Time dissemination and synchronization methods to support Galileo timing interfaces

Precise timing is an important factor in the modern information-oriented society and culture. Timing is one of the key technologies for such basic and everyday things, like cellular communications, Internet, satellite navigation and many others. Satellite navigation systems offer cost-efficient and high-performance timing services, and GPS is presently the unchallenged market leader. However, GPS is under military control and does not offer availability and performance guarantees. From a user perspective, this situation will change with the advent of the European satellite navigation system Galileo which shall be operated on a commercial basis by civil entities and shall accept certain liabilities for its services providing also guaranteed service performances. This work is motivated by the new opportunities and challenges related to Galileo timekeeping and applications, and in particular by the necessity to (a) produce and maintain a stable, accurate and robust system timescale which can serve for both accurate prediction of satellite clocks and for the metrological purposes, (b) establish accurate and reliable timing interface to GPS to facilitate Galileo interoperability, (c) maximize user benefits from the new system features like service guarantees and support application development by enabling their certification. The thesis starts with overview of atomic clocks, timekeeping and timing applications. Further Galileo project and system architecture are described and details on Galileo timekeeping concept are given. In addition, the state-of-the-art timekeeping and time dissemination methods and algorithms are presented. Main findings of the thesis focus on (a) Galileo timekeeping. Various options for generation of Galileo system time are proposed and compared with respect to the key performance parameters (stability and reliability). Galileo System Time (GST) stability requirements driven by its navigation and metrological functions are derived. In addition, achievable level of GST stability (considering hardware components) is analyzed. Further, optimization of the present baseline with respect to the design of Galileo Precise Timing Facility (PTF), and its redundancy and switching concepts is undertaken. Finally, performance analysis of different options for generation of the ensemble time is performed and considerations with respect to the role of the ensemble time in Galileo are provided, (b) GPS Galileo timing interface. The magnitude and statistical properties of the time offset are investigated and the impact of the time offset onto the user positioning and timing accuracy is studied with the help of simulated GPS and Galileo observations. Here a novel simulation concept which is based on utilization of GPS data and their scaling for Galileo is proposed. Both GPS and Galileo baseline foresees that the GPS/Galileo time offset shall be determined and broadcast to users in the navigation messages. For this purposes, the offset shall be predicted using available measurement data. Simulations of GPS Galileo time offset determination and prediction are presented. The prediction is made relying on both traditional method and on the advanced techniques like Box-Jenkins prediction (based on the autoregressive moving average approach) and Kalman filter. The end-to-end budgets for different options of GPS Galileo time offset determination are also presented. (c) Galileo interface to timing users (Galileo timing service). The relevance of GST restitution from the metrological point of view is discussed and recognition of GST as a legal time reference is proposed. Assessment of the accuracy of the Galileo timing service is presented. Finally, recommendations for Galileo are provided based on the findings of the thesis

A pulsed-Laser Rb atomic frequency standard for GNSS applications

We present the results of 10 years of research related to the development of a Rubidium vapor cell clock based on the principle of pulsed optical pumping (POP). Since in the pulsed approach, the clock operation phases take place at different times, this technique demonstrated to be very effective in curing several issues affecting traditional Rb clocks working in a continuous regime, like light shift, with a consequent improvement of the frequency stability performances. We describe two laboratory prototypes of POP clock, both developed at INRIM. The first one achieved the best results in terms of frequency stability: an Allan deviation of σy(τ) = 1.7 × 10−13 τ−1/2, being τ the averaging time, has been measured. In the prospect of a space application, we show preliminary results obtained with a second more recent prototype based on a loaded cavity-cell arrangement. This clock has a reduced size and exhibited an Allan deviation of σy(τ) = 6 × 10−13 τ−1/2, still a remarkable result for a vapor cell device. In parallel, an ongoing activity performed in collaboration with Leonardo S.p.A. and aimed at developing an engineered space prototype of the POP clock is finally mentioned. Possible issues related to space implementation are also briefly discussed. On the basis of the achieved results, the POP clock represents a promising technology for future GNSSs

Timing Experiments with Global Navigation Satellite System Clocks

The science of timekeeping is crucial in many dierent applications around the world. One of the most signicative applications in which time and frequency metrology has an essential role are Global Navigation Satellite Systems (GNSS). Any satellite navigation system indeed, is based on the transmission of signals from a constellation of satellites: processing these signals it is possible to estimate the position of a user, provided that the time of transmission is indicated with extremely high accuracy. In fact, being the distance measured from a time, any error in the measure of time will be directly mapped into an error in the user position, which has to be kept below its specied limits. The positioning accuracy is widely determined by the clocks quality. It is why all the satellites need to y very accurate atomic clocks: fundamental for their excellent stability. An agreement between the European Community and the European Space Agency (ESA) gave rise to a new European satellite system: Galileo. The Istituto Nazionale di Ricerca Metrologica (INRiM) is deeply involved in the Galileo project, mainly concerning the activities related to the experimental phases, such as the generation of an experimental reference time scale for the system and the metrological characterization of atomic clocks employed onboard satellites. This thesis will describe the timing experiments carried out in these years of doctorate with GNSS clocks, both with space and ground clocks, within the experimental phases of the Galileo project

The 25th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting

Papers in the following categories are presented: recent developments in rubidium, cesium, and hydrogen-based frequency standards, and in cryogenic and trapped-ion technology; international and transnational applications of precise time and time interval (PTTI) technology with emphasis on satellite laser tracking networks, GLONASS timing, intercomparison of national time scales and international telecommunication; applications of PTTI technology to the telecommunications, power distribution, platform positioning, and geophysical survey industries; application of PTTI technology to evolving military communications and navigation systems; and dissemination of precise time and frequency by means of GPS, GLONASS, MILSTAR, LORAN, and synchronous communications satellites

Coherent fibre-optic link: applications in Time and Frequency metrology, Geodesy, Radio Astronomy and Seismology

L'abstract è presente nell'allegato / the abstract is in the attachmen

Proceedings of the 8th Precise Time and Time Interval (PTTI) Applications and Planning Meeting

The Proceedings contain the papers presented at the Eight Annual Precise Time and Tme Interval PTTI Applications and Planning Meeting. The edited record of the discussions following the papers and the panel discussions are also included. This meeting provided a forum for the exchange of information on precise time and frequency technology among members of the scientific community and persons with program applications. The 282 registered attendees came from various U.S. Government agencies, private industry, universities and a number of foreign countries were represented. In this meeting, papers were presented that emphasized: (1) definitions and international regulations of precise time sources and users, (2) the scientific foundations of Hydrogen Maser standards, the current developments in this field and the application experience, and (3) how to measure the stability performance properties of precise standards. As in the previous meetings, update and new papers were presented on system applications with past, present and future requirements identified

Proceedings of the Eleventh Annual Precise Time and Time Interval (PTTI) Application and Planning Meeting

Thirty eight papers are presented addressing various aspects of precise time and time interval applications. Areas discussed include: past accomplishments; state of the art systems; new and useful applications, procedures, and techniques; and fruitful directions for research efforts