SUBROUTINE iau_APCG ( DATE1, DATE2, EBPV, EHP, ASTROM )
*+
* - - - - - - - - -
* i a u _ A P C G
* - - - - - - - - -
*
* For a geocentric observer, prepare star-independent astrometry
* parameters for transformations between ICRS and GCRS coordinates.
* The Earth ephemeris is supplied by the caller.
*
* The parameters produced by this routine are required in the parallax,
* light deflection and aberration parts of the astrometric
* transformation chain.
*
* This routine is part of the International Astronomical Union's
* SOFA (Standards of Fundamental Astronomy) software collection.
*
* Status: support routine.
*
* Given:
* DATE1 d TDB as a 2-part...
* DATE2 d ...Julian Date (Note 1)
* EBPV d(3,2) Earth barycentric position/velocity (au, au/day)
* EHP d(3) Earth heliocentric position (au)
*
* Returned:
* ASTROM d(30) star-independent astrometry parameters:
* (1) PM time interval (SSB, Julian years)
* (2-4) SSB to observer (vector, au)
* (5-7) Sun to observer (unit vector)
* (8) distance from Sun to observer (au)
* (9-11) v: barycentric observer velocity (vector, c)
* (12) sqrt(1-|v|^2): reciprocal of Lorenz factor
* (13-21) bias-precession-nutation matrix
* (22) unchanged
* (23) unchanged
* (24) unchanged
* (25) unchanged
* (26) unchanged
* (27) unchanged
* (28) unchanged
* (29) unchanged
* (30) unchanged
*
* Notes:
*
* 1) The TDB date DATE1+DATE2 is a Julian Date, apportioned in any
* convenient way between the two arguments. For example,
* JD(TDB)=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. For most
* applications of this routine the choice will not be at all
* critical.
*
* TT can be used instead of TDB without any significant impact on
* accuracy.
*
* 2) All the vectors are with respect to BCRS axes.
*
* 3) This is one of several routines that inserts into the ASTROM
* array star-independent parameters needed for the chain of
* astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.
*
* The various routines support different classes of observer and
* portions of the transformation chain:
*
* routines observer transformation
*
* iau_APCG iau_APCG13 geocentric ICRS <-> GCRS
* iau_APCI iau_APCI13 terrestrial ICRS <-> CIRS
* iau_APCO iau_APCO13 terrestrial ICRS <-> observed
* iau_APCS iau_APCS13 space ICRS <-> GCRS
* iau_APER iau_APER13 terrestrial update Earth rotation
* iau_APIO iau_APIO13 terrestrial CIRS <-> observed
*
* Those with names ending in "13" use contemporary SOFA models to
* compute the various ephemerides. The others accept ephemerides
* supplied by the caller.
*
* The transformation from ICRS to GCRS covers space motion,
* parallax, light deflection, and aberration. From GCRS to CIRS
* comprises frame bias and precession-nutation. From CIRS to
* observed takes account of Earth rotation, polar motion, diurnal
* aberration and parallax (unless subsumed into the ICRS <-> GCRS
* transformation), and atmospheric refraction.
*
* 4) The context array ASTROM produced by this routine is used by
* iau_ATCIQ* and iau_ATICQ*.
*
* Called:
* iau_ZPV zero pv-vector
* iau_APCS astrometry parameters, ICRS-GCRS, space observer
*
* This revision: 2017 March 12
*
* SOFA release 2021-05-12
*
* Copyright (C) 2021 IAU SOFA Board. See notes at end.
*
*-----------------------------------------------------------------------
IMPLICIT NONE
DOUBLE PRECISION DATE1, DATE2, EBPV(3,2), EHP(3), ASTROM(30)
DOUBLE PRECISION PV(3,2)
* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* Geocentric observer.
CALL iau_ZPV ( PV )
* Compute the star-independent astrometry parameters.
CALL iau_APCS ( DATE1, DATE2, PV, EBPV, EHP, ASTROM )
* Finished.
*+----------------------------------------------------------------------
*
* Copyright (C) 2021
* Standards Of Fundamental Astronomy Board
* of the International Astronomical Union.
*
* =====================
* SOFA Software License
* =====================
*
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*-----------------------------------------------------------------------
END