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# apco

erfa.apco(date1, date2, ebpv, ehp, x, y, s, theta, elong, phi, hm, xp, yp, sp, refa, refb)

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

  Parameters:
    date1 double array
    date2 double array
    ebpv double array
    ehp double array
    x double array
    y double array
    s double array
    theta double array
    elong double array
    phi double array
    hm double array
    xp double array
    yp double array
    sp double array
    refa double array
    refb double array
  Returns:
    astrom eraASTROM array

  Notes

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

      - - - - - - - -
       e r a A p c o
      - - - - - - - -

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

      Given:
         date1  double       TDB as a 2-part...
         date2  double       ...Julian Date (Note 1)
         ebpv   double[2][3] Earth barycentric PV (au, au/day, Note 2)
         ehp    double[3]    Earth heliocentric P (au, Note 2)
         x,y    double       CIP X,Y (components of unit vector)
         s      double       the CIO locator s (radians)
         theta  double       Earth rotation angle (radians)
         elong  double       longitude (radians, east +ve, Note 3)
         phi    double       latitude (geodetic, radians, Note 3)
         hm     double       height above ellipsoid (m, geodetic, Note 3)
         xp,yp  double       polar motion coordinates (radians, Note 4)
         sp     double       the TIO locator s' (radians, Note 4)
         refa   double       refraction constant A (radians, Note 5)
         refb   double       refraction constant B (radians, Note 5)

      Returned:
         astrom eraASTROM         star-independent astrometry parameters:
          pmt    double       PM time interval (SSB, Julian years)
          eb     double[3]    SSB to observer (vector, au)
          eh     double[3]    Sun to observer (unit vector)
          em     double       distance from Sun to observer (au)
          v      double[3]    barycentric observer velocity (vector, c)
          bm1    double       sqrt(1-|v|^2): reciprocal of Lorenz factor
          bpn    double[3][3] bias-precession-nutation matrix
          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) 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

             2450123.7           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.  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 function the choice will not be at all
         critical.

         TT can be used instead of TDB without any significant impact on
         accuracy.

      2) The vectors eb, eh, and all the astrom vectors, are with respect
         to BCRS axes.

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

         The adjusted longitude stored in the astrom array takes into
         account the TIO locator and polar motion.

      4) xp and yp are the coordinates (in radians) of the Celestial
         Intermediate Pole with respect to the International Terrestrial
         Reference System (see IERS Conventions), measured along the
         meridians 0 and 90 deg west respectively.  sp is the TIO locator
         s', in radians, which positions the Terrestrial Intermediate
         Origin on the equator.  For many applications, xp, yp and
         (especially) sp can be set to zero.

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

      5) 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.

      6) 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.

      7) In cases where the caller does not wish to provide the Earth
         Ephemeris, the Earth rotation information and refraction
         constants, the function eraApco13 can be used instead of the
         present function.  This starts from UTC and weather readings etc.
         and computes suitable values using other ERFA functions.

      8) 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.

      9) The context structure astrom produced by this function is used by
         eraAtioq, eraAtoiq, eraAtciq* and eraAticq*.

      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
         eraC2ixys    celestial-to-intermediate matrix, given X,Y and s
         eraPvtob     position/velocity of terrestrial station
         eraTrxpv     product of transpose of r-matrix and pv-vector
         eraApcs      astrometry parameters, ICRS-GCRS, space observer
         eraCr        copy r-matrix

      This revision:   2021 February 24

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.