source: branches/FACT++_lidctrl_new_eth/erfa/src/pap.c@ 18795

Last change on this file since 18795 was 18711, checked in by tbretz, 9 years ago
Updated to ERFA 1.3.0 (no relevant code change except the leap second at the beginning of 2017)
File size: 5.2 KB
Line 
1#include "erfa.h"
2
3double eraPap(double a[3], double b[3])
4/*
5** - - - - - - -
6** e r a P a p
7** - - - - - - -
8**
9** Position-angle from two p-vectors.
10**
11** Given:
12** a double[3] direction of reference point
13** b double[3] direction of point whose PA is required
14**
15** Returned (function value):
16** double position angle of b with respect to a (radians)
17**
18** Notes:
19**
20** 1) The result is the position angle, in radians, of direction b with
21** respect to direction a. It is in the range -pi to +pi. The
22** sense is such that if b is a small distance "north" of a the
23** position angle is approximately zero, and if b is a small
24** distance "east" of a the position angle is approximately +pi/2.
25**
26** 2) The vectors a and b need not be of unit length.
27**
28** 3) Zero is returned if the two directions are the same or if either
29** vector is null.
30**
31** 4) If vector a is at a pole, the result is ill-defined.
32**
33** Called:
34** eraPn decompose p-vector into modulus and direction
35** eraPm modulus of p-vector
36** eraPxp vector product of two p-vectors
37** eraPmp p-vector minus p-vector
38** eraPdp scalar product of two p-vectors
39**
40** Copyright (C) 2013-2016, NumFOCUS Foundation.
41** Derived, with permission, from the SOFA library. See notes at end of file.
42*/
43{
44 double am, au[3], bm, st, ct, xa, ya, za, eta[3], xi[3], a2b[3], pa;
45
46
47/* Modulus and direction of the a vector. */
48 eraPn(a, &am, au);
49
50/* Modulus of the b vector. */
51 bm = eraPm(b);
52
53/* Deal with the case of a null vector. */
54 if ((am == 0.0) || (bm == 0.0)) {
55 st = 0.0;
56 ct = 1.0;
57 } else {
58
59 /* The "north" axis tangential from a (arbitrary length). */
60 xa = a[0];
61 ya = a[1];
62 za = a[2];
63 eta[0] = -xa * za;
64 eta[1] = -ya * za;
65 eta[2] = xa*xa + ya*ya;
66
67 /* The "east" axis tangential from a (same length). */
68 eraPxp(eta, au, xi);
69
70 /* The vector from a to b. */
71 eraPmp(b, a, a2b);
72
73 /* Resolve into components along the north and east axes. */
74 st = eraPdp(a2b, xi);
75 ct = eraPdp(a2b, eta);
76
77 /* Deal with degenerate cases. */
78 if ((st == 0.0) && (ct == 0.0)) ct = 1.0;
79 }
80
81/* Position angle. */
82 pa = atan2(st, ct);
83
84 return pa;
85
86}
87/*----------------------------------------------------------------------
88**
89**
90** Copyright (C) 2013-2016, NumFOCUS Foundation.
91** All rights reserved.
92**
93** This library is derived, with permission, from the International
94** Astronomical Union's "Standards of Fundamental Astronomy" library,
95** available from http://www.iausofa.org.
96**
97** The ERFA version is intended to retain identical functionality to
98** the SOFA library, but made distinct through different function and
99** file names, as set out in the SOFA license conditions. The SOFA
100** original has a role as a reference standard for the IAU and IERS,
101** and consequently redistribution is permitted only in its unaltered
102** state. The ERFA version is not subject to this restriction and
103** therefore can be included in distributions which do not support the
104** concept of "read only" software.
105**
106** Although the intent is to replicate the SOFA API (other than
107** replacement of prefix names) and results (with the exception of
108** bugs; any that are discovered will be fixed), SOFA is not
109** responsible for any errors found in this version of the library.
110**
111** If you wish to acknowledge the SOFA heritage, please acknowledge
112** that you are using a library derived from SOFA, rather than SOFA
113** itself.
114**
115**
116** TERMS AND CONDITIONS
117**
118** Redistribution and use in source and binary forms, with or without
119** modification, are permitted provided that the following conditions
120** are met:
121**
122** 1 Redistributions of source code must retain the above copyright
123** notice, this list of conditions and the following disclaimer.
124**
125** 2 Redistributions in binary form must reproduce the above copyright
126** notice, this list of conditions and the following disclaimer in
127** the documentation and/or other materials provided with the
128** distribution.
129**
130** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
131** the International Astronomical Union nor the names of its
132** contributors may be used to endorse or promote products derived
133** from this software without specific prior written permission.
134**
135** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
136** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
137** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
138** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
139** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
140** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
141** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
142** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
143** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
144** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
145** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
146** POSSIBILITY OF SUCH DAMAGE.
147**
148*/
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