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The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons. Very long baseline interferometry is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources. This approach overcomes limitations of the performance of satellite transfer techniques.

We have developed a broadband VLBI (very long baseline interferometry) system inspired by the concept of the VLBI Global Observing System (VGOS). The new broadband VLBI system was implemented in the Kashima 34 m antenna and in two transportable stations utilizing 2.4 m diameter antennas. The transportable stations have been developed as a tool for intercontinental frequency comparison but are equally useful for geodesy. To enable practical use of such small VLBI stations in intercontinental VLBI, we have developed the procedure of node-hub style VLBI: In joint observation with a large, high sensitivity ‘hub’ antenna, the closure delay relation provides a virtual delay observable between ‘node’ stations. This overcomes the limited sensitivity of the small diameter node antennas, while error sources associated with large diameter antennas, such as gravitational deformation and delay changes in necessarily long signal cables, are eliminated. We show that this scheme does not result in an increased sensitivity to radio source structure if one side of the baseline triangle is kept short. We have performed VLBI experiments utilizing this approach over both short range and intercontinental distance. This article describes the system components, signal processing procedure, experiment, and results in terms of baseline repeatability. Our measurements reveal signatures of structure effects in the correlation amplitude of several of the observed radio sources. We present a model of the frequency-dependent source size for 1928+738 derived from correlation amplitude data observed in four frequency bands.

For the aim to enable precise frequency transfer over earth radius scale distance, NICT developed a broadband VLBI system observing 3-14 GHz radio frequency range, and that was implemented in a pair of transportable 2.4m diameter VLBI stations and Kashima 34m diameter VLBI station. The small diameter VLBI stations installed in Medicina observatory of INAF (Italy) and NICT headquarters in Koganei (Japan) were used compare the atomic frequency standards. The poor sensitivity of VLBI observation with the small antenna pair was compensated by participation of the large diameter antenna for boosting the signal to noise ratio (SNR) of the VLBI observation. Owing to the broad observing frequency range, a few pico seconds precision of group delay is achieved on intercontinental baseline even with small diameter antenna. Advanced digital data acquisition with highspeed 16 GHz sampling enabled stable group delay measurement. A series of frequency transfer experiments was conducted during Oct. 2018 – Feb. 2019 by using the broadband VLBI system between ytterbium (Yb) optical lattice clock of INRiM (Italy) and strontium (Sr) lattice clock of NICT(Japan) separated by 9000km. The deviation of fractional frequency ratio between Yb and Sr was obtained by VLBI observation as y(Yb/Sr)=2.5(2.8)x10-16. The uncertainty in this experiment is evaluated not to be limited by the VLBI link itself, and further improvement is expected. Prospect of VLBI frequency link and uncertainty budget in VLBI observation is discussed in this report.

We present a comparison of time transfer results over European baselines obtained with two space-based techniques: Precise Point Positioning with integer ambiguity resolution (IPPP) using GPS satellites and Time Transfer by Laser Link (T2L2) using the low orbiting satellite Jason-2. The data was obtained in a 3-month experiment in Autumn 2013 for baselines between Paris, Herstmonceux and Grasse. We find that the difference between the 30-s IPPP link results and the closest 1-s T2L2 data points has a standard deviation below 100 ps for all three baselines. These results provide the basis for a continuous time transfer technique with accuracy at the 100 ps level for baselines over which T2L2 can operate in CV. Occasional T2L2 measurements provide the absolute time difference with a high accuracy and a continuous IPPP link maintains the accuracy for weeks to months between calibrations.

The Precise Point Positioning (PPP) technique using GPS phase and code observations is widely used to compute time and frequency links. We have shown that a PPP technique where the integer nature of phase ambiguities is preserved provides significant improvement over the classical use of floating ambiguities. We show examples that all indicate that this Integer-PPP (IPPP) technique allows frequency comparisons with 1×10−16 accuracy in a few days, improving with longer averaging, and can be readily operated with existing products.

We report on the initial use of Galileo signals in the Time and Frequency Department of Physikalisch-Technische Bundeanstalt (PTB). To this end PTB operated five GNSS receivers capable of receiving Galileo Open Service signals with calibrated signal delays for the two frequencies E1 and E5a.

