#include "sofam.h" double iauPap(double a[3], double b[3]) /* ** - - - - - - - ** i a u P a p ** - - - - - - - ** ** Position-angle from two p-vectors. ** ** This function is part of the International Astronomical Union's ** SOFA (Standards Of Fundamental Astronomy) software collection. ** ** Status: vector/matrix support function. ** ** Given: ** a double[3] direction of reference point ** b double[3] direction of point whose PA is required ** ** Returned (function value): ** double position angle of b with respect to a (radians) ** ** Notes: ** ** 1) The result is the position angle, in radians, of direction b with ** respect to direction a. It is in the range -pi to +pi. The ** sense is such that if b is a small distance "north" of a the ** position angle is approximately zero, and if b is a small ** distance "east" of a the position angle is approximately +pi/2. ** ** 2) The vectors a and b need not be of unit length. ** ** 3) Zero is returned if the two directions are the same or if either ** vector is null. ** ** 4) If vector a is at a pole, the result is ill-defined. ** ** Called: ** iauPn decompose p-vector into modulus and direction ** iauPm modulus of p-vector ** iauPxp vector product of two p-vectors ** iauPmp p-vector minus p-vector ** iauPdp scalar product of two p-vectors ** ** This revision: 2008 May 25 ** ** Copyright (C) 2008 IAU SOFA Review Board. See notes at end. */ { double am, au[3], bm, st, ct, xa, ya, za, eta[3], xi[3], a2b[3], pa; /* Modulus and direction of the a vector. */ iauPn(a, &am, au); /* Modulus of the b vector. */ bm = iauPm(b); /* Deal with the case of a null vector. */ if ((am == 0.0) || (bm == 0.0)) { st = 0.0; ct = 1.0; } else { /* The "north" axis tangential from a (arbitrary length). */ xa = a[0]; ya = a[1]; za = a[2]; eta[0] = -xa * za; eta[1] = -ya * za; eta[2] = xa*xa + ya*ya; /* The "east" axis tangential from a (same length). */ iauPxp(eta, au, xi); /* The vector from a to b. */ iauPmp(b, a, a2b); /* Resolve into components along the north and east axes. */ st = iauPdp(a2b, xi); ct = iauPdp(a2b, eta); /* Deal with degenerate cases. */ if ((st == 0.0) && (ct == 0.0)) ct = 1.0; } /* Position angle. */ pa = atan2(st, ct); return pa; /*----------------------------------------------------------------------- ** ** Copyright (C) 2008 ** Standards Of Fundamental Astronomy Review Board ** of the International Astronomical Union. ** ** ===================== ** SOFA Software License ** ===================== ** ** NOTICE TO USER: ** ** BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING TERMS AND CONDITIONS ** WHICH APPLY TO ITS USE. ** ** 1. The Software is owned by the IAU SOFA Review Board ("the Board"). ** ** 2. Permission is granted to anyone to use the SOFA software for any ** purpose, including commercial applications, free of charge and ** without payment of royalties, subject to the conditions and ** restrictions listed below. ** ** 3. You (the user) may copy and adapt the SOFA software and its ** algorithms for your own purposes and you may copy and distribute ** a resulting "derived work" to others on a world-wide, royalty-free ** basis, provided that the derived work complies with the following ** requirements: ** ** a) Your work shall be marked or carry a statement that it (i) uses ** routines and computations derived by you from software provided ** by SOFA under license to you; and (ii) does not contain ** software provided by SOFA or software that has been distributed ** by or endorsed by SOFA. ** ** b) The source code of your derived work must contain descriptions ** of how the derived work is based upon and/or differs from the ** original SOFA software. ** ** c) The name(s) of all routine(s) that you distribute shall differ ** from the SOFA names, even when the SOFA content has not been ** otherwise changed. ** ** d) The routine-naming prefix "iau" shall not be used. ** ** e) The origin of the SOFA components of your derived work must not ** be misrepresented; you must not claim that you wrote the ** original software, nor file a patent application for SOFA ** software or algorithms embedded in the SOFA software. ** ** f) These requirements must be reproduced intact in any source ** distribution and shall apply to anyone to whom you have granted ** a further right to modify the source code of your derived work. ** ** 4. In any published work or commercial products which includes ** results achieved by using the SOFA software, you shall acknowledge ** that the SOFA software was used in obtaining those results. ** ** 5. You shall not cause the SOFA software to be brought into ** disrepute, either by misuse, or use for inappropriate tasks, or by ** inappropriate modification. ** ** 6. The SOFA software is provided "as is" and the Board makes no ** warranty as to its use or performance. The Board does not and ** cannot warrant the performance or results which the user may obtain ** by using the SOFA software. The Board makes no warranties, express ** or implied, as to non-infringement of third party rights, ** merchantability, or fitness for any particular purpose. In no ** event will the Board be liable to the user for any consequential, ** incidental, or special damages, including any lost profits or lost ** savings, even if a Board representative has been advised of such ** damages, or for any claim by any third party. ** ** 7. The provision of any version of the SOFA software under the terms ** and conditions specified herein does not imply that future ** versions will also be made available under the same terms and ** conditions. ** ** Correspondence concerning SOFA software should be addressed as ** follows: ** ** Internet email: sofa@rl.ac.uk ** Postal address: IAU SOFA Center ** Rutherford Appleton Laboratory ** Chilton, Didcot, Oxon OX11 0QX ** United Kingdom ** **---------------------------------------------------------------------*/ }