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erfasrc/src/atoc13.c  view on Meta::CPAN

**  - - - - - - - - - -
**
**  Observed place at a groundbased site to to ICRS astrometric RA,Dec.
**  The caller supplies UTC, site coordinates, ambient air conditions
**  and observing wavelength.
**
**  Given:
**     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)
**
**  Returned:
**     rc,dc  double   ICRS astrometric RA,Dec (radians)
**
**  Returned (function value):
**            int      status: +1 = dubious year (Note 4)
**                              0 = OK
**                             -1 = unacceptable date
**
**  Notes:
**
**  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 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 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
**      described.
**
**  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
**      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.
**
**  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,