The method and experiment analysis of two-way common-view satellite time transfer for compass system

Two-way satellite time and frequency transfer has become an important technical component in the process of the realization of International Atomic Time and also for comparisons among the best primary frequency standards. This paper contains results of studies aimed at improving the knowledge of the uncertainty for true time transfer between remote sites. Our study addresses mostly the collection of so-called time transfer triplets between sites 1, 2, and 3. Systematic deviations from zero in the combination (1-2) + (2-3) + (3-1) would point to incomplete understanding and handling of systematic offsets in the individual time transfers. We show that such offsets exist, but that they are only in specific cases significant (up to about 2 ns), and we discuss possible causes.

In a manner different from the two-way satellite time and frequency transfer (TWSTFT) via geostationary satellites which has been employed for subnanosecond time transfer between widely separated earth stations, TWSTFT via non-geostationary satellites has a problem attaining subnanosecond accuracy because of satellite motion. The impact of satellite motion on the accuracy of TWSTFT via non-geostationary satellites is wholly attributed to the non-reciprocal geometric path. We have analysed the non-reciprocal geometric path caused by satellite motion in order to evaluate the accuracy of TWSTFT via non-geostationary satellites. Our analysis shows that the non-reciprocal geometric path introduces significant error to the accuracy of subnanosecond time transfer. Using a numerical model, we have demonstrated that ordinary orbit estimation can correct the non-reciprocal geometric path in TWSTFT via non-geostationary satellites to subnanosecond accuracy. Residuals of the non-reciprocal geometric path correction with respect to the orbit estimation accuracy and baseline length are presented. As an example of a non-geostationary satellite orbit, we employed an orbit of a quasi-zenith satellite orbiting the Earth in a period of one day, in synchronization with the Earth's rotation and with an orbital inclination of 45°.

Two-way satellite time and frequency transfer (TWSTFT) has been used operationally by the international time and frequency community for several years. This paper describes analysis techniques being developed at the National Physical Laboratory for the processing of TWSTFT measurements.The measurements are modelled in terms of phase and normalized frequency offsets together with random errors considered to be a linear combination of the well-known noise types of white phase modulation, white frequency modulation and random walk frequency modulation encountered in time and frequency measurements. A method is described for solving the model to provide estimates of the phase and normalized frequency offsets, together with their associated uncertainties, at any measurement epoch. The random errors associated with the measurements are first characterized through the use of second-difference statistics. An analysis of the second-difference statistics is undertaken to estimate a covariance matrix for the time series of measurements. Finally, with the covariance matrix so obtained, generalized least-squares regression is used to provide unbiased and efficient estimates of the required offset parameters.

The Calibration Device is used to measure the time delay difference between the transmission and receiving paths inside Two-Way Satellite Time and Frequency Transfer (TWSTFT) earth station, it can improve the time comparison precision. A new calibration device for TWSTFT has been developed recently at Beijing Institute of Radio Metrology and Measurement (BIRMM). The signal paths inside calibration device is bidirectional which allow both the transmission and receiving signal passed, so the delay difference of the transmission and receiving inside calibration device almost be zero. The calibration device works on Ku-band. To evaluate the performance of the BIRMM calibration device, a local TWSTFT experiments was done with SATRE modem. The measurement results show that the time delay difference was quite small instability with standard deviation (1 σ) equal to 0.17ns.

Application of Vondrak Filtering Method to the Two-Way Satellite Time and Frequency Transfer Based on Software Defined Receiver

