The recent breakthroughs in the distribution of quantum information and high-precision time and frequency (T F) signals over long-haul optical fibre networks have transformative potential for physically secure communications, resilience of timing infrastructure (such as that supporting Global Navigation Satellite Systems (GNSS)) and fundamental physics. To date, these capabilities remain confined to isolated testbeds, with quantum and T F signals accessible, for example in Germany, to only a few institutions. In this white paper we propose the QTF Backbone: a dedicated national fibre-optic infrastructure in Germany for the networked distribution of Quantum and T F signals using dark fibres and specialised hardware. The QTF Backbone is planned as a four-phase deployment over ten years to ensure scalable, sustainable access for research institutions and industry. The concept builds on successful demonstrations of time and frequency distribution at high Technology Readiness Levels (TRLs) across Europe, including PTB–MPQ links in Germany, REFIMEVE in France, and the Italian LIFT network. The QTF Backbone will enable transformative Research and Development (R D), support a nationwide QTF ecosystem, and ensure the transition from innovation to deployment. As a national and European hub, it will position Germany and Europe at the forefront of quantum networking, as well as T F transfer.

A system for monitoring and analyzing GNSS performance is being developed at DLR’s Galileo Competence Centre. Various parameters are monitored in order to characterize the performance of the four global satellite navigation systems. The focus of this contribution is on the extension of our system with monitoring and analysis capabilities for the Galileo HAS and specifically for the Internet Data Distribution component of the service. An initial analysis of the content of the HAS messages and the positioning performance of the service at example IGS station locations is discussed. Our analysis shows the continuous availability of the internet distribution component of the service and a horizontal positioning performance of the order of 20 cm or better for 95% of the epochs of the selected time periods and for the selected station locations. In addition to HAS, we also examined an additional set of performance parameters, which show similar performance levels between the Galileo, GPS and BeiDou systems.

Global Navigation Satellite Systems (GNSSs), such as GPS and Galileo, provide precise time and space coordinates globally and constitute part of the critical infrastructure of modern society. To reliably operate GNSS, a highly accurate and stable system time is required, such as the one provided by several independent clocks hosted in Precise Timing Facilities (PTFs) around the world. The relative clock offset between PTFs is periodically measured to have a fallback system to synchronize the GNSS satellite clocks. The security and integrity of the communication between PTFs is of paramount importance: if compromised, it could lead to disruptions to the GNSS service. Therefore, securing the communication between PTFs is a compelling use-case for protection via Quantum Key Distribution (QKD), since this technology provides information-theoretic security. We have performed a field trial demonstration of such a use-case by sharing encrypted time synchronization information between two PTFs, one located in Oberpfaffenhofen (Germany) and one in Matera (Italy)—more than 900 km apart. To bridge this large distance, a satellite-QKD system is required, plus a “last-mile” terrestrial link to connect the optical ground station (OGS) to the actual location of the PTF. In our demonstration, we have deployed two full QKD systems to protect the last-mile connection at both locations and have shown via simulation that upcoming QKD satellites will be able to distribute keys between Oberpfaffenhofen and Matera, exploiting already existing OGSs.

Global Navigation Satellite Systems (GNSSs), such as GPS and Galileo, provide precise time and space coordinates globally and constitute part of the critical infrastructure of modern society. To reliably operate GNSS, a highly accurate and stable system time is required, such as the one provided by several independent clocks hosted in Precise Timing Facilities (PTFs) around the world. Periodically, the relative clock offset between PTFs is measured to have a fallback system to synchronize the GNSS satellite clocks. The security and integrity of the communication between PTFs is of paramount importance: if compromised, it could lead to disruptions to the GNSS service. Therefore, it is a compelling use-case for protection via Quantum Key Distribution (QKD), since this technology provides information-theoretic security. We have performed a field trial demonstration of such use-case by sharing encrypted time synchronization information between two PTFs, one located in Oberpfaffenhofen (Germany) and one in Matera (Italy) - more than 900km apart as the crow flies. To bridge this large distance, a satellite-QKD system is required, plus a "last-mile" terrestrial link to connect the optical ground station (OGS) to the actual location of the PTF. In our demonstration we have deployed two full QKD systems to protect the last-mile connection at both the locations and have shown via simulation that upcoming QKD satellites will be able to distribute keys between Oberpfaffenhofen and Matera exploiting already existing OGSs.

