HR: 0800h
AN: G41B-0361    [Abstracts]
TI: Using IGS Clock Products to Monitor GPS Station Performance
AU: * Senior, K
EM: Ken.Senior@nrl.navy.mil
AF: U.S. Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375 United States
AU: Ray, J
EM: jimr@ngs.noaa.gov
AF: U.S. National Geodetic Survey, SSMC-3, #9202 1315 East-West Highway, Silver Spring, MD 20910-3282 United States
AB: Since 22 Feb. 2004 the IGS official satellite and station clock products have been aligned to a new, highly stable timescale realized by an internal ensemble of the available frequency standards dynamically weighted based on their individual instabilities. (Comparable, unofficial clock products are archived starting 29 Oct. 2000.) All results and a variety of associated plots can be accessed at https://goby.nrl.navy.mil/IGStime/index.php. In addition to their value for clock diagnostics, these products can be used to monitor the general health of GPS tracking stations. This is only minimally true for stations not using H-maser external frequency standards since the large epoch-to-epoch clock variations limit the sensitivity to detect all but the most severe types of problems affecting data quality. With H-masers, though, many subtle effects on the pseudorange observables can be observed. This is because the overall clock bias for a given processing arc (normally 24 hours) is set by the average code data, while the higher frequency variations are determined by the carrier phase data. Assuming an average uncertainty of 1 m for code data and 5-min sampling, the formal accuracy of each clock estimate should be near 120 ps. A quantitative test of the actual clock accuracy can be made by comparing clock estimates at the boundaries between independent analysis arcs (i.e., at midnights between consecutive days). This is analogous to the classic geodetic repeatability test for a time series of positioning results. The test is only feasible when H-masers are used since the instabilities of lesser frequency standards dominate over the day-boundary jumps caused by code data quality. Applied to IGS clocks, we have found a very wide dispersion in station performances, from RMS clock jumps near the expected level of 120 ps to values >1 ns. Results are posted in plots available at https://goby.nrl.navy.mil/IGStime/daybdy/. In a number of cases, there are discrete changes in the magnitude of clock day-jumps correlated with equipment changes or malfunctions of various types. Sometimes the cause of changes in performance is unknown. Three Canadian stations show large annual variations, being far worse in winters than in summers. In general, the clock jumps presumably reflect primarily the level of local code multipath, especially the longest wavelength type due to near-field reflections. It is likely that there are associated effects in the carrier phase data, which are much more difficult to detect since they are largely absorbed into the estimated ambiguity parameters. The same techniques can be used to monitor data performance at stations not in the IGS network, provided that they are equipped with an H-maser frequency standard. A time series of clock estimates must be determined with the precise point positioning method using fixed IGS satellite orbits and aligned clocks. The high stability of the IGS timescale ensures that day-boundary jumps in the station clock can be detected to better than 100 ps RMS.
DE: 1229 Reference systems
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
SC: Geodesy [G]
MN: Fall Meeting 2005