The Science of Timekeeping

H The Science of Timekeeping Application Note 1289 • UTC: Official World Time • GPS: A Time Distribution Utility • Time: An Historical and Future Perspective • From Laboratory to Practical Use • Broad Applications Across Society 1 2 Table of Contents Introduction to the Science of Timekeeping .......................................... 8 Precise Timing Applications Pervade Our Society ................................ 10 Clocks and Timekeeping............................................................................ 14 The Definition of the Second and its General Importance .................. 19 Historical Perspective ................................................................................ 19 An Illustrative Timekeeping Example ..................................................... 22 UTC, Official Time for the World ............................................................. 26 GPS Time and UTC ..................................................................................... 30 Accuracy and Stability of UTC.................................................................. 31 Einstein’s Relativity and Precise Timekeeping ...................................... 36 How to Access UTC .................................................................................... 39 Future Timing Techniques......................................................................... 44 Global Navigation Satellite System Developments ................................ 47 UTC and the Future .................................................................................... 52 Conclusions ................................................................................................. 53 Acknowledgments ...................................................................................... 54 Appendix A: Time and Frequency Measures Accuracy, Error, Precision, Predictability, Stability, and Uncertainty ............................. 56 Appendix B: Stability Analysis of Harrison-like Chronometers ........... 66 Appendix C: Time and Frequency Transfer, Distribution and Dissemination Systems....................................................................... 72 Glossary and Definitions............................................................................ 82 Bibliography ................................................................................................ 85 3 Hewlett-Packard is grateful to the three authors of this application note for sharing their expertise. Their combined knowledge offers a resource which will undoubtedly be considered a classic reference on the science of timekeeping for decades to come. David W. Allan David W. Allan was born in Mapleton, Utah on September 25, 1936. He received the B.S. and M.S. degrees in physics from Brigham Young Univer- sity, Provo, Utah and from the University of Colorado, respectively. From 1960 until 1992 he worked at the U.S. National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS). His work with time and frequency research, development and generation has been with colleagues throughout the U.S. as well as in many other coun- tries. In cooperation with several colleagues, his principal contributions have been: 1) development of internationally adopted methods of charac- terizing the performance of clocks, oscillators and time and frequency distribution systems — known as the Allan variance, the Modified Allan variance and the Time variance; 2) development of a time-scale algorithm technique which combines clock readings for optimum and robust performance of the output — being better than the best clock in the ensemble and which has been used for more than 28 years to generate official time from NBS/NIST; 3) development of the dual-mixer time difference tech- David W. Allan nique, which allows clock measurements at the sub-picosecond level; 4) development of the GPS common-view time transfer technique, which is used to transfer clock times from around the world to the International Bureau of Weights and Measures for the generation of International Atomic Time and UTC, and for the comparison of the frequencies of the best primary standards in the world, and which provides a major benefit to the timing for NASA Jet Propulsion Laboratory’s (JPL’s) Deep Space tracking Network; 5) development of improved timing in support of the measurements of millisecond pulsars and the discovery, in this regard, of a random- walk effect of the total electron content in the galactic interstellar medium; 6) the development of a method of separating out the various error compo- nents causing inaccuracies in the Global Positioning System (GPS) performance; 7) the measurement of the relativistic Sagnac effect for the rotating Earth using GPS to transport time around the world; and 8) the invention of techniques (algorithms and measurement procedures) which allow a clock or set of clocks to always be correct; a patent associated therewith was later licensed by Hewlett-Packard and formed the basis of the HP SmartClock technology. Since retiring from NIST, Dave has developed a method of defeating the degradation (Selective Availability) on the GPS for timing purposes. Most recently, he led an R&D effort which has the potential of real-time satellite orbit determination at the centimeter level. He has published well over a hundred papers, has contributed chapters in several books, chaired several international timing committees, organized several tutorials and seminars, and participated on several other committees — including being the U.S. representative from NBS/NIST for two decades on the international Consultative Committee for the Definition of the Second. His greatest satisfaction is that his work is used and referenced 4 often — hopefully for the benefit of a better world. Neil Ashby Neil Ashby was born in Dalhart, Texas on March 5, 1934. He received the B.A. degree (Summa Cum Laude) in physics from the University of Colorado, Boulder, in 1955, and the M.S. and Ph.D. degrees from Harvard University, Cambridge, Massachusetts in 1956 and 1961, respectively. After spending a year in Europe as a postdoctoral fellow, he joined the faculty of the Department of Physics at the University of Colorado in 1962. He has been a Professor of Physics there since 1970, and was Department Chair from 1984–1988. He consults for the Time and Frequency Division of the National Institute of Standards and Technology, working on relativistic effects on clocks and global time synchronization. His work was the basis of general relativistic correc- tion being properly included in the Global Positioning System. He has been a member of the International Committee on General Relativity and Gravitation from 1989–1995. He serves on several international working groups on relativistic effects in geodesy and in metrology. Neil Ashby Clifford C. Hodge Cliff Hodge received the B.Sc. degree with First Class Honours in Physics from the Imperial College of Science, Technology and Medi- cine in South Kensington, London, in 1985. He continued his studies at Imperial College by joining the newly-formed Semiconductor Physics Group. In 1990, he was awarded the Ph.D. degree for his work on “Induced absorption of novel semiconductor superlattices” for optical and magneto-optical studies on a range of narrow- (indium arsenide- based) and wide- (gallium arsenide-based) band gap materials. In 1990, Cliff worked at the NASA Jet Propulsion Laboratory in Pasadena, California, as a Resident Research Associate in the Atmospheric and Oceanographic Sciences Division developing high-frequency modula- tion techniques using lead-salt semiconductor lasers for the observa- tion in real-time of a range of gaseous species in the stratosphere, as part of the Airborne Arctic Stratospheric Expedition (AASE II) Programme. In 1992, he joined the National Physical Laboratory (NPL) in Teddington, Middlesex, as Project Leader developing a cryogenic all- sapphire Super Mirror resonator for the intercomparison of the Allan Cliff Hodge variance of frequency standards in the microwave and optical region of the electromagnetic spectrum. Dr. Hodge has recently advised the European Space Research and Technology Centre (ESTEC) on “stable clocks and time-transfer techniques for GNSS2” as part of the Thomson-CSF-led Consortium. In March 1996, he organized an ESTEC- sponsored GNSS2 Workshop at NPL which brought together the Time and Frequency specialists with the GNSS experts to a one-day Work- shop entitled: “Next-Generation GNSS and The Frontiers of Time and Frequency Measurement.” Cliff remains actively involved in the exciting developments throughout Europe, the United States, the Russian Federation and Asia, towards a seamless international Global Navigation Satellite System (GNSS). Cliff’s interest in promoting the new science and novel applications that could be reaped from the latest improvements in accurate clocks and two-way time-transfer techniques has continued with the recent submission of a collaborative proposal to the ESA for an Atomic Clock Ensemble in Space (ACES) on board the International Space Stations ALPHA (ISSA). 5 2000 -1500 BC 3500 BC Mayan calendar Egyptian obelisks and sundials 400 BC -1600 AD Aztec calendar 1900 -1600 BC Stonehenge 1583 Galileo discovers pendulum period constancy 1094 Sung Su’s Chinese water clock perfected 1656 Huygens pendulum clock 1918 Quartz crystal oscillator developed 1955 Essen amd Parry start keeping time with cesium atomic clock 1964 HP’s First Cesium Clock (HP 5060A)

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