DOUBLE PRECISION FUNCTION iau_GST06 ( UTA, UTB, TTA, TTB, RNPB )
*+
*  - - - - - - - - - -
*   i a u _ G S T 0 6
*  - - - - - - - - - -
*
*  Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.
*
*  This routine is part of the International Astronomical Union's
*  SOFA (Standards of Fundamental Astronomy) software collection.
*
*  Status:  support routine.
*
*  Given:
*     UTA, UTB     d      UT1 as a 2-part Julian Date (Notes 1,2)
*     TTA, TTB     d      TT as a 2-part Julian Date (Notes 1,2)
*     RNPB       d(3,3)   nutation x precession x bias matrix
*
*  Returned:
*     iau_GST06    d      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
*
*         2450123.7D0        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 (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 routine, called internally:  maximum accuracy (or,
*     at least, minimum noise) 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 routine 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:
*     iau_BPN2XY   extract CIP X,Y coordinates from NPB matrix
*     iau_S06      the CIO locator s, given X,Y, IAU 2006
*     iau_ANP      normalize angle into range 0 to 2pi
*     iau_ERA00    Earth rotation angle, IAU 2000
*     iau_EORS     equation of the origins, given NPB matrix and s
*
*  Reference:
*
*     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
*
*  This revision:  2008 January 2
*
*  SOFA release 2018-01-30
*
*  Copyright (C) 2018 IAU SOFA Board.  See notes at end.
*
*-----------------------------------------------------------------------

      IMPLICIT NONE

      DOUBLE PRECISION UTA, UTB, TTA, TTB, RNPB(3,3)

      DOUBLE PRECISION X, Y, S

      DOUBLE PRECISION iau_S06, iau_ANP, iau_ERA00, iau_EORS

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

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

*  The CIO locator, s.
      S = iau_S06 ( TTA, TTB, X, Y )

*  Greenwich apparent sidereal time.
      iau_GST06 = iau_ANP ( iau_ERA00 ( UTA, UTB ) -
     :                      iau_EORS ( RNPB, S ) )

*  Finished.

*+----------------------------------------------------------------------
*
*  Copyright (C) 2018
*  Standards Of Fundamental Astronomy Board
*  of the International Astronomical Union.
*
*  =====================
*  SOFA Software License
*  =====================
*
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*
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*-----------------------------------------------------------------------

      END