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atco13

sofars::astro

Function atco13 

Source 
pub fn atco13(
    rc: f64,
    dc: f64,
    pr: f64,
    pd: f64,
    px: f64,
    rv: f64,
    utc1: f64,
    utc2: f64,
    dut1: f64,
    elong: f64,
    phi: f64,
    hm: f64,
    xp: f64,
    yp: f64,
    phpa: f64,
    tc: f64,
    rh: f64,
    wl: f64,
) -> Result<(f64, f64, f64, f64, f64, f64), i32>
Expand description

ICRS −> observed

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

SOFA models are used for the Earth ephemeris, bias-precession- nutation, Earth orientation and refraction.

This function is part of the International Astronomical Union’s SOFA (Standards of Fundamental Astronomy) software collection.

Status: support function.

Given:

   rc,dc  double   ICRS right ascension at J2000.0 (radians, Note 1)
   pr     double   RA proper motion (radians/year, Note 2)
   pd     double   Dec proper motion (radians/year)
   px     double   parallax (arcsec)
   rv     double   radial velocity (km/s, +ve if receding)
   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   latitude (geodetic, 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

   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)
   eo     double*  equation of the origins (ERA-GST, radians)

Returned (function value):

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

Notes:

  1. Star data for an epoch other than J2000.0 (for example from the Hipparcos catalog, which has an epoch of J1991.25) will require a preliminary call to iauPmsafe before use.

  2. The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.

  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 iauDtf2d 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 iauDat 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 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 Atan(z) + Btan^3(z) model. Providing the meteorological parameters are known accurately and there are no gross local effects, the predicted observed 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 iauAtco13 and iauAtoc13 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. “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.

  12. 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:

   iauApco13    astrometry parameters, ICRS-observed, 2013
   iauAtciq     quick ICRS to CIRS
   iauAtioq     quick CIRS to observed