From jimr@maia.usno.navy.mil Thu Jul 23 15:26:34 EDT 1998 Received: (from jimr@localhost) by maia.usno.navy.mil (8.7.5/8.7.3) id PAA20925 for gpst@maia; Thu, 23 Jul 1998 15:24:38 -0400 (EDT) From: Jim Ray (USNO 202-762-1444)Message-Id: <199807231924.PAA20925@maia.usno.navy.mil> Subject: Clock Resets at USNO To: gpst@maia.usno.navy.mil Date: Thu, 23 Jul 1998 15:24:38 EDT X-Mailer: Elm [revision: 212.4] Status: RO Clock Resets at USNO -- Comparison of 1 pps Data with Geodetic Analysis Jim Ray U.S. Naval Observatory, Washington, DC, USA Summary ======= AOA TurboRogue receivers have a well known tendency to reset their internal clocks occasionally, even when driven from a very stable external frequency standard. While this feature is necessary for the receiver to maintain tracking when using a poor quality frequency standard, it is quite annoying for high-precision timing applications. The JPL group (Larry Young) has advised users that the TurboRogue 1 pps output can be monitored to determine quantitatively the magnitude of such clock jumps. For the USNO IGS receiver installation, such a measurement system was set up by the Time Service Dept. (Wendy King and Paul Wheeler) to compare the 1 pps output from the TurboRogue against the 5 MHz signal input (which comes from a steered H-maser aligned to UTC(USNO)). Recently, we have observed two resets of the USNO receiver clock (one spontaneous and the other induced by a power outage) which provide data to compare the 1 pps monitor performance with independent geodetic analyses. An examination of these data show that: 1) the 1 pps measurement system provides very stable monitor data; 2) the 1 pps data can be used to calibrate spontaneous GPS receiver clock resets probably to the few ps level; 3) the calibration value for one such event has been confirmed using GPS geodetic analyis to within 0.1 to 0.2 ns. The 1 pps measurement system provides the means to monitor and calibrate GPS receiver resets, well within the uncertainty of the geodetic results. Spontaneous Reset on 1998 July 06 at ~12:22:35 UTC (MJD 51000.515683) ===================================================================== Results from 1 pps measurements ------------------------------- The 1 pps monitor data (at about 15-minute intervals) from the period before the clock glitch are summarized below for the entire measurement period and for the last day beforehand: all data before last day before glitch glitch ---------------------- ---------------------- 409.166 ns mean 409.166 ns mean 0.090 ns RMS 0.082 ns RMS 2626 # 100 # 50969.000243 MJD start 50999.479213 MJD start (1998 06 05 00:00:21) (1998 07 05 11:30:04) 51000.510475 MJD stop 51000.510475 MJD stop (1998 07 06 12:15:05) (1998 07 06 12:15:05) This shows that the measurement system is capable of monitoring the 1 pps signal with a stability of <100 ps per measurement and that there is no evidence for a drift. It also suggests that if the interval between resets is sufficiently long, then the clock jumps can probably be determined to the few-ps level (assuming Gaussian statistics). Using the mid-point between measurements where the jump occurred, we can estimate the epoch of the event as 1998 07 06 at ~12:22:35 UTC (MJD 51000.515683). Similar 1 pps monitor data for the period after the clock glitch are summarized below: all data after first day after glitch glitch ---------------------- ---------------------- 946.848 ns mean 946.864 ns mean 0.100 ns RMS 0.097 ns RMS 683 # 100 # 51000.520880 MJD start 51000.520880 MJD start (1998 07 06 12:30:04) (1998 07 06 12:30:04) 51007.625012 MJD start 51001.552141 MJD start (1998 07 13 15:00:01) (1998 07 07 13:15:05) The more recent 1 pps data are slightly less stable than the previous period but insignificantly. Thus, if we use the two longest periods and difference the mean values then the following estimate for the clock jump is obtained: 1 pps estimate for clock jump +537.682 ns standard error 0.004 ns epoch 1998 07 06 at 12:22:35 UTC +/- 07:30 (MJD 51000.515683 +/- 0.005208) Results from geodetic analysis ------------------------------ >From the USNO geodetic analysis performed for the IGS Rapid products, we can examine the estimated clock time series for the same event. The geodetic analysis uses the USNO receiver as the clock solution reference (i.e., all other receiver and satellite clocks are determined holding USNO fixed at zero) and estimates are made at 7.5-minute intervals. Thus a clock jump at USNO shows up as a discontinuity for all the other clocks. Since the actual epoch of the clock jump is not resolvable better than some time between 1998 07 06 12:11:03 and 1998 07 06 12:26:03 GPS Time (the intermediate data epoch was apparently affected by the reset and did not produce usable results), stations