Two-way time transfer experiments via meteor burst communication

Optical two-way time and frequency transfer (O-TWTFT) is an enabling technology that has applications ranging from fundamental investigations of relativity to the operation of global navigation satellite systems. Linear-optical-sampling (LOS) between optical frequency combs has been used to create very stable optical two-way time and frequency transfer links over free-space. Here, we demonstrate two-way time and frequency transfer using LOS between electro-optic frequency combs. This two-way electro-optic time and frequency transfer system demonstrated instabilities as low as 15 fs at 1 s of averaging time. These results show a pathway to highly stable, frequency agile, and low SWaP-C time transfer networks.

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.

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.

GPS time transfer is usually carried out with C/A code measurements using the common-view technique. An uncertainty of a few nanoseconds can be obtained for an integration time of about 1000 seconds when ionospheric measurements and precise orbits are used. This limits the capability of frequency comparison to the level of 3 parts in 10/sup 14/ for an integration time of one day. Recently new frequency standards have been developed that claim an accuracy at or below 1/spl times/10/sup -14/. For existing time transfer techniques it is a challenge to compare their frequency over a sufficiently short interval. Previous studies have already shown the potential of carrier-phase and P-code measurements from geodetic GPS receivers. This paper draws up tentative uncertainty budgets for clock frequency comparisons using phase and code measurements from commercially available dual-frequency and multi-channel GPS receivers. Although the level of noise involved in carrier phase measurements is two or three orders of magnitude lower than that of code measurements in the short term, other error sources such as tropospheric delay of GPS signals, multipath propagation and the phase ambiguity problem lead to a limitation in the accuracy of frequency transfer at the 10/sup -15/ level over an averaging time of one day. Carrier phase measurements, however, provide an improvement over the performance expected from code measurements for all averaging times smaller than one day.

Precise point positioning (PPP) technology has been widely used for more than a decade in timing and time transfer. Pseudorange bias caused by signal distortion will reach up ±3 ns, which cannot be neglected in sub-nanosecond level PPP timing and time transfer. We propose to improve the performance of PPP timing and time transfer by considering signal distortion bias (SDB). In principle, SDB will affect PPP timing and time transfer by influencing the initial clock offset. Then, it is verified by theoretical and experimental analyses. The theoretical results show that the impact of SDB on PPP timing can reach up 1.8 ns in different regions. Also, the impact of SDB on time transfer can reach up 0.8 ns in different regions even if the baseline length and receiver types are the same. For experimental results, the accuracy of PPP timing and time transfer improves with SDB correction. For time transfer, the RMSs are reduced by 54.8%, 61.1% and 17.6% for GPS, BDS3 and Galileo in all links, respectively. The clock frequency consistency obtained from different systems has been improved. And the frequency stability of time link with Hydrogen clocks is improved by 53.0% at 10000s interval with SDB correction.

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.

Combining GPS and TWSTFT considerably increases the robustness of the accurate time transfer result due to the complete independence between these two spatial techniques. In this paper we present the last improvements of a time and frequency transfer method based on a least-square analysis of GPS code and carrier phase measurements in common view, constrained by TWSTFT data. The combined method developed here provides a time transfer solution which benefits from the high short-term stability and high resolution of the GPS data and the high accuracy of the TWSTFT data. It allows to solve the large day boundary discontinuities that exist in some clock solutions obtained with geodetic time transfer, but some discontinuities appear occasionally in the CV+TW solution, due to the noise and diurnal perturbation in the TWSTFT data.

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.

More recently, GPS carrier phase measurement using geodetic receivers is widely conducted for accurate time and frequency comparisons. Time transfer experiments were conducted using the Ashtech Z12T receivers whose internal frequency is provided by an external 20 MHz clock and an external 1 pps (pulse per second) of a reference clock. The results of time comparison has shown that the stability of time transfer using carrier phase is better than that of GPS common-view time transfer using GPS C/A or P3. The self-developed processing software was used to the cycle slip detection and ambiguity resolution determination. The objective of this paper is to compare the GPS P3 and GPS L3 time transfer. The GPS P3 and L3 data have been obtained using an Ashtech Z12T at NTSC and an Ashtech ZIIX3 at NICT. The main results and analysis are given in Section 3 and Section 4. For a long baseline comparison, the RMS of L3 is better than that of P3. The stability of frequency of L3 is better than that of P3 for short term tau>6d, however, for tau< 6d the stability of L3 and P3 is almost at the same level

A three-cornered hat analysis has been performed on three independent time transfer systems between the United States Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST). These include (1) a direct Two-Way Satellite Time and Frequency Transfer (TWSTFT) link provided by the USNO, (2) an indirect TWSTFT link, and (3) GPS carrier phase (GPSCP) links. Time transfer instabilities for individual links, as quantified with the time deviation and TOTAL time deviation, have been measured for the first time for averaging periods between 2 h and 85 days. Time transfer instabilities beyond one day are roughly flicker phase in nature and generally range between 40 ps and 120 ps. Frequency transfer instabilities, as quantified by the modified Allan deviation, are observed to be as low as 1 × 10−16 for a 20 day comparison and into the mid 10−17 range at longer comparison times.

