This article reports on the characterization of cryogenic sapphire oscillators (CSOs), and on the first test of a CSO in a real field installation, where ultimate frequency stability and continuous operation are critical issues, with no survey. Thanks to low-vibration liquid-He cryocooler design, Internet monitoring, and a significant effort of engineering, these oscillators could bridge the gap from an experiment to a fully reliable machine. The cryocooler needs scheduled maintenance every 2 years, which is usual for these devices. The direct comparison of two CSOs demonstrates a frequency stability of 5 × 10 −16 for 30 s ≤ τ ≤ 300 s integration time, and 4.5 × 10 −15 at 1 day (1 × 10 −14 typical). Two prototypes are fully operational, codenamed ELISA and ULISS. ELISA has been permanently installed the new deep space antenna station of the European Space Agency in Malargüe, Argentina, in May 2012. ULISS is a transportable version of ELISA, modified to fit in a small van (8.5 m 2 footprint). Installation requires a few hours manpower and 1 day of operation to attain full stability. ULISS, intended for off-site experiments and as a technology demonstrator, and has successfully completed two long-distance travels.

The French atomic time scale TA(F), which is computed with data from about 20 industrial caesium standards located in nine French institutions, aims to provide a stable national frequency reference to the contributing institutions. To improve its stability, it was decided a few years ago to steer the time scale, which up to that date was free running, on the LNE-SYRTE primary frequency standards (PFS). The frequency of TA(F) was first slowly corrected monthly by an arbitrary given frequency offset with respect to TAI, to compensate the drift without disturbing the 30 d relative frequency stability of the time scale. Once close enough to the SI second, the time scale was steered monthly to the frequency data issued from the LNE-SYRTE PFS, in that way providing a more stable reference. We describe the steering applied to TA(F) and show the results in terms of relative stability with respect to TAI, or by comparing TA(F) with the SI second on the geoid as published monthly by the BIPM in its Circular T. When applying this steering during recent years, the departure over 30 d intervals of TA(F) from the SI second on the geoid was maintained inside the limits ±3 × 10 −15 . Within these limits, the TA(F) scale unit interval is in agreement with the SI second, a result which was made possible thanks to the four PFS currently in operation in the LNE-SYRTE.

The use of global navigation satellite systems (GNSS) for time transfer is widespread, in particular for the computation of the coordinated universal time (UTC). It is an inexpensive, easy-to-implement method to compare distant clocks. This time transfer method can also be applied to the monitoring of the GNSS time scales and of the broadcast UTC conversion parameters. One mission of the French space agency (CNES) is to perform this task for Galileo. For that purpose it is mandatory to carry out an absolute calibration of the propagation delay of a GNSS receiver chain. This propagation delay is the one between the GNSS antenna phase centre and the time reference that feeds the receiver chain. In order to allow a link to UTC, the time reference must be a UTC(k). CNES has developed a method of absolute calibration of GNSS receiver chains, in which the delays of the GNSS antenna, the antenna cable and the GNSS receiver are determined separately, with a total uncertainty in the range of 1 ns (1 – σ). The purpose of this paper is to present the method developed by CNES along with its uncertainty budget for GPS, Galileo and BeiDou signals. A validation in common-clock with actual GPS and Galileo signals is also provided. The absolute calibration is finally applied to an accurate monitoring of the GNSS time scales, the Galileo broadcast UTC conversion parameters and the broadcast Galileo to GPS time offset (GGTO), with associated uncertainties.

No abstract