Skip to main content

Full text of "DTIC ADA554685: Merging Maser and Cesium Clocks in Timescales"

See other formats


Report Documentation Page 


Form Approved 
OMB No. 0704-0188 


Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and 
maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, 
including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington 
VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it 
does not display a currently valid OMB control number. 


1. REPORT DATE 

OCT 2011 


2. REPORT TYPE 


3. DATES COVERED 

00-00-2011 to 00-00-2011 


4. TITLE AND SUBTITLE 

Merging Maser And Cesium Clocks In Timescales 


6. AUTHOR(S) 


7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Naval Observatory,3450 Massachusetts Avenue, 
N.W.,Washington,DC,20392 


5a. CONTRACT NUMBER 


5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

5d. PROJECT NUMBER 


5e. TASK NUMBER 


5f. WORK UNIT NUMBER 

8. PERFORMING ORGANIZATION 
REPORT NUMBER 


9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 


10. SPONSOR/MONITOR'S ACRONYM(S) 


11. SPONSOR/MONITOR'S REPORT 
NUMBER(S) 


12. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution unlimited 

13. SUPPLEMENTARY NOTES 

General Assembly and Scientific Symposium, 2011 XXXth URSI 

14. ABSTRACT 


15. SUBJECT TERMS 


16. SECURITY CLASSIFICATION OF: 

17. LIMITATION OF 

ABSTRACT 

18. NUMBER 

OF PAGES 

19a. NAME OF 

RESPONSIBLE PERSON 

a. REPORT 

unclassified 

b. ABSTRACT 

unclassified 

c. THIS PAGE 

unclassified 

Same as 
Report (SAR) 

3 


Standard Form 298 (Rev. 8-98) 

Prescribed by ANSI Std Z39-18 





Merging Maser and Cesium Clocks In Timescales 

Demetrios Matsakis (U.S. Naval Observatory) and Gianna Panfilo (BIPM) 

A variety of ways exist to combine clocks with different characteristics in a timescale. At the US Naval 
Observatory, for example, a process called dynamic weighting is used [1], which generates a timescale 
from a frequency scale that weighs recent maser data relatively higher than older maser data. A 
Kalman Filter could assign different phase, frequency, and frequency drift process noises to the different 
clock types [2], In this paper we use an ensemble of cesium and maser clocks to construct a timescale 
that is based upon an average of masers that are detrended against a cesium-only frequency scale, and 
also one which is based upon an average of masers being steered to the cesium timescale. 

The cesium timescale will be modeled with the BIPM's timescale algos [3], with a Kalman filter [2], and 
with an ARIMA model [4]. The masers will be detrended with a Kalman filter. For comparisons, the 
three algorithms used for the cesium timescale will be applied to the combined maser plus cesium 
ensemble with no distinction made between the types of clocks. 

EAL is generated as a free-running average of secondary standards. All clocks are detrended with a 
predictive model which assumes each clock's behavior with respect to EAL over any given month is a 
linear extrapolation of its behavior during the previous month. EAL is generated from a weighted 
average of the difference between the predictions and observations. Weights are according to their 
inverse variance of their mean-removed frequency with regards to EAL over the past year; however no 
clock is allowed to exceed a maximum weight. TAI is generated by frequency steers of EAL to primary 
frequency standards; historically these are all in the same direction and of approximately equal 
magnitude. One of the strong points of this algorithm is that it is robust in the presence of time 
transfer noise, and through its ultimate steering to the primary standards it meets the key requirement 
of realizing the SI second. However, the algorithm would require adjustment because it is noted that 
the linear predictor is not optimal for masers, and has been shown that a parabolic predictor is better 
for them [5], 

In combining clocks from remote laboratories, a key issue is the time transfer noise. Most of the clocks 
contributing to TAI do so via a Two Way Satellite Time Transfer (TWSTT) link, however an increasing 
number are also being used with a technique that is far more precise on subdaily scales - Precise Point 
Positioning (PPP)[6j. This currently uses GPS Carrier Phase to achieve precisions of 20 ps at 5-minutes 
and 100 ps at a day; with the advent of improved receivers, other GNSS systems, new signals, and new 
frequencies it will improve and be more robust. 

A final consideration would be the complexity and robustness of the algorithm. This can be and will be 
studied in simulations, although the authors fully recognize that operations in the field often reveal 
failures modes not anticipated in the simulations. 


978-1-4244-6051-9/11/$26.00 ©2011 IEEE 



[1] L. A. Breakiron, 1992, "Timescale Algorithms Combining Cesium Clocks and Hydrogen Masers," in 
Proceedings of the 23 rd Annual Precise Time and Time Interval (PTTI) Applications and Planning 
Meeting, 3-5 December 1991, Pasadena, California, USA (NASA CP-3159), pp. 297-305. 

[2] C. Zucca and P. Tavella, 2005, "The clock model and its relationship with the Allan and related 
variances," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, UFFC-52, 289- 
296. 

[3] G. Petit, "Towards an Optimal Weighting Scheme for Tai Computation", Metrologia, 2003, 40, N03 
S252-S256 

[4] Vandaele, "Applied Time Series and Box-Jenkins Models", 1983, Academic Press, Inc. 

[5] G. Panfilo and E. Arias, 2010,"Studies and Possible Improvements on the EAL Algorithm," in IEEE 
Trans. UFFC, 57, No. 1, January 2010, pp 154-160. 

[6] Guyennon, Cerretto, Tavella, and Lahaye, 2009, "Further Characterization of the Time Transfer 
Capabilities of Precise Point Positioning (PPP)", IEEE Trans. U.F.F.C, 56, No. 8, p 1634.