The recent activities in the field of time and frequency dissemination and applications at PTB resulted in improved quality and a wider reach of the offered services. The basis of these activities remains PTB’s local realization of Coordinated Universal Time UTC(PTB). PTB’s time services comprise the operation of NTP time servers, a telephone time service and the standard frequency and time dissemination by the low frequency transmitter DCF77. The number of requests of the public NTP service approaches 7000 per second. The number of accesses to the telephone service remains steady at about 1800 per day. The standard frequency and time signal transmitter DCF77 is still the most prominent of PTB’s time services. The number of operational receivers in Germany and Europe is estimated to be more than 100 Million. In summer 2016 a calibrated GNSS receiver was installed at the transmitter site. Time comparisons between the caesium clock which serves as the reference for the transmitted signal and UTC(PTB) have since then been made with an uncertainty of 5 ns via GPS and Galileo common view. Time and frequency comparisons with institutes all over the world represent an important part of the activities. PTB continues to be the pivot in all GPS and GLONASS comparisons by which the metrology and research institutes world-wide contribute to the realization of UTC. Data from four different GNSS receivers are regularly provided to BIPM. PTB’s “golden receiver”, a legacy Ashtech ZXII-T (acronym PT02), has been found stable and reliable in the very long term, but shows annual delay variations with a peak-to-peak amplitude of about 1 ns, likely related to the outdoor temperature affecting the antenna cable delay. A possible new reference receiver was identified and is going to be tested before eventually taking over the status of PT02. PTB has accepted the role of a so-called “Group1” laboratory that should offer (in cooperation with BIPM and together with French and Spanish institutes) calibrations of GPS time links to other European NMIs and other research institutes. A so-called G1G2 campaign was finished in August 2016, during which relative receiver internal delays of in total 11 receivers belonging to the institutes DLR, METAS, VSL, and BEV were determined with reference to the PT02 delays determined by BIPM in 2014. Draft results of the 2016 BIPM campaign showed that PT02 delay changes amount to not more than 0.5 ns. A similar exercise using travelling GPS receivers was organized by SYRTE/Observatoire de Paris in the frame of the Galileo Time and Geodetic Validation Facility (TGVF) project managed by GMV, Spain, providing similar results. A TWSTFT calibration campaign was once more undertaken by the U.S. Naval Observatory and revealed the excellent long-term stability of the link between USNO and PTB via TWSTFT. The difference between successive calibration values is at the 1 ns level.

GNSS timing is currently used worldwide in critical real-time systems that require precise synchronisation or time-stamping at geographically dispersed sites, such as wireless telephone stations, electrical power grids and financial services. For applications where multiple timing equipment needs to be deployed, it is desirable to use low-cost, single-frequency receivers. Together with calibration issues, the ionospheric delay is the major limiting factor for accurate timing in single-frequency setups. Increasing demands in cost-saving and accuracy will make the availability of precise single-frequency solutions more and more interesting.

Global satellite navigation systems (GNSS) are a standard measurement device for deformation monitoring. In many applications, double-differences are used to reduce distance dependent systematic effects, as well as to eliminate the receiver and satellites clock errors. However, due to the navigation principle of one way ranging used in GPS, the geometry of the subsequent adjustment is weakened. As a result, the height component is generally determined three times less precisely than the horizontal coordinates. In addition, large correlations between the height and elevation dependent effects exist such as tropospheric refraction, mismodelled phase center variations, or multipath which restricts the attainable accuracy. However, for a kinematic analysis, i. e. for estimating high rate coordinate time series, the situation can be significantly improved if a common clock is connected to different GNSS receivers in a network or on a baseline. Consequently, between-station single-differences are sufficient to solve for the baseline coordinates. The positioning geometry is significantly improved which is reflected by a reduction of the standard deviation of kinematic heights by about a factor 3 underlining the benefits of this new approach. Real data from baselines at the Physikalisch-Technische Bundesanstalt campus at Braunschweig where receivers are connected over 290 m via an optical fiber link to a common clock was analysed. © 2016 Walter de Gruyter GmbH, Berlin/Munich/Boston.

GMV is the prime contractor for the Time and Geodetic Validation Facility (TGVF) in the Galileo FOC phase (Full Operational Capability), a contract of the European Space Agency (ESA). Within the TGVF, the Time Validation Facility (TVF) is the subsystem in charge of steering Galileo System Time (GST) to UTC, among other duties. The TVF is operated at GMV headquarters near Madrid, Spain. Calibrated Galileo receivers are needed in the frame of TVF activities to, among other tasks, assess the UTC-GST offset broadcast in the Galileo navigation message. Absolute receiver calibration is a complex activity involving the availability of a signal simulator and of a calibrated reference antenna. An alternative data-based method to evaluate the Galileo receiver delay in E5 signals relative to GPS is also possible. The key feature of such method is the cancellation of the GPS and Galileo ionospheric delays when combining pseudoranges from two satellites with a close position in the sky. A software tool called gecal has been developed by GMV within the Galileo TVF, implementing such method and processing RINEX 3 observation files. Satellite positions are read from a SP3 orbit file. The tool allows the rapid E5 calibration of a new or existing Galileo receiver. This paper describes the tool and the calibration of three Galileo receivers used in the TVF. Galileo Signal-In-Space validation results obtained using such receivers are also presented.

We characterize a GPS carrier-phase link with 450 km baseline, using a long-distance phase-stabilized optical fiber link. Active hydrogen masers at both ends are compared via both links simultaneously. This allows us to assess the instability and accuracy of the GPS link with sub-10^-15 resolution.