Testing Coordinate Frame Transformations NOVAS vs SOFA
NAVAL OBSERVATORY WASHINGTON DC
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The Standards of Fundamental Astronomy SOFA library is the official collection of approved software for positional astronomy, operating under the auspices of International Astronomical Union IAU Division 1 Fundamental Astronomy. Both Fortran and C libraries are available. An international SOFA Reviewing Board manages the collection. Generally, the Naval Observatory Vector Astrometry Software NOVAS is independent of SOFA although both software libraries include code that is similar to two International Earth Rotation and Reference System Service IERS Fortran modules. NU2000A and iau2000a Fortran and C, respectively, which evaluate the full 1,365-term IAU 2000A nutation series in NOVAS 3.0, are based on the IERS subroutine NU2000A. EECT2000 and eect, which evaluate the complementary terms in the equation of the equinoxes, are based on IERS function EECT2000. The corresponding modules in SOFA are iauNUT00A, iauNut00a, iauEECT00, and iauEect00. The document SOFA Tools for Earth Attitude, also known as the SOFA Cookbook, contains several Fortran examples of the transformation between terrestrial and celestial coordinate systems. This technical note examines how one of those examples plays out in both NOVAS and SOFA.