#include "sofa.h"

double iauGst06(double uta, double utb, double tta, double ttb,
                double rnpb[3][3])
/*
**  - - - - - - - - -
**   i a u G s t 0 6
**  - - - - - - - - -
**
**  Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards Of Fundamental Astronomy) software collection.
**
**  Status:  support function.
**
**  Given:
**     uta,utb  double        UT1 as a 2-part Julian Date (Notes 1,2)
**     tta,ttb  double        TT as a 2-part Julian Date (Notes 1,2)
**     rnpb     double[3][3]  nutation x precession x bias matrix
**
**  Returned (function value):
**              double        Greenwich apparent sidereal time (radians)
**
**  Notes:
**
**  1) The UT1 and TT dates uta+utb and tta+ttb respectively, are both
**     Julian Dates, apportioned in any convenient way between the
**     argument pairs.  For example, JD=2450123.7 could be expressed in
**     any of these ways, among others:
**
**            Part A        Part B
**
**         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 (in the case of UT;  the TT is not at all critical
**     in this respect).  The J2000 and MJD methods are good compromises
**     between resolution and convenience.  For UT, the date & time
**     method is best matched to the algorithm that is used by the Earth
**     rotation angle function, called internally:  maximum precision is
**     delivered when the uta argument is for 0hrs UT1 on the day in
**     question and the utb argument lies in the range 0 to 1, or vice
**     versa.
**
**  2) Both UT1 and TT are required, UT1 to predict the Earth rotation
**     and TT to predict the effects of precession-nutation.  If UT1 is
**     used for both purposes, errors of order 100 microarcseconds
**     result.
**
**  3) Although the function uses the IAU 2006 series for s+XY/2, it is
**     otherwise independent of the precession-nutation model and can in
**     practice be used with any equinox-based NPB matrix.
**
**  4) The result is returned in the range 0 to 2pi.
**
**  Called:
**     iauBpn2xy    extract CIP X,Y coordinates from NPB matrix
**     iauS06       the CIO locator s, given X,Y, IAU 2006
**     iauAnp       normalize angle into range 0 to 2pi
**     iauEra00     Earth rotation angle, IAU 2000
**     iauEors      equation of the origins, given NPB matrix and s
**
**  Reference:
**
**     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
**
**  This revision:  2013 June 18
**
**  SOFA release 2020-07-21
**
**  Copyright (C) 2020 IAU SOFA Board.  See notes at end.
*/
{
   double x, y, s, era, eors, gst;


/* Extract CIP coordinates. */
   iauBpn2xy(rnpb, &x, &y);

/* The CIO locator, s. */
   s = iauS06(tta, ttb, x, y);

/* Greenwich apparent sidereal time. */
   era = iauEra00(uta, utb);
   eors = iauEors(rnpb, s);
   gst = iauAnp(era - eors);

   return gst;

/*----------------------------------------------------------------------
**
**  Copyright (C) 2020
**  Standards Of Fundamental Astronomy Board
**  of the International Astronomical Union.
**
**  =====================
**  SOFA Software License
**  =====================
**
**  NOTICE TO USER:
**
**  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
**  CONDITIONS WHICH APPLY TO ITS USE.
**
**  1. The Software is owned by the IAU SOFA Board ("SOFA").
**
**  2. Permission is granted to anyone to use the SOFA software for any
**     purpose, including commercial applications, free of charge and
**     without payment of royalties, subject to the conditions and
**     restrictions listed below.
**
**  3. You (the user) may copy and distribute SOFA source code to others,
**     and use and adapt its code and algorithms in your own software,
**     on a world-wide, royalty-free basis.  That portion of your
**     distribution that does not consist of intact and unchanged copies
**     of SOFA source code files is a "derived work" that must comply
**     with the following requirements:
**
**     a) Your work shall be marked or carry a statement that it
**        (i) uses routines and computations derived by you from
**        software provided by SOFA under license to you; and
**        (ii) does not itself constitute software provided by and/or
**        endorsed by SOFA.
**
**     b) The source code of your derived work must contain descriptions
**        of how the derived work is based upon, contains and/or differs
**        from the original SOFA software.
**
**     c) The names of all routines in your derived work shall not
**        include the prefix "iau" or "sofa" or trivial modifications
**        thereof such as changes of case.
**
**     d) The origin of the SOFA components of your derived work must
**        not be misrepresented;  you must not claim that you wrote the
**        original software, nor file a patent application for SOFA
**        software or algorithms embedded in the SOFA software.
**
**     e) These requirements must be reproduced intact in any source
**        distribution and shall apply to anyone to whom you have
**        granted a further right to modify the source code of your
**        derived work.
**
**     Note that, as originally distributed, the SOFA software is
**     intended to be a definitive implementation of the IAU standards,
**     and consequently third-party modifications are discouraged.  All
**     variations, no matter how minor, must be explicitly marked as
**     such, as explained above.
**
**  4. You shall not cause the SOFA software to be brought into
**     disrepute, either by misuse, or use for inappropriate tasks, or
**     by inappropriate modification.
**
**  5. The SOFA software is provided "as is" and SOFA makes no warranty
**     as to its use or performance.   SOFA does not and cannot warrant
**     the performance or results which the user may obtain by using the
**     SOFA software.  SOFA makes no warranties, express or implied, as
**     to non-infringement of third party rights, merchantability, or
**     fitness for any particular purpose.  In no event will SOFA be
**     liable to the user for any consequential, incidental, or special
**     damages, including any lost profits or lost savings, even if a
**     SOFA representative has been advised of such damages, or for any
**     claim by any third party.
**
**  6. The provision of any version of the SOFA software under the terms
**     and conditions specified herein does not imply that future
**     versions will also be made available under the same terms and
**     conditions.
*
**  In any published work or commercial product which uses the SOFA
**  software directly, acknowledgement (see www.iausofa.org) is
**  appreciated.
**
**  Correspondence concerning SOFA software should be addressed as
**  follows:
**
**      By email:  sofa@ukho.gov.uk
**      By post:   IAU SOFA Center
**                 HM Nautical Almanac Office
**                 UK Hydrographic Office
**                 Admiralty Way, Taunton
**                 Somerset, TA1 2DN
**                 United Kingdom
**
**--------------------------------------------------------------------*/
}
