Christine Hackman

A straightforward frequency-estimation technique for GPS carrier-phase time transfer

Although Global Positioning System (GPS) carrier-phase time transfer (GPSCPTT) offers frequency stability approaching I0-15 at averaging times of 1 d, a discontinuity occurs in the time-transfer estimates between the end of one processing batch (1-3 d in length) and the beginning of the next. The average frequency over a multiday analysis period often has been computed by first estimating and removing these discontinuities, i.e., through concatenation. We present a new frequency-estimation technique in which frequencies are computed from the individual batches then averaged to obtain the mean frequency for a multiday period. This allows the frequency to be computed without the uncertainty associated with the removal of the discontinuities and requires fewer computational resources. The new technique was tested by comparing the fractional frequency-difference values it yields to those obtained using a GPSCPTT concatenation method and those obtained using two-way satellite time-and-frequency transfer (TWSTFT). The clocks studied were located in Braunschweig, Germany, and in Boulder, CO. The frequencies obtained from the GPSCPTT measurements using either method agreed with those obtained from TWSTFT at several parts in 1016. The frequency values obtained from the GPSCPTT data by use of the new method agreed with those obtained using the concatenation technique at 1 - 4 middot 10-16.

Common-clock two-way satellite time transfer experiments

We report the results of a series of measurements designed to determine the magnitudes and noise types of the various sources of instability in the two-way satellite time and frequency transfer (TWSTFT) process. We find that, for our equipment, the earth-station noise predominates at the 1 day time scale. We also report results from a maser-to-maser time transfer conducted between NIST-Boulder and USNO (Washington DC), which show a stability at 1 day of about 1 ns.

Towards Sub-10 -16 Transcontinental GPS Carrier-Phase Frequency Transfer: a Simulation Study

A simulation study is performed using GIPSY software in order to determine the impact of site-based and satellite-based systematic errors on the accuracy of between-site GPS carrier-phase frequency comparisons. The data are analyzed using both the precise point positioning (ppp) and network methods: in the former, the time differences between the satellite clocks and system time are fixed to predetermined values. In the latter, the time differences of both the satellite clocks and the receiver clocks are estimated relative to some reference clock (usually a ground-based receiver clock). We also analyze data both with and without the added constraint of double-difference ambiguity fixing. We find that between-site frequency comparisons are largely unaffected by site-based and satellite-based systematic errors when 100% of the double-difference ambiguities are fixed. We also find that in the ppp method, although fixing ambiguities removes between-site frequency errors, it can cause errors in the values of the individual receiver clocks relative to system time. Finally, we find that when a network solution is performed and ambiguities are not fixed, an error made at site A may adversely affect frequency comparisons between sites B and C

New frequency comparisons using GPS carrier-phase time transfer

In October 2002, experiments were conducted to assess the frequency accuracy available from GPS carrier-phase time transfer (GPSCPTT). In these experiments, two GPS receivers, NIST/spl I.bar/A and NIST/spl I.bar/B, were operated in close proximity with one clock, UTC(NIST), serving as the receiver clock for both. Because the same clock was used for both receivers, the resultant time series Clk(NIST/spl I.bar/A)-Clk(NIST/spl I.bar/B) is expected to have a slope (frequency difference) of zero. The experiment yielded mixed results. Data recorded 22-25 October 2002 yielded slopes in the 12-24 ps/d range with both positive and negative signs, indicating that if one averaged long enough, the frequency error obtained from GPSCPTT might average out to zero. However, data recorded 5-9 October 2002 yielded slopes in the 45-77 ps/d range, with all of the slopes having the same sign. Thus, it appears that some sort of systematic frequency error is arising in either the measurement or data-analysis process. Attempts have been made to determine the cause of this systematic error. Potential sources investigated include (a) unequal sampling rates at the two receivers, (b) the analysis technique of fixing satellite-clock corrections to predetermined values rather than estimating them, and (c) errors in the estimation of the tropospheric delay. None of these appear be the root cause of the problem. Future work will include the investigation of site- and receiver-specific effects such as temperature sensitivity and multipath.

A new technique for estimating frequency from GPS carrier-phase time transfer data

GPS carrier-phase time transfer (GPSCPTT) offers good frequency stability at short averaging times, approaching a fractional frequency stability of 10/sup -15/ at an averaging time of 1 d. However, a discontinuity occurs in the time transfer estimates between the end of one processing batch (typically 1-3 d in length) and the beginning of the next. We present a new technique in which a frequency is computed from each batch solution and then these frequency values are averaged to obtain a mean frequency for the epoch of interest. The advantages of the new technique are (a) robustness in the event of a data outage and (b) the ability to compute frequency in the absence of the uncertainty introduced by the removal of the discontinuities. The frequency values obtained from the GPSCPTT data using the new method agreed with those obtained using the conventional method at 2-3/spl middot/10/sup -16/, thus, it may be feasible to replace the conventional method with the new one.