Two-Way Satellite Time and Frequency (TWSTFT) is increasingly used for the routine intercom-parison of time scales at primary timing laboratories. The performance of a two-way time transfer link is, however, limited by delay instabilities in the earth station hardware. Several European laboratories have been operation near field satellite simulators to measure the delay instabilities of their TWSTFT earth stations. At the National Physical Laboratory (NPL) we have adopted a different characterization configuration, where two independent characterization loops are used to measure the TWSTFT earth station uplink and downlink delay instabilities separately. This allows an assessment of the performance of the difference loops in the measurement of the delay instabilities. The cables connecting the earth station hardware have also been investigated. Direct comparisons are made between PTFE coaxial cable propagating Ku-band and L-band signals. Furthermore, the performance of a higher chip rate code has been investigated. Finally, the results presented in this paper are used to discuss future improvements in TWSTFT at NPL.
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