Improved GPS-based time link calibration involving ROA and PTB

1999

Over the last years there has been a growing interest in GPS carrier phase as a tool for time and frequency transfer. This technique, also known as Geodetic Time Transfer (GeTT), has been studied intensively in the frame of a collaboration between the Swiss Federal Office of Metrology (OFMET) and the Astronomical Institute of the University of Bern (AIUB). The results obtained over European baselines and presented at previous EFTF meetings have demonstrated the suitability of GeTT as method of frequency comparison between high performance clocks since much shorter averaging times would be required than with GPS Common View. In this study the focus is on a campaign between the Physikalisch Technische Bundesanstalt in Germany (PTB) and the US Naval Observatory, Washington (USNO). Besides the longer baseline the. Choice of these two sites offers also the possibility to compare frequently GeTT and TWSTFT. First results indicate that sub-nanosecond time transfer is possible by this new technique. The PTB-USNO campaign is the longest GeTT campaign carried out to this elate. The long term behaviour of the terminals will, therefore, also be addressed

Experiments in precise clock comparisons using t h e two-way time transfer technique via satellite began in 1962. Experience gained from a variety of experiments since t h a t time steadily improved the precision a n d accuracy of such comparisons. Recent growth in the fixed satellite service, or FSS, has created new opportunities at moderate costs for high-accuracy time transfers using geostationary satellites. We discuss fundamental aspects of two-way timing a n d show a n implementation of a satellite two-way time transfer system which has been used for two-years between USNO, Washington, D.C. and NIST, Boulder, C0:The raw d a t a collection procedure will be discussed. We also outline the rationale for t h e choice of satellite uplink/downlink frequencies, signal structure, and reduction of data. Short-term noise in the time transfer limits the precision t o about 300 ps in a 300 s average. Uncertainty in accuracy is due t o uncertainty in t h e non-reciprocity of the two-way signal path. Accuracy limits due t o the atmosphere, earth-satellite rotating system (Sagnac effect), and the equipment are discussed. The goal is t o achieve a n accuracy level of 1 ns after a suitable calibration of earth-station (differential) equipment delays.

The comparison of clocks located in different laboratories, called Time and Fre-quency Transfer, can be obtained from the common view of GPS satellites. This method consists in determining the clock offsets between the laboratory clocks and a common time scale associ-ated with the GPS system; this can be either the satellite clocks, or the GPS time scale, or the IGS time scale. The last developments in GPS data analysis for time transfer, in collaboration with the geodetic community associated with the IGS, have improved significantly the capabili-ties of the technique. The paper presents the state of the art of the performances and limitations of the GPS time and frequency transfer.

2007 8th International Conference on Electronic Measurement and Instruments, 2007

A possibility to obtain a new time reference constantly locked to the Caesium-derived GPS satellites carriers was investigated by means of a new GPS receiver architecture able to lock the two GPS carriers and mix them to obtain a reference frequency with an accuracy close to the GPS one. This is very important in Perceived Power Quality distributed measurements systems where the exact knowledge of the events on the geographic compass is strongly joined to the capability to associate them an accurate time.

For about oneyear the time scales UTC(0CA) and UTC(TUG) were compared by means of GPS and two-way safeme time transfeer. At the end of the experiment both links were independently "calibrated" by measuring the differential delays of the GPS receivers and of the satellite earth statwns by transportation of a GPS receiver andof one of the satellite terminuk. The results obtained by both methods differ by about 3 ns, but revealaseasonal variation of about 8 nspeak-to-peak which is Skeh the result of a temperaturedependence of the delays of the GPS receivers used. For the comparison of both methods the stabilities of the timescales are of great importance. Unfortunately during the last three months of the experiment a less stable clock had to be used for the generatwn of UTC(TUG).

