erfa.apco13(utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)[source]

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

utc1double array
utc2double array
dut1double array
elongdouble array
phidouble array
hmdouble array
xpdouble array
ypdouble array
phpadouble array
tcdouble array
rhdouble array
wldouble array
astromeraASTROM array
eodouble array


Wraps ERFA function eraApco13. The ERFA documentation is:

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 e r a A p c o 1 3
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For a terrestrial observer, prepare star-independent astrometry
parameters for transformations between ICRS and observed
coordinates.  The caller supplies UTC, site coordinates, ambient air
conditions and observing wavelength, and ERFA models are used to
obtain the Earth ephemeris, CIP/CIO and refraction constants.

The parameters produced by this function are required in the
parallax, light deflection, aberration, and bias-precession-nutation
parts of the ICRS/CIRS transformations.

   utc1   double     UTC as a 2-part...
   utc2   double     ...quasi Julian Date (Notes 1,2)
   dut1   double     UT1-UTC (seconds, Note 3)
   elong  double     longitude (radians, east +ve, Note 4)
   phi    double     latitude (geodetic, radians, Note 4)
   hm     double     height above ellipsoid (m, geodetic, Notes 4,6)
   xp,yp  double     polar motion coordinates (radians, Note 5)
   phpa   double     pressure at the observer (hPa = mB, Note 6)
   tc     double     ambient temperature at the observer (deg C)
   rh     double     relative humidity at the observer (range 0-1)
   wl     double     wavelength (micrometers, Note 7)

   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       longitude + s' (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)
   eo     double          equation of the origins (ERA-GST, radians)

Returned (function value):
          int        status: +1 = dubious year (Note 2)
                              0 = OK
                             -1 = unacceptable date


1)  utc1+utc2 is quasi Julian Date (see Note 2), apportioned in any
    convenient way between the two arguments, for example where utc1
    is the Julian Day Number and utc2 is the fraction of a day.

    However, JD cannot unambiguously represent UTC during a leap
    second unless special measures are taken.  The convention in the
    present function is that the JD day represents UTC days whether
    the length is 86399, 86400 or 86401 SI seconds.

    Applications should use the function eraDtf2d to convert from
    calendar date and time of day into 2-part quasi Julian Date, as
    it implements the leap-second-ambiguity convention just

2)  The warning status "dubious year" flags UTCs that predate the
    introduction of the time scale or that are too far in the
    future to be trusted.  See eraDat for further details.

3)  UT1-UTC is tabulated in IERS bulletins.  It increases by exactly
    one second at the end of each positive UTC leap second,
    introduced in order to keep UT1-UTC within +/- 0.9s.  n.b. This
    practice is under review, and in the future UT1-UTC may grow
    essentially without limit.

4)  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.

5)  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.

6)  If hm, the height above the ellipsoid of the observing station
    in meters, is not known but phpa, the pressure in hPa (=mB), is
    available, an adequate estimate of hm can be obtained from the

          hm = -29.3 * tsl * log ( phpa / 1013.25 );

    where tsl is the approximate sea-level air temperature in K
    (See Astrophysical Quantities, C.W.Allen, 3rd edition, section
    52).  Similarly, if the pressure phpa is not known, it can be
    estimated from the height of the observing station, hm, as

          phpa = 1013.25 * exp ( -hm / ( 29.3 * tsl ) );

    Note, however, that the refraction is nearly proportional to
    the pressure and that an accurate phpa value is important for
    precise work.

7)  The argument wl specifies the observing wavelength in
    micrometers.  The transition from optical to radio is assumed to
    occur at 100 micrometers (about 3000 GHz).

8)  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.

9)  In cases where the caller wishes to supply his own Earth
    ephemeris, Earth rotation information and refraction constants,
    the function eraApco can be used instead of the present function.

10) 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.

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

   eraUtctai    UTC to TAI
   eraTaitt     TAI to TT
   eraUtcut1    UTC to UT1
   eraEpv00     Earth position and velocity
   eraPnm06a    classical NPB matrix, IAU 2006/2000A
   eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
   eraS06       the CIO locator s, given X,Y, IAU 2006
   eraEra00     Earth rotation angle, IAU 2000
   eraSp00      the TIO locator s', IERS 2000
   eraRefco     refraction constants for given ambient conditions
   eraApco      astrometry parameters, ICRS-observed
   eraEors      equation of the origins, given NPB matrix and s

This revision:   2022 May 3

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