SummaryIn this mansucript we present a setup and first results for long-term operation of an optical clock based on an existing Doppler-free iodine vapor cell spectroscopy unit [1] acting as an optical frequency reference system and an optical frequency comb. The combination of comb and optical frequency reference represents an ultra-stable radiofrequency source, as the comb is transferring the stability of the optical frequency reference to the radiofrequency regime. The radiofrequency is therefore generated directly from the pulse train of the optical frequency comb. The frequency comb is optically phase locked to the optical frequency reference based on molecular iodine, resulting in a full optical-to-radiofrequency chain. We will present the phase and amplitude noise performance of this radiofrequency source, i.e. an optical clock. The short-term stability in terms of Allan deviation is determined in an optical comparison measurement with a cavity stabilized laser system with a sub-Hz linewidth via the frequency comb. However, our main focus lies on the investigation of the long-term stability of the clock chain in the radiofrequency domain in comparison to the UTC realization of the German Aerospace Center (represented by the Galileo Competence Center) and the continuous operation of an optical clock.

A bi-directional optical link is established over 10.45 km to perform Two-Way Frequency and Time Transfer (TWFT, TWTT) to validate the system design for Lower Earth Orbit (LEO) to ground links through the atmospheric channel. Binary phase shift keying with a homodyne reception scheme is used to recover the carrier frequency. The time transfer is enabled by optical correlation and dynamic tracking of a 25.6 Gcps transmission spread-sequence with a locally generated reference sequence. An additional data transmission of 50 Mbps is performed on the same channel. Optical references and frequency combs on both sides enable the frequency measurements. Ultra-stable oscillators generate Radio Frequency (RF) references for the time transfer. The performance of the system is evaluated under different atmospheric conditions. Frequency transfer with a stability of 5 · 10^?15 at 1 s gate time. Simultaneous two-way time transfer with a stability of 1 · 10^?13 at 1 s gate time prove the feasibility of the system even through the atmospheric channel.

A fundamental component in each Global Navigation Satellite System is the timing facility, which is responsible for the synchronization of all elements within the ground segment as well as for the satellites in space.Different concepts exist to provide a time scale that fulfils all requirements, e.g.master clock principle, weighted clocks or composite clock algorithm.In the European Global Navigation Satellite System Galileo, the so-called Precise Timing Facility (PTF), located in Oberpfaffenhofen, Germany, and Fucino, Italy, has the task to provide the Galileo System Time (GST).The actual published design of the PTF depends on a master clock principle and is therefore sensitive to failures of individual units within the GST generation [1].At DLR Galileo Competence Center we propose an alternative design for such a timing facility, which is called Robust Precise Timing Facility (RPTF).The focus of the concept is on mitigating technical vulnerabilities and increasing the tolerance to the failure of any of the components of the existing PTF.In this paper we present the concept, current status and future plans for such a RPTF to generate robust system timescales for the next generation of the European satellite navigation systems.The aim is to build, test and characterize the RPTF under the aspect of a 24/7 operational service.The key element is the combination of all the individual atomic clocks in the facility via the composite clock approach to generate a weighted average, the so-called Implicit Ensemble Mean (IEM), and to provide redundancy and test opportunities in case of hardware and software failures.

