Updates for SOFA Release 15 : 2019 July 22
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Summary of Changes
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The changes fall into the following categories:

1. Wrong sign in the ANSI C function iauTdbtcb corrected.

2. Update DAT release year.

3. Implementation of four new routines that have been added to the
   Star Catalog Conversions section dealing with the transformation
   between the FK4 and the FK5 reference systems.  These routines
   have have been included partly for completeness but mainly so that
   positions in publications pre-1984 can be properly handled.  They
   cover conversions between B1950.0 FK4 and J2000.0 FK5, with and
   without proper motion.

4. Enhancement of 17 routines that compare the two components of
   given date/time arguments to minimize rounding errors, so that
   optimum results are achieved even when one of the arguments is
   negative.  (SOFA is grateful to the Astropy group for drawing
   attention to the deficiency.)

5. Due to introducing these new routines, the Astrometry Tools
   Cookbook, the test program and other supporting files have also
   been updated.

6. Miscellaneous typographical corrections and improvements to
   various other documents.

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FORTRAN 77 Library
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1.  iau_DAT      Year ....

2.  iau_FK425    Convert B1950.0 FK4 star catalog data to J2000.0 FK5.
    iau_FK524    Convert J2000.0 FK5 star catalog data to B1950.0 FK4.
    iau_FK45Z    Convert a B1950.0 FK4 star position to J2000.0 FK5,
                 assuming zero proper motion in the FK5 system.
    iau_FK54Z    Convert a J2000.0 FK5 star position to B1950.0 FK4,
                 assuming zero proper motion in FK5 and zero parallax.

3. iau_JD2CAL    In all these routines the Fortran function ABS was
   iau_JD2CALF   included when comparing the two argument data/time
   iau_TAITT     parameter.
   iau_TAIUT1
   iau_TAIUTC
   iau_TCBTDB
   iau_TCGTT
   iau_TDBTCB
   iau_TDBTT
   iau_TTTAI
   iau_TTTCG
   iau_TTTDB
   IAU_TTUT1
   iau_UT1TAI
   iau_UT1TT
   iau_UT1UTC
   iau_UTCTAI

4. t_sofa_f.for     Addition of new routines.

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ANSI C Library
--------------

1.  iauTdbtcb.c Replace
                    *tcb1 = f - ( d - ( f - t77tf ) ) * elbb;
                by
                    *tcb1 = f + ( d - ( f - t77tf ) ) * elbb;

                n.b. the sign error affects only one of two paths
                through the code.

2. iauDat       Release year.

3. iauFk425     Convert B1950.0 FK4 star catalog data to J2000.0 FK5.
   iauFk524     Convert J2000.0 FK5 star catalog data to B1950.0 FK4.
   iauFk45z     Convert a B1950.0 FK4 star position to J2000.0 FK5,
                assuming zero proper motion in the FK5 system.
   iauFk54z     Convert a J2000.0 FK5 star position to B1950.0 FK4,
                assuming zero proper motion in FK5 and zero parallax.
   sofa.h       Inclusion of the above routines' prototype
                declarations.

4. iauJd2cal    In these 17 routines the ANSI C function fabs() was
   iauJd2calf   included when comparing the two argument data/time
   iauJd2calf   parameter.
   iauTaitt
   iauTaiut1
   iauTaiutc
   iauTcbtdb
   iauTcgtt
   iauTdbtcb
   iauTdbtt
   iauTttai
   iauTttcg
   iauTttdb
   IAUTtut1
   iauUt1tai
   iauUt1tt
   iauUt1utc
   iauUtctai

5. t_sofa_c.c   Addition of new routines.

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+ SOFA thanks all those who have reported the various issues that go
+ to ensuring the libraries and documentation are kept up-to-date and
+ relevant.
+
+ End of updates
+ 2019 June 30
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