... PUL VLBI Analysis Center . ... Pulkovo EOP and Reference Systems Analysis Center (PERSAC) is located at the Pulkovo Observatory . ... Processing of VLBI , GPS, and optical observations aimed at analysis of the Earth rotation, station displacement, and celestial reference system. ... Celestial Pole Offset (CPO) and Free Core Nutation (FCN) . Four daily updated CPO and FCN series (models) are provided. ... File fcn1.dat contains the ZM1 FCN model with variable amplitude and phase. ...