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# bp06

erfa.bp06(date1, date2)

  Frame bias and precession, IAU 2006.

  Parameters
    date1 double array
    date2 double array
  Returns
    rb double array
    rp double array
    rbp double array

  Notes

  Wraps ERFA function `eraBp06`. The ERFA documentation is:

      - - - - - - - -
       e r a B p 0 6
      - - - - - - - -

      Frame bias and precession, IAU 2006.

      Given:
         date1,date2  double         TT as a 2-part Julian Date (Note 1)

      Returned:
         rb           double[3][3]   frame bias matrix (Note 2)
         rp           double[3][3]   precession matrix (Note 3)
         rbp          double[3][3]   bias-precession matrix (Note 4)

      Notes:

      1) The TT date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TT)=2450123.7 could be expressed in any of these ways,
         among others:

                 date1         date2

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in
         cases where the loss of several decimal digits of resolution
         is acceptable.  The J2000 method is best matched to the way
         the argument is handled internally and will deliver the
         optimum resolution.  The MJD method and the date & time methods
         are both good compromises between resolution and convenience.

      2) The matrix rb transforms vectors from GCRS to mean J2000.0 by
         applying frame bias.

      3) The matrix rp transforms vectors from mean J2000.0 to mean of
         date by applying precession.

      4) The matrix rbp transforms vectors from GCRS to mean of date by
         applying frame bias then precession.  It is the product rp x rb.

      5) It is permissible to re-use the same array in the returned
         arguments.  The arrays are filled in the order given.

      Called:
         eraPfw06     bias-precession F-W angles, IAU 2006
         eraFw2m      F-W angles to r-matrix
         eraPmat06    PB matrix, IAU 2006
         eraTr        transpose r-matrix
         eraRxr       product of two r-matrices
         eraCr        copy r-matrix

      References:

         Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855

         Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.