.help aoppa Jun99 "Slalib Package"
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      SUBROUTINE slAOPA (DATE, DUT, ELONGM, PHIM, HM,
     :                      XP, YP, TDK, PMB, RH, WL, TLR, AOPRMS)

     - - - - - -
      A O P A
     - - - - - -

  Precompute apparent to observed place parameters required by
  slAOPQ and slOAPQ.

  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=273.155D0)
     PMB    d      local atmospheric pressure (mB; std=1013.25D0)
     RH     d      local relative humidity (in the range 0D0-1D0)
     WL     d      effective wavelength (micron, e.g. 0.55D0)
     TLR    d      tropospheric lapse rate (DegK/metre, e.g. 0.0065D0)

  Returned:
     AOPRMS d(14)  star-independent apparent-to-observed parameters:

       (1)      geodetic latitude (radians)
       (2,3)    sine and cosine of geodetic latitude
       (4)      magnitude of diurnal aberration vector
       (5)      height (HM)
       (6)      ambient temperature (TDK)
       (7)      pressure (PMB)
       (8)      relative humidity (RH)
       (9)      wavelength (WL)
       (10)     lapse rate (TLR)
       (11,12)  refraction constants A and B (radians)
       (13)     longitude + eqn of equinoxes + sidereal DUT (radians)
       (14)     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, improper, 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
       slOBS 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=0D0.  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 slOBS.  If P,
       the pressure in millibars, is available, an adequate
       estimate of HM can be obtained from the expression

             HM ~ -29.3D0*TSL*LOG(P/1013.25D0).

       where TSL is the approximate sea-level air temperature in
       deg K (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
       station, HM as follows:

             P ~ 1013.25D0*EXP(-HM/(29.3D0*TSL)).

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

   7)  Repeated, computationally-expensive, calls to slAOPA for
       times that are very close together can be avoided by calling
       slAOPA just once and then using slAOPT for the subsequent
       times.  Fresh calls to slAOPA will be needed only when changes
       in the precession have grown to unacceptable levels or when
       anything affecting the refraction has changed.

  Called:  slGEOC, slRFCO, slEQEX, slAOPT

  P.T.Wallace   Starlink   9 June 1998

  Copyright (C) 1998 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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