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74 lines
2.0 KiB
Fortran
74 lines
2.0 KiB
Fortran
SUBROUTINE sla_DTP2V (XI, ETA, V0, V)
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*+
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* - - - - - -
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* D T P 2 V
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* - - - - - -
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*
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* Given the tangent-plane coordinates of a star and the direction
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* cosines of the tangent point, determine the direction cosines
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* of the star.
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*
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* (double precision)
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*
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* Given:
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* XI,ETA d tangent plane coordinates of star
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* V0 d(3) direction cosines of tangent point
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*
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* Returned:
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* V d(3) direction cosines of star
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*
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* Notes:
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*
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* 1 If vector V0 is not of unit length, the returned vector V will
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* be wrong.
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*
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* 2 If vector V0 points at a pole, the returned vector V will be
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* based on the arbitrary assumption that the RA of the tangent
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* point is zero.
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*
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* 3 This routine is the Cartesian equivalent of the routine sla_DTP2S.
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*
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* P.T.Wallace Starlink 11 February 1995
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*
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* Copyright (C) 1995 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION XI,ETA,V0(3),V(3)
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DOUBLE PRECISION X,Y,Z,F,R
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X=V0(1)
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Y=V0(2)
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Z=V0(3)
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F=SQRT(1D0+XI*XI+ETA*ETA)
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R=SQRT(X*X+Y*Y)
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IF (R.EQ.0D0) THEN
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R=1D-20
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X=R
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END IF
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V(1)=(X-(XI*Y+ETA*X*Z)/R)/F
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V(2)=(Y+(XI*X-ETA*Y*Z)/R)/F
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V(3)=(Z+ETA*R)/F
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END
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