erfa.atoc13(type, ob1, ob2, utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)[source]

Observed place at a groundbased site to to ICRS astrometric RA,Dec. The caller supplies UTC, site coordinates, ambient air conditions and observing wavelength.

typeconst char array
ob1double array
ob2double array
utc1double array
utc2double array
dut1double array
elongdouble array
phidouble array
hmdouble array
xpdouble array
ypdouble array
phpadouble array
tcdouble array
rhdouble array
wldouble array
rcdouble array
dcdouble array


Wraps ERFA function eraAtoc13. The ERFA documentation is:

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 e r a A t o c 1 3
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Observed place at a groundbased site to to ICRS astrometric RA,Dec.
The caller supplies UTC, site coordinates, ambient air conditions
and observing wavelength.

   type   char[]   type of coordinates - "R", "H" or "A" (Notes 1,2)
   ob1    double   observed Az, HA or RA (radians; Az is N=0,E=90)
   ob2    double   observed ZD or Dec (radians)
   utc1   double   UTC as a 2-part...
   utc2   double   ...quasi Julian Date (Notes 3,4)
   dut1   double   UT1-UTC (seconds, Note 5)
   elong  double   longitude (radians, east +ve, Note 6)
   phi    double   geodetic latitude (radians, Note 6)
   hm     double   height above ellipsoid (m, geodetic Notes 6,8)
   xp,yp  double   polar motion coordinates (radians, Note 7)
   phpa   double   pressure at the observer (hPa = mB, Note 8)
   tc     double   ambient temperature at the observer (deg C)
   rh     double   relative humidity at the observer (range 0-1)
   wl     double   wavelength (micrometers, Note 9)

   rc,dc  double   ICRS astrometric RA,Dec (radians)

Returned (function value):
          int      status: +1 = dubious year (Note 4)
                            0 = OK
                           -1 = unacceptable date


1)  "Observed" Az,ZD means the position that would be seen by a
    perfect geodetically aligned theodolite.  (Zenith distance is
    used rather than altitude in order to reflect the fact that no
    allowance is made for depression of the horizon.)  This is
    related to the observed HA,Dec via the standard rotation, using
    the geodetic latitude (corrected for polar motion), while the
    observed HA and (CIO-based) RA are related simply through the
    Earth rotation angle and the site longitude.  "Observed" RA,Dec
    or HA,Dec thus means the position that would be seen by a
    perfect equatorial with its polar axis aligned to the Earth's
    axis of rotation.

2)  Only the first character of the type argument is significant.
    "R" or "r" indicates that ob1 and ob2 are the observed right
    ascension (CIO-based) and declination;  "H" or "h" indicates
    that they are hour angle (west +ve) and declination;  anything
    else ("A" or "a" is recommended) indicates that ob1 and ob2 are
    azimuth (north zero, east 90 deg) and zenith distance.

3)  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

4)  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.

5)  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.

6)  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.

7)  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.

8)  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.

9)  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).

10) The accuracy of the result is limited by the corrections for
    refraction, which use a simple A*tan(z) + B*tan^3(z) model.
    Providing the meteorological parameters are known accurately and
    there are no gross local effects, the predicted astrometric
    coordinates should be within 0.05 arcsec (optical) or 1 arcsec
    (radio) for a zenith distance of less than 70 degrees, better
    than 30 arcsec (optical or radio) at 85 degrees and better
    than 20 arcmin (optical) or 30 arcmin (radio) at the horizon.

    Without refraction, the complementary functions eraAtco13 and
    eraAtoc13 are self-consistent to better than 1 microarcsecond
    all over the celestial sphere.  With refraction included,
    consistency falls off at high zenith distances, but is still
    better than 0.05 arcsec at 85 degrees.

11) 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.

   eraApco13    astrometry parameters, ICRS-observed
   eraAtoiq     quick observed to CIRS
   eraAticq     quick CIRS to ICRS

This revision:   2022 August 30

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