W. Lewandowski

BIPM comparison of time transfer techniques

Clock comparison for International Atomic Time (TAI) is based on common views of satellites of the GPS constellation and on the technique of two-way satellite time and frequency transfer (TWSTFT). All the TWSTFT links are backed up by the GPS ones. Due to the duplicity of techniques in many baselines, the international network of time links is today highly redundant. The improvement of GPS satellite ephemerides and clock values produced by the International GNSS Service (IGS) might lead to the utilisation of the all-in-view (AV) method for the computation of GPS links instead of the common-view (CV) one currently used for TAI. Studies started at the BIPM to validate the GPS AV method for introduction in the calculation of TAI. To assist related studies, we developed a procedure to compare the links measured with different techniques and calculated with different methods. Results of these comparisons obtained since January 2005 are published on the BIPM ftp site, with monthly updates after the calculation of BIPM circular T.

UTCr: A rapid realization of UTC

Considering the evolving needs of time metrology and the convenience of allowing the contributing laboratories access to a realization UTC more frequently than through the monthly Circular T, the BIPM Time Department has started to implement the computation of UTCr, a rapid realization of UTC published every week and based on daily clock and time transfer data. Results of the first weeks of a pilot experimentation of this new product are presented.

Recent progress in GLONASS time transfer

Unlike GPS, the GLONASS P-code is broadly accessible. This paper discuss GLONASS capabilities and prospects in terms of precise time transfer. We have tested GLONASS common-view time transfer using the C/A- and P-code, over time links varying in length from about 800 km to 9200 km. The raw GPS and GLONASS data were collected using 3S navigation receivers, and were corrected using IGS precise orbit data and IGS ionosphere maps. It is proposed that GLONASS time links be calculated monthly, initially as backup links for TAI calculation, and later as possible official time links

Toward new procedures in TWSTFT and GNSS delay characterization for UTC time transfer

UTC generation includes the computation of UTC-UTC(k) and its uncertainty estimation. A significant part of the uncertainty of the UTC approximations UTC(k) in national contributing laboratories is based on accurate metrological measurement of time transfer equipment delays (so called “equipment calibration”). Organizing and maintaining the calibration of the time transfer facilities contributing to UTC is among the responsibilities of the BIPM. At present, the time transfer techniques used for UTC generation are based on the two-way satellite time and frequency transfer (TWSTFT or TW) and the global navigation satellite systems (GNSS), i.e. GPS and GLN (Glonass). They are used for calculating the differences [UTC(k) - UTC (1)] between any participating laboratory and that chosen as a pivot (at present the PTB). In the 1980s, GPS C/A technique dominated the UTC time transfer. Since 2000, TW and GPS MC, P3 and PPP techniques as well as GLN have been successively introduced in the UTC generation. In consequence, the calibrations of the different time transfer equipment were introduced and are performed separately. Today, there are four parallel types of independent calibrations based on different strategies that can be defined either as site-based or link-based. The BIPM has assigned values of uB of about 1 ns for the link-based TW calibrations, based on the values reported by those performing the calibration. However, for the sited-based GNSS calibrations, a conventional value of 5 ns has generally been assigned. This choice has been motivated by studies which found that values of [UTC(k) - UTC (1)] bigger the respective uB may exist when the link as calculated by different techniques, and because long-term instability of the standard receivers may cause inconstancy in individual calibrations carried out in different periods. On the other side, due to the development in technology, the statistical uncertainty uA has been reduced by a factor of 10 since a dozen of years. The state of the art of uA is 0.5 ns for TW and 0.3 ns for GPS PPP. The uB calibration uncertainty is dominant in the total uncertainty of [UTC-UTC(k)], and several authors have investigated how to improve the calibration of time transfer equipment to decrease its value. Also the BIPM has undertaken studies for improving the current calibration policy. The goal of this studies are: 1) to reduce the inconsistency between different techniques by making a combined use of the respective calibrations; 2) to reduce influence of the long-term instability of the BIPM standards by a special designed schedule; 3) to obtain more realistic uB values than the conventional 5-ns and, in consequence, to reduce the total uncertainty of UTC-UTC(k); 4) to easy the calibration organization and reduce its cost; 5) to simplify the calibration monitoring and the combination of different time transfer techniques. Uncertainty estimation is an important part of the UTC computation and hence carefully discussed. We present hereafter a study, which in no way means the adoption of a new calibration policy at the BIPM.