: Satellite navigation has become a vital part of our daily lives by ensuring navigation on land, in air and at sea, and by providing precise timing information for the energy, communications and finance sector. It is therefore essential to monitor the performance of the four main global navigation satellite systems (GNSS) Galileo, GPS, GLONASS and BeiDou. The Galileo Competence Center (GK), part of the German Aerospace Center (DLR), is dedicated to the further development of the European GNSS consisting of Galileo and EGNOS. Within the SigPerMon project, the GK monitors the reliability and quality of navigation signals with comparable metrics for all four GNSS, and detects deviations from the nominal state of navigation systems. Necessary data are sourced from a global network of GNSS receiver stations. These data are used to compute performance indicators to monitor and analyse the availability and health status of navigation signals, and the precision of positioning and timing solutions. In the future, machine learning models will be used to detect anomalies in the satellite signals. A summary of the results will be presented on a dedicated webpage, which provides both detailed analyses for authorized researchers and personnel, and interactive data visualizations for the general public.

The application of precise point positioning with broadcast ephemerides (PPP-BCE) is discussed as an alternative to the established all-in-view technique for multi-GNSS time transfer. It combines the use of broadcast ephemerides with low-noise carrier-phase observations for accessing GNSS system time scales and Coordinated Universal Time (UTC) with improved precision, and can be employed on stationary as well as mobile receivers in offline or real-time analyses. Using calibrated timing receivers, the method is shown to provide estimates of the GNSS-to-GNSS time offsets (XYTOs) with an accuracy at the 2 ns level. In the absence of prior calibrations, 0.5 ns consistency across different stations is achieved for GPS, Galileo, and BeiDou-3 after adjustment of systematic biases in comparison with calibrated reference stations or broadcast XYTO values. Furthermore, access to GNSS-specific UTC realizations can be obtained through predictions of the UTC offset from GNSS system time as provided in the broadcast ephemerides of individual constellations. The overall quality of the PPP-BCE-derived receiver clock offsets from UTC is assessed using calibrated receivers at various timing laboratories along with BIPM-provided UTC-UTC(k) measurements. Over the 1.5 years covered in the study, an accuracy of 1.8 ns for GPS and 2.5 ns for Galileo is demonstrated. For BeiDou, a slightly worse accuracy of 3 ns is obtained for a single timing laboratory over 9 months.

Global Navigation Satellite Systems (GNSSs) are indispensable for numerous daily life applications. They provide essential services for both civil and military uses. Other than enabling global navigation, GNSSs also allow the distribution of a stable time scale used in various sectors such as aviation, ground and maritime traffic, surveying, synchronization of power grids and stock trading activities. Thus, one key element of such systems is the generation of a stable and robust system time, which can be referenced to a global time scale, i.e. Coordinated Universal Time (UTC). There are several different approaches to ensure an appropriate system time for operational systems, like Global Positioning System (GPS) as an example for ensemble system time or Galileo for a master clock approach. In this paper we will present our latest results on composite clock realization with a set of cesium clocks. One of these clocks is used as the UTC realization by DLR. For our composite clock solution, we used a Kalman filter to predict the clock states, integrated differential clock measurements and produced an Implicit Ensemble Mean (IEM). A second Kalman filter was used to calculate the steering commands necessary to align a clock to the IEM of the ensemble. A regulator was used to implement the required steering. Simulation, long term measurements of an ensembles of cesium clocks and the realization of the IEM have been performed over a period larger than 10(5)s. This demonstration can be seen as the next step of DLR's contribution to UTC with a physical realization of a robust clock ensemble.

