era.fw2xy.md

eraFw2xy

[x, y] = ERFA.fw2xy(gamb, phib, psi, eps)

CIP X,Y given Fukushima-Williams bias-precession-nutation angles.

Given:

   gamb     double    F-W angle gamma_bar (radians)
   phib     double    F-W angle phi_bar (radians)
   psi      double    F-W angle psi (radians)
   eps      double    F-W angle epsilon (radians)

Returned:

   x,y      double    CIP unit vector X,Y

Notes:

1) Naming the following points:

         e = J2000.0 ecliptic pole,
         p = GCRS pole
         E = ecliptic pole of date,
   and   P = CIP,

the four Fukushima-Williams angles are as follows:

      gamb = gamma = epE
      phib = phi = pE
      psi = psi = pEP
      eps = epsilon = EP

2. The matrix representing the combined effects of frame bias, precession and nutation is:

      NxPxB = R_1(-epsA).R_3(-psi).R_1(phib).R_3(gamb)

The returned values x,y are elements [2][0] and [2][1] of the matrix. Near J2000.0, they are essentially angles in radians.

Called:

   eraFw2m      F-W angles to r-matrix
   eraBpn2xy    extract CIP X,Y coordinates from NPB matrix

Reference:

Hilton, J. et al., 2006, Celest.Mech.Dyn.Astron. 94, 351

This revision: 2021 May 11

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.