Satellite Time Transfer Research Articles

Design and Evaluation of an Open-Loop Receiver for TWSTFT Applications

The two-way satellite time and frequency transfer (TWSTFT) technique provides uncertainty at the nanosecond level for time differences measured over an intercontinental baseline. The time-difference results occasionally reveal daily variations of a few nanoseconds due to multiple effects. These daily variations are called diurnals, and they increase the uncertainty of the time-difference measurements. Since a time difference is obtained by combining two time-of-arrival (TOA) measurements of two receivers, the receivers may contribute the diurnals if they are not reciprocal. To improve reciprocities, a TWSTFT receiver based on the open-loop (OL) method was proposed and implemented on a software-defined radio receiver (SDR). For evaluation, the OL-based SDR was compared with a conventional receiver in a short-baseline TWSTFT experiment. The experimental results indicate that the diurnals disappeared with the OL-based SDRs while a diurnal of ±0.2 ns could be observed with conventional receivers. These results imply that the diurnals partially depend on TOA measurement methods used in the TWSTFT receiver.

2-GEO TWSTFT for BDS ground stations: Confidence interval of clock difference Allan variance

Two-way satellite time and frequency transfer(TWSTFT) is important for time and frequency transfer between ground stations in Chinese Bei Dou Navigation Satellite System(BDS). Different from ordinary TWSTFT links with single geostationary(GEO) satellite between each pair of ground stations, there are 2-GEO TWSTFT links between ground stations in BDS. Based on the 2-GEO TWSTFT links, we have proposed a separation method of estimating Allan variance of clock difference apart from the influence of Allan variance of noise measurement. This paper analyzes the confidence interval of the clock difference Allan variance obtained by this method with the condition that the noise measurement is white phase noise and the clock difference is affected by white frequency noise. The results applied to the BDS measurement data show that the obtained confidence interval upper bound of clock difference Allan deviation(the square root of Allan variance) is 3.248×1012 with 0.95 confidence. Compared with the single link TWSTFT with Allan deviation of(8.59–8.91)×1011(=1 s), it proves further that the separation method has ability to weaken the influence of noise measurement on the evaluation of Allan deviation of clock difference measured by two independent TWSTFT links.

Study of optical frequency transfer via fiber

Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.

Comparison of Two-Way and BeiDou Common View Time Transfer between NTSC and BEIJ Station

The data obtained by the two-way satellite time and frequency transfer (TWSTFT) and the BeiDou common view (BDCV) experiments between the National Time Service Center (NTSC) and Beijing station (BEIJ) will be compared in this paper. Results with high agreement between the two techniques will be described. Data analysis results demonstrated that the two-way satellite time and frequency transfer (TWSTFT) and the BeiDou common view (BDCV) can back up each other.

Direct comparison of optical lattice clocks with an intercontinental baseline of 9000 km.

We have demonstrated a direct frequency comparison between two ⁸⁷Sr lattice clocks operated in intercontinentally separated laboratories in real time. Two-way satellite time and frequency transfer technique, based on the carrier-phase, was employed for a direct comparison, with a baseline of 9000 km between Japan and Germany. A frequency comparison was achieved for 83,640 s, resulting in a fractional difference of (1.1±1.6)×10⁻¹⁵, where the statistical part is the largest contributor to the uncertainty. This measurement directly confirms the agreement of the two optical frequency standards on an intercontinental scale.

Carrier-phase two-way satellite frequency transfer over a very long baseline

In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Carrier-Doppler-based real-time two way satellite frequency transfer and its application in BeiDou system

Two-way satellite time and frequency transfer (TWSTFT) method is valuable for precise time and frequency transfer. The frequency could be directly transferred by utilizing two-way carrier Doppler measurement. This method requires each station observing both its own and associated station’s transponder signals transferred by satellite. In BeiDou system (BDS), the reference station and the field station construct a two-way link via a GEO satellite, and the field station sends pseudorange and carrier phase information to the reference station by transmission signal, in order to achieve real time TWSTFT. This paper analyzes the theory of carrier Doppler based TWSTFT (called CD-TWSTFT), and validates it by using on line data of BDS. The results demonstrated that the performance of CD TWSTFT is much better than code based TWSTFT in short-term frequency transfer and almost same at long-term frequency transfer.

