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SUBROUTINE sla_PDA2H (P, D, A, H1, J1, H2, J2)
*+
* - - - - - -
* P D A 2 H
* - - - - - -
*
* Hour Angle corresponding to a given azimuth
*
* (double precision)
*
* Given:
* P d latitude
* D d declination
* A d azimuth
*
* Returned:
* H1 d hour angle: first solution if any
* J1 i flag: 0 = solution 1 is valid
* H2 d hour angle: second solution if any
* J2 i flag: 0 = solution 2 is valid
*
* Called: sla_DRANGE
*
* P.T.Wallace Starlink 6 October 1994
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*
* License:
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program (see SLA_CONDITIONS); if not, write to the
* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307 USA
*
*-
IMPLICIT NONE
DOUBLE PRECISION P,D,A,H1
INTEGER J1
DOUBLE PRECISION H2
INTEGER J2
DOUBLE PRECISION DPI
PARAMETER (DPI=3.141592653589793238462643D0)
DOUBLE PRECISION D90
PARAMETER (D90=DPI/2D0)
DOUBLE PRECISION TINY
PARAMETER (TINY=1D-12)
DOUBLE PRECISION PN,AN,DN,SA,CA,SASP,QT,QB,HPT,T
DOUBLE PRECISION sla_DRANGE
* Preset status flags to OK
J1=0
J2=0
* Adjust latitude, azimuth, declination to avoid critical values
PN=sla_DRANGE(P)
IF (ABS(ABS(PN)-D90).LT.TINY) THEN
PN=PN-SIGN(TINY,PN)
ELSE IF (ABS(PN).LT.TINY) THEN
PN=TINY
END IF
AN=sla_DRANGE(A)
IF (ABS(ABS(AN)-DPI).LT.TINY) THEN
AN=AN-SIGN(TINY,AN)
ELSE IF (ABS(AN).LT.TINY) THEN
AN=TINY
END IF
DN=sla_DRANGE(D)
IF (ABS(ABS(DN)-ABS(P)).LT.TINY) THEN
DN=DN-SIGN(TINY,DN)
ELSE IF (ABS(ABS(DN)-D90).LT.TINY) THEN
DN=DN-SIGN(TINY,DN)
ELSE IF (ABS(DN).LT.TINY) THEN
DN=TINY
END IF
* Useful functions
SA=SIN(AN)
CA=COS(AN)
SASP=SA*SIN(PN)
* Quotient giving sin(h+t)
QT=SIN(DN)*SA*COS(PN)
QB=COS(DN)*SQRT(CA*CA+SASP*SASP)
* Any solutions?
IF (ABS(QT).LE.QB) THEN
* Yes: find h+t and t
HPT=ASIN(QT/QB)
T=ATAN2(SASP,-CA)
* The two solutions
H1=sla_DRANGE(HPT-T)
H2=sla_DRANGE(-HPT-(T+DPI))
* Reject unless h and A different signs
IF (H1*AN.GT.0D0) J1=-1
IF (H2*AN.GT.0D0) J2=-1
ELSE
J1=-1
J2=-1
END IF
END
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