gst06.c

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#include "erfa.h"

double eraGst06(double uta, double utb, double tta, double ttb,
                double rnpb[3][3])
/*
**  - - - - - - - - -
**   e r a G s t 0 6
**  - - - - - - - - -
**
**  Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.
**
**  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:
**     eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
**     eraS06       the CIO locator s, given X,Y, IAU 2006
**     eraAnp       normalize angle into range 0 to 2pi
**     eraEra00     Earth rotation angle, IAU 2000
**     eraEors      equation of the origins, given NPB matrix and s
**
**  Reference:
**
**     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
**
**  Copyright (C) 2013-2015, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double x, y, s, era, eors, gst;

/* Extract CIP coordinates. */
   eraBpn2xy(rnpb, &x, &y);

/* The CIO locator, s. */
   s = eraS06(tta, ttb, x, y);

/* Greenwich apparent sidereal time. */
   era = eraEra00(uta, utb);
   eors = eraEors(rnpb, s);
   gst = eraAnp(era - eors);

   return gst;

}
/*----------------------------------------------------------------------
**  
**  
**  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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