Long-term uncertainty in time transfer using GPS and TWSTFT techniques

The techniques of GPS time and frequency transfer (code based and carrier phase) and TWSTFT are widely used in remote clock comparison and in the computation of TAI and UTC. Many timing laboratories in the world utilize both techniques (GPS and TWSTFT transfer links) to compare each other's clocks. A time link must be calibrated to assure the time transfer accuracy. In many cases, calibration campaigns have been very infrequent due to the expense and lack of suitable equipment. In lieu of repeated calibrations, some information regarding the long-term stability of these links can be obtained through comparisons between the two links (a so called double difference). Without frequent calibrations it is impossible to tell where the instabilities originate, but information regarding the magnitude of the instabilities can be obtained from double difference data. We have been investigating the combined variations of GPS and TWSTFT links for a number of laboratory pairs, including both long and short baselines. Our results show that the relative change between GPS and TWSTFT transfer links can be as large as 6 to 7 ns over a few years and that all of the laboratory pairs that have been investigated show similar magnitudes in the double difference data. Currently the longest set of good double difference data is about 7 years. The study results point out the need for frequent calibration campaigns if accuracies at the nanosecond level are required.