A Revised Way of Fixing an Upper Limit to Clock Weights in TAI Computation

J. Azoubib

Abstract:	The free atomic time scale Echelle Atomique Libre (EAL), from which International Atomic Time (TAl) is derived by frequency steering, is obtained as a weighted average of a large number of free-running and independent atomic clocks spread worldwide, using the algorithm ALGOS which is optimized for long-term stability. Since January 1998, a new procedure for implementing an upper limit of clock weights has been used. The use of an absolute maximum weight PMAX, was replaced by the choice of a relative maximum weight, WMAX. This new technique is more robust than the former one and it optimizes the stability of the time scale at the expense of a more complicated computation. The chosen value WMAX = 7.00~10-~ corresponded to the value of the maximum relative weight assigned to clocks in the EAL computation, with p~u = 2500, in the 60-day interval November/December 1997. In this paper, we show that WMAX = 7.00~10-~ is no longer appropriate. No efficient discrimination is made between the HP 5071A units: more than 80% of such clocks reach the maximum relative weight. The value of omegaMAX really needs to be updated from time to time in order to obtain an efficient discrimination between the HP 5071A units and to improve the stability of EAL. To avoid frequent redefinition of omegaMAX, we suggest here making omegaMAX a function of the number, N, of clocks that participate in T'. A relation such as WMAX = A/N, where A is an empirical constant, could be used. Such a relation has been tested using ALGOS with values 3.0, 2.5, and 2.0 for A. The resulting computed time scales (over 2.5 years) using real data show that all the HP 5071A units are not equivalent. We also obtain an improved stability for the computed time scales, which is the underlying aim of this study.
Published in:	Proceedings of the 32th Annual Precise Time and Time Interval Systems and Applications Meeting November 28 - 30, 2000 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	195 - 210
Cite this article:	Azoubib, J., "A Revised Way of Fixing an Upper Limit to Clock Weights in TAI Computation," Proceedings of the 32th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, November 2000, pp. 195-210.
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