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By Means of Portable Cesium-Beam Clocks, Time Has Been Cor- Related to 1 Microsecond at Many of the World's Timekeep- Ing Centers

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By Means of Portable Cesium-Beam Clocks, Time Has Been Cor- Related to 1 Microsecond at Many of the World's Timekeep- Ing Centers By means of portable cesium-beam clocks, time has been cor- related to 1 microsecond at many of the world's timekeep- ing centers. A comparison of four of the world's best-known 'long-beam' frequency standards has also been made. Fig. 2 (at left). Dr. Yoshikazu Saburi (cen- ter) of Radio Research Labs, home of Stand- ards Broadcast Station JJY, Japan, discusses time and frequency measurement results with -hp- engineers Richard Baugh and Lee Bodily. Fig. 3. Operation of trau- eling cesium-beam clock @ checked by -hp- En- gineer Richard Baugh before start of ocean- spanning journey. Cesi- um-Beam Frequency Standard (middle instrument) has self-check- ing feature that enables adjustment of cesium-beam resonator for correct operation without reference to any other frequency standard. THEFEASIBILITY of transporting com- scales maintained by widely-separated microseconds of the first comp pact ‘atomic’ clocks in continuous timekeeping laboratories have been made eight months ago. This is e operation to distant points by com- correlated within a microsecond, an mercial airliner or other conventional accuracy far higher than that possible transport was established by a flying by high-frequency radio broadcasts of Another accomplishment of clock experiment performed in the time signals which, at intercontinental periment was the verification summer of 1964.l That experiment distances, have only been able to correlated the time of day within a achieve accuracies in time comparisons nization experiment cond microsecond between the Observatoire of about 1 millisecond at best. Indica- of the satellite Relay 11. de NeuchLtel in Switzerland and both tive of the accuracy achieved in the lat- the U. S. Naval Observatory and the est flying-clock experiment, one of the track Radio Tracking Station at the National Bureau of Standards in the portable clocks changed less than Goldstone tracking site in the Califor- United States. It also established 10 pec during the 29 days between nia Mojave Desert and the trackingsta- within 200 microseconds the propaga- two measurements made with respect tion operated for NASA at Kashima, tion time of radio timing signals from to the Master Clock at the Naval Ob- Japan, by the Japanese Radio Re- Beltsville, Md., to NeuchLtel, and it servatory, which establishes Universal search Laboratories. Preliminary re- provided an opportunity for intercom- (Earth rotational) time in the United sults from the U. s. Naval Observator paring laboratory-constructed ‘long- States. The other clock changed less indicate that the exp beam’ cesium frequency standards by than 1 pec in a 23-day period with re- also used VLF tran way of the traveling cesium-beam spect to the NBS UA Time Scale, a standards. time scale based on Atomic time but A new flying clock experiment has offset slightly to approximate Univer- now been performed with the travel- sal time. ing cesium-beam clocks, an experi- The experiment repeated the com- ment that involved more time-keeping parison made last summer between The experiment provided an oppor- centers than any clock correlation ex- time scales in the United States and tunity for frequency comparisons be- periment conducted so far. Standards that maintained by the Observatoire tween the traveling standards and four facilities in Japan, Hawaii, Canada, de NeuchLtel in Switzerland, but in so laboratory-constructed “long-beam” and most of the countries of western doing, it provided further information cesium standards, five commerci Europe were visited in the experiment. by disclosing any relative drift in the built cesium-beam standards, and In all, time and frequency were corre- time scales. As described later, the com- one hydrogen maser. The inte lated at 21 different faciIities in 11 parison shows excellent agreement be- different countries. tween the time scales of the Observa- As a result of the experiment, time toire de Newhiitel and the National - Bureau of Standards in Boulder, Colo- 1 Alan S. Bagley and Leonard S. Cutler, “A New Perform- ance of the Flying Clock Experiment,” Hewlett-Packard rado, the two scales being within 50 Annual Symposium an Frequency Control, 1 Journal, Val. 15, No. 11, July, 1964. Iished. 