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116 lines
3.0 KiB
Fortran
116 lines
3.0 KiB
Fortran
SUBROUTINE sla_PDQ2H (P, D, Q, H1, J1, H2, J2)
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*+
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* - - - - - -
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* P D Q 2 H
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* - - - - - -
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*
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* Hour Angle corresponding to a given parallactic angle
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*
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* (double precision)
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*
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* Given:
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* P d latitude
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* D d declination
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* Q d parallactic angle
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*
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* Returned:
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* H1 d hour angle: first solution if any
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* J1 i flag: 0 = solution 1 is valid
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* H2 d hour angle: second solution if any
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* J2 i flag: 0 = solution 2 is valid
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*
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* Called: sla_DRANGE
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*
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* P.T.Wallace Starlink 6 October 1994
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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 P,D,Q,H1
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INTEGER J1
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DOUBLE PRECISION H2
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INTEGER J2
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DOUBLE PRECISION DPI
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PARAMETER (DPI=3.141592653589793238462643D0)
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DOUBLE PRECISION D90
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PARAMETER (D90=DPI/2D0)
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DOUBLE PRECISION TINY
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PARAMETER (TINY=1D-12)
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DOUBLE PRECISION PN,QN,DN,SQ,CQ,SQSD,QT,QB,HPT,T
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DOUBLE PRECISION sla_DRANGE
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* Preset status flags to OK
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J1=0
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J2=0
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* Adjust latitude, declination, parallactic angle to avoid critical values
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PN=sla_DRANGE(P)
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IF (ABS(ABS(PN)-D90).LT.TINY) THEN
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PN=PN-SIGN(TINY,PN)
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ELSE IF (ABS(PN).LT.TINY) THEN
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PN=TINY
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END IF
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QN=sla_DRANGE(Q)
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IF (ABS(ABS(QN)-DPI).LT.TINY) THEN
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QN=QN-SIGN(TINY,QN)
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ELSE IF (ABS(QN).LT.TINY) THEN
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QN=TINY
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END IF
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DN=sla_DRANGE(D)
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IF (ABS(ABS(D)-ABS(P)).LT.TINY) THEN
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DN=DN-SIGN(TINY,DN)
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ELSE IF (ABS(ABS(D)-D90).LT.TINY) THEN
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DN=DN-SIGN(TINY,DN)
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END IF
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* Useful functions
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SQ=SIN(QN)
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CQ=COS(QN)
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SQSD=SQ*SIN(DN)
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* Quotient giving sin(h+t)
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QT=SIN(PN)*SQ*COS(DN)
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QB=COS(PN)*SQRT(CQ*CQ+SQSD*SQSD)
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* Any solutions?
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IF (ABS(QT).LE.QB) THEN
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* Yes: find h+t and t
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HPT=ASIN(QT/QB)
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T=ATAN2(SQSD,CQ)
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* The two solutions
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H1=sla_DRANGE(HPT-T)
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H2=sla_DRANGE(-HPT-(T+DPI))
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* Reject if h and Q different signs
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IF (H1*QN.LT.0D0) J1=-1
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IF (H2*QN.LT.0D0) J2=-1
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ELSE
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J1=-1
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J2=-1
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END IF
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END
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