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Installation of GNSS antennas on the rooftop of PTB’s clock hall. The direction of view is toward the northeast. Starting from the new PTBB antenna in the foreground, we see toward the background the antennas of receivers PT09, PT10 of type MESIT GTR51 and GRCP of type MESIT GTR55, respectively
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The realization of Coordinated Universal Time, one of the tasks of the International Bureau of Weights and Measures, relies on a network of international time links which currently is organized in a star-like scheme that links all contributing laboratories. GPS signal reception is the technique most widely employed by the laboratories. The PTB curr...
Citations
... Global Navigation Satellite Systems (GNSSs) have been an essential part of frequency dissemination, UTC traceability and comparison systems at many national metrological institutes [1][2][3][4][5]. The realized frequency instability is not yet fulfilling the requirements for all geodetic applications. ...
Realizing a clock-based geodetic network with a relative uncertainty level of 10^-18 has been a significant objective for the scientific community. This network can be utilized for realizing more accurate geodetic reference frames and for testing the fundamental laws of physics, such as the theory of relativity. Typically, optical fibers are connecting optical clocks in such a network. For the last decades, Global Navigation Satellite Systems (GNSSs) have built a trustful and easy-setup method for frequency and time transfer. However, recently optical fiber link networks showed better frequency instability. In this study, we investigate the limits of GNSS-based frequency transfer links with the help of an optical fiber link as ground truth. Therefore, we analyze the GNSS data acquired in a dedicated common-clock experiment over a 52 km baseline. We focus on developing two algorithms to estimate the receiver clock differences, hence the frequency instability. These are the single difference (SD) approach with ambiguity fixing as a common view technique, and precise point positioning as an all in-view technique. We discuss the frequency instability achieved by the optical fiber link as well. We evaluate further the performance by computing the modified Allan deviation for both cases. The results show that the ambiguity-fixed solution of SD-CV improves the relative frequency instability via GNSS to reach the order of 3–5 · 10^-17 at one day averaging time. In the optical fiber link, which is the basis of the common clock setup, the round-trip instability shows better performance for all averaging times.
... All receivers were manufactured from Septentrio (Belgium), whose equipments are widely used in time and frequency determination. Considering that the station PTBB, associated with the Physikalisch-Technische Bundesanstalt (PTB), was usually set as the central station in the practical work of the time and frequency transfer, we used it as a reference station in the experiment [26]. Therefore, time links were formed, denoted by UTC(Lab2)-UTC(Lab1) or Station1-Station2, referring to the basic quantity that was generally used to analyze the performance of the time and frequency transfer. ...
The carrier-phase (CP) technique based on the BeiDou global satellite navigation system (BDS-3) has proven to be an important spatial tool for remote time and frequency transfer. The current CP technique models the receiver clock offset as a white noise stochastic process, and easily absorbs some unmodeled errors, which compromises time and frequency transfer performance. To further improve the performance of time and frequency transfer, a new BDS-3 receiver clock estimation algorithm based on the epoch-difference (ED) model is presented, and the mathematical principle and applied mode are discussed. The algorithm makes full use of both observation of current epoch and practical variation of receiver clock offset, further improving the performance of time and frequency transfer. Five MGEX network stations equipped with various types of receivers and antennas with dual-frequency BDS-3 signals were used to establish four time transfer links (i.e., AMC4–PTBB, BRUX–PTBB, OP71–PTBB, and WTZS–PTBB) to evaluate their effectiveness. The ED model improved all four time links in terms of noise level, with improvements of 17.0%, 18.3%, 20.3%, and 5.9% for AMC4–PTBB, BRUX–PTBB, OP71–PTBB, and WTZS–PTBB, respectively, when compared with the results from a non-ED model. ED model outputs were better than raw solutions in terms of frequency stability at all time links, particularly for average time intervals (tau) < 1,000 s. The mean improvement was 8.1% for AMC4–PTBB, 16.1% for BRUX–PTBB, 10.0% for OP71–PTBB, and 18.6% for WTZS–PTBB when the average time (tau) was less than 1,000 s.
... Significant documentation on traceability to US standards was published earlier [21,22]. Those form together with previous papers of this work's authors [23][24][25] the foundation of this report. Based on already available guidelines [26,27] we illustrate in the next section a full scheme for traceable frequency measurements with counters in a calibration laboratory. ...
