Implementation of SDR TWSTFT in UTC Computation

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.

The International Bureau of Weights and Measures and the Consultative Committee for Time and Frequency Working Group on Two-Way Satellite Time and Frequency Transfer (TWSTFT) have stimulated a pilot study on using software defined radio (SDR) receivers for TWSTFT in the framework of the realization of Coordinated Universal Time. The SDR receivers based on a software developed by TL have been set up at PTB, OP and NIST during the summer and fall of 2016. Continuous SDR TWSTFT measurements have been established for the OP/PTB, NIST/OP and NIST/PTB links. From the SDR TWSTFT clock comparison results (TWSTFT difference), we observed that the commonly present but undesired diurnal pattern in the OP/PTB SDR TWSTFT difference is greatly reduced compared to the diurnal pattern in the difference obtained from the regular (nonSDR) TWSTFT equipment. However, the diurnal reduction in the NIST/OP and NIST/PTB SDR TWSTFT differences is not as significant as in the OP/PTB link. This is a strong indication that the application of SDR receivers has the ability to significantly improve the stability of some TWSTFT links, but is not a solution for all links. In this paper, we will analyze the diurnal reduction using the SDR receivers for the three links and study the dominant causes for the diurnal effect in the EuropeEurope and transatlantic TWSTFT links.

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].

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.

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°.

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.

Two-Way Satellite Time and Frequency Transfer (TWSTFT) links were first introduced to Coordinated Universal Time (UTC) generation in 1999. These TWSTFT links were calibrated by alignment with the corresponding GPS time links, of which the nominal uncertainty was 5 ns. In the past decade, the primary calibration technique for TWSTFT link calibrations was based on a TWSTFT mobile ground station (MS) with uncertainty at the level of 1 ns. The use of an MS for TWSTFT link calibration is limited by the availability of an MS, a common satellite transponder, transportation, and high cost. For example, due to the lack of a common transponder, a MS cannot be used for a stand-alone calibration of the NIST-PTB link (the link between the National Institue of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB)). Therefore, alternative techniques have been proposed and validated in recent years. Investigations for improving GPS time link calibrations have been performed since 2008. In 2011 this triggered the International Bureau of Weights and Measures (BIPM) to carry out a pilot study on using GPS link calibrations for the UTC TWSTFT time links. The study concluded that a link calibration uncertainty of 1.5 ns is attainable. Based thereon, the ‘TWSTFT Calibration Guidelines for the UTC Time Links’ recognize the GPS link calibration as an alternative technique for the calibration of TWSTFT links. In this paper, we first outline the new TWSTFT Calibration Guidelines for UTC Time Links (v3.0), which was approved at the 23rd meeting of the Consultative Committee for Time and Frequency (CCTF) Working Group on TWSTFT. It authorizes several techniques, including the GPS link calibration and the Triangle Closure Calibration (TCC). Then we discuss the attainable uncertainty of the GPS calibration. Finally we provide in the Annex an example report of using a GPS calibrator for a typical UTC time link calibration based on the US Naval Observatory (USNO) and PTB, UTC(USNO)-UTC(PTB) TWSTFT link calibration. Here we describe the characteristics of BIPM’s Measurement of Total Delay (METODE) Global Navigation Satellite System (GNSS) calibrator, its setup at a UTC laboratory, Lab(k), measurements, and its results. In this calibration, the GPS result differs from that of the TWSTFT MS by 0.9 ns with an uncertainty of 1.5 ns.

The time difference calibration between stations is a hot research topic in two-way satellite time and frequency transfer (TWSTFT). According to the disadvantage of the traditional TWSTFT calibration methods using a portable station(PS) that baseband modem and RF transceiver of the portable station must be compatible with TWSTFT earth station's, a new TWSTFT calibration method is proposed based on two TWSTFT portable stations. Two TWSTFT portable stations' prototypes are developed. The prototypes' system compositions are introduced and the performance of prototypes is tested. The results show that type A uncertainty of time difference calibration between stations is 0.077ns based on the zero baseline comparison. The type A uncertainty of ground station time delay of portable station 1 and 2 respectively is 0.043ns and 0.027ns. The measuring system can be applied to the satellite navigation systems and the comparisons of time and frequency among different laboratories.

The two-way satellite time and frequency transfer (TWSTFT) is the most accurate time synchronization method currently. Antenna is an indispensable element and plays an important role in the earth station of TWSTFT, its direction has a definite effect on the signal quality and performance in TWSTFT. In this paper, some preliminary test results are introduced by using the new TWSTFT system of Ku band with a 2.4 meters tracking antenna in National Time Service Center (NTSC). Through the experiment, we have obtained some data according to different antenna direction, by the analysis we can know: the change of azimuth angle is more sensitive than elevation angle during receiving the signal; in theory, the biggest deviation of azimuth angle should be symmetrical in both sides and asymmetric actually, the deviation of elevation angle is also. The deviation is the angle apart from the pointing of antenna at which receiving signal is the best. It can offer a powerful reference for antenna pointing deviation in earth station of TWSTFT and provide theoretical foundation for the antenna direction and high precision time transfer and synchronization.

