The measurement of time is central to advanced engineering and communication systems. The precision of time dissemination is now at the nanosecond level and is expected to reach a picosecond within a decade. The history of clocks will be summarized, various time scales will be described, and the origin of the leap second will be discussed. The effects of the special and general theories of relativity on time transfer algorithms are significant in modern timekeeping systems and their magnitudes will be illustrated. The Global Positioning System is a significant universal source of time. Its principles of measurement will be given and its current status and future growth will be discussed.
Robert A Nelson is president of Satellite Engineering Research Corporation, an engineering consulting firm in Bethesda, Maryland. Mr. Nelson holds the degree of Ph.D. in physics from the University of Maryland and is a licensed Professional Engineer. He is also a lecturer in the Department of Aerospace Engineering at the University of Maryland. Mr. Nelson is co-author of the textbook Satellite Communication Systems Engineering (Prentice-Hall, 1993) and is Technical Editor of Via Satellite magazine. His publications include a variety of technical papers and reports on the application of relativity to high precision time transfer systems.