Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/iau-comm31/pdf/c31_bus_manchester.pdf Äàòà èçìåíåíèÿ: Thu Oct 9 13:25:41 2008 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 05:45:08 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ion drive

(ed.), IA U T r an sact ion s, V ol. X X IV , xx{xx. c °2000 IA U .

COM M I SSI ON (TE M P S)

31: T I M

E

PRESIDENT: T. Fukushima VICE-PRESIDENT: G. Petit ORGANIZING COMMITTEE: D.C. Backer, G. Beutler, V.A. Brumberg, H.F. Fliegel, S.M. Leschiutta, J. Luck, P. Paquet, E. Provebio, G.R. Qi, C. Thomas, C. Veillet, G.M.R. Winkler, Z.C. Zhai

1.

App ointment of O±cials for 2000-2003

Drs. G. Petit and D. Matsakis have been elected as the President and Vice President of the Commission for the next term, 2000-2003, resp ectively. As for the Organizing Committee Members, we welcome D. Matsakis as a new memb er and appreciate the outgoing memb ers, Drs Brumberg, Fliegel, Luck, Paquet, and Thomas, for their contributions and e®orts. Also T. Fukushima was nominated as the IAU Representative to the CIPM/CCTF for the next triennium. 2. Rep ort of the BIPM Time Section (communicated by F. Arias)

Reference time scales International Atomic Time (TAI) and Universal Co ordinated Time (UTC) have been computed regularly and have been published in the monthly Circular T. De¯nitive results for 1997, 1998 and 1999 have b een available, in the form of computerreadable ¯les in the BIPM home-page and on printed volumes of the respective Annual Reports of the BIPM Time Section. The printed version of the last volume has been substantially shortened with the aim to progress to an electronic version in the next future. Work is done to automate the calculation of TAI and UTC, this allowing a shorter delay in the publication of Circular T. Research concerning time scale algorithms includes studies to improve the long-term stability of the free atomic time scale EAL and the accuracy of TAI. Studies are undertaken to evaluate the feasibility of providing a prediction of UTC in quasi-real time. Some 80 % of the clo cks are now either commercial caesium clocks of the type HP5071A or active, auto-tuned active hydrogen masers, and together they contribute 86 % of the total weight with consequent improvement in the stability of EAL. Since most HP5071A clo cks have at present the maximum relative weight, the weighting pro cedure of clo cks in TAI is under revision. The medium-term stability of EAL, expressed in terms of the Allan deviation, is estimated to be 0:6 ¸ 10¡15 for averaging times of 20 to 40 days over the perio d. Nine primary frequency standards reported their measures to the BIPM. The global treatment of these individual measurements led to a relative departure of the duration of the TAI scale unit from the SI second on the geoid ranging, in the last year, from +0:2 ¸ 10¡14 to +0:6 ¸ 10¡14 , with an uncertainty of 0:4 ¸ 10¡14 . Following the recommendations of the Consultative Committee on Time and Frequency, changes were implemented to render the data used in TAI, as well as the results, more accessible to the users and to make the pro cedures of calculation even more transparent and traceable. Since April 2000 two mo di¯cations were implemented: a new mo del to characterise the instability of the free atomic scale EAL, and a more complete representation of the uncertainty of the deviation of the TAI scale interval relative to that of the Terrestrial Time TT. In the last decade the time links computed at the BIPM used the classical GPS common-view technique based on C/A-co de measurements obtained from one-channel re1


2

COMMISSION 31

ceivers. The commercial availability of newly developed receivers has stimulated interest in extending the classical common-view technique for use of multichannel dual-code dualsystem (GPS and GLONASS) observations, with the aim of improving the accuracy of time transfer. The two-way time transfer via geostationary satellites (TWSTFT) proved to be p otentially better that GPS. Since July 1999 GPS multichannel links and TWSFTF links are being progressively intro duced in TAI. In addition, the BIPM Time section carries on testing other time and frequency comparison metho ds, such as phase measurements . Ionospheric parameters and precise ephemerides provided by the IGS (International GPS Service) are now routinely used to correct all links in regular TAI calculations. The BIPM/IAU Joint Committee on general relativity for space-time reference systems and metrology (JCR), created in 1997, continued its work. Two studies have b een conducted at the BIPM in collaboration with other memb ers of the JCR. One concerns the extension of the relativistic framework to allow a correct treatment for time transformations and the realisation of barycentric co ordinate time at the full post Newtonian level. The second study concerns the realisation of geo centric coordinate times. Following a Call for Participation of the IERS, the BIPM, jointly with the USNO, will provide its Conventions Pro duct Centre since January 2001. 3. Rep ort of USNO Time Serivice Dept. (communicated by D. Matsakis)

The U.S. Naval Observatory (USNO) Time Service Department has a real-time mission to keep the USNO Master Clo ck as stable and as close to UTC as p ossible, and to disseminate its time via GPS and other means, such as Two-Way Satellite Time Transfer (TWSTT) and Loran. To achieve the required stability and robustness, we maintain a large ensemble of clo cks - about 65 HP5071 cesiums and 15 cavity-tuned Datum masers, distributed in three buildings at two sites, kept in environmentally benign conditions, and intercompared using state-of-the-art measurement systems. Although most of our 28-person sta® is involved with maintaining the hardware, software, and data °ow needed for our op erations, we are also ¯nding ways to improve things. We are ab out to switch to a more stable and robust measurement system; have recently improved our clo ck steering algorithms; are testing steering algorithms that are b etter still; are constructing improved chamb ers to house our clo cks; are developing better GPS receivers; have improved timescale and clo ck-characterization algorithms; are building atomic fountains; and are purchasing trapped-ion mercury clo cks from Lute Maleki's group at Jet Propulsion Lab oratory (JPL). The driver for many of our upgrade plans is the anticipated needs of speci¯c users, among them GPS. But it is also worthy of note that we cannot do it alone. We need the help of the timekeeping community, and we are seeking to improve co operation in many ways. To help improve the short-term stability of TAI, we are co op erating with the BIPM in making TWSTT the operational mo de for TAI generation, and are interested in improving TWSTT technology through carrier-phase receive systems and more stable hardware. Another very exciting mo de of time transfer is carrier-phase GPS. Under the leadership of the BIPM and the IGS, we are again working on improvements to b oth the software and hardware. With Allan Osborne Asso ciates we have found a hardware ¯x that promises to make their widely used receivers calibratable and robust in a timekeeping sense; with Ron Muellerscho en of JPL we have developed a continuous Kalman ¯lter so that day-boundary processing discontinuities can be avoided; with data from many IGS sites created and published a frequency scale based only on GPS carrier-phase data; and with Jo e White and Ron Beard of Naval Research Lab oratory (NRL) we are conducting absolute calibration studies on many hardware comp onents. Toshio Fukushima President of the Commission