Despite the improvement in the model of the motion of the Earth’s pole in space provided by the IAU 2000A precession-nutation, the IERS provides Celestial pole offsets that are time-dependent corrections to the model, determined by astronomical observations. They are given daily in the form of the differences, dX and dY, of the CIP coordinates in the GCRS with respect to the IAU 2000A precession-nutation model (i.e. the Celestial Intermediate Pole is realized by the IAU 2000A precession-nutation plus these celestial pole offsets). In parallel the IERS also provides the offsets, dψ and dε, in longitude and obliquity with respect to the IAU 1976/1980 precession/nutation model. Magnitudes are generally less than 1 mas and due largely to free core nutation (or nearly diurnal free wobble), caused by the rotation of the fluid core of the Earth with respect to the mantle. It appears as an unpredictable very small nutation component with a period of approximately 430 days.

The NGA 5-liner input file contains no information about celestial pole offsets. Therefore, this option should not be chosen with NGA 5-liner
inputs chosen. If both inputs are selected, an error flag will be generated, and no EO matrix results will appear -- only error notices will appear.