1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
|
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
*-
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
|
