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*     wl = double (Given)
*        Effective wavelength of the source (micrometre)
*     phi = double (Given)
*        Latitude of the observer (radian, astronomical)
*     tlr = double (Given)
*        Temperature lapse rate in the troposphere (K/metre)
*     eps = double (Given)
*        Precision required to terminate iteration (radian)
*     ref = double * (Returned)
*        Refraction: in vacuao ZD minus observed ZD (radian)

*  Description:
*     Calculates the atmospheric refraction for radio and optical/IR
*     wavelengths.

*  Authors:
*     PTW: Patrick T. Wallace
*     TIMJ: Tim Jenness (JAC, Hawaii)
*     {enter_new_authors_here}

*  Notes:
*     - A suggested value for the TLR argument is 0.0065.  The
*     refraction is significantly affected by TLR, and if studies
*     of the local atmosphere have been carried out a better TLR
*     value may be available.  The sign of the supplied TLR value
*     is ignored.
*
*     - A suggested value for the EPS argument is 1E-8.  The result is
*     usually at least two orders of magnitude more computationally
*     precise than the supplied EPS value.
*
*     - The routine computes the refraction for zenith distances up
*     to and a little beyond 90 deg using the method of Hohenkerk
*     and Sinclair (NAO Technical Notes 59 and 63, subsequently adopted
*     in the Explanatory Supplement, 1992 edition - see section 3.281).
*
*     - The code is a development of the optical/IR refraction subroutine
*     AREF of C.Hohenkerk (HMNAO, September 1984), with extensions to
*     support the radio case.  Apart from merely cosmetic changes, the
*     following modifications to the original HMNAO optical/IR refraction
*     code have been made:
*
*     .  The angle arguments have been changed to radians.
*
*     .  Any value of ZOBS is allowed (see note 6, below).
*
*     .  Other argument values have been limited to safe values.
*
*     .  Murray's values for the gas constants have been used
*        (Vectorial Astrometry, Adam Hilger, 1983).
*
*     .  The numerical integration phase has been rearranged for
*        extra clarity.
*
*     .  A better model for Ps(T) has been adopted (taken from
*        Gill, Atmosphere-Ocean Dynamics, Academic Press, 1982).
*
*     .  More accurate expressions for Pwo have been adopted
*        (again from Gill 1982).
*
*     .  The formula for the water vapour pressure, given the
*        saturation pressure and the relative humidity, is from
*        Crane (1976), expression 2.5.5.

*     .  Provision for radio wavelengths has been added using
*        expressions devised by A.T.Sinclair, RGO (private
*        communication 1989).  The refractivity model currently
*        used is from J.M.Rueger, "Refractive Index Formulae for
*        Electronic Distance Measurement with Radio and Millimetre
*        Waves", in Unisurv Report S-68 (2002), School of Surveying
*        and Spatial Information Systems, University of New South
*        Wales, Sydney, Australia.
*
*     .  The optical refractivity for dry air is from Resolution 3 of
*        the International Association of Geodesy adopted at the XXIIth
*        General Assembly in Birmingham, UK, 1999.
*
*     .  Various small changes have been made to gain speed.
*
*     - The radio refraction is chosen by specifying WL > 100 micrometres.
*     Because the algorithm takes no account of the ionosphere, the
*     accuracy deteriorates at low frequencies, below about 30 MHz.
*
*     - Before use, the value of ZOBS is expressed in the range +/- pi.
*     If this ranged ZOBS is -ve, the result REF is computed from its
*     absolute value before being made -ve to match.  In addition, if
*     it has an absolute value greater than 93 deg, a fixed REF value
*     equal to the result for ZOBS = 93 deg is returned, appropriately
*     signed.
*
*     - As in the original Hohenkerk and Sinclair algorithm, fixed values
*     of the water vapour polytrope exponent, the height of the
*     tropopause, and the height at which refraction is negligible are
*     used.
*
*     - The radio refraction has been tested against work done by
*     Iain Coulson, JACH, (private communication 1995) for the
*     James Clerk Maxwell Telescope, Mauna Kea.  For typical conditions,
*     agreement at the 0.1 arcsec level is achieved for moderate ZD,
*     worsening to perhaps 0.5-1.0 arcsec at ZD 80 deg.  At hot and
*     humid sea-level sites the accuracy will not be as good.
*
*     - It should be noted that the relative humidity RH is formally
*     defined in terms of "mixing ratio" rather than pressures or
*     densities as is often stated.  It is the mass of water per unit
*     mass of dry air divided by that for saturated air at the same
*     temperature and pressure (see Gill 1982).

*     - The algorithm is designed for observers in the troposphere. The
*     supplied temperature, pressure and lapse rate are assumed to be
*     for a point in the troposphere and are used to define a model
*     atmosphere with the tropopause at 11km altitude and a constant
*     temperature above that.  However, in practice, the refraction
*     values returned for stratospheric observers, at altitudes up to
*     25km, are quite usable.

*  History:
*     2012-08-24 (TIMJ):
*        Initial version, direct port of SLA Fortran source.
*        Adapted with permission from the Fortran SLALIB library.
*     {enter_further_changes_here}