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eraAtio13

[rv, aob, zob, hob, dob, rob] = ERFA.atio13(ri, di, utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)

CIRS RA,Dec to observed place. The caller supplies UTC, site coordinates, ambient air conditions and observing wavelength.

Given:

   ri     double   CIRS right ascension (CIO-based, radians)
   di     double   CIRS declination (radians)
   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   geodetic latitude (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)

Returned:

   aob    double*  observed azimuth (radians: N=0,E=90)
   zob    double*  observed zenith distance (radians)
   hob    double*  observed hour angle (radians)
   dob    double*  observed declination (radians)
   rob    double*  observed right ascension (CIO-based, radians)

Returned (function value):

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

Notes:

  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 described.

  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.

  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 expression

      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 follows:

          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.

  1. 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).

  2. "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 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.

  3. The accuracy of the result is limited by the corrections for refraction, which use a simple Atan(z) + Btan^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.

  4. The complementary functions eraAtio13 and eraAtoi13 are self- consistent to better than 1 microarcsecond all over the celestial sphere.

  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.

Called:

   eraApio13    astrometry parameters, CIRS-observed, 2013
   eraAtioq     quick CIRS to observed

This revision: 2021 February 24

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.