s00(jdtt, x, y)
[SOFA] Returns the CIO locator in radians for the specified TT Julian
jdtt given the CIP’s
y coordinates, using IAU 2000A precession-nutation.
The CIO locator is the difference between the right ascensions of the same point in two systems. The two systems are the GCRS and the CIP, CIO, and the point is the ascending node of the CIP equator. The CIO locator remains below 0.1 arcsec throughout the years 1900 - 2100.
The CIP coordinates must be appropriate for the given date.
See also: Astronomical Coordinate Calculations