Frequency Reference Stability and Coherence Loss in Radio Astronomy Interferometers Application to the SKA

The requirements on the stability of the frequency reference in the square Kilometer array (SKA), as a radio astronomy interferometer, are given in terms of the maximum accepted degree of coherence loss caused by the instability of the frequency reference. In this paper, we analyze the relationship between the characterization of the instability of the frequency reference in the radio astronomy array and the coherence loss. The calculation of the coherence loss from the instability characterization given by the Allan deviation is reviewed. A model of a typical frequency distribution system is presented. The verification of the coherence and frequency reference stability requirements is discussed. Some practical aspects and limitations relevant to the SKA are analyzed.