source: trunk/FACT++/sofa/src/pap.c@ 18375

Last change on this file since 18375 was 18346, checked in by tbretz, 11 years ago
File size: 6.6 KB
Line 
1#include "sofa.h"
2
3double iauPap(double a[3], double b[3])
4/*
5** - - - - - - -
6** i a u P a p
7** - - - - - - -
8**
9** Position-angle from two p-vectors.
10**
11** This function is part of the International Astronomical Union's
12** SOFA (Standards Of Fundamental Astronomy) software collection.
13**
14** Status: vector/matrix support function.
15**
16** Given:
17** a double[3] direction of reference point
18** b double[3] direction of point whose PA is required
19**
20** Returned (function value):
21** double position angle of b with respect to a (radians)
22**
23** Notes:
24**
25** 1) The result is the position angle, in radians, of direction b with
26** respect to direction a. It is in the range -pi to +pi. The
27** sense is such that if b is a small distance "north" of a the
28** position angle is approximately zero, and if b is a small
29** distance "east" of a the position angle is approximately +pi/2.
30**
31** 2) The vectors a and b need not be of unit length.
32**
33** 3) Zero is returned if the two directions are the same or if either
34** vector is null.
35**
36** 4) If vector a is at a pole, the result is ill-defined.
37**
38** Called:
39** iauPn decompose p-vector into modulus and direction
40** iauPm modulus of p-vector
41** iauPxp vector product of two p-vectors
42** iauPmp p-vector minus p-vector
43** iauPdp scalar product of two p-vectors
44**
45** This revision: 2013 June 18
46**
47** SOFA release 2015-02-09
48**
49** Copyright (C) 2015 IAU SOFA Board. See notes at end.
50*/
51{
52 double am, au[3], bm, st, ct, xa, ya, za, eta[3], xi[3], a2b[3], pa;
53
54/* Modulus and direction of the a vector. */
55 iauPn(a, &am, au);
56
57/* Modulus of the b vector. */
58 bm = iauPm(b);
59
60/* Deal with the case of a null vector. */
61 if ((am == 0.0) || (bm == 0.0)) {
62 st = 0.0;
63 ct = 1.0;
64 } else {
65
66 /* The "north" axis tangential from a (arbitrary length). */
67 xa = a[0];
68 ya = a[1];
69 za = a[2];
70 eta[0] = -xa * za;
71 eta[1] = -ya * za;
72 eta[2] = xa*xa + ya*ya;
73
74 /* The "east" axis tangential from a (same length). */
75 iauPxp(eta, au, xi);
76
77 /* The vector from a to b. */
78 iauPmp(b, a, a2b);
79
80 /* Resolve into components along the north and east axes. */
81 st = iauPdp(a2b, xi);
82 ct = iauPdp(a2b, eta);
83
84 /* Deal with degenerate cases. */
85 if ((st == 0.0) && (ct == 0.0)) ct = 1.0;
86 }
87
88/* Position angle. */
89 pa = atan2(st, ct);
90
91 return pa;
92
93/*----------------------------------------------------------------------
94**
95** Copyright (C) 2015
96** Standards Of Fundamental Astronomy Board
97** of the International Astronomical Union.
98**
99** =====================
100** SOFA Software License
101** =====================
102**
103** NOTICE TO USER:
104**
105** BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
106** CONDITIONS WHICH APPLY TO ITS USE.
107**
108** 1. The Software is owned by the IAU SOFA Board ("SOFA").
109**
110** 2. Permission is granted to anyone to use the SOFA software for any
111** purpose, including commercial applications, free of charge and
112** without payment of royalties, subject to the conditions and
113** restrictions listed below.
114**
115** 3. You (the user) may copy and distribute SOFA source code to others,
116** and use and adapt its code and algorithms in your own software,
117** on a world-wide, royalty-free basis. That portion of your
118** distribution that does not consist of intact and unchanged copies
119** of SOFA source code files is a "derived work" that must comply
120** with the following requirements:
121**
122** a) Your work shall be marked or carry a statement that it
123** (i) uses routines and computations derived by you from
124** software provided by SOFA under license to you; and
125** (ii) does not itself constitute software provided by and/or
126** endorsed by SOFA.
127**
128** b) The source code of your derived work must contain descriptions
129** of how the derived work is based upon, contains and/or differs
130** from the original SOFA software.
131**
132** c) The names of all routines in your derived work shall not
133** include the prefix "iau" or "sofa" or trivial modifications
134** thereof such as changes of case.
135**
136** d) The origin of the SOFA components of your derived work must
137** not be misrepresented; you must not claim that you wrote the
138** original software, nor file a patent application for SOFA
139** software or algorithms embedded in the SOFA software.
140**
141** e) These requirements must be reproduced intact in any source
142** distribution and shall apply to anyone to whom you have
143** granted a further right to modify the source code of your
144** derived work.
145**
146** Note that, as originally distributed, the SOFA software is
147** intended to be a definitive implementation of the IAU standards,
148** and consequently third-party modifications are discouraged. All
149** variations, no matter how minor, must be explicitly marked as
150** such, as explained above.
151**
152** 4. You shall not cause the SOFA software to be brought into
153** disrepute, either by misuse, or use for inappropriate tasks, or
154** by inappropriate modification.
155**
156** 5. The SOFA software is provided "as is" and SOFA makes no warranty
157** as to its use or performance. SOFA does not and cannot warrant
158** the performance or results which the user may obtain by using the
159** SOFA software. SOFA makes no warranties, express or implied, as
160** to non-infringement of third party rights, merchantability, or
161** fitness for any particular purpose. In no event will SOFA be
162** liable to the user for any consequential, incidental, or special
163** damages, including any lost profits or lost savings, even if a
164** SOFA representative has been advised of such damages, or for any
165** claim by any third party.
166**
167** 6. The provision of any version of the SOFA software under the terms
168** and conditions specified herein does not imply that future
169** versions will also be made available under the same terms and
170** conditions.
171*
172** In any published work or commercial product which uses the SOFA
173** software directly, acknowledgement (see www.iausofa.org) is
174** appreciated.
175**
176** Correspondence concerning SOFA software should be addressed as
177** follows:
178**
179** By email: sofa@ukho.gov.uk
180** By post: IAU SOFA Center
181** HM Nautical Almanac Office
182** UK Hydrographic Office
183** Admiralty Way, Taunton
184** Somerset, TA1 2DN
185** United Kingdom
186**
187**--------------------------------------------------------------------*/
188}
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