27-10-2012, 02:20 PM
A New Realization of Terrestrial Time
ABSTRACT
Terrestrial Time TT is a time coordinate in a geocentric reference system. It is realized through International Atomic Time
TAI, which gets its stability from some 200 atomic clocks worldwide and its accuracy from a small number of primary
frequency standards (PFSs), whose frequency measurements are used to steer the TAI frequency. Because TAI is computed
in real time and has operational constraints, it does not provide an optimal realization of TT. The BIPM, therefore, computes
another realization TT (BIPM) in postprocessing, which is based on a weighted average of the evaluations of TAI frequency
by the PFSs. The procedures to process PFS data have been recently updated and we consequently propose an updated
computation of TT (BIPM). We use all recently available data from new Cs fountain PFS and a revised estimation of the
stability of the free-running atomic time scale EAL on which TAI is based. The performance of the new realization of TT is
discussed and is used to assess the accuracy of recent PFS measurements.