Improved two-way satellite time and frequency transfer with Multi-GEO in BeiDou navigation system

Due to the limitation of Geostationary Earth Orbit (GEO) satellite resources, only very few TWSTFT links with parallel counterpart operating via different GEO satellites between two ground stations are available. There are 5 GEO satellites in BeiDou Navigation System, which enables an improved TWSTFT with Multi-GEO. Two algorithms are put forward to take the advantage of Multi-GEO links to improve the performance of time and frequency transfer respectively. The dynamic weight average algorithm based on a newly proposed dynamic time variance aiming at improving the performance of time transfer is shown to be able to reduce the dynamic time deviation of the two-way measurement result to 1/\(\sqrt N \) dynamic time deviation of the single link using N-GEO links, if each link has similar level of link noises. The algorithm of cancelling out the link noise Allan variances with 2-GEO links is expected to obtain the Allan variance of clock difference between two ground stations. The experiment results using 2-GEO links in BeiDou Navigation System show that the dynamic weight average algorithm can improve the dynamic time deviation of the two-way measurement to around 0.6 of that of the single link in an average concept. And the estimated Allan deviation of the clock difference using the algorithm for cancelling the noise Allan variances of both links can reach 7×10−12(τ −0.65) for the averaging time 1–105 s which is within the same order of magnitude as the estimated Allan deviation based on carrier Doppler measurement for the averaging time 10 s–104 s, despite the noisy links with the Allan deviation follows 3 × 10−10(τ −1).

A New Method of Data Pre-Processing to Two-Way Satellite Time and Frequency Transfer Experiments

Based on the basic theory of two-way satellite time and frequency transfer (TWSTFT), the mathematical model on real errors of observations is established in this paper. However, the model is rank-deficient. In order to solve this problem effectively, the paper introduces the algorithm of combining parameters, and applies Quasi-Accurate Detection of Gross Errors (QUAD) proposed in reference [ to data pre-processing. The method programs opportune algorithms and resolves the problem of detecting gross errors. In the end, the method has been verified to be successful by calculating and analysing simulated data and practical measured data.

Experimental and Simulation Study for a Time Transfer Service via a Commercial Geostationary Satellite

Time transfer over satellite links has been explored since the satellite era began. Currently, Two Way Satellite Time and Frequency Transfer (TWSTFT) is routinely used between national timing labor...

Carrier-phase TWSTFT experiments using the ETS-VIII satellite

The National Institute of Information and Communications Technology (NICT) has developed and tested carrier-phase two-way satellite time and frequency transfer (TWSTFT) as a next-generation technique applicable to greater distances and resulting in higher precision. A method different from that used for code-based TWSTFT is required for carrier-phase TWSTFT. We propose a new method based on variations of carrier phases for the propagation of signals.Using the ETS-VIII satellite launched by the Japan Aerospace Exploration Agency (JAXA) and the Time Comparison Equipment (TCE) system developed by NICT, carrier-phase TWSTFT experiments were carried out between two ground-based hydrogen masers separated by a baseline of 110 km. Our tests showed that the frequency difference between two hydrogen masers can be measured for averaging times larger than 1000 s and the precision of carrier-phase TWSTFT is about 100 times better than that of code-based TWSTFT.

