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diff --git a/src/slalib/atms.f b/src/slalib/atms.f
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+      SUBROUTINE sla__ATMS (RT, TT, DNT, GAMAL, R, DN, RDNDR)
+*+
+*     - - - - -
+*      A T M S
+*     - - - - -
+*
+*  Internal routine used by REFRO
+*
+*  Refractive index and derivative with respect to height for the
+*  stratosphere.
+*
+*  Given:
+*    RT      d    height of tropopause from centre of the Earth (metre)
+*    TT      d    temperature at the tropopause (deg K)
+*    DNT     d    refractive index at the tropopause
+*    GAMAL   d    constant of the atmospheric model = G*MD/R
+*    R       d    current distance from the centre of the Earth (metre)
+*
+*  Returned:
+*    DN      d    refractive index at R
+*    RDNDR   d    R * rate the refractive index is changing at R
+*
+*  P.T.Wallace   Starlink   14 July 1995
+*
+*  Copyright (C) 1995 Rutherford Appleton Laboratory
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION RT,TT,DNT,GAMAL,R,DN,RDNDR
+
+      DOUBLE PRECISION B,W
+
+
+      B = GAMAL/TT
+      W = (DNT-1D0)*EXP(-B*(R-RT))
+      DN = 1D0+W
+      RDNDR = -R*B*W
+
+      END