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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).