equipped with very stable frequency standards are preferred to estimate a value for the clock discontinuity. Three such geodetic estimates are listed below based on differences of clock estimates immediately before and after the reset: FORT +537.688 ns (Sigma Tau H-maser) formal error 0.204 ns GRAZ +537.383 ns (Cesium) formal error 0.197 ns TSKB +537.242 ns (Cesium) formal error 0.202 ns The formal error is merely the RSS of the errors from the GIPSY Kalman filter. Because such errors are time-correlated, they can over-estimate the uncertainty for a differential quantity such as this. On the other hand, the formal error does not include any contribution due to intrinsic variations in each clock during the 15-minute gap (including all clock-like effects, not only the underlying frequency standard). Thus, the FORT result, using an auto-tuned H-maser, is probably the most reliable. This conclusion is supported by the following statistical observations for the 3 hours before and after the clock jump: FORT 0.073 ns RMS for 3 hr before jump 0.072 ns RMS for 3 hr after jump GRAZ 0.208 ns RMS for 3 hr before jump 0.381 ns RMS for 3 hr after jump TSKB 0.917 ns RMS for 3 hr before jump 1.074 ns RMS for 3 hr after jump To attempt to verify the FORT result, we can examine results from other H-masers with large drifts provided that rate corrections are applied. This has been done for the H-maser stations below using first-differences immediately before and after the glitch interpolated to the mid-point (1998 07 06 12:18:33 GPS Time): ALGO +537.890 ns estimated error 0.304 ns DRAO +536.846 ns estimated error 0.503 ns FAIR +537.907 ns estimated error 0.339 ns GOLD +538.097 ns estimated error 0.223 ns KOKB +537.981 ns estimated error 0.258 ns NLIB +537.888 ns estimated error 0.215 ns TIDB +537.844 ns estimated error 0.216 ns WTZR +538.244 ns estimated error 0.348 ns YELL +537.573 ns estimated error 0.261 ns The error estimates here include the formal errors from the Kalman filter as well as the full effect of the clock drift over the 15-minute window. The variation in error estimates is due to different drifts. If we compute the weighted mean of all 12 estimates above for the clock jump, the result below is obtained. In weighting the cesium clock results, the observed variance of the clocks before/after the glitch is also included, although the weighted mean is not sensitive to whether the cesium results are included or not. Indeed, the overall mean itself is not sensitive to whether weighting is used or not: weighted mean for clock jump +537.827 ns weighted RMS 0.270 ns unweighted mean for clock jump +537.715 ns unweighted RMS 0.395 ns Based on all the geodetic results, it appears that the estimate derived from the FORT data alone is the best overall. Hence we conclude: geodetic estimate for clock jump +537.688 ns estimated error 0.204 ns epoch 1998 07 06 at 12:18:33 GPS Time +/- 07:30 (MJD 51000.512882 +/- 0.005208) Comparison of results --------------------- The geodetic estimate agrees very well with the 1 pps measurement value, better than expected from the error estimates which is probably fortuitous. Using the very stable FORT data, the geodetic error estimate could well be pessimistic although it is unlikely to be better than ~0.1 ns. Much more sophisticated analyses of the geodetic results could be performed but it is unlikely that the conclusions will be greatly affected. Clearly, the 1 pps measurement system provides the means to monitor and calibrate GPS receiver resets, well within the uncertainty of the geodetic results. Power Reset on 1998 July 13 shortly after 15:00:01 UTC (MJD 51007.625012) ========================================================================= Some time within 15 minutes after 1998 07 13 15:00:01, the UPS power for the GPS receiver, 1 pps measurement system, and other equipment was interrupted. Because manual intervention was required to restart the various systems, about a day elapsed before everything was fully restored. For this reason it is not feasible to perform a quantitative comparison of clock offset determinations as with the 1998 July 06 event. However, from the 1 pps measurement data available, before and after the power interruption, a precise estimate for the clock offset can be determined. For the period since the power outage, we have: all data after power break ---------------------- 224.252 ns mean 0.099 ns RMS 589 # 51008.886794 MJD start (1998 07 14 21:16:59) 51014.989618 MJD start (1998 07 20 23:45:03) which, together with the previous offset for the period from 1998 July 06 to 13 gives the following estimate for the clock offset: 1 pps estimate for clock jump -722.596 ns standard error 0.006 ns epoch 1998 07 13 after 15:00:01 UTC (MJD 51007.625012)