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 2021-05-12
*
* Copyright (C) 2021 IAU SOFA 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) 2021
* Standards Of Fundamental Astronomy Board
* of the International Astronomical Union.
*
* =====================
* SOFA Software License
* =====================
*
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*
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*
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*
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*
*-----------------------------------------------------------------------
END