This Recommendation outlines the requirements for enhanced primary reference clocks (ePRCs) suitable for frequency synchronization. These requirements apply under normal environmental conditions specified for the equipment. The ePRC provides the reference signal for the timing or synchronization of other clocks within a network or section of a network. In particular@ the ePRC in its autonomous form can provide a frequency input to an enhanced primary reference time clock (ePRTC). This Recommendation defines the ePRC output@ but it does not apply to the output of a slave clock directly fed by an ePRC. The long-term accuracy of the ePRC should be maintained at 1 part in 1012 or better@ with verification to coordinated universal time (UTC). An ePRC may be realized as an autonomous clock@ operating independently of other sources. Alternatively@ the ePRC may be realized as a non-autonomous clock which is disciplined by UTC-derived precision signals received from a radio or satellite system. In either case@ the requirements for long-term accuracy and short-term stability@ as specified in this Recommendation@ apply. Several classes of ePRCs are being considered. This version specifies ePRC class A. Other classes are for further study. The long-term accuracy specified in this Recommendation is adequate when a single ePRC provides the reference synchronization signal to all other clocks within a network (synchronous mode of operation). In the pseudo-synchronous mode@ i.e.@ not all clocks in the network have timing traceable to the same ePRC@ the long-term accuracy is dependent on the number of ePRCs in that network. When more than one ePRC is used in a network@ a statistical approach is needed to determine the long-term accuracy of each ePRC in that network