Zhiheng Jiang

GPS All in View time transfer for TAI computation

Since the 1980s, GPS time links have been essential to the TAI computation and, until 2006, the Common View (CV) technique has been used for this purpose. Recent advances in obtaining precise satellite orbits and clock parameters now permit us to obtain better results using another technique, which we name All in View (AV). By comparing the GPS CV and AV with the independent and more accurate TW and PPP time transfer techniques, we quantify the gain that can be obtained on a given time link. The AV technique also allows us to choose a more efficient network of GPS links between the tens of laboratories participating in TAI, which further improves the uncertainty in the access to UTC. The BIPM TAI software has been updated and the AV technique has been effectively used since the computation for the month of September 2006.

Precise Point Positioning for TAI Computation

We discuss the use of some new time transfer techniques for computing TAI time links. Precise point positioning (PPP) uses GPS dual frequency carrier phase and code measurements to compute the link between a local clock and a reference time scale with the precision of the carrier phase and the accuracy of the code. The time link between any two stations can then be computed by a simple difference. We show that this technique is well adapted and has better short-term stability than other techniques used in TAI. We present a method of combining PPP and two-way time transfer that takes advantage of the qualities of each technique, and shows that it would bring significant improvement to TAI links.

Results of the Sixth International Comparison of Absolute Gravimeters, ICAG-2001

Like all the previous International Comparisons of Absolute Gravimeters (ICAGs) the sixth, ICAG-2001, was held at the Bureau International des Poids et Mesures (BIPM). Major improvements in the 2001 campaign were a new measurement strategy using the absolute gravimeters to measure the ties of the gravity network, new sites constructed at the BIPM, improved relative measurements of the ties and gravity gradients, and combined adjustment of the absolute and relative data, realized using new software with a novel data weighting and rejection scheme. The g-values at four sites of the BIPM were measured with an uncertainty of 6 μGal. Good agreement was obtained between the results of the absolute and relative measurements of the ties of the gravity network. The final mean gvalue obtained at the reference site A was 7 μGal less than that obtained in the previous comparison, ICAG-97.

Absolute Calibration of an Ashtech Z12-T GPS Receiver

Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated. We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate uncertainty of order 1 ns in the absolute calibration seems to be attainable.

Use of GPS ASHTECH Z12T receivers for accurate time and frequency comparisons

The GPS phase measurements described in this paper were obtained using two similar multichannel GPS ASHTECH Z12T receivers belonging to the Bureau International des Poids et Mesures, BIPM, and the Laboratoire Primaire du Temps et des Frequences, BNM-LPTF. These receivers are based on the conventional geodetic ASHTECH Z12 unit, which has been modified to meet the stability requirements of time and frequency comparisons. Comparison of the two receivers operated side by side in different antenna configurations shows typical short-term noise of 1.1 to 3.5 ps. Longer term variations indicate a temperature sensitivity in the equipment, which limits the performance of the GPS phase method. One of the receivers was successfully operated using a temperature-stabilized antenna TSA from 3S Navigation, and the ASHTECH antenna, which feeds the second receiver, was placed in a home-built oven maintained at a constant temperature. These precautions made it possible to reduce a number of systematic effects. A separate study of frequency comparison was carried out between two hydrogen-masers located at the BNM-LPTF (Paris, France) and the PTB (Braunschweig, Germany) using receivers similar to ASHTECH Z12T receivers. The relative frequency stability obtained was about 3.3/spl times/10/sup -15/ for an average time of 15 000 s, an interesting result comparable with the outstanding performance of new ultrastable frequency standards.

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

Accurate GLONASS Time Transfer for the Generation of the Coordinated Universal Time

The spatial techniques currently used in accurate time transfer are based on GPS, TWSTFT, and GLONASS. The International Bureau of Weights and Measures (BIPM) is mandated for the generation of Coordinated Universal Time (UTC) which is published monthly in the BIPM Circular T. In 2009, the international Consultative Committee for Time and Frequency (CCTF) recommended the use of multitechniques in time transfer to ensure precision, accuracy, and robustness in UTC. To complement the existing GPS and TWSTFT time links, in November 2009 the first two GLONASS time links were introduced into the UTC worldwide time link network. By November 2011, 6 GLONASS time links are used in the UTC computation. In the frame of the application in the UTC computation, we establish the technical features of GLONASS time transfer: the short- and long-term stabilities, the calibration process, and in particular the impact of the multiple GLONASS frequency biases. We then outline various considerations for future developments, including the uses of P-codes and carrier-phase information.

Interpolation of TW time transfer from measured points onto standard MJD for UTC generation

We often need to determine the time transfer result for an epoch at which no direct measurement was made. For example, the time tag used in Circular T is 0 h UTC of the standard MJD, which is not generally a measuring epoch. Usual time transfer practice is to first smooth the raw data to filter out white measurement noise, and then interpolate the smoothed data to determine the time transfer value for the required epoch. However, up until the end of 2009, for two-way (TW) time transfer only simple linear interpolation (but any high-order interpolation method) was used. This was because, when TW was introduced into the computation of UTC in 1998, there were not enough measured points to allow a mathematically rigorous smoothing-interpolation process. Since the end of 2005 the number of measurements has increased to between 12 and 24 per day. To exploit the redundancy of this increased number of data points, in 2005 it was proposed that a higher order smoothing-interpolation method be used. It was concluded that: (1) a high-order smoothing interpolation is better than a simple linear interpolation; and (2) Vondrak smoothing is preferable to other high-order smoothing-interpolation techniques. This paper builds on the previous study, examining the Vondrak smoothing-interpolation method with the benefit of a new powerful tool: the GPS PPP time-transfer solution. This provides an objective measure of the performance of different smoothing-interpolation methods. We conclude that in most cases Vondrak smoothing to the power 105 leads to the best interpolation results.

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.