Algorithms
The algorithms used by NOVAS are based on vector astrometry theories and the IAU resolutions. Instead of using trigonometric formulae from spherical astrometry, NOVAS uses the matrix and vector formulation which is more rigorous. This version implements the resolutions on astronomical reference systems and Earth rotation models passed at the IAU General Assemblies in 1997, 2000, and 2006. According to the Astronomical Applications Department, the algorithms used in NOVAS are identical to those used in the production of the US part of the Astronomical Almanac.
A detailed description of the algorithms can be found here: Kaplan, et al. (1989) Astron. J. 97, 1197.
Read more about this topic: Naval Observatory Vector Astrometry Subroutines