Two-way Satellite Time Transfer Research Articles

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.

An Evaluation of the Beidou Time System (BDT)

The evaluation of Beidou system time is one of the important tasks in constructing and developing the Beidou satellite navigation system. The evaluation methods based on two-way satellite time and frequency transfer method (TWSTFT) and GPS common-view method are proposed in this paper. Tests and performance evaluation of the Beidou time system (BDT) were conducted. Results show that BDT is stable and reliable. The BDT frequency deviation, Allan deviations and clock offsets, relative to UTC (NTSC), are 10e-14, 10e-15 and less than 30 ns respectively.

Sagnac Effect and Diurnal Correction on Two-Way Satellite Time Transfer

Precision of two-way satellite time and frequency transfer (TWSTFT) highly depends on the residual nonreciprocity delays; one of them is caused by the Sagnac effect. In this paper, we employ the satellite ephemeris data to compute the precise Sagnac effect and discuss the possible additive effect related to the Sagnac effect. We also describe a method to calculate the diurnal correction based on the time derivative of the Sagnac effect. Through the analysis of the link between the National Institute of Information and Communications Technology in Japan and the Telecommunication Laboratories in Taiwan, we demonstrate that the correction was useful to improve the TWSTFT data. The time deviation at an 8-h averaging time is reduced from 346 to 156 ps after the correction.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

Astrometric positioning and orbit determination of geostationary satellites

In the project titled “Astrometric Positioning of Geostationary Satellite” (PASAGE), carried out by the Real Instituto y Observatorio de la Armada (ROA), optical observation techniques were developed to allow satellites to be located in the geostationary ring with angular accuracies of up to a few tenths of an arcsec. These techniques do not necessarily require the use of large telescopes or especially dark areas, and furthermore, because optical observation is a passive method, they could be directly applicable to the detection and monitoring of passive objects such as space debris in the geostationary ring. By using single-station angular observations, geostationary satellite orbits with positional uncertainties below 350 m (2 sigma) were reconstructed using the Orbit Determination Tool Kit software, by Analytical Graphics, Inc. This software is used in collaboration with the Spanish Instituto Nacional de Técnica Aeroespacial. Orbit determination can be improved by taking into consideration the data from other stations, such as angular observations alone or together with ranging measurements to the satellite. Tests were carried out combining angular observations with the ranging measurements obtained from the Two-Way Satellite Time and Frequency Transfer technique that is used by ROA’s Time Section to carry out time transfer with other laboratories. Results show a reduction of the 2 sigma uncertainty to less than 100 m.

Improvement of precision on two-way satellite time and frequency transfer

Multi-channel modems have been used in Asian two-way satellite time and frequency transfer (TWSTFT). To make full use of data in multi-channel TWSTFT and improve the precision of TWSTFT, a new method to process data is presented. The precision of the multi-channel TWSTFT is compared with that of the single-channel TWSTFT, and the results show that it achieves an improvement.

Numerical prediction methods for clock deviation based on two-way satellite time and frequency transfer data

Three functional models, polynomial, spectral analysis, and modified AR model, are studied and compared in fitting and predicting clock deviation based on the data sequence derived from two-way satellite time and frequency transfer. A robust equivalent weight is applied, which controls the significant influence of outlying observations. Some conclusions show that the prediction precision of robust estimation is better than that of LS. The prediction precision calculated from smoothed observations is higher than that calculated from sampling observations. As a count of the obvious period variations in the clock deviation sequence, the predicted values of polynomial model are implausible. The prediction precision of spectral analysis model is very low, but the principal periods can be determined. The prediction RMS of 6-hour extrapolation interval is 1 ns or so, when modified AR model is used.

Time and Frequency Activities at the U.S. Naval Observatory

The US Naval Observatory (USNO) has provided timing for the navy since 1830 and, in cooperation with other institutions, has also provided timing for the United States and the international community. Its Master Clock (MC) is the source of UTC(USNO), the USNO′s realization of Coordinated Universal Time (UTC), which has stayed within 5 nanoseconds RMS of UTC since 1999. The data used to generate UTC(USNO) are based upon 73 cesium and 21 hydrogen maser frequency standards in three buildings at two sites. The USNO disseminates time via voice, telephone modem, LORAN, Network Time Protocol (NTP), GPS, and two‐way satellite time transfer (TWSTT). This paper describes some of the changes being made to meet the future needs for precision, accuracy, and robustness.

