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# apio

erfa.apio(sp, theta, elong, phi, hm, xp, yp, refa, refb)

  For a terrestrial observer, prepare star-independent astrometry parameters for transformations between CIRS and observed coordinates.

  Parameters
    sp double array
    theta double array
    elong double array
    phi double array
    hm double array
    xp double array
    yp double array
    refa double array
    refb double array
  Returns
    astrom eraASTROM array

  Notes

  Wraps ERFA function `eraApio`. The ERFA documentation is:

      - - - - - - - -
       e r a A p i o
      - - - - - - - -

      For a terrestrial observer, prepare star-independent astrometry
      parameters for transformations between CIRS and observed
      coordinates.  The caller supplies the Earth orientation information
      and the refraction constants as well as the site coordinates.

      Given:
         sp     double      the TIO locator s' (radians, Note 1)
         theta  double      Earth rotation angle (radians)
         elong  double      longitude (radians, east +ve, Note 2)
         phi    double      geodetic latitude (radians, Note 2)
         hm     double      height above ellipsoid (m, geodetic Note 2)
         xp,yp  double      polar motion coordinates (radians, Note 3)
         refa   double      refraction constant A (radians, Note 4)
         refb   double      refraction constant B (radians, Note 4)

      Returned:
         astrom eraASTROM        star-independent astrometry parameters:
          pmt    double       unchanged
          eb     double[3]    unchanged
          eh     double[3]    unchanged
          em     double       unchanged
          v      double[3]    unchanged
          bm1    double       unchanged
          bpn    double[3][3] unchanged
          along  double       adjusted longitude (radians)
          xpl    double       polar motion xp wrt local meridian (radians)
          ypl    double       polar motion yp wrt local meridian (radians)
          sphi   double       sine of geodetic latitude
          cphi   double       cosine of geodetic latitude
          diurab double       magnitude of diurnal aberration vector
          eral   double       "local" Earth rotation angle (radians)
          refa   double       refraction constant A (radians)
          refb   double       refraction constant B (radians)

      Notes:

      1) sp, the TIO locator s', is a tiny quantity needed only by the
         most precise applications.  It can either be set to zero or
         predicted using the ERFA function eraSp00.

      2) The geographical coordinates are with respect to the ERFA_WGS84
         reference ellipsoid.  TAKE CARE WITH THE LONGITUDE SIGN:  the
         longitude required by the present function is east-positive
         (i.e. right-handed), in accordance with geographical convention.

      3) The polar motion xp,yp can be obtained from IERS bulletins.  The
         values are the coordinates (in radians) of the Celestial
         Intermediate Pole with respect to the International Terrestrial
         Reference System (see IERS Conventions 2003), measured along the
         meridians 0 and 90 deg west respectively.  For many applications,
         xp and yp can be set to zero.

         Internally, the polar motion is stored in a form rotated onto the
         local meridian.

      4) The refraction constants refa and refb are for use in a
         dZ = A*tan(Z)+B*tan^3(Z) model, where Z is the observed
         (i.e. refracted) zenith distance and dZ is the amount of
         refraction.

      5) It is advisable to take great care with units, as even unlikely
         values of the input parameters are accepted and processed in
         accordance with the models used.

      6) In cases where the caller does not wish to provide the Earth
         rotation information and refraction constants, the function
         eraApio13 can be used instead of the present function.  This
         starts from UTC and weather readings etc. and computes suitable
         values using other ERFA functions.

      7) This is one of several functions that inserts into the astrom
         structure star-independent parameters needed for the chain of
         astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.

         The various functions support different classes of observer and
         portions of the transformation chain:

              functions         observer        transformation

           eraApcg eraApcg13    geocentric      ICRS <-> GCRS
           eraApci eraApci13    terrestrial     ICRS <-> CIRS
           eraApco eraApco13    terrestrial     ICRS <-> observed
           eraApcs eraApcs13    space           ICRS <-> GCRS
           eraAper eraAper13    terrestrial     update Earth rotation
           eraApio eraApio13    terrestrial     CIRS <-> observed

         Those with names ending in "13" use contemporary ERFA 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.

      8) The context structure astrom produced by this function is used by
         eraAtioq and eraAtoiq.

      Called:
         eraIr        initialize r-matrix to identity
         eraRz        rotate around Z-axis
         eraRy        rotate around Y-axis
         eraRx        rotate around X-axis
         eraAnpm      normalize angle into range +/- pi
         eraPvtob     position/velocity of terrestrial station

      This revision:   2021 February 24

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.