USNO MASTER CLOCK DESIGN ENHANCEMENTS

Transcription

1 USNO MASTER CLOCK DESIGN ENHANCEMENTS P. Koppang, J. Skinner, and D. Johns U.S. Naval Observaory Absrac We have implemened several enhancemens o he US Naval Observaory (USNO) Maser Clock sysem. Design changes o he sysem include he use of a Kalman filer for phase and frequency esimaes, decreasing he ime inerval beween seers, and he redesign of conrol parameers. The presen conrol sysem uilizes realime daa esimaes of he differences beween he Maser Clock and a imescale ha combines hydrogen masers and commercial cesium frequency sandards wih a imevarying weighing scheme. We are researching a Maser Clock sysem design ha uilizes as is reference a hydrogen maser ensemble ha is seered o an ensemble of cesium sandards. We presen sysem designs, simulaions, and performance daa. INTRODUCTION We are implemening and invesigaing echniques o increase he robusness of he U.S. Naval Observaory (USNO) Maser Clock (MC) sysem while mainaining and/or improving overall performance. There are roughly 6 cesium sandards and hydrogen masers conribuing o he operaional imescale a any given ime. These sandards are spread hroughou several locaions and housed in 5 separae environmenal chambers a USNO in Washingon, DC. Hydrogen masers are approximaely an order of magniude more sable han highperformance commercial cesium sandards in he shor erm, while he cesium sandards show beer longerm characerisics. Sraegies on how o combine hese frequency sandards o bes benefi from heir respecive srenghs are shown along wih he evoluion of he MC sysem design. We also discuss he minimal conrol energy echnique and is implemenaion in seering he MC o UTC as derived by BIPM. MASTER CLOCK SYSTEM The USNO MC sysem consiss of aomic frequency sandards, signal measuremen and disribuion componens, frequency synhesizers, imescales, and conrol algorihms. The overall concep of he MC sysem is o creae a physical realizaion of a robus and sable imescale ha represens our bes realime esimae of UTC. PREVIOUS SYSTEM DESIGN The previous MC sysem used dynamic weighing o combine he cesium and hydrogen maser frequency sandards ino a imescale (also referred o as a mean or ensemble) [,]. This imescale weighed masers highly in he recen pas wih he cesiums receiving a higher weigh furher ino he pas. This creaed a 85

2 Repor Documenaion Page Form Approved OMB No Public reporing burden for he collecion of informaion is esimaed o average hour per response, including he ime for reviewing insrucions, searching exising daa sources, gahering and mainaining he daa needed, and compleing and reviewing he collecion of informaion. Send commens regarding his burden esimae or any oher aspec of his collecion of informaion, including suggesions for reducing his burden, o Washingon Headquarers Services, Direcorae for Informaion Operaions and Repors, 5 Jefferson Davis Highway, Suie 4, Arlingon VA 43. Respondens should be aware ha nowihsanding any oher provision of law, no person shall be subjec o a penaly for failing o comply wih a collecion of informaion if i does no display a currenly valid OMB conrol number.. REPORT DATE JAN 7. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE USNO Maser Clock Design Enhancemens 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Naval Observaory 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES). SPONSOR/MONITOR S ACRONYM(S). DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, disribuion unlimied 3. SUPPLEMENTARY NOTES See also ADM9., The original documen conains color images. 4. ABSTRACT 5. SUBJECT TERMS. SPONSOR/MONITOR S REPORT NUMBER(S) 6. SECURITY CLASSIFICATION OF: 7. LIMITATION OF ABSTRACT UU a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified 8. NUMBER OF PAGES 8 9a. NAME OF RESPONSIBLE PERSON Sandard Form 98 (Rev. 898) Prescribed by ANSI Sd Z398

3 noncausal imescale ha was sable in he shor erm due o he higher weighed hydrogen masers and gained longerm sabiliy from he cesiums. Clock models were used o mahemaically remove bes esimaes of he characerisic frequency and drif (for masers) from he freerunning clock daa. In order o creae a physical oupu represening he paper clock imescale, a hydrogen maser was seered o he imescale via a lownoise, highresoluion frequency synhesizer. A block diagram of he sysem is shown in Figure. Seers were made daily based on phase and frequency esimaes of he dynamic imescale a he ime of he seer. The frequency esimaes were paricularly sensiive o discrepancies beween he cesium and hydrogen maser means. Masers UTC(USNO) Σ Mean Maser Clock(MC) Cesiums Daily Seer MeanMC Calculaion Phase Frequency Esimaes Figure. Previous design block diagram. PRESENT OPERATIONAL SYSTEM DESIGN The presen operaional sysem uilizes he dynamic mean as described in he previous secion, bu uses a Kalman filer o esimae he phase and frequency offses beween he MC and he mean [3,4]. This gives an opimal realime esimae ha approximaes an all hydrogen maserbased mean and solves he frequency bias issue of he previous design. Wih he goal of increasing robusness, he seering rae was increased from daily o hourly (see Figure ). The lowernoise esimaes and increased daa rae allow for a igher conrol design ha can beer reac o perurbaions ha can occur in he reference, or seered, frequency sandard. These perurbaions may be caused by inernal physical changes in he reference sandard or exernal facors such as environmenal disurbances. Masers UTC(USNO) Σ Mean Maser Clock(MC) Cesiums MeanMC Kalman Filer Phase Frequency Esimaes Hourly Seer Calculaion Figure. Presen design block diagram. 86

