We review the development and status of GPS geodetic methods for high-precision global time and frequency comparisons. A comprehensive view is taken, including hardware effects in the transmitting satellites and tracking receiver stations, data analysis and interpretation, and comparisons with independent results. Other GPS techniques rely on single-frequency data and/or assume cancellation of most systematic errors using differences between simultaneous observations. By applying the full observation modelling of modern geodesy to dual-frequency observations of GPS carrier phase and pseudorange, the precision of timing comparisons can be improved from the level of several nanoseconds to near 100 ps. For an averaging interval of one day, we infer a limiting Allan deviation of about 1.4 × 10 −15 for the GPS geodetic technique. The accuracy of time comparisons is set by the ability to calibrate the absolute instrumental delays through the GPS receiver and antenna chain, currently about 3 ns. Geodetic clock measurements are available for most of the major timing laboratories, as well as for many other tracking stations and the satellites, via the routine products of the International GPS Service.
Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize the timing performance of the GPS satellites. Using 5-minute and 30-second observational samples and focusing only on the sub-daily regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day (24 hr UTC), for n =1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 hours, which surprisingly differs from the reported mean GPS orbital period of 11.9659 ± 0.0007 hours by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval. REPORT TYPE 1. REPORT DATE (DD-MM-YYYY) TITLE AND SUBTITLE AUTHOR(S) PERFORMING ORGANIZATION REPORT NUMBER PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S)
The development within the International GPS Service (IGS) of a suite of clock products, for both satellites and tracking receivers, offers some experiences which mirror the operations of the Bureau International des Poids et Mesures (BIPM) in its formation of TAI/UTC but some aspects differ markedly. The IGS relies exclusively on the carrier phase-based geodetic technique whereas BIPM time/frequency transfers use only common-view and two-way satellite (TWSTFT) methods. The carrier-phase approach has the potential of very high precision but suitable instrumental calibration procedures are only in the initial phases of deployment; the current BIPM techniques are more mature and widely used among timing labs, but are either less precise (common-view) or much more expensive (TWSTFT). In serving its geodetic users, the essential requirement for IGS clock products is that they be fully self-consistent in relative terms and also fully consistent with all other IGS products, especially the satellite orbits, in order to permit an isolated user to apply them with accuracy of a few centimetres. While there is no other strong requirement for the IGS timescale except to be reasonably close to broadcast GPS time, it is nonetheless very desirable for the IGS clock products to possess additional properties, such as being highly stable and being accurately relatable to UTC. These qualities enhance the value of IGS clock products for applications other than pure geodesy, especially for timing operations. The jointly sponsored 'IGS/BIPM Pilot Project to Study Accurate Time and Frequency Comparisons using GPS Phase and Code Measurements' is developing operational strategies to exploit geodetic GPS methods for improved global time/frequency comparisons to the mutual benefit of both organizations. While helping the IGS to refine its clock products and link them to UTC, this collaboration will also provide new time transfer results for the BIPM that may eventually improve the formation of TAI and allow meaningful comparisons of new cold atom clocks. Thus far, geodetic receivers have been installed at many timing labs, a new internally realized IGS timescale has been produced using a weighted ensemble algorithm, and instrumental calibration procedures developed. Formulating a robust frequency ensemble from a globally distributed network of clocks presents unique challenges compared with intra-laboratory timescales. We have used these products to make a detailed study of the observed time transfer performance for about 30 IGS stations equipped with H-maser frequency standards. The results reveal a large dispersion in quality which can often be related to differences in local station factors. The main elements of the Project's original plan are now largely completed or in progress. In major ways, the experiences of this joint effort can serve as a useful model for future distributed timing systems, for example, Galileo and other GNSS operations.
Figure 7 in this paper is inaccurate. The correct figure 7 is shown below. Figure 7. Allan deviation plot of each Block IIR satellite clock as well as a plot of the average Allan deviation of the Blocks II/IIA cesium and II/IIA rubidium satellites during December 2001. All clocks are referenced to IGST and no data editing has been applied.
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