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# bp00

erfa.bp00(date1, date2)

  Frame bias and precession, IAU 2000.

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

  Notes

  Wraps ERFA function `eraBp00`. The ERFA documentation is:

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

      Frame bias and precession, IAU 2000.

      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 J2000.0 mean equator and
         equinox to mean equator and equinox of date by applying
         precession.

      4) The matrix rbp transforms vectors from GCRS to mean equator and
         equinox 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:
         eraBi00      frame bias components, IAU 2000
         eraPr00      IAU 2000 precession adjustments
         eraIr        initialize r-matrix to identity
         eraRx        rotate around X-axis
         eraRy        rotate around Y-axis
         eraRz        rotate around Z-axis
         eraCr        copy r-matrix
         eraRxr       product of two r-matrices

      Reference:
         "Expressions for the Celestial Intermediate Pole and Celestial
         Ephemeris Origin consistent with the IAU 2000A precession-
         nutation model", Astron.Astrophys. 400, 1145-1154 (2003)

         n.b. The celestial ephemeris origin (CEO) was renamed "celestial
              intermediate origin" (CIO) by IAU 2006 Resolution 2.

      This revision:  2021 May 11

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.