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96 lines
2.5 KiB
Fortran
96 lines
2.5 KiB
Fortran
SUBROUTINE sla_DV2TP (V, V0, XI, ETA, J)
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*+
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* - - - - - -
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* D V 2 T P
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* - - - - - -
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*
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* Given the direction cosines of a star and of the tangent point,
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* determine the star's tangent-plane coordinates.
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*
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* (double precision)
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*
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* Given:
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* V d(3) direction cosines 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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* XI,ETA d tangent plane coordinates of star
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* J i status: 0 = OK
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* 1 = error, star too far from axis
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* 2 = error, antistar on tangent plane
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* 3 = error, antistar too far from axis
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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, or if vector V is of zero
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* length, the results will be wrong.
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*
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* 2 If V0 points at a pole, the returned XI,ETA will be based on the
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* arbitrary assumption that the RA of the tangent point is zero.
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*
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* 3 This routine is the Cartesian equivalent of the routine sla_DS2TP.
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*
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* P.T.Wallace Starlink 27 November 1996
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*
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* Copyright (C) 1996 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 V(3),V0(3),XI,ETA
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INTEGER J
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DOUBLE PRECISION X,Y,Z,X0,Y0,Z0,R2,R,W,D
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DOUBLE PRECISION TINY
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PARAMETER (TINY=1D-6)
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X=V(1)
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Y=V(2)
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Z=V(3)
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X0=V0(1)
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Y0=V0(2)
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Z0=V0(3)
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R2=X0*X0+Y0*Y0
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R=SQRT(R2)
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IF (R.EQ.0D0) THEN
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R=1D-20
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X0=R
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END IF
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W=X*X0+Y*Y0
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D=W+Z*Z0
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IF (D.GT.TINY) THEN
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J=0
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ELSE IF (D.GE.0D0) THEN
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J=1
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D=TINY
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ELSE IF (D.GT.-TINY) THEN
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J=2
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D=-TINY
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ELSE
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J=3
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END IF
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D=D*R
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XI=(Y*X0-X*Y0)/D
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ETA=(Z*R2-Z0*W)/D
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END
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