Metrologia, 1994

m e role of the NASA Tracking and Data Relay Satellite System (TDRSS) is to increme the volume and frequency oJcommunicution between un orbiting spucecruft and the Eurth, while at the same time providing command and trackingjknctions with extended coveruge via a network oJorbiting sulellites und one or more ground stutions. The same concept i s currently being studied and i s planned by the European Space Agency (ESA) under the nume of Data Relay Satellite System (DRSS). TDRSS i s an answer to the irzcreusing complexity of new satellites und spuce missions that, especiully in the$eltl of scientijic und upplicution sutellites, ure plucing increasing requirements in terms of mission support. New sutellites, designed for scientijc missions, such us ustronomicul observuturies, or eurth applications, for remote sensing, geadesy and precise rtavigution, ure relying on precise onboard clocks to accompbh their missions. All these spueborne c h h require precise synchronizution to some external ground reference, synchronization that must be provided as apart of t h stunuhrd mission suppod. Since mission support is the primary rok fir the TDRS systems, synchronization must be provided through the same links used for telemetry, command and data acquisition. There have been many time truqkr experiments, and the techniques are well known and established throughout the yeurs. A number of experiments have evolved into operutionul services now availabk worMwide and, in the case of the GPS, even to satellites in low earth orbit. However, the requirement to provide timing support as a part of t h standard support to the space missions, has resulted in NASA providing timing services to user spacecraft directly via the TDRSS. The same service is being considered for t h new Advunced TDRSS (ATDRSS) and the ESA Dutu Rehy Satellites (DRS). We will stud with u brieJreview of the well known time trunsfer techniques that have been studied and tested throughout the years. We will t h n discuss the applicability of time trun,$er techniques to a timing service as provided through a TDRSIDRS System, the problems rebted to the choice of the timing signal within the constraints imposed by the existing systems, and the possible practical implementations, including a description of the time synchronization support via TDRSS to the Gamma Ray Obsewutory (GRO).

IEEE International Frequency Control Symposium, 2007

The Royal Observatory of Belgium (ROB) developed the software Atomium to perform GPS-based time and frequency transfer. Originally dedicated to perform Precise Point Positioning (PPP) based on a combined analysis of dualfrequency carrier phase and code measurements, Atomium has recently been adapted to allow a phase-only analysis, providing a continuous solution independent of the GPS codes. In this paper, the analysis strategy used in Atomium is described and the clock solutions obtained through the phase-only approach are compared to the results from the PPP mode. It is shown that the continuous solution improves the stability of the time link for averaging times smaller than 7 days, but that the phase-only solution is drifting with respect to the combined code-carrier phase solution; this drift is station-dependent.

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 1987

The National Bureau of Standards (NBS) Boulder Lahoratory is assembling a system for time transfer with a projected accuracy of 1 ns to Incations that can access a Ku-band geostationary communications satellite in common view with Boulder. The system includes a 6.1-m diameter antenna and satellite earth station at Boulder, two transportable earth stations, each with a 1.8-m dish, and modems designed for two-way timing. The elements of the system are described and a method of using them to achieve high-accuracy time transfer is presented. A satellite simulator has been used in measurements of the stability of the loop delay for different configurations. The day-today variations exhibit peak deviations of less than 1 ns and offer encouragement that similar stability will he found for the ground segments in two-way time transfer configurations. Allan-variance stability plots are presented for sample times of 1 S to several days at various carrier-to-noise (C / N ,) density ratios. The results are compared to theoretical limits obtained from the model of phase jitter given for the spread spectrum modem. The 1.8-m earth stations have full duplex capability to permit signal turnaround at a remote location while using a geostationary satellite in common view with the NBS 6.1-m earth station. The use of one portable earth station to calibrate the critical differential delays between two other earth stations is discussed. The calibration offers a method of converting the previously demonstrated nanosecond and suhnanosecond time transfer precision to nanosecond time transfer accuracy between widely separated locations on Earth.

For the dissemination of precise time and frequency, the use of the Two-Way Satellite Time and Frequency Transfer (TWSTFT) method has, in recent years, become increasingly valuable. By the application of spread-spectrum technology, a client station, located anywhere within a common satellite s footprint, can link to a reference station and compare or synchronize its clock to the reference clock within nanoseconds or better with high levels of confidence. But as the frequency stabilities of today's sophisticated atomic-level clocks improve, so must the stability of a Two-Way system s method of time and frequency transfer. Carrier-phase information holds the promise of improving the stabilities of TWSTFT measurements, because of the great precision at which frequency transfers can be achieved. The technique requires that each site observe both its own satellite-translated signal and that of the cooperating site. As was reported in earlier papers, the translation frequency of the s...