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.

In recent years, a certain correlation detected between clocks at time and frequency laboratories has been the main reason behind the development of a revised version of the classical 'Three-Cornered Hat' method as a mathematical tool to estimate the frequency instability of a set of clocks. The method has been formulated keeping in mind the possibility of dependence among clocks and has been generalized to an ensemble of N clocks [1][2][3][4][5]. It basically consists of the minimization of an objective function, the quadratic sum of cross-correlation coefficients, with the constraint condition established by the positive definiteness of the (estimated) absolute covariance matrix related to the clocks.Later on, a matrix formulation to calculate the optimal weights in the case of correlated clocks was presented in [6]. The impact of this weighting procedure on the instability of the ensemble time was evaluated, on the basis of simulated clocks, when correlation was appropriately taken into account and in the case when it was neglected. That paper established, as a main conclusion, the mathematical treatment to be used in the usual case of weak dependence.In this paper, we present a summary of the theory involved, and then apply the procedures to optimal weighting of clocks using real data clocks at Real Observatorio de la Armada (San Fernando, Spain). Finally, we discuss the results obtained.

For a GPS calibration campaign between ROA and PTB a GTR50 time and frequency transfer receiver from ROA was chosen. The operation performance of GTR50 type receivers has been validated in long term operation during the last years. The travelling GTR50 was also tested in a sequence of shutdowns and restarts to detect time jumps and malfunctions just after the restart and during the following days of operation.In September 2008, the receiver was shipped to PTB and then was operated there for eleven days. One week after it had returned back at ROA, the common-view data showed only a 100 ps difference with respect to the same configuration carried out one week before the trip.In this paper we will present the calibration results of this GPS link, and an evaluation of its type A and B uncertainty. The time transfer results were achieved by using C/A, P3 and also carrier phase PPP comparison techniques. We finally extrapolate these results in order to calibrate the two-way satellite time and frequency transfer (TWSTFT) link between ROA and PTB, using one month of data, and involving one GTR50 from ROA and one Ashtech Z-12T from PTB.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.

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Au/TiO 2 and Au/TiO 2 -CeO 2 catalysts were prepared by the sol-gel method and carbon monoxide, hydrogen chemisorption and TEM spectroscopy have been exploited to determine the size of gold particles. The gold nanoparticles (8.1 to 2.1 nm) were deposited by using the deposition-precipitation method. The XRD characterization shows the presence of anatase as the TiO 2 crystalline phase; while by XPS spectroscopy, the presence of Au°, Au 2 O 3 , Ce 3+ and Ce 4+ species co-existing in the Au/TiO 2 -CeO 2 catalysts is shown. The characterizations by TPD-CO as well as by TPD-H 2 (temperature programmed desorption) showed that on catalysts containing cerium, the gold particle size can be determined with great accuracy by using these chemisorption methods. The gold particle size calculated from either the CO or H 2 thermodesorption values is in good agreement with that obtained by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy (STEM) analyses. It was proposed that the TPD-CO and/or TPD-H 2 techniques could be helpful for the characterization of the gold particles by TEM; especially when the high contrast in the pictures of the supports containing CeO 2 prevents the particle size from being determined.