xys00a.c

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#include "erfa.h"

void eraXys00a(double date1, double date2,
               double *x, double *y, double *s)
/*
**  - - - - - - - - - -
**   e r a X y s 0 0 a
**  - - - - - - - - - -
**
**  For a given TT date, compute the X,Y coordinates of the Celestial
**  Intermediate Pole and the CIO locator s, using the IAU 2000A
**  precession-nutation model.
**
**  Given:
**     date1,date2  double   TT as a 2-part Julian Date (Note 1)
**
**  Returned:
**     x,y          double   Celestial Intermediate Pole (Note 2)
**     s            double   the CIO locator s (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
**
**         2450123.7           0.0       (JD method)
**         2451545.0       -1421.3       (J2000 method)
**         2400000.5       50123.2       (MJD method)
**         2450123.5           0.2       (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 Celestial Intermediate Pole coordinates are the x,y
**     components of the unit vector in the Geocentric Celestial
**     Reference System.
**
**  3) The CIO locator s (in radians) positions the Celestial
**     Intermediate Origin on the equator of the CIP.
**
**  4) A faster, but slightly less accurate result (about 1 mas for
**     X,Y), can be obtained by using instead the eraXys00b function.
**
**  Called:
**     eraPnm00a    classical NPB matrix, IAU 2000A
**     eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
**     eraS00       the CIO locator s, given X,Y, IAU 2000A
**
**  Reference:
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**  Copyright (C) 2013-2015, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double rbpn[3][3];

/* Form the bias-precession-nutation matrix, IAU 2000A. */
   eraPnm00a(date1, date2, rbpn);

/* Extract X,Y. */
   eraBpn2xy(rbpn, x, y);

/* Obtain s. */
   *s = eraS00(date1, date2, *x, *y);

   return;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2015, 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
**  
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**  modification, are permitted provided that the following conditions
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**  
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**  
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