ATIO13 Subroutine

public subroutine ATIO13(ri, di, utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl, aob, zob, hob, dob, rob, j)

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

Status: support routine.

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 routine is that the JD day represents UTC days whether the length is 86399, 86400 or 86401 SI seconds.

   Applications should use the routine DTF2D 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 DAT 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 WGS84 reference ellipsoid. TAKE CARE WITH THE LONGITUDE SIGN: the longitude required by the present routine 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.

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. "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.

9. 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.

10. The complementary routines ATIO13 and ATOI13 are self- consistent to better than 1 microarcsecond all over the celestial sphere.

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.

History

• IAU SOFA revision: 2016 February 2

Arguments

    subroutine ATIO13 ( ri, di, utc1, utc2, dut1, &
                        elong, phi, hm, xp, yp, phpa, tc, rh, wl, &
                        aob, zob, hob, dob, rob, j )

    implicit none

    real(wp),intent(in) :: ri !! CIRS right ascension (CIO-based, radians)
    real(wp),intent(in) :: di !! CIRS declination (radians)
    real(wp),intent(in) :: utc1 !! UTC as a 2-part...
    real(wp),intent(in) :: utc2 !! ...quasi Julian Date (Notes 1,2)
    real(wp),intent(in) :: dut1 !! UT1-UTC (seconds, Note 3)
    real(wp),intent(in) :: elong !! longitude (radians, east +ve, Note 4)
    real(wp),intent(in) :: phi !! geodetic latitude (radians, Note 4)
    real(wp),intent(in) :: hm !! height above ellipsoid (m, geodetic Notes 4,6)
    real(wp),intent(in) :: xp !! polar motion coordinates (radians, Note 5)
    real(wp),intent(in) :: yp !! polar motion coordinates (radians, Note 5)
    real(wp),intent(in) :: phpa !! pressure at the observer (hPa = mB, Note 6)
    real(wp),intent(in) :: tc !! ambient temperature at the observer (deg C)
    real(wp),intent(in) :: rh !! relative humidity at the observer (range 0-1)
    real(wp),intent(in) :: wl !! wavelength (micrometers, Note 7)
    real(wp),intent(out) :: aob !! observed azimuth (radians: N=0,E=90)
    real(wp),intent(out) :: zob !! observed zenith distance (radians)
    real(wp),intent(out) :: hob !! observed hour angle (radians)
    real(wp),intent(out) :: dob !! observed declination (radians)
    real(wp),intent(out) :: rob !! observed right ascension (CIO-based, radians)
    integer,intent(out) :: j !! status: +1 = dubious year (Note 2)
                             !!  0 = OK
                             !! -1 = unacceptable date

    integer :: js
    real(wp) :: astrom(30)

    !  Star-independent astrometry parameters for CIRS->observed.
    call APIO13 ( utc1, utc2, dut1, elong, phi, hm, xp, yp, &
                  phpa, tc, rh, wl, astrom, js )

    !  Abort if bad UTC.
    if ( js>=0 ) then

      !  Transform CIRS to observed.
      call ATIOQ ( ri, di, astrom, aob, zob, hob, dob, rob )

    end if

    !  Return OK/warning status.
    j = js

    end subroutine ATIO13