Long-term comparisons between two-way satellite and geodetic time transfer systems

Global Positioning System (GPS) observations recorded in the United States and Europe were used to evaluate time transfer capabilities of GETT (geodetic time transfer). Timing estimates were compared with two-way satellite time and frequency transfer (TWSTFT) systems. A comparison of calibrated links at the U.S. Naval Observatory, Washington, D.C., and Colorado Springs, CO, yielded agreement of 2.17 ns over 6 months with a standard deviation of 0.73 ns. An uncalibrated link between the National Institute of Standards and Technology (NIST) and Physikalisch-Technische Bundesanstalt, Braunschweig, Germany, has a standard deviation of 0.79 ns over the same time period.

Accuracy of two-way satellite time and frequency transfer via non-geostationary satellites

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°.

衛星双方向時刻比較用地球局のための局内遅延校正システムの開発

衛星双方向時刻比較用地球局のための局内遅延校正システムの開発

Least-squares analysis of time series data and its application to two-way satellite time and frequency transfer measurements

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.

Timekeeping at the US naval observatory

No one can stop time, but time can stop a system. Users need to be aware of the different flavors of time, depending upon their needs. Most systems require that all their components be on common time (synchrony), but they cannot keep them on time unless they are also on a common frequency (syntony). Other systems simply require syntony. The US Naval Observatory (USNO) is the timing reference for GPS. Its master clock (MC) is based upon 72 HP5071 cesium and 17 hydrogen maser frequency standards in three buildings at two sites, and their data are used to generate UTC (USNO). The USNO disseminates and distributes the time not only via GPS, but also with Loran, network time protocol (NTP), and two-way-satellite time transfer (TWSTT). Our emphasis is on robustness through repeated calibration and multiplicity of systems, and we recommend this for our users as well. Further details and explanations of our services can be found on: http://tycho.usno.navy.mil, or by contacting the author directly.

Impact of satellite motion on two-way satellite time and frequency transfer

The impact of satellite motion on two-way satellite time and frequency transfer (TWSTFT) is analysed. Due to the two distances from two stations to the satellite being different, the non-reciprocal paths are produced, which impact the accuracy of TWSTFT. Choosing a satellite for which both elevations are about the same can minimise the effect, or offsetting the transmission time can eliminate this. In addition, due to Earth's rotation and the finite signal velocity, the non-reciprocal paths are also produced, and the error mainly results from the aviation of the satellite longitude and altitude.

Performance of Two-way Satellite Time and Frequency Transfer in Asia-Pacific Region

Performance of Two-way Satellite Time and Frequency Transfer in Asia-Pacific Region

Continuous time transfer using GPS carrier phase.

The Astronomical Institute of the University of Berne is hosting one of the Analysis Centers (AC) of the International GPS Service (IGS). A network of a few GPS stations in Europe and North America is routinely analyzed for time transfer purposes, using the carrier phase observations. This work is done in the framework of a joint project with the Swiss Federal Office of Metrology and Accreditation (METAS). The daily solutions are computed independently. The resulting time transfer series show jumps of up to 1 ns at the day boundaries. A method to concatenate the daily time transfer solutions to a continuous series was developed. A continuous time series is available for a time span of more than 4 mo. The results were compared with the time transfer results from other techniques such as two-way satellite time and frequency transfer. This concatenation improves the results obtained in a daily computing scheme because a continuous time series better reflects the characteristics of continuously working clocks.

Two-way satellite time and frequency transfer networks in Pacific Rim region

A two-way satellite time and frequency transfer (TWSTFT) network in the Pacific Rim region is under construction to contribute to the calculation of the international atomic time (TAI). Four major time and frequency institutes in this region have been conducting long-term TWSTFT experiments. In addition to these institutes, several others in the region are planning to join the network. A new type of time transfer modem for TWSTFT is also described.

Assessment of GPS carrier-phase stability for time-transfer applications

We have conducted global positioning system (GPS) carrier-phase time-transfer experiments between the master clock (MC) at the U.S. Naval Observatory (USNO) in Washington, DC and the alternate master clock (AMC) at Schriever Air Force Base near Colorado Springs, Colorado. These clocks are also monitored on an hourly basis with two-way satellite time-transfer (TWSTT) measurements. We compared the performance of the GPS carrier phase and TWSTT systems over a 236-d period. Because of power problems and data outages during the carrier-phase experiment, the longest continuous time span is 96 d. The data from this period show agreement with TWSTT within +/-1 ns, apart from an overall constant time offset (caused by unknown delays in the GPS hardware at both ends). For averaging times of a day, the carrier-phase and TWSTT systems have a frequency uncertainty of 2.5 and 5.5 parts in 10(15), respectively.