4 The saespace model for a frequency sandard seered by discree frequency seps is given as: X + + τ = ΦX Bu, () x phase offse τ x τ where X = =, Φ =,, and. y frequency offse u = GX = [ g x g y ] B = y Given he gain funcion G, he saespace equaion can be wrien as: x y + τ g xτ ( g y ) τ x = g x g y y. () Figure 4 compares he conrols of he daily seers of he previous sysem o he hourly seers of he presen sysem. As expeced, he hourly seers creaed a smooher response in he shor erm. The experimenal seup used a common hydrogen maser wih wo separae frequency synhesizers and is shown in Figure 3. Hourly seer Daily seer Figure 3. Common maser reference signal. Accumulaed Frequency Seers (xe3) Hourly Daily Days Figure 4. Comparison of accumulaed frequencies from hourly and daily seering. Acual performance of he presen seup versus he realime reference and a cesium mean are shown in Figure 5. The small (~< ns) phase seps characerisic of he posprocessed model changes can be seen in he daa. The MC seers are given in Figure 6 and are dominaed by values wihin a band of ± 6. 87

5 Time Difference (ns) 3 Maser Clock vs. Mean reference Maser Clock vs. Cesium Mean (bias removed) Frequency Seers (xe6) MJD MJD Figure 5. Time difference daa. Figure 6. Maser Clock seers. FUTURE SYSTEM DESIGN We are esing a design wih separae means from he respecive collecions of hydrogen masers and cesiums [5]. The maser mean (MM) is seered o he cesium mean (CM) wih a ime consan of several weeks (see Figure 7). The seered MM preserves he shorerm sabiliy of he maser mean and gains he longerm performance of he CM. Like he previously described sysems, he physical oupu is creaed by seering a hydrogen maserbased signal. In his design, models are used o remove iniial characerisic raes and drifs from a maser when i is firs inroduced ino he MM. The conrol design of he MM o CM is hen expeced o remove any nominal clock divergence from he iniial model ha would normally creae a posprocessed recharacerizaion of he clock in he dynamic mean. Large changes in model or significan performance degradaion resul in he maser being removed from he mean. The smooher realime MM allows for a igher, more robus, conrol design for he physical oupu. The MM is creaed wih daa from a recenly upgraded measuremen sysem ha is approximaely an order of magniude quieer han he measuremen sysem used in he dynamic mean calculaions and previous experimens [5]. Filer Seer Calculaion UTC(USNO) CMMM Maser Mean(MM) Σ Maser Clock(MC) Cesium Mean (CM) MMMC Seer Calculaion Σ Filer Figure 7. Fuure sysem block diagram. The saespace model for he sysem described in Figure 7 is: 88

6 X = ΦX + Bu + τ xmm AOG 6 τ τ τ y = MM AOG 6 X, Φ =, and B = (3) xcm MM τ τ ycm MM where x is phase difference, y is frequency difference, he conrol u ( ) The above equaion can be rewrien as X = Φ BG X. + τ = GX, and G is he conrol gain. Figure 8 shows he performance of he frequency synhesizer AOG6 versus he MM reference and also he MM versus he CM (see Figure ). The small disurbance in he MMAOG6 daa near 5396 was caused by an issue wih he environmenal chamber ha housed he reference maser. The Allan deviaions of AOG6 and is reference maser are shown in Figure 9. The plo shows ha he conrol has effecively removed he drif of he maser. There were 7 maser recharacerizaions and weigh changes over his period in he dynamic imescale compared o wo maser deweighings in he MM over he same inerval. The new design is more robus and exhibis excellen performance compared o he MC. This sysem is presenly operaing as a backup MC and is planned o ransiion o he primary MC afer furher evaluaion. 3 Maser Mean Aog6 Cesium Mean Maser Mean 4 Maser ClockAog6 Maser ClockNAV Time Difference (ns) ADEV MJD Figure 8. Measured ime difference daa Averaging Time (Sec) Figure 9. Sabiliy comparison. Maser Mean AOG6 NAV Cesium Mean MMAOG6 Filer Seer Calculaion Figure. Sysem seup for Figures 8 and 9. 89