Accurate time transfers are now fully based on space techniques. The recently developed optical fibre technique is expected to be used in the UTC generation in the future. Some UTC labs operate optical fibre links using different approaches. In particular, the here called Two-Way Optical Fibre Time (and Frequency) Transfer (TWOFT/TWOTFT) is of special interest. The symmetry of the propagation conditions in both directions allows to substantially reducing the instability due to fluctuations of the propagation, and its capability of self-calibration. The first operational TWOTT between the UTC laboratories has been established in Poland between AOS and GUM since January 2012. The result has being submitted to the BIPM since March 2013. The BIPM started the monthly data processing and compares to the GNSS links. The TWOTT can reach an accuracy of hundred ps in a few minutes and therefore provide an effective new tool for the assessment of GNSS and TWSTT CP time link calibration and bring substantial improvement to the [UTC-UTC(k)]. However, time transfer by optical fibres today does not match the structure of the UTC time transfer network with a unique pivot. This requires the new network configuration, algorithm and will provoke fundamental changes in the UTC time link computation.

GAGAN (GPS - Aided Geo Augmented Navigation) is an Indian SBAS (Satellite Based Augmentation Systems). In contrast to the European EGNOS, this system already supports the navigation function and it transmits signals both in L1 and L5 frequency channels. We used the GAGAN signals for an experimental common-view time transfer between IPE Prague and PTB Braunschweig which is a distance of 370 km and we also tested the time transfer properties using a single clock common-view. The L1 and ionosphere-free code measurements have markedly lower accuracy compared to a GPS common-view because of rather narrow bandwidth of the SBAS signal in L1 frequency channel. L5 code measurement provides much better precision. It results from the single clock common-view that the observed fluctuations can be described as white noise with standard deviations of 6 ns, 1.3 ns and 14 ns for L1, L5 and ionosphere-free combination. The results obtained from the carrier phase measurements are promising. The single clock common-view precision was approximately 30 ps RMS even for the ionosphere-free combination. Interesting results followed from the comparison at the long distance where relatively fast fluctuations induced by ionosphere are obvious in the single frequency measurements. We believe that using GAGAN and other SBASs for permanent carrier phase frequency transfer could be an ideal solution for continuous comparisons of precise frequency sources.

The first two-way time transfer experiments using an INTELSAT Ku-band satellite in eastern Asia in cooperation between the Communications Research Laboratory (CRL, Japan), Korea Research Institute of Standard and Science (KRISS, Korea), and the Telecommunication Laboratory (TL, Taiwan) were carried out during 1992. The CRL-KRISS experiment attained a short term precision of better than 1 ns. The CRL-TL experiment suffered from a low C/N/sub 0/ ratio which made the short term precision greater than 1 ns. The long term averaged residuals contained a periodic noise signal which we believe was the result of a Doppler shift in the frequency of the signal caused by the relative motion of the satellite. A MITREX compatible modem under development at CRL exhibited a linear correlation with frequency offsets that were similar to the noise in the data.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Over the past few years, atomic clocks have been improving and are now reaching stabilities and accuracies of a few parts in 1018 in fractional frequency. Fiber-based time and frequency transfer techniques have demonstrated excellent performance in the comparison of state-of-art optical atomic clocks over thousands of kilometers. For highly accurate time and frequency transfers, relativistic effects which affect the signal propagation in optical fibers, need to be taken into account. The most important is the Sagnac correction because of the computation complexity and also the non-reciprocity in the two-way time transfer. Sagnac correction is an important source of uncertainty in fiber-based time and frequency transfer. Besides, not all important parameters are known with sufficient precision when we compute the Sagnac correction such as the large position error of fiber nodes and also the sparse fiber nodes. It is necessary to evaluate the Sagnac correction due to imperfect knowledge of parameters. In our work, several simulation fiber links in China are analyzed as specific examples to evaluate the influences of imperfect knowledge on the accuracy of the time transfer. The results show that in order to ensure one picosecond precision of time transfer using optical fiber, position accuracy of nodes should be higher than 500 m when the information of enough number of nodes can be obtained.

The model of BDS/GPS carrier phase time and frequency transfer is given in this paper, and the transformation method of time and coordinate reference are researched. Adopting precise satellite orbits and clocks provided by GFZ, data from MGEX are processed to analysis the performance of BDS/GPS carrier phase time and frequency transfer. The results show that the precision of BDS/GPS carrier phase time and frequency transfer is not better than the single system obviously when the visible satellites are more than 7. BDS convergence time is significantly longer than GPS and BDS/GPS, due to fewer MEO satellites. The influence for single system time and frequency transfer is more obvious than BDS/GPS under complex environment. When the elevation mask angle is over 30°, BDS have better performance than GPS due to BDS’s special constellation in asian-pacific region, and GPS time transfer results are nonsequence. When the elevation mask angle is over 40°, BDS time transfer is also nonsequence at sometime, and GPS cannot be used for time transfer, but BDS/GPS is still get the high precision results; the convergence speed and stability are better than the single system.