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SUBROUTINE sla_PDQ2H (P, D, Q, H1, J1, H2, J2)
*+
* - - - - - -
* P D Q 2 H
* - - - - - -
*
* Hour Angle corresponding to a given parallactic angle
*
* (double precision)
*
* Given:
* P d latitude
* D d declination
* Q d parallactic angle
*
* 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
*-
IMPLICIT NONE
DOUBLE PRECISION P,D,Q,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,QN,DN,SQ,CQ,SQSD,QT,QB,HPT,T
DOUBLE PRECISION sla_DRANGE
* Preset status flags to OK
J1=0
J2=0
* Adjust latitude, declination, parallactic angle 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
QN=sla_DRANGE(Q)
IF (ABS(ABS(QN)-DPI).LT.TINY) THEN
QN=QN-SIGN(TINY,QN)
ELSE IF (ABS(QN).LT.TINY) THEN
QN=TINY
END IF
DN=sla_DRANGE(D)
IF (ABS(ABS(D)-ABS(P)).LT.TINY) THEN
DN=DN-SIGN(TINY,DN)
ELSE IF (ABS(ABS(D)-D90).LT.TINY) THEN
DN=DN-SIGN(TINY,DN)
END IF
* Useful functions
SQ=SIN(QN)
CQ=COS(QN)
SQSD=SQ*SIN(DN)
* Quotient giving sin(h+t)
QT=SIN(PN)*SQ*COS(DN)
QB=COS(PN)*SQRT(CQ*CQ+SQSD*SQSD)
* Any solutions?
IF (ABS(QT).LE.QB) THEN
* Yes: find h+t and t
HPT=ASIN(QT/QB)
T=ATAN2(SQSD,CQ)
* The two solutions
H1=sla_DRANGE(HPT-T)
H2=sla_DRANGE(-HPT-(T+DPI))
* Reject if h and Q different signs
IF (H1*QN.LT.0D0) J1=-1
IF (H2*QN.LT.0D0) J2=-1
ELSE
J1=-1
J2=-1
END IF
END
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