... In this work, PTB's reference time scale UTC(PTB) plays this part. As already described in previous publications [23,25], monitoring results are published in the PTB TSB and the operator should collect them for his own analysis of the contributing uncertainties. All three parts together form the basis for the uncertainty budget, which is developed step by step in the subsequent sections. ...
... From the observations, daily averages of UTC(PTB)-GPS time are computed and reported in the PTB TSB. Its current format is described in [25] (for availability information see section 2). How the data are prepared for the TSB is described in [24] and here summarized as follows. ...
Received signals from Global Navigation Satellite Systems (GNSS) are nowadays widely used by industry laboratories for ensuring metrological traceability for their respective range of calibration services in the field of time and frequency. Usually, a local frequency standard is steered by continuous GNSS signal reception providing at its output stable and accurate reference signals for the laboratory measurement equipment, in general for synthesizers and counters. Reception of GNSS signals is surely an adequate and practical tool for the purpose, however further steps are needed to establish traceability in a strict metrological sense. Based on already available guidelines and publications, this paper is a contribution to the discussion how metrological traceability to internationally accepted standards can be established in a calibration laboratory. We restrict the discussion to equipment in common use which may not necessarily be of the highest sophistication. In this spirit, we develop a detailed scheme for an uncertainty budget comprising all links of the traceability chain from the device under test to the SI second, the scale-unit of Coordinated Universal Time. Then we go through and apply this scheme step by step to a demonstration setup for frequency measurements with a counter with varying operational parameters. In this framework, a novel approach to distinguish between components of statistical measurement uncertainty is introduced. Furthermore, the limiting uncertainty contributions are discussed and based on a suitable set of parameters an expression for the best measurement capability is given. With this scheme at hand a user may develop an uncertainty budget adapted to his own setup, especially if acceptance from a national accreditation body is sought.
... On the one hand, the receiver hardware delays are related to code and frequency [6], and might not be identical for signals from different constellations [7]. Fortunately, research indicates that most receiver hardware delays between different constellations are stable over a long time [8][9][10]. ...
... But the firmware upgrade of receivers may change the hardware delay difference [11,12]. Since receiver hardware delays are almost constant without firmware updating [13], receiver calibration is a common approach to determine hardware delays [6]. Moreover, to model the ISB, Tian et al. [10] divided receivers to different groups according to receiver manufacturers and firmware versions for GPS/ Galileo/BDS overlapping frequencies and provided the empirical constant corrections for each group. ...
... Based on precise GNSS orbit/clock products and observable specific biases from the Center of Orbit Determination in Europe (CODE; Dach et al. 2020;Schaer et al. 2021), the static antenna position and epoch-wise receiver clock offsets relative to the inherent system time scale of the CODE products were determined in a least-squares PPP adjustment with single-receiver ambiguity fixing. In the same way, clock offsets of the PTBB GNSS reference stations at the timing laboratory of the Physikalisch-Technische Modelled with GPT (Boehm et al. 2007) and GMF (Boehm et al. 2006); epoch-wise estimation of the wet tropospheric zenith delay correction Ionosphere 1st-order effects compensated using ionosphere-free dual-frequency combinations Bundesanstalt (PTB; Bauch et al. 2020) in Braunschweig, Germany, were computed. This station is connected to an active hydrogen maser, which offers a 10 −15 to 10 −13 stability on sub-daily time scales (Bauch et al. 2012) and is steered to Correlated Universal Time (UTC) with better than 2 ns accuracy through long-term adjustments (BIPM 2022). ...
Over the last decade, chip-scale atomic clocks (CSACs) have emerged as stable time and frequency references with small size, weight, and power (SWaP). While the short-term stability of these devices clearly outperforms other oscillators with similar power consumption, their stability over longer time intervals is notably limited by frequency noise. Such long-term deviations can effectively be compensated by disciplining the clock with respect to a stable time and frequency reference such as Coordinated Universal Time (UTC) or a time scale based on GNSS observations. In view of the limited accuracy of GPS pseudorange observations and broadcast ephemerides, the performance of GNSS-disciplined atomic clocks is commonly limited to the few-nanosecond level. For further improvement, this study combines the use of carrier phase-based precise-point-positioning (PPP) techniques and high-performance broadcast ephemerides to discipline the phase of a CSAC with respect to GNSS broadcast time. Making use of a dual-frequency, dual-constellation GPS/Galileo receiver, a sub-nanosecond time interval error with respect to a national UTC timing laboratory is demonstrated over time intervals from 1 s to several days.