Global navigation satellite systems need a stable and robust system time in order to provide services with high precision. A common approach for its generation is the composite clock method, where an ensemble of clocks contributes to the definition of the system time as a weighted average of the single clocks. In this way the robustness of the so generated time scale is enhanced, since a failure in one clock can be compensated by the others. The core part of the composite clock method is a Kalman filter, which estimates the future states of each clock depending on the relative measurements between all clocks. The Kalman filter implicitly provides the system time of the ensemble, in terms of the implicit ensemble mean (IEM) - a so called paper clock. However, this quantity is not directly available, since it requires knowing the exact state of each clock at each time step which is not directly measurable. A solution consists in realizing the IEM by steering a clock signal towards the IEM. Different steering techniques can be used, for instance the pole placement method and the linear quadratic Gaussian regulator. Both the Kalman filter and the regulator are based on a clock model, which describes the clock as a linear dynamic system. Hence, the performance of the used clock has implications in every step of the ensembling and realization algorithms. This paper discusses the advantage and disadvantages of different clock models and gives a recommendation on which one is most promising for future investigations. The 3-state clock model and the 2-state clock model with and without drift are described and expanded with additional Gauss-Markov processes, to account for floors of flicker noise. These models are then compared by considering four properties, namely their precision, the number of states, their theoretical formulation and controllability. The precision refers to which noise characteristics a model is able to describe and how well each model is suited to fit a given clock behavior. The theoretical formulation is studied by means of the theoretical expression of the overlapping Allan deviation (OADEV), which is used to fit a model to real measurements. Finally, the controllability of the dynamic model is needed in order to use the aforementioned control techniques. In light of these criteria, the 2-state clock model with drift and Gauss-Markov processes has been chosen, since it is the one providing a good modeling precision and controllability while being simple and flexible. Furthermore, in order to more easily determine the model parameters, a semi-automatic fitting procedure has been developed. Previously, once a clock measurement was given in terms of OADEV, the parameters were found by trial and error, meaning that the model's dynamics was simulated, its OADEV computed and compared with the measured one. This procedure was repeated until the simulation described the measurement in a reasonable way. However, it is possible to describe the OADEV with a function which can be directly used to fit the measurement data. Several techniques are here described and have been implemented into a graphical interface which eases the fitting procedure.

Clock steering is an important discipline within the time and frequency community as well as in cognate disciplines such as global navigation satellite systems (GNSSs). In the latter, clock steering is a mandatory tool for realizations of time scales, which first are typically created by algorithms forming a paper clock. Often, this software solution of a time scale is already sufficient. However, sometimes a realization in the form of a frequency signal is feasible or even necessary. Future GNSSs concepts plan to use composite clocks consisting of several different clocks to achieve synchronization of satellites within one orbit with the system time. In this approach, a real clock output in the microwave regime is indispensable. Therefore, a real clock has to be steered against the paper clock to realize the composite clock. Consequently, steering techniques are an essential piece for obtaining a realization. Besides the number of algorithms to choose from, there are several parameters that need to be defined by the user to achieve the best performance for distinct scenarios. In this paper we compare the realization performance and applicability of two different steering techniques, namely linear-quadratic Gaussian control and pole placement, in a simplified hardware setup. The studied scenario consists of a steerable clock consisting of a micro phase stepper which changes the frequency of the output of an oven-controlled quartz oscillator (OCXO) and a rubidium atomic clock (RB). The OCXO in turn is steered to follow the free-running RB as closely as possible. Steering the output of the OCXO introduces a new process in the clock. As a result, the Allan deviation of the steered clock shows a bump which location is dependent on the chosen control interval and steering technique. This has to be considered for the designated operation. We compare simulations of the clock steering with both techniques to real-data results from our laboratory.

This manuscript reviews recent progress in optical frequency references and optical communication systems and discusses their utilizations in global satellite navigation systems and satellite geodesy. Lasers stabilized with optical cavities or spectroscopy of molecular iodine are analyzed, and a hybrid architecture is proposed to combine both forms of stabilization with the aim of achieving a target frequency stability of 10(-15) [s/s] over a wide range of sampling intervals. The synchronization between two optical frequency references in real-time is realized by means of time and frequency transfer on optical carriers. The technologies enabling coherent optical links are reviewed, and the development of an optical communication system for synchronization, ranging and data communication in space is described. An infrastructure exploiting the capabilities of both optical technologies for the realization of a modernized constellation of navigation satellites emitting highly synchronized signals is reviewed. Such infrastructure, named Kepler system, improves satellite navigation in terms intra-system synchronization, orbit determination accuracy, as well as system monitoring and integrity. The potential impact on geodetic key parameters is addressed. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

