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

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

spdouble array
thetadouble array
elongdouble array
phidouble array
hmdouble array
xpdouble array
ypdouble array
refadouble array
refbdouble array
astromeraASTROM array


Wraps ERFA function eraApio. The ERFA documentation is:

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 e r a A p i o
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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.

   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)

   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)


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

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.

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