UTC.

                 'ET'     Ephemeris seconds past J2000 TDB,
                          also known as barycentric dynamical
                          time (TDB).

Detailed_Output

     DELTA    is the value of

                 "delta ET" = ET - UTC

              at the input epoch. This is added to UTC to give
              ET, or subtracted from ET to give UTC. The routine
              is reversible: that is, given the following calls,

                 CALL DELTET ( UTC,      'UTC', DEL1 )
                 CALL DELTET ( UTC+DEL1, 'ET',  DEL2 )

              the expression

                 ( DEL1 .EQ. DEL2 )

              is always .TRUE.

Parameters

     None.

Exceptions

     1)  If the input epoch is not recognized, the error
         SPICE(INVALIDEPOCH) is signaled.

     2)  If the variables necessary for the computation of DELTA
         have not been loaded into the kernel pool, the error
         SPICE(KERNELVARNOTFOUND) is signaled.

     3)  If the number of leapseconds in the pool is greater than
         the local leapseconds buffer size, the error
         SPICE(BUFFEROVERFLOW) is signaled.

Files

     None.

Particulars

     The constants necessary for computing the offset are taken
     from the kernel pool, where they are assumed to have been
     loaded from a kernel file.

     The tables are consulted to determine the number of leap seconds
     preceding the input epoch. Also, an approximation to the periodic
     yearly variation (which has an amplitude of just under two
     milliseconds) in the difference between ET and TAI (Atomic Time)
     is computed. The final value of Delta ET is given by

        Delta ET = ( ET - TAI ) + leap seconds

Examples

     The following example shows how DELTET may be used to convert
     from UTC seconds past J2000 to ephemeris seconds past J2000.

        CALL DELTET ( UTCSEC, 'UTC', DELTA )
        ET = UTCSEC + DELTA

     The following example shows how DELTET may be used to convert
     from ephemeris seconds past J2000 to UTC seconds past J2000.

        CALL DELTET ( ET, 'ET', DELTA )
        UTCSEC = ET - DELTA

     See the Time required reading time.req for further examples.

Restrictions

     1)  The routines STR2ET and ET2UTC are preferred for conversions
         between UTC and ET. This routine is provided mainly as a
         utility for STR2ET and ET2UTC.

     2)  A leapseconds kernel containing leapseconds and relativistic
         terms MUST be loaded prior to calling this subroutine.
         Examples demonstrating how to load a kernel pool are included
         in the Required Reading file time.req and in the $Examples
         section of this header. For more general information about
         kernel pools, please consult the Required Reading file
         kernel.req.

Literature_References

     [1]  "The Astronomical Almanac for the Year 1990," United States
          Naval Observatory, U.S. Government Printing Office,
          Washington, D.C., 1989.

Author_and_Institution

     N.J. Bachman       (JPL)
     J. Diaz del Rio    (ODC Space)
     W.M. Owen          (JPL)
     B.V. Semenov       (JPL)
     W.L. Taber         (JPL)
     I.M. Underwood     (JPL)

Version

    SPICELIB Version 1.3.0, 24-AUG-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard. Removed
        unnecessary entries in $Revisions section.

        Replaced reference to UTC2ET with STR2ET in $Restrictions
        section.

    SPICELIB Version 1.2.2, 18-APR-2014 (BVS)

        Minor header edits.

    SPICELIB Version 1.2.1, 18-MAY-2010 (BVS)

        Removed "C$" marker from text in the header.

    SPICELIB Version 1.2.0, 24-AUG-1998 (WLT)

        The previous upgrade introduced an error in the fetch
        of the variable DELTET/M from the kernel pool. This
        error was corrected.

    SPICELIB Version 1.1.0, 20-APR-1998 (NJB)

        Calls to RTPOOL were replaced with calls to GDPOOL, which
        does more robust error checking. Check for buffer overflow
        was added. Local declarations were re-organized.

    SPICELIB Version 1.0.1, 10-MAR-1992 (WLT)

        Comment section for permuted index source lines was added
        following the header.

    SPICELIB Version 1.0.0, 31-JAN-1990 (WMO) (IMU)