An Algorithm of Inter Satellite Two-Way Time Transfer Based on Mobile Satellite

Two-way time transfer is one of the most accurate time synchronization methods applied to spacecrafts and ground stations to carry out time transfer. As this method doesn’t require the knowledge of locations of two satellites in advance and it offsets the negative influence of transmission path and other additional delays, this method has boosted the time synchronization accuracy. However, in the process of time synchronization, this method demands that the aircrafts, who conduct time synchronization, could be relatively static. So it is mainly used in GEO satellites for satellite-ground two-way time transfer. Based on the establishment of mobile satellite mutual visual model, the simulation of satellite mutual visual time on mobile satellite, including IGSO (Inclined Geo Synchronous Orbit) satellite and MEO (Medium Earth Orbit) satellite, has been conducted. The visual time and the variation range of IGSO-MEO link distance have been gained. The characteristics of the propagation delay of two- way time transfer signals between IGSO satellite and MEO satellite varying with inter satellite range were analyzed and the rule of inter satellite clock offset varying with inter satellite range obtained with this algorithm was deduced. This study presents a inter satellite dynamic two-way time transfer algorithm based on mobile satellite. The high- accuracy inter satellite clock offset is solved through the combination of inter satellite pseudo-range polynomial fitting and clock-offset polynomial fitting. Simulation results showed that with the algorithm the inter satellite time transfer error can be controlled within 1ns. The algorithm can be used high-accuracy time transfer between mobile satellites.

Carrier-phase-based two-way satellite time and frequency transfer

We performed measurements of carrier-phase-based two-way satellite time and frequency transfer (TWST-FT) with an A/D sampler and conventional TWSTFT system. We found that an instability resulting from a local signal at the satellite transponder was negligible. The short-term stability of 4 × 10(-13) at 1 s was achieved in a short-baseline measurement. The results showed good agreement with the GPS carrier phase.

Time transfer through optical fibres over a distance of 73 km with an uncertainty below 100 ps

We demonstrate the capability of accurate time transfer using optical fibres over long distances utilizing a dark fibre and hardware which is usually employed in two-way satellite time and frequency transfer (TWSTFT). Our time transfer through optical fibre (TTTOF) system is a variant of the standard TWSTFT by employing an optical fibre in the transmission path instead of free-space transmission of signals between two ground stations through geostationary satellites. As we use a dark fibre there are practically no limitations to the bandwidth of the transmitted signals so that we can use the highest chip rate of the binary phase-shift modulation available from the commercial equipment. This leads to an enhanced precision compared with satellite time transfer where the occupied bandwidth is limited for cost reasons. The TTTOF system has been characterized and calibrated in a common-clock experiment at PTB, and the combined calibration uncertainty is estimated as 74 ps. In a second step the remote part of the system was operated at Leibniz Universität Hannover, Institut für Quantenoptik (IQ) separated by 73 km from PTB in Braunschweig. In parallel, a GPS time transfer link between Braunschweig and Hannover was established, and both links connected a passive hydrogen maser at IQ with the reference time scale UTC(PTB) maintained in PTB. The results obtained with both links agree within the 1-σ uncertainty of the GPS link results, which is estimated as 0.72 ns. The fibre link exhibits a nearly ten-fold improved stability compared with the GPS link, and assessment of its performance has been limited by the properties of the passive maser.

Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network

In this work, we demonstrate for the first time that it is possible to transfer simultaneously an ultra-stable optical frequency and precise and accurate timing over 540 km using a public telecommunication optical fiber network with Internet data. The optical phase is used to carry both the frequency information and the timestamps by modulating a very narrow optical carrier at 1.55 μm with spread spectrum signals using two-way satellite time transfer modems. The results in terms of absolute time accuracy (250 ps) and long-term timing stability (20 ps) well outperform the conventional Global Navigation Satellite System or geostationary transfer methods.

Time and frequency transfer with the ESA/CNES ACES-PHARAO mission

AbstractWe have written a theoretical description of one-way and two-way satellite time and frequency transfer and developed a model of the micro-wave link in the frame of the ACES/PHARAO mission. This is used to write a data analysis software and a simulation to test it. A very short description of the mission and of the micro-wave link is given here. A detailed description can be found in Delva et al., 2012, Proceedings of the EFTF, Gothenburg, Sweden, arXiv:1206.6239.

