To establish the accuracy limits of frequency and time transfer methods one has to study the measurement stability of each single element of the measurement system. This seemingly trivial statement became immediately of renewed interest when two-way satellite time transfer between European time laboratories (covered by EUROMET Project 93) became of wider use, and results proved to be occasionally corrupted by the quality of the local measurement equipment rather than by the satellite time transfer itself.
In the course of the activities of this Project, a variety of equipment was systematically studied, like cables including connectors, mixers, amplifiers, high-resolution electronic counters, and more TWSTFT-specific equipment. By exchange of the results, the awareness of the problems inherent with some of the equipment utilized until then was increased among the participating institutions. Some of the results were discussed during informal TWSTFT experimenters meetings and Sessions of the CCTF Working Group on TWSTFT. The instrumentation in the laboratories has been considerably improved after stimulation by the results shown.
What has not been achieved – contrary to the initial plans – was the development and publication of an equipment data base. It turned out that the utilization of equipment is in some cases very specific and difficult to compare, and that some institutes utilize home-made, purpose-built equipment, whose characteristics are of little relevance for others. Another obstacle was the variety of equipment which could “in principle” be of interest. In some occasions this prevented a clear strategy about what should be studied at what time.
The recently completed assessment of the Calibration and Measurement Capabilities tables within Euromet, and the approval of the tables by another RMO may serve as an indication that the instrumentation in the laboratories is typically well understood and well characterized. The currently active Project “828 Euromet Supplementary Comparison TF-TI.K1” can to a certain extent be understood as a continuation of Project 321: It focuses specifically on one well defined measurement problem and has a clear time line to be observed.
Rather than advocating for the continuation of Project 321 we would like to stimulate another RMO comparison of similar kind as that done under Project “828” when this has been successfully terminated.
Two-way time transfer via communicaion satellites using pn-modulated signals provides highest precision with the potential of high accuracy. The basic instrumentation employed using this technique are high resolution electronic counters, pn-modems and small satellite earth stations. In order to establish the accuracy limits of this technique and to identify the limiting elements, the measurement stability of the single elements has to be studied carefully. This will be done employing unified pre-agreed measurement and data processing procedures. The obtained results should provide a data base relevant to the properties of presently commercially available equipment and indicate were improvements are necessary to exploit the accuracy potential of the two-way system.