International audience

Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). About 20 timing laboratories around the world continuously operate TWSTFT using satellite time and ranging equipment (SATRE) modems for remote time and frequency comparisons in this context. The precision of the SATRE TWSTFT as observed today is limited by an apparent daily variation pattern (diurnal) in the TWSTFT results. The observed peak-to-peak variation have been found as high as 2 ns in some cases. Investigations into the origins of the diurnals have so far provided no complete understanding about the cause of the diurnals. One major contributor to the diurnals, however, could be related to properties of the receive part in the modem. In 2014 and 2015, it was demonstrated that bypassing the receive part and the use of software-defined radio (SDR) receivers in TWSTFT ground stations (SDR TWSTFT) instead could considerably reduce both the diurnals and the measurement noise.In 2016, the International Bureau of Weights and Measures (BIPM) and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the TWSTFT network for UTC computation.The first results of the pilot study were reported to the CCTF WG on TWSTFT annual meeting in May 2017, demonstrating that SDR TWSTFT shows superior performance compared to that of SATRE TWSTFT for practically all links between participating stations. In particular, for continental TWSTFT links, in which the strongest diurnals appear, the use of SDR TWSTFT results in a significant suppression of the diurnals by a factor of between two and three. For the very long inter-continental links, e.g. the Europe-to-USA links where the diurnals are less pronounced, SDR TWSTFT achieved a smaller but still significant gain of 30%. These findings are supported by an evaluation of some of the links with an alternate technique based on GPS signals (GPS IPPP) as reported in this paper.Stimulated by these results, the WG on TWSTFT prepared a recommendation for the 21st CCTF meeting, which proposed the introduction of SDR TWSTFT in UTC generation. With CCTF approval of the recommendation, a roadmap was developed for the implementation of SDR TWSTFT in UTC generation. In accordance with the roadmap, most of the stations that participated in the pilot study have updated the SDR TWSTFT settings to facilitate the use of SDR TWSTFT data in UTC generation. In addition, the BIPM conducted a final evaluation to validate the long-term stability of SDR TWSTFT links, made test runs using the BIPM standard software for the calculation of UTC, now including SDR TWSTFT data, and started to calculate SDR TWSTFT time links as backup from October 2017. The use of SDR TWSTFT in UTC generation will begin in 2018.

This paper describes the carrier-phase two-way satellite time and frequency transfer experiment performed between the metrology institutes PTB (Braunschweig) and LNE-SYRTE/OP (Paris) using ground stations equipped with dedicated modem equipment developed by NICT. The frequency instability of the PTB-OP link reached 4×10−16 at 1 day, a performance of about one order of magnitude better than that obtained by the code-phase two-way satellite method.

Global navigation satellite system (GNSS) can realize global time synchronization with tens of nanoseconds precision. For more precise time synchronization applications, two-way satellite time and frequency transfer is commonly used despite its high cost and requiring a geostationary satellite. Considering the high cost, people started to use the GNSS signals to realize high accuracy time transfer, named common-view or all-in-view, which can achieve several nanoseconds according to the analyses and verifications of previous studies. However, time transfer cannot realize real-time timing in a large area as a common GNSS one-way timing receiver does. In such a timing system, a highly stable reference and a matched high-precision timing receiver system are necessary. To solve this problem, we have established a one-way timing system based on a ground-based augmentation system. The system estimates real-time high-precision satellite clocks using the ground-based augmentation network as the time reference. Moreover, the time reference is transferred to the terminal through real-time precise point positioning (PPP). Finally, the timing receiver synchronizes its local clock to the reference by real-time clock adjustment, resulting in high-precision timing. We present the details of this system, including the time reference, the PPP algorithm, and the timing receiver technologies. Finally, we give the precision and stability evaluation of the whole system, especially for one pulse per second (1 PPS) outputs of the terminal. The results show that the stability of the 1 PPS output is better than 1 ns in a day.

Two-way time transfer is a common method applied to spacecrafts and ground stations to carry out time transfer. This method improved the accuracy of time transfer by eliminating the impact of propagation paths and extra delay caused by atmosphere, so the algorithm of two-way satellite time transfer is presently one of the time transfer methods with high accuracy. But this method requires the spacecrafts and ground stations, on which time transfer is to be performed, to keep a relatively static state in time transfer. So it is mainly used in GEO satellites for satellite-ground two-way time transfer. On the basis of two-way satellite time transfer algorithm, the characteristics of the propagation delay of two-way time transfer signals between MEO satellite and GEO satellite varying with intersatellite range were analyzed, and the rule of intersatellite clock offset varying with intersatellite range obtained with this algorithm was deduced. This paper presents a dynamic two-way time transfer algorithm(DTWTTA) with which high-accuracy intersatellite clock offset is solved through the combination of intersatellite pseudo-range polynomial fitting and clock-offset polynomial fitting. Simulation data of actual satellites showed that with the algorithm the intersatellite time transfer error can be controlled within 5ns provided with minimum square fitting, the two-way time transfer periods beginning at moments basically symmetrical with the occurrence of minimum intersatellite range relatively, and the impact of other factors being taken into account. As a result the algorithm can be used for intersatellite high-accuracy time transfer.

