Procedure

     IRFNUM ( Inertial reference frame number )

     ENTRY IRFNUM ( NAME, INDEX )

Abstract

     Return the index of one of the standard inertial reference
     frames supported by IRFROT.

Required_Reading

     FRAMES

Keywords

     CONVERSION
     COORDINATES
     EPHEMERIS
     FRAMES
     MATRIX
     ROTATION
     TRANSFORMATION
     VECTOR

Declarations

    CHARACTER*(*)         NAME
    INTEGER               INDEX

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     NAME       I   Name of standard inertial reference frame.
     INDEX      O   Index of frame.

Detailed_Input

     NAME     is the name of one of the standard inertial
              reference frames supported by IRFROT, or
              'DEFAULT'.

Detailed_Output

     INDEX    is the index of the frame specified by NAME.

Parameters

     None.

Exceptions

     1)  If NAME is not recognized, INDEX is zero.

     2)  If no default frame has been specified, INDEX is zero.

Files

     None.

Particulars

     IRFNUM is supplied as a convenience, to allow users to refer to
     the various standard inertial reference frames by name.

Examples

     In the following example, the rotation from DE-118 to FK4 is
     computed without knowing the indices of these frames.

        CALL IRFNUM ( 'DE-118', A )
        CALL IRFNUM ( 'FK4',    B )

        CALL IRFROT ( A, B, ROTAB )

     IRFNUM can be used to rotate vectors into the default frame,
     as illustrated by the following code fragment.

        CALL IRFNUM ( 'FK4',     A )
        CALL IRFNUM ( 'DEFAULT', B )

        CALL IRFROT ( A, B, ROTAB                 )
        CALL MXV    (       ROTAB, OLDVEC, NEWVEC )

Restrictions

     None.

Literature_References

     [1]  J. Lieske, "Precession Matrix Based on IAU (1976) System of
          Astronomical Constants," Astron. Astrophys. 73, 282-284,
          1979.

     [2]  E. M. Standish, Jr., "Orientation of the JPL Ephemerides,
          DE 200/LE 200, to the Dynamical Equinox of J2000," Astron.
          Astrophys. 114, 297-302, 1982.

     [3]  E. M. Standish, Jr., "Conversion of Ephemeris Coordinates
          from the B1950 System to the J2000 System," JPL IOM
          314.6-581, 24 June 1985.

     [4]  E. M. Standish, Jr., "The Equinox Offsets of the JPL
          Ephemeris," JPL IOM 314.6-929, 26 February 1988.

     [5]  J. Lieske, "Expressions for the Precession Quantities Based
          upon the IAU (1976) System of Astronomical Constants,"
          Astron. Astrophys. 58, 1-16, 1977.

     [6]  L. Bass and R. Cesarone, "Mars Observer Planetary Constants
          and Models," JPL D-3444, November 1990.

     [7]  P. Kenneth Seidelmann (Ed.), "Explanatory Supplement to the
          Astronomical Almanac," University Science Books, 1992.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     B.V. Semenov       (JPL)
     W.L. Taber         (JPL)
     I.M. Underwood     (JPL)
     E.D. Wright        (JPL)

Version

    SPICELIB Version 4.4.0, 17-JUN-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard. Added
        $Literature_References.

    SPICELIB Version 4.3.0, 24-SEP-2013 (BVS)

        Updated to treat J2000 as a special case and to not
        CHKIN/CHOUT to increase efficiency.

    SPICELIB Version 4.2.1, 04-JAN-2002 (EDW)

        Added DE-143 to header description for IRFROT.

    SPICELIB Version 4.2.0, 10-APR-1997 (WLT)

        A descriptive diagnostic was added to the entry points
        IRFROT and IRFDEF. Before they simply signaled the error
        with no diagnostic.

    SPICELIB Version 4.1.0, 14-OCT-1996 (WLT)

        The number of inertial frames recognized is now stored
        in the include file ninert.inc.

    SPICELIB Version 4.0.0, 20-MAY-1996 (WLT)

        The inertial frame DE-143 was added to the list of recognized
        inertial frames.

    SPICELIB Version 3.0.0, 20-MAR-1995 (WLT)

        The inertial frames DE-140 and DE-142  were added to the
        list of recognized inertial frames.

    SPICELIB Version 2.0.0, 30-JUL-1993 (WLT)

        The transformation from J2000 to B1950 was upgraded
        so that the transformation matrix produced matches
        the matrix given in [1].

        The frame MARSIAU was added to the list
        of recognized frames. This is the standard mars
        referenced inertial frame used by the Mars Observer
        project.

        Values for the obliquity of the ecliptic were taken
        from the Explanatory Supplement [7] to the Astronomical
        Almanac (1992) at both the epochs J2000 and B1950 and
        used to define the mean ecliptic and equinox frames
        ECLIPJ2000 and ECLIPB1950.

    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 (WLT) (IMU)