GPS Common-view Time Transfer Research Articles

Comparison of VLBI and GNSS common view for time transfer

With the rapid development of optical clock, the stability and system uncertainty of optical clocks has reached a 1.0e–18 level. Optical clocks will likely constitute the next generation of time-frequency standards for redefining the SI second. Because time and frequency transfer services that rely on satellite systems are not always reliable and currently available technologies are insufficient for comparing the next generation of frequency standards, high-precision time and transfer techniques are strongly desired. Very Long Baseline Interferometry (VLBI) is one of the space geodetic techniques that measure the arrival time delays between multiple stations utilizing radio signals from distant celestial radio sources. Not only can VLBI obtain the angle position measurement of the radio source with sub-millisecond accuracy and the station coordinate measurement with millimeter accuracy, but also, it can provide high-precision information regarding inter-station atomic clock differences. Therefore, it is theoretically feasible to use the VLBI technology to do the remote time transfer. Because of this characteristic of VLBI technology, VLBI has significant application potential in the field of remote time transfer. To confirm the suitability of VLBI to time-frequency transfer for future practical applications, the results of VLBI and GPS common view time transfer were compared using a Kunming-Urumqi baseline. The performance characteristics of time transfer based on VLBI are then analyzed. Experimental results show that VLBI technology can accurately measure the variation of clock differences between stations as same as the GPS common view time comparison technology. It briefly describes the challenges of future VLBI technology for practical applications of time transfer.

Around-the-world GPS common-view closures with conventional P3 and smoothed P3 codes

The GPS common-view method has recently been utilised with P3 code rather than C/A code in TAI computation. Cases using both conventional and smoothed P3 code are investigated to understand the characteristics of GPS common-view time transfer closures with IGS precise ephemeris. Analytical results reveal that smoothed P3 code yields a lower uncertainty than conventional P3 code.

Impact of satellite position error on GPS common-view time transfer

The impact of satellite position error on GPS common-view time transfer is analysed. The impact varies depending on the elevation of users and the baseline distance between two users. When the two elevations are the same, the impact is approximately directly proportional to the baseline. When the baseline is less than about 2500 km, the impact varies greatly with any difference in the two elevations, while the variation of the impact is low for longer baselines.

Carrier-phase time transfer

We have conducted several time-transfer experiments using the phase of the GPS carrier rather than the code, as is done in current GPS-based time-transfer systems. Atomic clocks were connected to geodetic GPS receivers; we then used the GPS carrier-phase observations to estimate relative clock behavior at 6-minute intervals. GPS carrier-phase time transfer is more than an order of magnitude more precise than GPS common view time transfer and agrees, within the experimental uncertainty, with two-way satellite time-transfer measurements for a 2400 km baseline. GPS carrier-phase time transfer has a stability of 100 ps, which translates into a frequency uncertainty of about two parts in 10(-15) for an average time of 1 day.

Preliminary evaluation of time scales based on hydrogen masers

Two experimental time scales based on ensembles of hydrogen masers were generated and compared with a variety of other references both internal and external to NIST. The masers all had some type of active cavity control to reduce frequency drift due to cavity changes, The first experimental time scale, TA2M, was generated over the interval from MJD 49050 to 49190 (March 4, 1993-July 22, 1993) using measurements between the masers every 6 hours, TA2M was compared to the NIST AT1 time scale, primarily based on commercial cesium frequency standards, and via once-per-day GPS common-view measurements to the USNO unsteered master clock and to UTC. The linear frequency drift of TA2M relative to UTC was small compared to the uncertainty of 2/spl middot/10/sup -16//d in estimating linear frequency drift. The masers at NIST were then linked to two masers at USNO using GPS common-view time transfer to study the performance of a maser ensemble, TA2M1, at measurement times from a few days to a few months. Again we found that the frequency drift of TA2M1 relative to UTC was smaller than the estimation uncertainty of 0.6/spl middot/10/sup -16//d. The stability of the ensemble at 1 month appeared to be about 1-2 parts in 10/sup 15/. From this work we see that the use of autotuned hydrogen masers in time scales is very promising. A single such maser rivals the 1993 stability of UTC. The typical fractional frequency stability of the masers showed flicker frequency modulation at 4/spl middot/10/sup -15/ or less for measurement times of 10 d-20 d. Linear frequency drift was measured for the masers against the TA2M1 time scale at levels under 1/spl middot/10/sup -16//d with uncertainties of 0.3/spl middot/10/sup -16//d. Measuring frequency drift against the definition of the SI second at this level pushes the limits of current technology.

Technical Directives for Standardization of GPS Time Receiver Software: to be implemented for improving the accuracy of GPS common-view time transfer

Technical Directives for Standardization of GPS Time Receiver Software: to be implemented for improving the accuracy of GPS common-view time transfer