Aurelie Harmegnies

A new prediction algorithm for the generation of International Atomic Time

In this paper we present a new prediction algorithm for the generation of International Atomic Time (TAI). The new prediction algorithm takes into account the frequency drift which affects most of the participating atomic clocks. In particular, we focus on the effect of the application of the new model on the prediction term for the frequency drift affecting the free atomic time scale (EAL). We also present its effect on TAI performance and on atomic clock weights.

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 of UTC more frequently than through the monthly Circular T, the BIPM Time Department started in 2012 to implement the computation of UTCr, a rapid realization of UTC published every week and based on daily data. After 18 months of pilot experiment, this new product has been declared operational and is now an official publication of the BIPM. This paper presents the main characteristics and properties of UTCr.

The time stability of PPP links for TAI

In the last few years the BIPM has started using GNSS phase and code observations and the Precise Point Positioning technique to compute time links for the generation of TAI. The estimated instability of such links for averaging time up to 1 month has been taken as 0.3 ns. In this paper, we investigate methods to estimate this instability, following several approaches.

Combining GPS and GLONASS in all-in-view for time transfer

GPS code measurements have been used for three decades for remote clock comparison, also called Time Transfer. Initially based on a technique using common-view (CV) single-frequency measurements, GPS time transfer now mostly uses dual-frequency measurements from geodetic receivers processed in all-in-view (AV). With the completion of the GLONASS constellation, it has been possible to readily use it in the CV single-frequency mode, providing results similar to GPS for short-distance time links. However GLONASS results are not readily equivalent to GPS in the dual-frequency AV mode, necessary for any moderate- to long-distance link, and this paper shows how to achieve this. We first present the GLONASS upgrade of the R2CGGTTS software, a tool to provide dual-frequency measurements in a format dedicated to time transfer named CGGTTS (Common GPS GLONASS Time Transfer Standard). The GLONASS navigation files are used to determine satellite clocks and positions, and dual-frequency pseudorange measurements are linearly combined to compute the CGGTTS results in a similar way as for GPS. In a second part, we present the combination of GPS and GLONASS into one unique time transfer solution based on AV. The results are first corrected using precise satellite orbit and clock products delivered by the IGS analysis centre ESOC, and characterized by the same reference for the GPS and GLONASS satellite clocks. Then, the need to introduce satellite-dependent hardware delays in GLONASS results is emphasized, and a procedure is proposed for their determination. The time transfer solutions obtained for GPS-only and GPS+GLONASS are then compared. The combination of GPS and GLONASS results in AV provides a time transfer solution having the same quality as GPS only. Furthermore, comparisons show that even when increasing the number of observations in CV thanks to the combination of the two constellations, the AV remains superior to the CV solution in terms of noise and short term stability, especially for long baselines.

Time and frequency transfer combining GLONASS and GPS data

In order to allow the use of GLONASS data from geodetic type receivers providing only raw data in RINEX format, we updated the R2CGGTTS software developed initially at the Royal Observatory of Belgium (ROB) for GPS data. The GLONASS navigation files are used for the determination of satellite clocks and positions, and the computation procedure to get the CGGTTS data from the pseudorange measurements is applied similarly as for the GPS satellites. Using the satellite orbits and clock products provided by the IGS analysis center ESA, we combine GPS and GLONASS in All in view. Adding GLONASS decreases the noise of the clock solution with respect to the GPS-only solution by about 20% thanks to the increased number of satellite observations. We also quantify the added value of using GLONASS data in addition to GPS data for time and frequency transfer based on Precise Point Positioning (PPP). A particular attention is given to the determination of the frequency-dependent hardware delays in both approaches.

Advances in multi-GNSS time transfer

This paper proposes a method to combine GPS and GLONASS measurements for time transfer in All-in-View, using calibration data for both GPS and GLONASS. GLONASS has to date a complete constellation, but its use for time transfer is made difficult by the existence of biases for each satellite-receiver pair. These biases contain both a receiver contribution, due to the different frequencies emitted by the different GLONASS satellites, and a satellite contribution, varying with time, and coming from the satellite clock products. Up to recently, these biases were determined with respect to the calibrated GPS solution. We propose here to determine these biases for the two stations of a time link, in a constrained least square analysis, where the constraint is given by the GLONASS calibration data obtained from a link calibration with these two stations. With these biases, it is then possible to obtain an All-in View solution combining GPS and GLONASS measurements and calibrated for the GPS as well as GLONASS signals. The method was tested on the link Brussels-Paris for which a calibration was performed for GPS and GLONASS. Using one month of data, the impact of using or not the GLONASS calibration results is in that case smaller than 2.5 ns peak to peak.

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.

A new prediction algorithm for EAL

In this paper the new algorithm for the prediction of EAL (Echelle Atomique Libre) is presented. The effect of the application of the new model for the prediction term on the EAL frequency drift is quantified and presented together with its effect on TAI and on the atomic clock weights.

GPS frequency transfer with IPPP

Since many years, the time community has been using the Precise Point Positioning (PPP) technique using GPS phase and code observations to compute time and frequency links. However progresses in atomic clocks imply that the performance of PPP frequency comparisons is now a limiting factor in comparing the best frequency standards. One main limiting factor comes from the effect on the clock solution of the simultaneous resolution of floating phase ambiguities together with other parameters. In this paper we study how to improve the PPP frequency transfer using the Integer-PPP (IPPP) technique implemented by the CNES.

A new weighting procedure for UTC

This paper deals with the new weighting procedure studied and developed at the BIPM Time Department for the calculation of the Coordinated Universal Time (UTC). After the implementation of the quadratic prediction model in UTC algorithm, no effect has been observed on the clock weight distribution. To optimize the use of atomic clocks in order to improve the stability of UTC, a proposed review of the weighting algorithm is described.