Progress in the Link Calibration for UTC Time Transfer -- attainable uncertainty of the METODE (MEasurement of the TOtal DElay) Abstract The dominant part of the total uncertainty budget in the [UTC-UTC(k)] is from the type B uncertainty (u B ) in time link calibration. In 2011, the BIPM launched a pilot experiment to reduce the u B from 5 ns at present to ≤ 2 ns and has developed a standard calibration scheme, namely METODE, associated with an integrated portable GNSS calibrator and reference receivers stationary at BIPM. Similar setups are made in TL. Using these systems, we performed a series experiments at BIPM, OP and TL to study the reachable uncertainty of METODE. The results are compared to that of the traditional differential receiver calibrations. Keywords : UTC, Time transfer, Calibration ReceiversPPP±u A /ns OPMT vs. BP0C0.929±0.060 Prague EFTF2013 ID 515 -Background- Conclusion We estimate the total uncertainty of METODE U M in the total delay. It is composed of: 1. Measurement uncertainty: about 0.1 ns~0.3 ns (u A of PPP link); 2. Calibration uncertainty of the calibrator Std B : 0.3~1.0 ns; 3. Instabilities of reference and travel receivers: 0.5~1.0 ns (e.g. closures); 4. Others 1.0 ns (unexpected uncertainties)  The total uncertainty of METODE u M is hence 1.1~1.8 ns. The experiments were carefully designed to investigate the total uncertainty of the METODE. In conclusion, the reachable uncertainty is of the order 1.8 ns (1σ) in the worst without considering the long-term variations. The later might be up to a few ns. The BIPM calibrator Std B is suitable for such a task. Two DICOM GTR50 GPS time transfer receivers are thoroughly studied for the use of the METODE. It proves the stability, simplicity and precision for UTC time link calibration purpose. Compared with the 2008 classic calibration result with the BIPM BP0C as the reference, we obtained a total delay calibration correction of 0.9 ns to the OP OPMT. The BIPM Std B has visited PTB and is planned to travel to AOS, PL and TL to further tests of the METODE and the calibrator. -Methods- Uncertainty Type A / Type B /ns u B /CA/GNSS 16% u B /C|P/GNSS/TW 44% u B / TW 7% u B /CA/GNSS 26% Optic fibre etc. year TW+PPP MC/CV P3/AV TW/PPP GPS SA ON | Off X O.F. …TW uAuA GNSS TW Unbalance: u A vs. u B ?! Evolution of Time links and Uncertainties since % u A ≤ 1 ns vs. 93 % u B ≥ 5 ns  93% u > 5 ns The uncertainty in UTC is dominated by the u B.  The major challenge: Reduce the u B ! UTC Calibration Guidelines for RMO Reduce u B from 5 ns to 1~2 ns by Link Calibration - BIPM transportable time link calibrator (BIPM) - TWSTFT mobile calibrator (TimeTech) - GNSS receiver absolute calibration (BIPM-CNES) Z Jiang 1, G Petit 1, L Tisserand 1, P Uhrich 2, G D Rovera 2 and S Y Lin 3 1 Time Department, Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil F-92312, SÈVRES CEDEX, France 2 Laboratoire National de Métrologie et d’Essais - Systèmes de Référence Temps-Espace, CNRS, UPMC, Observatoire de Paris (OP), France 3 National Standard Time and Frequency Laboratory, Telecommunication Laboratories (TL), Chunghwa Telecom The type B uncertainty (u B ) in the time transfer calibration is the dominant part of the total uncertainty budget (u) in the [UTC-UTC(k)]. Reducing u B below 2 ns would lead to reducing u below 2 ns. This is the target of the time link calibration experiment. In Circular T, 7% of the u B of the total UTC links is 1.0~1.2 ns; that of 5~10 ns takes 65%. The rest of 28% have the u B 20 ns. While as the best of u A is now below 0.3 ns and more that 60% of u A < 1 ns. Further improving UTC must reduce the u B. Since 2007, the BIPM began to consider to unify the TW and GNSS calibrations and proposed a calibration method, namely, METODE (MEasurement of TOtal DElay) and developed a standard calibration scheme in which an integrated and pre-cabled portable calibrator (Std B ) is used. The calibrator is composed of a 10 MHz distributor, a PPS distributor and two or three GNSS geodetic setups each include a set of independent receiver, antenna and cables. The calibrator and the fixed reference receivers are like ‘black’ boxes for which we do not need to know their internal delays. All we require is their short-term stabilities during the calibration tours, typically 3 months. In this paper, we discuss the reachable uncertainty of the METODE through carefully designed experiments. Design of the experiments to evaluate the total uncertainty The basic idea is to measure a delay using different and independent methods then to compare the results. The RMS (the root of mean squares of the differences between the methods) will provide an estimate of the total uncertainty (u M ) which includes all the uncertainty sources of GPSs’, TICs’, and cable delay measurements’ etc. This plus the instabilities of the reference and the travel receivers make the total uncertainty of the METODE link calibration: U M (u B ). The 2013 setup of the BIPM portable calibrator with two GTR50 GPS receiver-antenna systems Four independent methods are used to test the BIPM calibrator (Std B ): (A) Use the Std B to measure the delay of the known delay cables; (B) Comparisons between the SR620 time interval counter (TIC) and the Std B ; (C) Comparisons between A mobile Cs standard and the Std B ; (D) Comparison between the earlier calibrations and the Std B The 4 offsets in the total delays designed on different MJDs measured by the TIC SR620 and the BIPM Calibrator (P3/PPP). The mean differences is -0.2±0.3ns __ Method (A) Dif. ≤ 0.3 ns__ ___ Method (B) _Dif. ≤ 0.7 ns ____ The 10 meter baseline between the UTC(TL) and the mobile Cs standard. The CCD are measured continuously by both METODE and a TIC SR 620. The difference is 0.7 ns. OPMT at OP BIPM Std B at OP UTC(OP) at OP Lab H889’ (Ref. Point) at OP Lab H889 in basement LZ Δ=2.67 ns Common reference point of the OPMT and the StdB BIPM PPS cable Δ=117.2 ns Δ= ns = BIPM PPS cable Setup Setup The CCD setups of the Std B at OP of two references: the maser H889 and UTC(OP). The difference is 0.5 ns A 25km CCD baseline experience was carried out at TL. A mobile Cs clock was transported together with the METODE calibrator developed by TL. The RMS of the two independent data sets is 0.92 ns on average. It should be pointed out that the RMS includes the instability of the mobile Cs clock which may be up to 1 ns depending on various conditions of temperatures, transports and power etc. Method (C) Dif. =0.9 ns _ Method (D)_Dif. = 0.9 ns _ The BIPM Z12T BP0C was absolutely calibrated in The related calibration uncertainty u B is about 2.3 ns. Table below gives the differences of the calibrations results by the METODE calibrator and by the 2008 classical receiver calibrations of OPMT vs. BP0C. By PPP, the difference of the calibrations is 0.9 ns between OP Z12T OPMT and BIPM BP0C. In the table, u A is the measurement uncertainty. Briefly, above the Tests A, B, C and D give separately the uncertainty estimates < 1.0 ns. The RMS is 0.7 ns GNSS PTB TW PTB PTB GNSS sat. Lab k GNSS Lab K TW k TW sat. Ref PTB Std B TW: Lab K -PTB GNSS Lab K -PTB: Ref k - Ref PTB Std B - Ref PTB An one for all link calibration The BIPM traveling Calibrator at OP and PTB GTR50 Polarx4 BIPM StdB antennas’ supporter BIPM Std B Setup at OP