Global navigation satellite systems need a stable and robust system time in order to provide services with high accuracy. A well established approach for the generation of such a time scale is the composite clock method. Differently from single clock approaches, a set of clocks is used to define a system time scale by generating a weighted average of their single contributions. The clocks are measured with respect to each other and the measured signals are fed into a Kalman filter, which implicitly provides the system time of the ensemble, in terms of the implicit ensemble mean (IEM), a so called paper clock. The IEM generally exhibits a better stability than every single clock in the ensemble for all sample intervals. However, this quantity is not directly available in hardware, but can be realized by steering a clock with a dedicated control loop containing a second Kalman filter and a regulator. The output of the steered clock provides in this way a physical realization of the IEM. The system realizing the IEM is quite complex and the resulting performances depend on a variety of factors. Furthermore, the resulting behavior can only be evaluated after a sufficient amount of data is collected, which can take a long time. For these reasons, it is desirable to have a tool for simulating this system and thus analyzing the resulting IEM before launching extensive measurements. This paper describes how such a system can be simulated, by providing a flexible environment to prove a given setup of clocks and settings. The simulation algorithm is tested with scenarios of increasing complexity: firstly, for validation purposes, the simulation results are compared with real measurements. Secondly, different compositions of mixed ensembles are simulated. Then, the simulation is run by using different regulators and different parameters. Finally, the effects of mismodelling on the resulting IEM and its realization are assessed. In light of the long term goal of setting up a mixed clock ensemble in hardware, these analyses can reduce the invested time, as well as pointing out which aspects of the composition algorithm must be researched with particular care.

Robust, reliable and stable time scales are needed in a wide field of applications, i.e. in the Global Navigation Satellite System (GNSS) sector. The performance of GNSSs for ground users in terms of navigation is directly connected with the accuracy and stability of the related time scales as errors in time directly propagate into the precision of positioning on earth. Hence, in order to achieve improved robustness as well as stability, future GNSS time concepts should be based on clock ensembles and not on a single clock signal. This concept is already used for prominent time scales such as GPS system time or Universal Time Coordinated. However, in order to generate a physical output of such clock ensembles, a single clock signal is steered towards the composite clock solution. Thus, the steering process has significant impact on the overall performance of the realization of a time scale generated by an ensemble of clocks. Hence, this paper will present our latest results on clock steering. We will compare simulations with real measurements where one clock is steered to another. We used a Kalman filter approach predicting future clock states and the linear quadratic regulator as well as pole placement to calculate the required steering parameters. The simulations for a variety of different steering parameters are then subsequently compared to real clock measurements to verify the applicability and the performance of the different steering techniques. This can be seen as an intermediate step towards realizing a composite clock consisting of different clock types in future investigations.

The quality of the positioning and time dissemination services provided by Global Navigation Satellite Systems is tightly coupled with the provision of a stable system time scale. The robustness of time generation is important to ensure the continuity of service without performance degradation. A well-established method to generate a stable and robust time scale is the composite clock approach. Here, the contributions from each clock are combined to generate a weighted average, the so-called Implicit Ensemble Mean. A clock ensemble offers an enhanced robustness with respect to a system depending on a master clock, since the IEM is less sensitive to failures of single units. The time scale generation can be monitored by employing Fault Detection and Identification techniques. These are based on statistical tests designed to detect and identify faults at single clocks. In this work, these techniques are applied to provide statistically rigorous fault detectors for an ensemble of clocks. The clock composition algorithm can be equipped with a number of statistical tests, which are applied to detect and identify abrupt events (e.g. phase and frequency jumps) and slow deviations from the nominal expected clock behavior.

