Improving two-way satellite time and frequency transfer with redundant links for UTC generation.

Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). At present, more than 12 timing laboratories around the world use SAtellite Time and Ranging Equipment (SATRE) modems in TWSTFT operations and contribute data for the realization of UTC. The advantages of TWSTFT are its small calibration uncertainty (≤1.0 ns if the link is calibrated with a TWSTFT mobile station) and its long-term link stability. However, the precision of SATRE TWSTFT in the operational networks is degraded by a daily variation pattern (diurnal) in the TWSTFT results. The diurnal with varying amplitude appears virtually in all SATRE TWSTFT links. The observed peak-topeak variation of the diurnals can reach 2.0 ns in some cases. So far, studies on the sources of the diurnal have not provided conclusive understanding of the diurnal's dominant origin. Therefore, efforts have been made to reduce the impact of the diurnal variation in TWSTFT for UTC computation. The BIPM has been using the combination of SATRE TWSTFT results and GPS carrier-phase precise point positioning solutions (GPSPPP) for UTC computation since 2010. The combination adjusts the GPSPPP results to long-term averages of TWSTFT and is effectively free from the diurnal variations because the GPSPPP results contain almost no diurnal. Lately, the use of software-defined radio receivers (SDR) in TWSTFT has shown one way of how to reduce the diurnal variations by a factor of two to three in most of the inner-continental SATRE TWSTFT links, and furthermore, how the short-term stability for all UTC SDR TWSTFT links can be improved. In addition, there has been research on the full use of the redundancy in the TWSTFT network to improve the TWSTFT link stability. Recent studies on evaluating indirect links revealed that it is possible to apply a simplified procedure to use the redundancy, in a most effective way, to reduce the diurnal variations in the Europeto-Europe SATRE TWSTFT links by a factor of two to three. Based on these findings, we gained new insights about the diurnals and its dominant origin(s) which are discussed in this paper. The methods of the combination of SATRE TWSTFT and GPSPPP as well as the indirect SATRE TWSTFT links utilize the redundancy in the UTC time transfer network. SDR TWSTFT can largely reduce the diurnal in SATRE TWSTFT, but noticeable residual diurnal remains. In this paper, we provide the analyses of using the combination of SDR TWSTFT and GPSPPP results, as well as using the indirect SDR TWSTFT links. This paper concludes that the use of SDR TWSTFT redundant links can further improve the stabilities of UTC TWSTFT links. In addition, the use of SDR TWSTFT indirect links is a pure TWSTFT solution. The independence of the TWSTFT results to GPS results can improve the robustness of UTC computation.

Improved GPS-based time link calibration involving ROA and PTB.

The calibration of time transfer links is mandatory in the context of international collaboration for the realization of International Atomic Time. In this paper, we present the results of the calibration of the GPS time transfer link between the Real Instituto y Observatorio de la Armada (ROA) and the Physikalisch-Technische Bundesanstalt (PTB) by means of a traveling geodetic-type GPS receiver and an evaluation of the achieved type A and B uncertainty. The time transfer results were achieved by using CA, P3, and also carrier phase PPP comparison techniques. We finally use these results to re-calibrate the two-way satellite time and frequency transfer (TWSTFT) link between ROA and PTB, using one month of data. We show that a TWSTFT link can be calibrated by means of GPS time comparisons with an uncertainty below 2 ns, and that potentially even sub-nanosecond uncertainty can be achieved. This is a novel and cost-effective approach compared with the more common calibration using a traveling TWSTFT station.

On measurement noise in the European TWSTFT network.

Two-way satellite time and frequency transfer (TWSTFT) using geostationary telecommunication satellites is widely used in the timing community today and has also been chosen as the primary means to effect synchronization of elements of the ground segment of the European satellite navigation system Galileo. We investigated the link performance in a multistation network based on operational parameters such as the number of simultaneously transmitting stations, transmit and receive power, and chip rates of the pseudorandom noise modulation of the transmitted signals. Our work revealed that TWSTFT through a "quiet" transponder channel (2 stations transmitting only) leads to a measurement noise, expressed by the 1 pps jitter, reduced by a factor of 1.4 compared with a busy transponder carrying signals of 12 stations. The frequency transfer capability expressed by the Allan deviation is reduced at short averaging times by the same amount. At averaging times of >1 d, no such reduction could be observed, which points to the fact that other noise sources dominate at such averaging times. We also found that higher transmit power increases the carrier-to-noise density ratio at the receive station and thus entails lower jitter but causes interference with other station's signals. In addition, the use of lower chip rates, which could be accommodated by a reduced assigned bandwidth on the satellite transponder, is not recommended. The 1 pps jitter would go up by a factor of 2.5 when going from 2.5 MCh/s to 1 MCh/s. The 2 Galileo precise timing facilities (PTFs) can be included in the currently operated network of 12 stations in Europe and all requirements on the TWSTFT performance can be met, provided that suitable ground equipment will be installed in the Galileo ground segment.

A straightforward frequency-estimation technique for GPS carrier-phase time transfer.

Although Global Positioning System (GPS) carrier-phase time transfer (GPSCPTT) offers frequency stability approaching 10-15 at averaging times of 1 d, a discontinuity occurs in the time-transfer estimates between the end of one processing batch (1-3 d in length) and the beginning of the next. The average frequency over a multiday analysis period often has been computed by first estimating and removing these discontinuities, i.e., through concatenation. We present a new frequency-estimation technique in which frequencies are computed from the individual batches then averaged to obtain the mean frequency for a multiday period. This allows the frequency to be computed without the uncertainty associated with the removal of the discontinuities and requires fewer computational resources. The new technique was tested by comparing the fractional frequency-difference values it yields to those obtained using a GPSCPTT concatenation method and those obtained using two-way satellite time-and-frequency transfer (TWSTFT). The clocks studied were located in Braunschweig, Germany, and in Boulder, CO. The frequencies obtained from the GPSCPTT measurements using either method agreed with those obtained from TWSTFT at several parts in 1016. The frequency values obtained from the GPSCPTT data by use of the new method agreed with those obtained using the concatenation technique at 1-4 x 10(-16).