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SIDTIM returns value of sidereal time, either mean or apparent. Internally can work by either of two
methods, set by previous call to SETMOD, EQINOX, or CEOTEO:
Equinox-based
method: Evaluates expression for sidereal time given in IERS Conventions
(2003), Chapter 5, eq. (35). For apparent sidereal time, last three terms are considered part of
equation of the equinoxes, obtained from ETILT. The Earth rotation angle
is obtained from
EROT.
CEO-based
method: Obtains sidereal time from eq. (6) given in Kaplan (2003), based on the
position of the equinox in the celestial intermediate system. The orthonormal basis of the celestial
intermediate system is obtained from CEOBAS and the Earth rotation angle
is obtained from
EROT. Mean sidereal time, when requested, is obtained by subtracting the equation of the
equinoxes, obtained from ETILT.
In either method, SIDTIM/EROT evaluates
using the input UT1 epoch, but other components of
sidereal time are evaluated using TDB (set equal to TT), with TT=UT1+T. Default value is
T=64 sec, applicable at or near 2000; for highest precision applications, T value can be set via
prior call to SETDT.
TERCEL performs the terrestrial-to-celestial transformation on a given vector, i.e., the total rotation
from the ITRS to the ICRS. Internally can work by either of two methods, set by previous call to
SETMOD, EQINOX, or CEOTEO:
Equinox-based
method: Evaluates the old-style transformation as per previous subroutine PNSW,
but with a call to FRAME added at the end to put final vector in ICRS. Uses apparent sidereal
time, obtained from SIDTIM.
CEO-based
method: Performs the transformation of eq. (4) given in Kaplan (2003), based on the
celestial intermediate system. The orthonormal basis of the celestial intermediate system is
obtained from CEOBAS and the Earth rotation angle
is obtained from EROT.
In either method, the "fast angle" (rotation about z axis) is evaluated using the input UT1 epoch,
but other components of the transformation are evaluated using TDB (set equal to TT), with
TT=UT1+T. Default value is T=64 sec, applicable at or near 2000; for highest precision
applications, T value can be set via prior call to SETDT.
ETILT now evaluates a more complete series for the complementary terms in the equation of the
equinoxes (formerly just the two largest terms). Internally works in either high- or low-accuracy
mode, set by previous call to SETMOD, HIACC, or LOACC:
High-accuracy
mode: Obtains the sum of the terms from IERS function EECT2000.
Low-accuracy
mode: Obtains the sum of the terms from a 9-term internal series.
ETILT uses the expression for the mean obliquity from Lieske et al. (1977), with a rate adjustment
as per IERS Conventions (2003), Chapter 5, eqs. (31)-(32).
PRECES now evaluates precession-angle polynomials from IERS Conventions (2003), Chapter 5,
eq. (33) (with extra significant digits added to coefficient values). Some code changes made to
ensure reversibility of transformation (to/from J2000.0).
NOD now just calls either of the nutation subroutines, NU2000A (from the IERS) or NU2000K (a
reduced-accuracy version of NU2000A), to do the hard work; does not contain nutation series
itself. Which of the two nutation subroutines is called depends on whether high-accuracy or low-
accuracy mode has been chosen for Earth rotation calculations (see new subroutines SETMOD,
LOACC, HIACC).