First international two-way satellite time and frequency transfer experiment employing dual pseudo-random noise codes

Two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. To both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. In this paper, we demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps are achieved for averaging times from 1 s to 1 d. Moreover, the DPN data has less diurnal variation than that of the conventional TWSTFT. Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.

Recent Development and Utilization of Two-Way Satellite Time and Frequency Transfer

TWSTFT (Two Way Satellite Time and Frequency Transfer) has been developed for a long time, and has become one of the most precise and accurate techniques for comparison of the frequency standards located at remote sites. Since 1999, TWSTFT has been used in TAI (International Atomic Time) generation. More than two-thirds of TAI clocks and almost all the primary frequency standards are transferred using TWSTFT. To increase the time transfer precision and stability, several calibration methods were developed and the possible instability sources were investigated. Due to the high redundancy of the time transfer links and quick developments of independent time transfer techniques (e.g. GPS), much utilization was proposed to enhance the robustness, to reduce the uncertainty, and to reduce the diurnal effect of TWSTFT. For example, one can adopt the concept of network time transfer to improve the short term stability, or combine the data of different time transfer techniques to take their advantages. The numerical results of network time transfer are very promising. For the future development, a newly developed DPN-based TWSTFT method shows competitive performance with the GPS PPP and much less diurnals than the conventional TWSTFT. It is a very promising method for the next-generation TWSTFT. This paper will give an overview of the above topics.

Improvement of the Asia-Pacific TWSTFT network solutions by using DPN results

Two major time and frequency transfer techniques, two-way satellite time and frequency transfer (TWSTFT) and global navigation satellite systems (GNSS: GPS, GALILEO, GLONASS, etc.), are used for the generation of Coordinated Universal Time (UTC)/International Atomic Time (TAI). These time and frequency transfer links comprise a worldwide network and the utilization of the highly redundant time and frequency data is an important topic. Two methods, either TW-only network (i.e., TWSTFT) or single-link combination of TW and Global Positioning System (GPS), have been developed for combining the redundant data from different techniques. In our previous study, we have proposed a feasible method, utilizing full time-transfer network data, to improve the results of TWSTFT network. The National Institute of Information and Communications Technology (NICT) has recently developed a software-based two-way time-transfer modem using a dual pseudo-random noise (DPN) signal. The first international DPN TWSTFT experiment, using these modems, was performed between NICT (Japan) and Telecommunication Laboratories (TL; Taiwan)and its ability to improve the time transfer precision was demonstrated. In comparison with the conventional NICT–TLTWSTFT link, the DPN time transfer results have higher precision and lower diurnal effects. The estimation also shows that DPN is comparable to GPS precise point positioning (PPP).Because the DPN results show better performance than the conventional TWSTFT results, we would adopt the DPN data for the NICT–TL link and solve the TW+DPN network solutions by using our proposed method. The concept of this application is similar to the so-called multi-technique-network time/frequency transfer. The encouraging results confirm that the TWSTFT network performance can benefit from DPN data by improving short-term stabilities and reducing diurnal effects.The results of TW+PPP network solutions are also illustrated.

Use of GLONASS for UTC time transfer

At present, the applicable spatial techniques used in UTC (Coordinated Universal Time) computation are GPS, TWSTFT (Two-Way Satellite Time and Frequency Transfers) and GLONASS. To enable accuracy and robustness for the generation of UTC, a multi-technique strategy for UTC time transfer is indispensable. Over the last two decades efforts have been made to use GLONASS for accurate time transfer. The first GLONASS time link that presents in UTC was introduced in November 2009, BIPM Circular T 263.For present and future accurate time transfers, GLONASS is comparable to GPS with the same types of observations. In this paper, we first recall principles of the GNSS Common-View and All in View time transfers; we present the technical issues for the use of GLONASS in UTC, i.e. short- and long-term stabilities, frequency biases, calibration and its practical implementation. Finally, we outline the prospects for the use of GLONASS in accurate time transfer.