1 of cesium-beam standards - s that were independently de- and constructed at widelysep- ed points of the globe - provided her evidence that cesium-beam res- ary frequency stand- accuracy. For instance, frequency comparison cesium-beam standard in e traveling clocks and the er cesium-beam standard of the s Horological Research Labora- Laboratoire Suisse de Recherches res) in Neuch2te1, considered ne of the most accurate fre- y standards in the world, agreed 1011 with a compari- etween the same traveling stand- and one of the long-beam cesium Fig. 4. “Long-beam“ cesium resonator at National Research Council, Ottawa, Canada, is typical of high-precision, laboratory-constructed frequency standards dards (NBS 11) at the National intercompared during flying-clock experiment. Cesium-beam standards in both of Standards in Boulder, Colo- traveling clocks agreed with this standard to better than 1 part in 10“. o separate comparisons made apart between the traveling developed compact cesium-beam fre- time. The Model 115BR also generates d and NBS I1 agreed within 1 quency standards, because of their rel- a 25-pec pulse once per second for 1012. This performance attests atively small size and modest power re- measurements of the time correspond- stability and reliability of port- quirements, now make it possible to ence between it and another clock. cesium standards in a traveling transport atomically-controlled clocks The Frequency Standard and Clock nment, as well as confirming the by conventional carriers. The clocks were powered by special power sup- ental accuracy of cesium res- used in the experiment described here plies, designed for the clock experi- were each controlled by an -hp- Model ment, which contain rechargeable CESIUM-BEAM CLOCKS 5060A Cesium-Beam Frequency Stand- nickel-cadmium batteries capable of s driven by quartz crystal os- ard, a relatively small, lightweight (65 powering the instruments for several tors have been used in many trav- lbs.) instrument requiring minimum hours. The power supplies can also eling-clock time synchronization ex- power (40W at 26 V dc) but neverthe- power the instruments and recharge periments. Higher accuracy is obtained less capable of high accuracy (&2 x the batteries when operating from ac with clocks controlled by atomic res- 10-l1) and long term stability (&1 x sources ranging from 100 volts to 280 onators, however, because the micro- 10-11).* volts at any frequency from 50 to 400 wave spectral lines characteristic of The traveling clocks each used an cis, or from 6 or 12-V dc sources. The ertain atoms are fundamental physi- * These Standards use a compact cesium-beam -hp- Model 115BR frequency divider tube developed especially for them by the cal constant3 that can be reproduced Quantum Electronics Division (Bomac) of Varian to integrate frequency and display Associates, under contract to Hewlett-Packard. by independent means anywhere on earth without reference to another pri- mary standard. Most of the world’s frequency standards now use cesium- beam resonztors as an absolute refer- ence since a certain cesium spectral line at present provides the most prac- 1 means for controlling frequency by a fundamental atomic constant. 5 10 15 20 25 2 7 I2 I7 FEE (Clock No 4 Reset)> MAR The International Fundamental Unit TIME + 0 Clock No 3 tick minus Clock No 4 tick. of Time (the Atomic Second) is based Phase difference between 100 kcls outputs ot CtockNo. 3 and ClockNo 4 on this particular cesium transition Fig. 5. Time displacement between once- dividers. Discontinuity on Feb. 20 results per-second “ticks” produced by flying from restart of Clock No. 4 (see text). Until recently, however, atomic clocks as measured during trip. Phase dis- Slope of graph during March indicates clocks required the use of cargo planes placement between 100-kcls outputs of that clocks agreed within 2.4 parts in IO”. transportation because of their Cesium Beam Frequency Standards was During February, slope is equivalent to 4 also measured to provide correction factor parts in lo“, well within specified d power requirements. Newly- for any phase shifts in clock frequency accuracy of clocks. *3* multipled to higher greater measurement peated measurements o the beat note provided tween the two standards. INITIAL ADJUSTMENTS Feb. 2, 1965 at Palo Alto, Califor where the Cesium-Beam Standard were aligned independently of one other. The cesium-beam “C” field, Fig. 6. Time kept by traveling clocks is compared to Master Clock at U. S. Naval Observatory, which establishes time in United States, by -hp engineers Baugh and Bodily and by William Markowitz, Director of Time Service Division of U. S. Naval Observatory. Time of once-per-second ticks of traveling clocks, which had been set according to -hp- House Standard three days earlier, differed from time of Naval Observatory ticks by less than 80 psec. cedure that uses the cesi to check on its own perfo instruments may also operate, without ated by the two clocks being compared. Cesium-Beam Frequency recharging the batteries, from 24-V dc Electronic counters were used to meas- sources. Thus it was possible to power ure the time interval between ticks the clocks from the aircraft on which with a resolution of approximately 0.1 required, without reference to an they were flown, from storage batteries psec. other standard. or automobile electrical systems, from Frequency comparisons were made commercial power lines anywhere in in some cases by measuring the phase which derive the 9192-Mcls c the world, or, when necessary, from difference between waveforms pro- resonance frequency from the the internal batteries. duced by the two standards being com- controlled 5-Mc/s oscillator MEASUREMENT TECHNIQUES pared.
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