... The details of the experiment setup are discussed previously at [16]. Figure 1a depicts the three stations used for our study. The IGS (International GNSS Service) station at PTB, named PTBB, is located at the Kopfermann building (see [17] for details). The two stations MEI1 and MEI2 are located at the Meitner building. ...
... GNSS is an important technique that can provide reliably high-precision positioning, navigation and timing (PNT) service (Bauch et al. 2020). Although accurate time transfer requires the calibration of the complete user systems, precise point positioning (PPP) has shown its potential for highprecision time and frequency transfer (Ray and Senior 2005). ...
Precise point positioning (PPP) can currently achieve time transfer to a precision of one hundred picoseconds. However, the long convergence time is the main drawback of this popular time and frequency transfer method. It is well known that the convergence of position and receiver clock offset parameters in PPP solution is related to the satellite geometry, in which the receiver clock offset is the key parameter in time and frequency transfer and its performance could be reflected by the time dilution of precision (TDOP). This study applies tropospheric augmentation to accelerate PPP initialization and improve PPP frequency transfer performance during the convergence period. With the tropospheric augmentation applied to BDS-3 PPP, the convergence speed of receiver clock offset has been greatly improved when there are few visible satellites. Since GPS and BDS-3/GPS combined PPP, the satellite geometry and TDOP are better than that of BDS-3, the improvement in convergence speed is limited correspondingly. In addition, due to a more stable receiver clock offset estimated with the tropospheric information constraints, the short-term stability of frequency transfer during the convergence period has been greatly improved for GPS, BDS-3 and BDS-3/GPS combined PPP. For the two time links selected in this study, the short-term stability has been improved by more than 60, 45 and 40% in BDS-3, GPS and BDS-3/GPS combined PPP frequency transfer, respectively. However, the contribution of tropospheric augmentation to PPP frequency transfer is no longer significant after convergence.
... Substantial work has been done in this field before [2,3]. Based on these papers, already available guidelines [4], and own work [5,6,7] we present in detail the development of an uncertainty budget including all relevant contributions to establish an unbroken traceability chain. A full paper is in preparation [8] and will be submitted to the journal Metrologia. ...
Frequency counters are widely used in industry laboratories to facilitate routine frequency measurement and calibration services. Such services require evidence of traceability to internationally recognized standards if an accreditation is sought. And it is the responsibility of the operator to demonstrate his ability and practices to comply with respective requirements from international standards as EN ISO/IEC 17025. In this report we present an example of a full traceability chain for such a service, in detail comprising all links from the measurements with the so-called device under test via the operation of the laboratory standard to the internationally agreed standard, which is Coordinated Universal Time UTC. Some technical aspects of frequency measurements with counters requiring a closer look are addressed and discussed. Finally, a proposal for summarizing and reporting the uncertainty contribution is being made.
In this chapter general aspects of atomic timescales (AT) are discussed. As an introduction, the history and importance of AT and their applications is presented. Then, the construction of AT from an ensemble of atomic clocks is exposed. Different types of AT exist depending on their applications. There are AT defined by a physical signal, typically an electrical signal, usually distributed in real time by different means. An example of this type of AT is the UTC(k) timescale. For most developed countries, the UTC(k) atomic timescale is the reference for the national official time. Meanwhile, there exist also AT that are not defined by a physical signal, alternatively defined by a mathematical algorithm. An example of this type of AT is the Coordinated Universal Time, UTC, timescale which is produced by the International Bureau of Weights and Measures (BIPM). The UTC has a remarkable role worldwide because, mainly, it is the international reference for the local UTC(k) timescales. This chapter discusses how the UTC atomic timescale is produced by the BIPM and how it and UTC(k) timescales provide traceability to the time unit of the International System of Units (SI), the second. Finally, the most important technical requirements in setting up an AT laboratory are listed.