Towards accurate optical fibre time transfer in UTC

Accurate time transfers are now fully based on space techniques. The recently developed optical fibre technique is expected to be used in the UTC generation in the future. Some UTC labs operate optical fibre links using different approaches. In particular, the here called Two-Way Optical Fibre Time (and Frequency) Transfer (TWOFT/TWOTFT) is of special interest. The symmetry of the propagation conditions in both directions allows to substantially reducing the instability due to fluctuations of the propagation, and its capability of self-calibration. The first operational TWOTT between the UTC laboratories has been established in Poland between AOS and GUM since January 2012. The result has being submitted to the BIPM since March 2013. The BIPM started the monthly data processing and compares to the GNSS links. The TWOTT can reach an accuracy of hundred ps in a few minutes and therefore provide an effective new tool for the assessment of GNSS and TWSTT CP time link calibration and bring substantial improvement to the [UTC-UTC(k)]. However, time transfer by optical fibres today does not match the structure of the UTC time transfer network with a unique pivot. This requires the new network configuration, algorithm and will provoke fundamental changes in the UTC time link computation.

Review and preview of two-way time transfer for UTC generation — From TWSTFT to TWOTFT

Bi-directional methods are widely used in scientific and productive measurements. By the symmetric principle, systematic errors are largely cancelled. The technique of Two-Way Satellite Time and Frequency Transfer (TWSTFT) is a typical example. Using the reciprocated radio signals emitted at two Earth laboratories and exchanged through a geostationary telecommunication satellite, the atmosphere delay effects are greatly reduced in the combination of the up and down signals. Applications to time transfer of the optical fibre technique, here under the acronym TWOTFT (Two-Way Optical fiber Time and Frequency Transfer) developed rapidly. The reciprocity of the signal in both the directions allows balancing the propagation path delays in the fibre since they average out almost entirely by employing the two-way method. Based on historical documents and the latest developments and in view of metrology, we briefly review and preview the two-way techniques for application in accurate time transfer for UTC.

BIPM calibration scheme for UTC time links — BIPM pilot expermiment to strenghen Asia-Europe very long baselines

The calibration uncertainty (uB) is the dominant part in the total uncertainty budget of the UTC generation [1,3]. There are two calibration strategies: (a) Receiver calibration and (b) Time link calibration. The state-of-the-art [1,4,5] of the uB is of order 5 ns for GNSS time transfer based on the strategy (a) and is 1 ns for TWSTFT based on the strategy (b) in which the absolute offset is canceled. In the BIPM Circular T, the uncertainty in UTC-UTC(k) is obtained from the uncertainties in the links [3]. To reduce the uncertainty in UTC-UTC(k), it is optimal to use the calibration strategy (b). Earlier investigations gave theoretical and practical proofs [6–12]. Based on these studies, we propose the BIPM calibration scheme using a calibration system composed of three GNSS geodesic receivers: one is stationary at BIPM and the other two travel between the laboratories. We describe the set-up of the system, the schedule, the uncertainty uB and aging of the uB. The uB is expected to be better than 2 ns for even very long inter-continental UTC baselines.

Comparison of the GLONASS orbit products for UTC time transfer

Accurate GNSS (Global Navigation Satellite System) time transfer needs accurate satellite orbit information. Since Nov. 2009, the GLONASS time transfer is introduced in UTC generation. The precise and timely GLONASS orbit information is indispensable for Circular T computation. In this paper, we compare the GLONASS orbits produced by different analysis centers. We study the differences of the products, their influences on the common view time transfers and the delays of the availability. The products to investigate are the rapid and final sp3 ephemeredes produced by ESA (Europe Space Agency) and IAC (Information-Analytical Center). IAC belongs to Russia Federal Space Agency. Its primary goal is to analyze the information on the satellite navigation based on the global systems, mainly the GLONASS. The delay of the IAC sp3 rapid products is 24–28 hours. This satisfies the rhythm of the BIPM monthly Circular T computation, of which the deadline of data collection is the 5th of every month. Since Nov. 2010, the BIPM started to use the IAC ephemeredes to compute the GLONASS satellite orbits for UTC time transfer. At present the IAC product is the most suitable for the UTC GLONASS time link computations.

Use of multi-technique combinations in UTC/TAI time and frequency transfers

This is a review paper. We revisited the key conceptions of the combined techniques used in the UTC generation: the combination of the GNSS and TWSTFT and that of the GPS and GLONASS code measurements.