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101 lines
3.2 KiB
Fortran
101 lines
3.2 KiB
Fortran
SUBROUTINE sla_DTPV2C (XI, ETA, V, V01, V02, N)
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*+
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* - - - - - - -
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* D T P V 2 C
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* - - - - - - -
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*
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* Given the tangent-plane coordinates of a star and its direction
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* cosines, determine the direction cosines of the tangent-point.
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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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* V d(3) direction cosines of star
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*
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* Returned:
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* V01 d(3) direction cosines of tangent point, solution 1
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* V02 d(3) direction cosines of tangent point, solution 2
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* N i number of solutions:
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* 0 = no solutions returned (note 2)
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* 1 = only the first solution is useful (note 3)
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* 2 = both solutions are useful (note 3)
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*
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* Notes:
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*
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* 1 The vector V must be of unit length or the result will be wrong.
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*
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* 2 Cases where there is no solution can only arise near the poles.
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* For example, it is clearly impossible for a star at the pole
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* itself to have a non-zero XI value, and hence it is meaningless
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* to ask where the tangent point would have to be.
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*
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* 3 Also near the poles, cases can arise where there are two useful
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* solutions. The argument N indicates whether the second of the
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* two solutions returned is useful. N=1 indicates only one useful
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* solution, the usual case; under these circumstances, the second
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* solution can be regarded as valid if the vector V02 is interpreted
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* as the "over-the-pole" case.
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*
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* 4 This routine is the Cartesian equivalent of the routine sla_DTPS2C.
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*
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* P.T.Wallace Starlink 5 June 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,V(3),V01(3),V02(3)
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INTEGER N
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DOUBLE PRECISION X,Y,Z,RXY2,XI2,ETA2P1,SDF,R2,R,C
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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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RXY2=X*X+Y*Y
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XI2=XI*XI
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ETA2P1=ETA*ETA+1D0
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SDF=Z*SQRT(XI2+ETA2P1)
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R2=RXY2*ETA2P1-Z*Z*XI2
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IF (R2.GT.0D0) THEN
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R=SQRT(R2)
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C=(SDF*ETA+R)/(ETA2P1*SQRT(RXY2*(R2+XI2)))
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V01(1)=C*(X*R+Y*XI)
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V01(2)=C*(Y*R-X*XI)
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V01(3)=(SDF-ETA*R)/ETA2P1
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R=-R
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C=(SDF*ETA+R)/(ETA2P1*SQRT(RXY2*(R2+XI2)))
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V02(1)=C*(X*R+Y*XI)
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V02(2)=C*(Y*R-X*XI)
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V02(3)=(SDF-ETA*R)/ETA2P1
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IF (ABS(SDF).LT.1D0) THEN
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N=1
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ELSE
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N=2
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END IF
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ELSE
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N=0
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END IF
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END
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