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erfasrc/src/apco13.c  view on Meta::CPAN

**  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.
**
**  Given:
**     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)
**
**  Returned:
**     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)
**
**  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.
**
**      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
**      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.
**
**  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