slaAoppa -

SYNOPSYS

PARAMETERS

double date

double dut

double elongm

double phim

double hm

double xp

double yp

double tdk

double pmb

double rh

double wl

double tlr

double *aoprms

DESCRIPTION

  Precompute apparent to observed place parameters required by
  slaAopqk and slaOapqk.

Given

     date   d      UTC date/time (Modified Julian Date, JD-2400000.5)
     dut    d      delta UT:  UT1-UTC (UTC seconds)
     elongm d      mean longitude of the observer (radians, east +ve)
     phim   d      mean geodetic latitude of the observer (radians)
     hm     d      observer's height above sea level (metres)
     xp     d      polar motion x-coordinate (radians)
     yp     d      polar motion y-coordinate (radians)
     tdk    d      local ambient temperature (DegK; std=273.155)
     pmb    d      local atmospheric pressure (mB; std=1013.25)
     rh     d      local relative humidity (in the range 0.0-1.0)
     wl     d      effective wavelength (micron, e.g. 0.55)
     tlr    d      tropospheric lapse rate (DegK/metre, e.g. 0.0065)

Returned

       (0)      geodetic latitude (radians)
       (1,2)    sine and cosine of geodetic latitude
       (3)      magnitude of diurnal aberration vector
       (4)      height (hm)
       (5)      ambient temperature (tdk)
       (6)      pressure (pmb)
       (7)      relative humidity (rh)
       (8)      wavelength (wl)
       (9)      lapse rate (tlr)
       (10,11)  refraction constants A and B (radians)
       (12)     longitude + eqn of equinoxes + sidereal DUT (radians)
       (13)     local apparent sidereal time (radians)

Notes

   1)  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.

   2)  The date argument is UTC expressed as an MJD.  This is,
       strictly speaking, wrong, because of leap seconds.  However,
       as long as the delta UT and the UTC are consistent there
       are no difficulties, except during a leap second.  In this
       case, the start of the 61st second of the final minute should
       begin a new MJD day and the old pre-leap delta UT should
       continue to be used.  As the 61st second completes, the MJD
       should revert to the start of the day as, simultaneously,
       the delta UTC changes by one second to its post-leap new value.

   3)  The delta UT (UT1-UTC) is tabulated in IERS circulars and
       elsewhere.  It increases by exactly one second at the end of
       each UTC leap second, introduced in order to keep delta UT
       within +/- 0.9 seconds.

   4)  IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION.
       The longitude required by the present routine is east-positive,
       in accordance with geographical convention (and right-handed).
       In particular, note that the longitudes returned by the
       slaObs routine are west-positive, following astronomical
       usage, and must be reversed in sign before use in the present
       routine.

   5)  The polar coordinates xp,yp can be obtained from IERS
       circulars and equivalent publications.  The maximum amplitude
       is about 0.3 arcseconds.  If xp,yp values are unavailable,
       use xp=yp=0.0.  See page B60 of the 1988 Astronomical Almanac
       for a definition of the two angles.

   6)  The height above sea level of the observing station, HM,
       can be obtained from the Astronomical Almanac (Section J
       in the 1988 edition), or via the routine slaObs.  If p,
       the pressure in millibars, is available, an adequate
       estimate of hm can be obtained from the expression

             hm = -8149.9415 * log(p/1013.25);

       (See Astrophysical Quantities, C.W.Allen, 3rd edition,
       section 52.)  Similarly, if the pressure p is not known,
       it can be estimated from the height of the observing

             p = 1013.25 * exp(-hm/8149.9415);

       Note, however, that the refraction is proportional to the
       pressure and that an accurate p value is important for
       precise work.

  Defined in slamac.h:  D2PI, DS2R

Called

 slaGeoc, slaRefco, slaEqeqx, slaAoppat

  P.T.Wallace   Starlink   31 October 1993

SEE ALSO

• slalib.3