No abstract

The national metrology institute BNM-SYRTE has developed an earth station for satellite communications in order to compare, by using radiofrequency link with other earth stations installed in Europe and USA, atomic clocks disseminated in the time laboratories. The goal of BNM-SYRTE is to contribute in the realization of International Atomic Time (TAI) calculated by the BIPM, by using an alternative technique to GPS, denoted TWSTFT (Two Way Satellite Time and Frequency Transfer).With this particular technique, atomic clocks are compared through a geostationary satellite using microwave links between ground clocks and the satellite, in the frequency bands 13,75-14,50 GHz (up-link) and 10,95-12,75 GHz (down-link). Such comparisons are made using the spread spectrum method with a 70 MHz Intermediate Frequency (IF) carrier modulated by a pseudo-noise (PN) code sequence, at 2,5 MChips/s, generated by a SATRE (SAtellite Time and Ranging Equipment) modem.

Important investigations in the time and frequency transfer have been done during the last four years at the Observatoire de Paris (OP). The goal was twofold: improvement of the GPS comparison technique by the use of dual frequency GPS multi-channel receivers in common-views (GPS P3) and the development of a full two-way satellite time and frequency transfer (TWSTFT) station in the Ku-Band.The advantage of the GPS P3 technique is the compensation of the ionospheric delays. The GPS P-Code is transmitted simultaneously on both carriers L1 and L2, and the dualfrequency multi-channel GPS receiver Ashtech Z12-T allows for the so-called P1 and P2 measurements. A simple linear combination gives the ionosphere free P3 measurements. The software developed by the Observatoire Royal de Belgique [1] builds from the geodetic RINEX files such P3 data in the CGGTTS format, and the Common-View method is applied. Due to the availability of several simultaneous Common-Views, a selection can be made which improves the short term frequency stability of the results. A typical result on European baselines gives an Allan Time Deviation σ x (τ) below 200 ps over an analysis period lower than 1 d between H-Masers.The TWSTFT technique, a direct comparison between ground clocks through a communication satellite, is a wellknown technique independent from GPS. In this field, OP developed a full Ku-band two-way station with a dual offset antenna, active up/down-converters and a SATRE modem. The station, calibrated in relative mode in 2004, is officially used for TAI since January 2005. A complete uncertainty budget on the two-way measurement has been evaluated giving a combined uncertainty of only 1 ns. Moreover, the realization of a satellite simulator based on an original design [2] is in progress. Such a simulator will permit mainly an absolute difference delay calibration of a two-way station. During the Primary Frequency Standard comparison done in 2004 over 27-days measurement period, it was observed that H-Masers can be reached at only 0,6 d, with a σ x (τ) = 80 ps on the OP-PTB two-way link.