stefan.countryman@LIGO.ORG - posted 17:11, Saturday 20 June 2015 - last comment - 13:24, Thursday 25 June 2015(19254)
1PPS Time Offset Histograms for EY/EX/MSR Time Code Generators and MSR Trimble GPS Clock
Data extracted for the three week period of ER7 (since the timing system would nominally be running steadily the entire time). The histograms show: - Tight grouping of the minute trends in the data; min and max values for each minute end up in discrete bands. - The Time Code Generators show nearly identical histograms for their mean minute trends. - The mean minute trends of the GPS clock are much more tightly grouped with a width of roughly 20ns; with minimum and maximum offsets included, the width roughly doubles. Issues: - There was a single second during which the MSR Time Code Generator was off from the timing system by approximately 0.4 seconds. The issue self-rectified before the start of the next second. Second-trend data was not available through dataview (or at least I couldn't get any out of it). This did not happen in the EY and EX time code generators. It did not happen again in the MSR time code generator. The anomaly happened at GPS time 1117315540. I've attached histograms as well as screenshots of data taken from Grace.
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Conclusion: The timing system is internally consistent and doesn't drift much relative to the atomic clock. We should look at this again once we hook up the Master's 1PPS input to the Symmetricom's 1PPS output; right now it's getting its 1PPS frpm the Master's built in GPS clock, which isn't as accurate as the Symmetricom's signal. The time code generator in MSR is connected to an atomic clock, which we'd expect to provide more accurate short-term timing, though GPS beats it in the long-run. So we're interested in short-term deviations from the atomic clock time, not the overall linear trend, which won't be flat unless the atomic clock itself is perfectly calibrated. For this reason, it's not surprising that the timeseries for the TCG and TCT show linear drift. The relevant metric (variation about the linear trend) is actually smaller than the above histograms would suggest, which is good. Even the naive measurement presented in these histograms shows variance of less than 100 ns.