DOUBLE PRECISION FUNCTION iau_S00A ( DATE1, DATE2 )
*+
*  - - - - - - - - -
*   i a u _ S 0 0 A
*  - - - - - - - - -
*
*  The CIO locator s, positioning the Celestial Intermediate Origin on
*  the equator of the Celestial Intermediate Pole, using the IAU 2000A
*  precession-nutation model.
*
*  This routine is part of the International Astronomical Union's
*  SOFA (Standards of Fundamental Astronomy) software collection.
*
*  Status:  support routine.
*
*  Given:
*     DATE1,DATE2    d      TT as a 2-part Julian Date (Note 1)
*
*  Returned:
*     iau_S00A       d      the CIO locator s in radians (Note 2)
*
*  Notes:
*
*  1) The TT date DATE1+DATE2 is a Julian Date, apportioned in any
*     convenient way between the two arguments.  For example,
*     JD(TT)=2450123.7 could be expressed in any of these ways,
*     among others:
*
*            DATE1          DATE2
*
*         2450123.7D0        0D0        (JD method)
*          2451545D0      -1421.3D0     (J2000 method)
*         2400000.5D0     50123.2D0     (MJD method)
*         2450123.5D0       0.2D0       (date & time method)
*
*     The JD method is the most natural and convenient to use in
*     cases where the loss of several decimal digits of resolution
*     is acceptable.  The J2000 method is best matched to the way
*     the argument is handled internally and will deliver the
*     optimum resolution.  The MJD method and the date & time methods
*     are both good compromises between resolution and convenience.
*
*  2) The CIO locator s is the difference between the right ascensions
*     of the same point in two systems.  The two systems are the GCRS
*     and the CIP,CIO, and the point is the ascending node of the
*     CIP equator.  The CIO locator s remains a small fraction of
*     1 arcsecond throughout 1900-2100.
*
*  3) The series used to compute s is in fact for s+XY/2, where X and Y
*     are the x and y components of the CIP unit vector;  this series is
*     more compact than a direct series for s would be.  The present
*     routine uses the full IAU 2000A nutation model when predicting the
*     CIP position.  Faster results, with no significant loss of
*     accuracy, can be obtained via the routine iau_S00B, which uses
*     instead the IAU 2000B truncated model.
*
*  Called:
*     iau_PNM00A   classical NPB matrix, IAU 2000A
*     iau_BNP2XY    extract CIP X,Y from the BPN matrix
*     iau_S00      the CIO locator s, given X,Y, IAU 2000A
*
*  References:
*
*     Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
*     "Expressions for the Celestial Intermediate Pole and Celestial
*     Ephemeris Origin consistent with the IAU 2000A precession-nutation
*     model", Astron.Astrophys. 400, 1145-1154 (2003)
*
*     n.b. The celestial ephemeris origin (CEO) was renamed "celestial
*          intermediate origin" (CIO) by IAU 2006 Resolution 2.
*
*     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
*     IERS Technical Note No. 32, BKG (2004)
*
*  This revision:  2010 January 18
*
*  SOFA release 2009-12-31
*
*  Copyright (C) 2009 IAU SOFA Review Board.  See notes at end.
*
*-----------------------------------------------------------------------

      IMPLICIT NONE

      DOUBLE PRECISION DATE1, DATE2

      DOUBLE PRECISION RBPN(3,3), X, Y

      DOUBLE PRECISION iau_S00

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

*  Bias-precession-nutation-matrix, IAU 2000A.
      CALL iau_PNM00A ( DATE1, DATE2, RBPN )

*  Extract the CIP coordinates.
      CALL iau_BPN2XY ( RBPN, X, Y )

*  Compute the CIO locator s, given the CIP coordinates.
      iau_S00A = iau_S00 ( DATE1, DATE2, X, Y )

*  Finished.

*+----------------------------------------------------------------------
*
*  Copyright (C) 2009
*  Standards Of Fundamental Astronomy Review Board
*  of the International Astronomical Union.
*
*  =====================
*  SOFA Software License
*  =====================
*
*  NOTICE TO USER:
*
*  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING TERMS AND CONDITIONS
*  WHICH APPLY TO ITS USE.
*
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*
*  2. Permission is granted to anyone to use the SOFA software for any
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*
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*
*     a) Your work shall be marked or carry a statement that it
*        (i) uses routines and computations derived by you from
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*
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*
*      By email:  sofa@rl.ac.uk
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*                 STFC Rutherford Appleton Laboratory
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*
*----------------------------------------------------------------------

      END