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

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

date1double array
date2double array
ebpvdouble array
ehpdouble array
xdouble array
ydouble array
sdouble array
thetadouble array
elongdouble array
phidouble array
hmdouble array
xpdouble array
ypdouble array
spdouble array
refadouble array
refbdouble array
astromeraASTROM array


Wraps ERFA function eraApco. The ERFA documentation is:

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

   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)

   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)


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

          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

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

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

   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

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*.

   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.