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diff --git a/ast/pal/palDtps2c.c b/ast/pal/palDtps2c.c
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+/*
+*+
+*  Name:
+*     palDtps2c
+
+*  Purpose:
+*     Determine RA,Dec of tangent point from coordinates
+
+*  Language:
+*     Starlink ANSI C
+
+*  Type of Module:
+*     Library routine
+
+*  Invocation:
+*     palDtps2c( double xi, double eta, double ra, double dec,
+*                double * raz1, double decz1,
+*                double * raz2, double decz2, int *n);
+
+*  Arguments:
+*     xi = double (Given)
+*        First rectangular coordinate on tangent plane (radians)
+*     eta = double (Given)
+*        Second rectangular coordinate on tangent plane (radians)
+*     ra = double (Given)
+*        RA spherical coordinate of star (radians)
+*     dec = double (Given)
+*        Dec spherical coordinate of star (radians)
+*     raz1 = double * (Returned)
+*        RA spherical coordinate of tangent point, solution 1 (radians)
+*     decz1 = double * (Returned)
+*        Dec spherical coordinate of tangent point, solution 1 (radians)
+*     raz2 = double * (Returned)
+*        RA spherical coordinate of tangent point, solution 2 (radians)
+*     decz2 = double * (Returned)
+*        Dec spherical coordinate of tangent point, solution 2 (radians)
+*     n = int * (Returned)
+*        number of solutions: 0 = no solutions returned (note 2)
+*                             1 = only the first solution is useful (note 3)
+*                             2 = both solutions are useful (note 3)
+
+
+*  Description:
+*     From the tangent plane coordinates of a star of known RA,Dec,
+*     determine the RA,Dec of the tangent point.
+
+*  Authors:
+*     PTW: Pat Wallace (STFC)
+*     TIMJ: Tim Jenness (JAC, Hawaii)
+*     {enter_new_authors_here}
+
+*  Notes:
+*     - The RAZ1 and RAZ2 values are returned in the range 0-2pi.
+*     - Cases where there is no solution can only arise near the poles.
+*       For example, it is clearly impossible for a star at the pole
+*       itself to have a non-zero XI value, and hence it is
+*       meaningless to ask where the tangent point would have to be
+*       to bring about this combination of XI and DEC.
+*     - Also near the poles, cases can arise where there are two useful
+*       solutions.  The argument N indicates whether the second of the
+*       two solutions returned is useful.  N=1 indicates only one useful
+*       solution, the usual case;  under these circumstances, the second
+*       solution corresponds to the "over-the-pole" case, and this is
+*       reflected in the values of RAZ2 and DECZ2 which are returned.
+*     - The DECZ1 and DECZ2 values are returned in the range +/-pi, but
+*       in the usual, non-pole-crossing, case, the range is +/-pi/2.
+*     - This routine is the spherical equivalent of the routine sla_DTPV2C.
+
+*  History:
+*     2012-02-08 (TIMJ):
+*        Initial version with documentation taken from Fortran SLA
+*        Adapted with permission from the Fortran SLALIB library.
+*     {enter_further_changes_here}
+
+*  Copyright:
+*     Copyright (C) 1995 Rutherford Appleton Laboratory
+*     Copyright (C) 2012 Science and Technology Facilities Council.
+*     All Rights Reserved.
+
+*  Licence:
+*     This program is free software: you can redistribute it and/or
+*     modify it under the terms of the GNU Lesser General Public
+*     License as published by the Free Software Foundation, either
+*     version 3 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 Lesser General Public License for more details.
+*
+*     You should have received a copy of the GNU Lesser General
+*     License along with this program.  If not, see
+*     <http://www.gnu.org/licenses/>.
+
+*  Bugs:
+*     {note_any_bugs_here}
+*-
+*/
+
+#include "pal.h"
+#include "pal1sofa.h"
+
+#include <math.h>
+
+void
+palDtps2c( double xi, double eta, double ra, double dec,
+           double * raz1, double * decz1,
+           double * raz2, double * decz2, int *n) {
+
+  double x2;
+  double y2;
+  double sd;
+  double cd;
+  double sdf;
+  double r2;
+
+  x2 = xi * xi;
+  y2 = eta * eta;
+  sd = sin(dec);
+  cd = cos(dec);
+  sdf = sd * sqrt(x2 + 1. + y2);
+  r2 = cd * cd * (y2 + 1.) - sd * sd * x2;
+  if (r2 >= 0.) {
+    double r;
+    double s;
+    double c;
+
+    r = sqrt(r2);
+    s = sdf - eta * r;
+    c = sdf * eta + r;
+    if (xi == 0. && r == 0.) {
+      r = 1.;
+    }
+    *raz1 = eraAnp(ra - atan2(xi, r));
+    *decz1 = atan2(s, c);
+    r = -r;
+    s = sdf - eta * r;
+    c = sdf * eta + r;
+    *raz2 = eraAnp(ra - atan2(xi, r));
+    *decz2 = atan2(s, c);
+    if (fabs(sdf) < 1.) {
+      *n = 1;
+    } else {
+      *n = 2;
+    }
+  } else {
+    *n = 0;
+  }
+  return;
+}