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.