Fibre-optic delivery of time and frequency to VLBI station

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.Comment: 8 pages, 8 figures, matches the version published in A&A, section Astronomical instrumentatio

Monitoring of phase jitter in fibre optic time and frequency transfer systems

The phase jitter enables to assess quality of signals transmitted in a bi-directional, long-distance fibre optic link dedicated for dissemination of the time and frequency signals. In the paper, we are considering measurements of jitter using a phase detector the detected frequency signal and the reference signal are supplied to. To cover the wideband jitter spectrum the detected signal frequency is divided and - because of the aliasing process - higher spectral components are shifted down. We are also examining the influence of a residual jitter that occurs in the reference signal generated by filtering the jitter occurring in the same signal, whose phase fluctuations we intend to measure. Then, we are discussing the evaluation results, which were obtained by using the target fibre optic time and frequency transfer system

Testing Time and Frequency Fiber-Optic Link Transfer by Hardware Emulation of Acoustic-Band Optical Noise

The low-frequency optical-signal phase noise induced by mechanical vibration of the base occurs in field-deployed fibers. Typical telecommunication data transfer is insensitive to this type of noise but the phenomenon may influence links dedicated to precise Time and Frequency (T&F) fiber-optic transfer that exploit the idea of stabilization of phase or propagation delay of the link. To measure effectiveness of suppression of acoustic noise in such a link, a dedicated measurement setup is necessary. The setup should enable to introduce a low-frequency phase corruption to the optical signal in a controllable way. In the paper, a concept of a setup in which the mechanically induced acoustic-band optical signal phase corruption is described and its own features and measured parameters are presented. Next, the experimental measurement results of the T&F transfer TFTS-2 system’s immunity as a function of the fibre-optic length vs. the acoustic-band noise are presented. Then, the dependency of the system immunity on the location of a noise source along the link is also pointed out

Testing Time and Frequency Fiber-Optic Link Transfer by Hardware Emulation of Acoustic-Band Optical Noise

The low-frequency optical-signal phase noise induced by mechanical vibration of the base occurs in field-deployed fibers. Typical telecommunication data transfer is insensitive to this type of noise but the phenomenon may influence links dedicated to precise Time and Frequency (T&F) fiber-optic transfer that exploit the idea of stabilization of phase or propagation delay of the link. To measure effectiveness of suppression of acoustic noise in such a link, a dedicated measurement setup is necessary. The setup should enable to introduce a low-frequency phase corruption to the optical signal in a controllable way. In the paper, a concept of a setup in which the mechanically induced acoustic-band optical signal phase corruption is described and its own features and measured parameters are presented. Next, the experimental measurement results of the T&F transfer TFTS-2 system’s immunity as a function of the fibre-optic length vs. the acoustic-band noise are presented. Then, the dependency of the system immunity on the location of a noise source along the link is also pointed out

Verification of nonlinearities in time interval measurement with a chosen type of time inteval counters

Celem niniejszego artykułu jest zaprezentowanie metody i wyników badań nad nieliniowością częstościomierzyczasomierzy w pomiarach precyzyjnych przedziałów czasu, prowadzonych w GUM przy współpracy z AGH. Dokonano porównań wyników pomiarów dla częstościomierzy-czasomierzy SR620 i HP 53132A zarówno stałych i zmiennych przedziałów czasu. Potwierdzono przydatność dwukanałowego generatora/dzielnika typu Dual Channel 1/100 PPS Generator oraz komparatora częstotliwości typu A7-MX.In this paper we present the method and the last results of verification of nonlinearities in time interval measurements with the usage of Time Interval Counters (TIC), performed at Central Office of Measures (GUM) with cooperation of AGH University of Science and Technology. We consider the non-ideal operation of TIC manifested by fluctuations of differences between the current indication of TIC and the real value of the measured time interval. We observed instabilities of indications of TIC, even if the measured time interval is keeping constant. For verification of TIC operation at continuously changed time intervals, it was set up a special measurement system that performs simultaneous measurements of the phasetime changes between two standard frequency signals (with the usage of A7-MX standard frequency comparator) and time intervals between two pulse signals (with the usage of TIC under test). Due to applying a special, developed by AGH, dual channel 1/100 pps generator, the measured 1 pps (1 pulse per second) and 10 MHz signals are standardized and the phasetime between 1 pps and 10 MHz signals are kept in pairs constant. Differences between the indications of TIC and the proper measurement results for frequency allow to verify internal nonlinearities of TIC in time interval measurements. Here, we show and shortly discuss the results obtained for SR620 and 53132A universal counters. The obtained range of fluctuations of indications of SR6200 with relation to the measured time interval approaches about 190 ps, and for 53132A – about 450 ps, whereas the observed short-term noise of measurement for SR620 is about 10 ps, and for 53132A – about 100 ps typically. It was confirmed the complex and random influence of many factors on operation of TIC. Our investigation will be continued with the usage of 100 pps (100 pulses per second) signals and with automatic switching of the measured pulse signals in pairs

Precise time and frequency transfer over optical fiber between GUM-AOS (420 km)

W artykule przedstawiono najnowsze wyniki, prowadzonych w GUM i AOS (Obserwatorium Astrogeodynamiczne CBK PAN) w ścisłej współpracy z AGH, badań nad precyzyjnym transferem czasu i częstotliwości przez włókna światłowodowe w oparciu o uruchomione pomiędzy GUM i AOS operacyjne łącze światłowodowe ze stabilizacją opóźnienia, o łącznej długości optycznej około 420 km. Dokonano kalibracji łącza na poziomie niepewności rozszerzonej 0,25 ns. Uzyskano zgodność wyników z wynikami uzyskanymi z metody GPS CV i GPS PPP. Ponadto potwierdzono poprawność przyjęcia poprawek kalibracyjnych w użytkowanych w GUM i AOS systemach TTS-4 i potwierdzono ogromną przydatność metod światłowodowych do rozwoju i oceny stabilności i dokładności satelitarnych metod transferu czasu i częstotliwości.In this paper there are presented the newest results of investigation carried on at Central Office of Measures (GUM - Główny Urząd Miar) and Astrogeodynamical Observatory of Space Research Center PAS (AOS) in close cooperation with AGH Technical University (Akademia Górniczo-Hutnicza), in the scope of precise time transfer over optical fibers [1-9]. Since January 2012 the constant operational optical link with stabilization of propagation delay for time and frequency transfer has been run between two UTC(k) time and frequency laboratories: GUM – maintaining UTC(PL), and AOS – maintaining UTC(AOS), over 420 km of optical link [10]. Si-multaneously, there are carried on continuous time transfer with TTS-4 dual-frequency GNSS systems and comparison of measure-ment results obtained from the fiber method and GPS CV or GPS PPP method is possible. It allows to verify calibration results and assess the relative accuracy and stability of the all methods. Calibration of the optical link was performed on the base of local measurements with expanded uncertainty of 0,25 ns, with no need of usage of other time transfer systems or travelling standards or receivers. Calibration results agreed with satellite methods: GPS CV and GPS PPP within the range of about ± 1 ns (Fig. 4, Section 4), which confirms the correctness of adopted calibration method in practice. Simultaneously, this confirms the correctness of adopted calibration corrections in TTS-4 systems used at GUM and AOS for remote time transfer. The obtained results confirmed also the huge useful-ness of our optical method of time and frequency transfer for development and investigation accuracy and stability of satellite methods within this domain (Fig. 5 and 6, Section 4)