Two-way satellite time and frequency transfer (TWSTFT), other than the GNSS related techniques, has been one of the most precise time transfer methods. Telecommunication laboratories (TL) has devoted to set up the TWSTFT links with several important Laboratories and achieved the goal of establishing TWSTFT links circling the world. The numerical results of several direct and connected TWSTFT links will be demonstrated in the paper. The results show that a well maintained connected TWSTFT links, even with more than 2 relay stations, can still have good performance.

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.

Time synchronization between ground and satellites is a key technology for satellite navigation system. With dual-channel satellite, a method called Two-Way Common-View (TWCV) satellite time transfer for Compass system is proposed, which combines both characteristics of satellite common-view and two-way satellite-ground time transfer. By satellite-ground two-way pseudo-range differencing and two stations common-view differencing, this TWCV method can completely eliminate the influence of common errors, such as satellite clock offset, ephemeris errors, troposphere delay and station coordinates errors. At the same time, ionosphere delay related to signal frequency is also weakened significantly. So the precision of time transfer is improved much more greatly than before. In this paper, the basic principle is introduced in detail, the effect of major errors is analyzed and the practical calculation model in the Earth-fixed coordinate system for this new method is provided. Finally, experiment analysis is conducted with actual Compass observing data. The results show that the deviation and the stability of the satellite dual channel can be better than 0.1 ns, and the accuracy of the two-way common-view satellite time transfer can achieve 0.4 ns. All these results have verified the correctness of this TWCV method and model. In addition, we compare this TWCV satellite time transfer with the independent C-band TWSTFT (Two-Way Satellite Time and Frequency Transfer). It shows that the result of the TWCV satellite time transfer is in accordance with the C-band TWSTFT result, which further suggests that the TWCV method is a remote high precision time transfer technique. The research results in this paper are very important references for the development and application of Compass satellite navigation system.

Two-way satellite time and frequency transfer (TWSTFT) using geo-stationary telecommunication satellites is widely used in the timing community nowadays and has also been chosen as primary means to effect synchronization of elements of the ground segment of the European satellite navigation system Galileo. We investigated the link performance in a multi-station network in dependence on operational parameters, such as the number of simultaneously transmitting stations, transmit and receive power, and chip rates of the pseudo random noise modulation of the transmitted signals with the following results: 1. TWSTFT through a quiet transponder channel (two stations transmitting only) leads to a measurement noise, expressed by the lpps jitter, reduced by a factor of 1.4 compared to a busy transponder carrying signals of 12 stations. 2. The frequency transfer capability expressed by the Allan deviation is reduced at short averaging times by the same amount. 3. At averaging times of > 1 day no such reduction could be observed, which points to the fact that other noise sources dominate at such averaging times. 4. Higher transmit power increases the carrier-to-noise density ratio at the receive station and thus entails lower jitter but causes interference with other stations' signals. 5. The use of lower chip rates which could be accommodated by a reduced assigned bandwidth on the satellite transponder is not recommended. The lpps jitter would go up by a factor of 2.5 when going from 2.5 MCh/s to 1 MCh/s. The two Galileo PTFs can be included in the currently operated network of 12 stations in Europe and all requirements on the TWSTFT performance can be met, provided that suitable ground equipment will be installed in the Galileo Ground Segment.

This paper estimated the local model to predict ionosphere TEC on a scale of day. Ionosphere TEC were obtained from the two-way satellite time and frequency transfer (TWSTFT), based on these data a sine function was fitted by iteration to get the prediction model of diurnal ionosphere TEC. This model simply and directly reflected the characteristic of the diurnal ionosphere behavior, and was consistent with the corresponding Ionosphere TEC obtained from TWSTFT.

Two-way satellite time and frequency transfer (TWSTFT) data are typically recorded on Monday, Wednesday and Friday. This produces an unevenly spaced time series on which it is difficult to perform an accurate two-sample variance analysis. We have investigated the effect of uneven data spacing on the computation of σx(τ) [1-2]. Evenly spaced simulated data sets were generated for noise processes ranging from white phase modulation to random walk frequency modulation. σx(τ) was then calculated for each noise type. Data were subsequently removed from each simulated data set to create two unevenly spaced sets with average intervals of 2,8 and 3,6 days. These sparse sets correspond to typical TWSTFT data patterns. σx(τ) was then calculated for each sparse data set using two different approaches. First, the missing data points were replaced by linear interpolation and σx(τ) calculated from this now full data set. The second approach ignored the fact that the data were unevenly spaced and calculated σx(τ) as if the data were equally spaced with average spacing of 2,8 or 3,6 days. The impact of uneven data spacing on the results of these two approaches is significant and is discussed. Finally, techniques are presented for correcting errors caused by uneven data spacing in simulated TWSTFT data sets, and the appropriate technique is applied to a real data set.