7 STEERING TO UTC One of he goals of he MC is o represen a highqualiy realime physical realizaion of UTC (he posprocessed inernaional paper imescale defined by he BIPM). A block diagram describing how he MC is seered o UTC, using he monh of December as an example, is shown in Figure. Clock daa are colleced during December USNO MC 45 day lag Our daa are sen o BIPM before Jan. 5 Daa are sen ou from BIPM on ~ Jan 5 h for UTCUTC(USNO) in Dec. USNO Mean Predic curren UTCUTC(USNO) Time and Frequency Calculae USNO Mean Conrol Sequence Figure. Example of conrol sequence calculaion for December daa. Daa from he BIPM are published monhly wih a ime lag ranging from approximaely 5 o 45 days. Presen ime and frequency offses are prediced from he given daa, and hen he sequence of frequency seers minimizing he conrol effor is deermined. The goal is o minimize he conrol effor, or socalled conrol energy, N k = u ( k ) (4) of he frequency seers u(k) used o seer he MC o UTC [6,7]. The soluion using he model given in () is τ 6 U = ( ) N(N + ) τ N M τ N 3 N + x() 3 y() M N (N ) + 3 or, (5) 6 k N u( k) = τ x() + k y() N(N + ), for k =... N. (6) N 3 9

8 Figure shows how he simulaed conrol sysem using he above equaion reacs o removing 3 ns of phase and 5 of frequency offses wih varying seering updae inervals. As expeced, he shorer inervals produced a smooher response. Time Offse (ns) Time Difference (ns) Frequency Seers (xe5) Hourly Seer Daily Seer 5 Day Seers 5 Day Seers Days Frequency Seers (xe5) MJD Figure. Response o removing 3 ns and 3 5 offses. (USNO). Figure 3. Top plo is UTC UTC (USNO); boom plo is he seers applied o UTC Figure 3 shows he ime difference beween UTC and UTC (USNO) and he associaed frequency seers. Prior o MJD 5396, seers were sofwarelimied o an updae rae of every 3 days; pas ha poin, daily seers have been implemened. The oulying poins on he plo are seers ha mach he TAI seers implemened by he BIPM. CONCLUSION Effors are underway o improve boh he robusness and performance of USNO Maser Clock sysem. Designs are decreasing he ime inervals beween updaes and ighening conrol parameers in boh he inernally derived sysems and hose referencing UTC. Performance ess verified he resuls ha were prediced by simulaions. Fuure work will include how o bes incorporae aomic founains ino an appropriae reference imescale. We will also look ino seering each maser independenly o he CM and improving he predicors uilized in calculaing he conrol sequences for seering o UTC. ACKNOWLEDGMENT The auhors graefully acknowledge he suppor provided by he Office of Naval Research (ONR) in he fuure sysem design work. 9

9 REFERENCES [] L. A. Breakiron, 99, Timescale algorihms combining cesium clocks and hydrogen masers, in Proceedings of he 3 rd Annual Precise Time and Time Inerval (PTTI) Applicaions and Planning Meeing, 35 December 99, Pasadena, California (NASA CP359), pp [] D. Masakis, M. Miranian, and P. Koppang, 999, Seering he U.S. Naval Observaory (USNO) Maser Clock, in Proceedings of he 999 Naional Technical Meeing of he Insiue of Navigaion, 57 January 999, San Diego, California, USA (ION, Alexandria, Virginia), pp [3] R. Brown and P. Hwang, 99, Inroducion o Random Signals and Applied Kalman Filering (second ediion, John Wiley & Sons, New York). [4] J. Skinner and P. Koppang,, Effecs of parameer esimaion and conrol limis on seered frequency sandards, in Proceedings of he 33rd Annual Precise Time and Time Inerval (PTTI) Sysems and Applicaions Meeing, 79 November, Long Beach, California, USA (U.S. Naval Observaory, Washingon, D.C.), pp [5] P. Koppang, D. Johns, and J. Skinner, 4, Applicaion of conrol heory in he formaion of a imescale, in Proceedings of he 35 h Annual Precise Time and Time Inerval (PTTI) Sysems and Applicaions Meeing, 4 December 3, San Diego, California, USA (U.S. Naval Observaory, Washingon, D.C.), pp [6] K. Ogaa, 995, DiscreeTime Conrol Sysems (second ediion, Prenice Hall, Englewood Cliffs, New Jersey). [7] P. Koppang and D. Masakis,, New seering sraegies for he USNO Maser Clock, in Proceedings of he 3 s Annual Precise Time and Time Inerval (PTTI) Sysems and Applicaions Meeing, 79 December 999, Dana Poin, California, USA (U.S. Naval Observaory, Washingon, D.C.), pp