occasionally either be discarded or replicated in order to maintain overall synchronization.

The appendix consists of nine sections, including this one. The following two sections describe the means for generating and distributing the national standards of time and frequency in the United States. In particular, Section 2 discusses general aspects of standard time and frequency scales used for navigation and space science, while Section 3 describes the primary services operated by the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), for the dissemination of standard time and frequency.

The next three sections describe how synchronizing information is distributed throughout the United States and utilized as timing sources by various switches and transmission systems in the U.S. telephone networks. In particular, in Section 4 several time and frequency distribution systems are presented, with special emphasis on LORAN-C, which is fast becoming the system of choice used by U.S. exchange and interexchange carriers. Section 5 discusses issues important for the understanding of synchronization errors and how they may affect the operation of the various switches and other components of the telephone network. Section 6 describes the synchronization networks operated by the various exchange and interexchange carriers in the United States, including AT&T, the Bell Operating Companies (BOCs), and selected independents.

The final three sections contain the committee assessment of the impact of synchronization impairments on the National-Level Program/National Security Emergency Preparedness (NLP/NSEP) programs. In particular, Section 7 analyzes the effects of these impairments on transmission, switching, and user applications, while Section 8 discusses the impact on the NLP/NSEP programs. Section 9 states the committee’s conclusion and recommendation.

2. DETERMINING STANDARD TIME AND FREQUENCY2

For many years the most important use of time information was for worldwide navigation and space science, which depend on astronomical observations of the moon and stars. Ephemeris time is based on the revolution of the earth about the sun with respect to the vernal equinox on the celestial sphere. In 1956 the tropical year, or one complete revolution, was standardized at its value at the beginning of this century, when it had a period of 31,556,925.9747 seconds, or



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