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For about one year the time scales UTC (OCA) and UTC (TUG) were compared by means of GPS common-view and two-way satellite time transfer. At the end of the experiment, both links were independently "calibrated" by measuring the differential delays of the GPS receivers and the satellite Earth stations by transportation of one GPS receiver and one satellite terminal to the other site. The results obtained by the two methods differ by about 3 ns, but reveal a seasonal variation of about 8 ns which, most likely, is mainly the result of temperature-dependent delays in the GPS receiving equipment used.

For a decade and a half GPS Common-View time transfer has greatly sewed the needs of primary timing laboratories for regular intercomparisons of remote atomic clocks. However, GPS as a oneway technique has natural limits and may not meet all chulollenges of the comparison of the coming new generations of atomic clocks. no-Way Satellite Time and Frequency nansfer (WSTFT) is a promising technique which may successfiUy complement GPS. For two years, regular WSTFT's have been performed between eighf laboratories situated in both Europe and North America, using INTELSAT satellites. This has enabled an extensive direct comparison to be mcrde behveen these two high performance time-transfer methods. The performance of the TWSTFT and GPS Common-View methods are compared over a number of time-transfer links. These links use a variety oftime-transfer hardware and atomic clocks and have baselines of substantially different lengths. The relative merits of the two time-transfer systems are discussed.

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