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slalib/refv.f

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+      SUBROUTINE sla_REFV (VU, REFA, REFB, VR)
+*+
+*     - - - - -
+*      R E F V
+*     - - - - -
+*
+*  Adjust an unrefracted Cartesian vector to include the effect of
+*  atmospheric refraction, using the simple A tan Z + B tan**3 Z
+*  model.
+*
+*  Given:
+*    VU    dp    unrefracted position of the source (Az/El 3-vector)
+*    REFA  dp    tan Z coefficient (radian)
+*    REFB  dp    tan**3 Z coefficient (radian)
+*
+*  Returned:
+*    VR    dp    refracted position of the source (Az/El 3-vector)
+*
+*  Notes:
+*
+*  1  This routine applies the adjustment for refraction in the
+*     opposite sense to the usual one - it takes an unrefracted
+*     (in vacuo) position and produces an observed (refracted)
+*     position, whereas the A tan Z + B tan**3 Z model strictly
+*     applies to the case where an observed position is to have the
+*     refraction removed.  The unrefracted to refracted case is
+*     harder, and requires an inverted form of the text-book
+*     refraction models;  the algorithm used here is equivalent to
+*     one iteration of the Newton-Raphson method applied to the above
+*     formula.
+*
+*  2  Though optimized for speed rather than precision, the present
+*     routine achieves consistency with the refracted-to-unrefracted
+*     A tan Z + B tan**3 Z model at better than 1 microarcsecond within
+*     30 degrees of the zenith and remains within 1 milliarcsecond to
+*     beyond ZD 70 degrees.  The inherent accuracy of the model is, of
+*     course, far worse than this - see the documentation for sla_REFCO
+*     for more information.
+*
+*  3  At low elevations (below about 3 degrees) the refraction
+*     correction is held back to prevent arithmetic problems and
+*     wildly wrong results.  For optical/IR wavelengths, over a wide
+*     range of observer heights and corresponding temperatures and
+*     pressures, the following levels of accuracy (arcsec, worst case)
+*     are achieved, relative to numerical integration through a model
+*     atmosphere:
+*
+*              ZD    error
+*
+*              80      0.7
+*              81      1.3
+*              82      2.5
+*              83      5
+*              84     10
+*              85     20
+*              86     55
+*              87    160
+*              88    360
+*              89    640
+*              90   1100
+*              91   1700         } relevant only to
+*              92   2600         } high-elevation sites
+*
+*     The results for radio are slightly worse over most of the range,
+*     becoming significantly worse below ZD=88 and unusable beyond
+*     ZD=90.
+*
+*  4  See also the routine sla_REFZ, which performs the adjustment to
+*     the zenith distance rather than in Cartesian Az/El coordinates.
+*     The present routine is faster than sla_REFZ and, except very low down,
+*     is equally accurate for all practical purposes.  However, beyond
+*     about ZD 84 degrees sla_REFZ should be used, and for the utmost
+*     accuracy iterative use of sla_REFRO should be considered.
+*
+*  P.T.Wallace   Starlink   10 April 2004
+*
+*  Copyright (C) 2004 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION VU(3),REFA,REFB,VR(3)
+
+      DOUBLE PRECISION X,Y,Z1,Z,ZSQ,RSQ,R,WB,WT,D,CD,F
+
+
+
+*  Initial estimate = unrefracted vector
+      X = VU(1)
+      Y = VU(2)
+      Z1 = VU(3)
+
+*  Keep correction approximately constant below about 3 deg elevation
+      Z = MAX(Z1,0.05D0)
+
+*  One Newton-Raphson iteration
+      ZSQ = Z*Z
+      RSQ = X*X+Y*Y
+      R = SQRT(RSQ)
+      WB = REFB*RSQ/ZSQ
+      WT = (REFA+WB)/(1D0+(REFA+3D0*WB)*(ZSQ+RSQ)/ZSQ)
+      D = WT*R/Z
+      CD = 1D0-D*D/2D0
+      F = CD*(1D0-WT)
+
+*  Post-refraction x,y,z
+      VR(1) = X*F
+      VR(2) = Y*F
+      VR(3) = CD*(Z+D*R)+(Z1-Z)
+
+      END