Astrometric positioning and orbit determination of geostationary satellites

The Software-Defined Radio (SDR) receiver [1-5] has been developed at the Telecommunications Laboratory (TL, Chinese Taipei) for implementation in two-way satellite time and frequency transfer (TWSTFT/TW) Earth stations, aiming at improving the stability of TW time transfer, with particular impact on the diurnal signature present in almost all the links, which is the major uncertainty source in TW time links. The BIPM and the Consultative Committee for Time and Frequency (CCTF) Working Group on TWSTFT launched in February 2016 a pilot study for validating the SDR receiver in view of its implementation for use in UTC time links. Participants to this pilot study are the laboratories contributing to UTC which operate the SDR receiver and the BIPM Time Department. Goals of the pilot study are first, to validate the efficacy of the SDR receiver for improving the TWSTFT uncertainty and significantly reduce the diurnal effects; second to implement routine TW code measurement data through SDR receiver to improve the UTC time comparisons. In the frame of the pilot study, the SDR receiver has been installed and is operational by the end of 2016 at TL, NICT, KRIS, NTSC in Asia, PTB, OP and SU in Europe and NIST in the US. The project to install the SDR receiver in all the TW laboratories is ongoing. We present the results obtained at the BIPM Time Department in the preliminary steps of the analysis and validation of the SDR measurement data by using different methods, such as the time deviation (TDev), the triangle closures, the comparison with GPSPPP [6]. We analysed the data obtained from the pilot study and observed that in some TW links there is significant improvement, while in others no noise reduction is visible. This preliminary result needs more investigation, together with other open points as the calibration, the discontinuities observed after restarting the receiver etc. If the conclusion of this pilot project is positive, a proposal could be discussed at the CCTF to include TW time links with the SDR receiver in the computation of UTC. We study also the potential strategies to further improve the TWSTFT SDR; its combination with GNSS links [7] and the computation of SDR indirect links [8, 9].

Time Transfer Link fusion algorithm based on wavelet multi-resolution analysis

Two-way satellite time and frequency transfer (TWSTFT) has become an important technical component in the process of the realization of International Atomic Time. To employ the full potential of the technique, especially for true time transfer, a dedicated calibration is necessary. This consists of the calibration either of the operational link at large, including every component involved, or of the involved ground stations' internal delays only. Both modes were successfully employed by circulating and operating a portable reference station between the sites involved. In this paper, we summarize the theoretical background for the different calibration modes applied and report examples of results from the 13 calibration campaigns performed up to now in Europe and between Europe and the United States. In all of these exercises, estimated uncertainties around 1 ns were achieved. Consecutive campaigns showed a very good reproducibility at the nanosecond level. Additionally, we address and briefly discuss sources that possibly limit the uncertainty for true time transfer employing TWSTFT.

Two-way satellite time and frequency transfer (TWSTFT) is extensively used by the Bureau International des Poids et Mesures (BIPM) in the calculation of coordinated universal time (UTC). The Asia-Europe TWSTFT links using the Express-80 satellite were reestablished in June 2021. This paper uses monthly measurements to evaluate the time comparison performance of the Asia-Europe TWSTFT network. We select TWSTFT data for analysis on the long and short baselines over the same period, respectively. Since the TWSTFT link has not been calibrated, we take the GPS precise point positioning (PPP) link, independent of TWSTFT, as a reference and align the TWSTFT results with it. Then, the results from the TWSTFT and GPS PPP solutions are compared on the different baselines. Finally, the modified Allan deviation (MDEV) and the double clock difference (DCD) between the TWSTFT link and the GPS PPP link are used to evaluate the performance of the TWSTFT results. The results show that the TWSTFT results are consistent with the results of CPS PPP time transfer, and the frequency stability of TWSTFT links on long and short baselines can reach 3E-15 or better with an averaging time of 1 d. The standard deviation of the DCD results is less than 0.4 ns on different baselines, which ensures the accuracy of the TWSTFT link. The Asia-Europe TWSTFT link can be used for UTC time transfer after calibration.