The ionospheric delay of global navigation satellite systems (GNSS) signals typically is compensated by adding a correction value to the pseudorange measurement. We examine the ionospheric signal distortion beyond a constant delay. These effects become increasingly significant with increasing signal bandwidth and hence more critical for the new broadband navigation signals. By simulation, we first demonstrate that the signal modulation constellation diagram is particularly susceptible to the influence of the ionosphere already at moderate electron content. Using high gain antenna measurements of the Galileo E5 signal, we then verify that the expected influence can indeed be observed and compensated. A new method based on a binned maximum likelihood estimator is derived to estimate the total electron content (TEC) from a single frequency high gain antenna measurement of a broadband GNSS signal. Results of the estimation process are presented and discussed comparing to common TEC products such as TEC maps and dual-frequency receiver estimates.

Clock ensembling is a well-established concept for creating robust and stable time frames, e.g. it is used to define the Universal Time Coordinated as well as the GPS system time. In order to reference Global Navigation Satellite Systems (GNSSs) to a terrestrial time scale, the provision of a robust, reliable and stable system time is a mandatory key aspect. Here, we propose a mixed clock ensemble for system time generation in future GNSSs that exploit inter-satellite optical links to synchronize all elements of the constellation. We consider a Kalman filter-based algorithm which combines several different clock types. The algorithm computes predictions based on advanced clock models and updates the predicted state vector by processing differential measurements between the clocks in the ensemble. The state vector, which includes all parameters describing the clock behavior (i.e. phase, frequency and potentially Gauss-Markov states) is then used to produce a weighted average, named Implicit Ensemble Mean (IEM). The so-formed Composite Clock (CC) typically performs at the level of the best frequency reference of the ensemble for all time intervals in terms of stability. However, the so computed IEM is only a software solution with no physical/electrical output signal. In some cases a real signal output of this IEM is desirable or even mandatory, i.e. if it is used for synchronization of signals broadcasted by navigation satellites. The proposed mixed clock ensemble consists of both optical frequency references and classical microwave clocks. With this ensemble it is possible to realize a CC with fractional frequency stability below 10-15 [s/s] for time intervals between 100 and 105 s, serving as system time as well as an essential component of the synchronization scheme.

The development of robust and accurate time scales is a key parameter for various future applications, i.e., in the global navigation satellite system sector. A promising concept is the composite clock approach which combines several clocks to produce a time scale which performs better than the individual inputs. One method to generate a composite time uses a Kalman filter to compute the Implicit Ensemble Mean (IEM). This produces only a software timescale but a physical realization may also be required, e.g. to create a Universal Coordinate Time contribution in a time lab. In a simplified manner, this can be achieved by combining the IEM with phase measurements of a real clock in a second Kalman filter. The second Kalman filter generates control values which can be applied to the real clock in turn steering its behavior to the generated IEM. To date, these control values are primarily calculated using the Linear Quadratic Gaussian (LQG) control technique. LQG control is a very flexible technique where the user can tune three different parameters to optimize the control values. However, this flexibility also makes it difficult to find the best parameter set for a given scenario. In this paper we investigate the use of an alternate technique to generate the aforementioned control values. This Pole Placement (PP) technique is designed for use in a closed loop system such as the one defined in the composite clock approach. One determines the poles of the system and moves them to desired pole locations. The poles are obtained by identifying the eigenvalues of the system's state transition matrix. The benefit of this technique is that only one parameter, the desired pole location, must be fed into the system. This allows for significant simplification of the control value optimization. To evaluate this technique, we performed simulations in which two fictional clocks are steered against each other. We demonstrate sufficient steering performance using PP thus allowing for easier optimization of the control parameters in future applications.

Clock ensembling is a promising concept for future time scale generation as robustness and stability can be considerably improved compared to master clock approaches. While ensembles consisting of the same clock types were already demonstrated to improve robustness of the generated time scale, ensembles using different clock types can clearly benefit from the advantages of the individual single clocks contributing to the time generation process. In particular, the performance of the ensemble time can be improved over a wide range of averaging times by combining clocks exhibiting superior stabilities for distinct time scales. In future, this approach could be used to integrate newly developed clock types such as e.g. optical clocks into a clock ensemble.