Subj: GBES guide to determining a clock model From: Toney Minter Date: 97 July 3 This document describes how a clock model is derived for the GBES. OVERVIEW ======== The GBES clock is driven by the SigmaTau maser located in the interferometer control building. The maser is about 2 km from the tracking station. The maser signal is sent via optical fibers from the interferometer control building to the GBES trailor. The length of the optical fiber connection is monitored. The GBES clock is compared to GPS at the tracking station. The SigmaTau maser clock is compared to GPS at the interferometer control building. Since the GBES 1PPS signal is driven by the output from the SigmaTau maser it is necessary to keep models of both the SigmaTau maser clock and the GBES clock. CLOCK MODEL =========== The clock model consists of eight parameters. These are the epoch of the GBES clock model and the SigmaTau clock model. The offset of the GBES clock. The drift rate and acceleration of both the GBES clock and the SigmaTau maser clock. Finally, the final parameter describes the quality of the GBES clock. Generally the SigmaTau maser is adjusted only once or twice a year and is very stable. The GBES clock, however, must be maintained within one microsecond of UTC and thus needs to be reset every few months. GBES CLOCK MODEL ================ The GBES clock model can be determined by using the clockModelPlot command. If you want to determine the clock model between June 26, 1997 and July 3, 1997 you just issue the following command: clockModelPlot 97Jun26 97Jul03 By just type clockModelPlot you get the following information on how to run this program: clockModelPlot where and are of the form 97Feb06 A plot of the clockModel will be output to the screen as well as to the printer so that the quality of the model can be determined. The epoch, offset, drift and acceleration of the model will also be output. Note that the acceleration term will only be determined if a baseline of 15 days is available where the clock monintoring information is valid. The resulting model from clockModelPlot is corrected for variations in the optical fiber connecting the SigmaTau maser with the GBES tracking station. SIGMA TAU MODEL =============== The SigmaTau clock model is determined using two seperate programs. The first is the program "logsum" maintained for USNO operations by Frank Ghigo. Here is a typical sample of how this program is run: logsum Select (1)85-1, (2)85-2, (3)85-3, (4)Kokee, (5)GB 20-M, (6)GBI, (7)Time/Maser (8)140ft VLBA, (9)GBT weather : 7 List (1)1PPS, (2)Maser, (3)GPS, (4)Clock Room environment: (5)clock corrections now, (6)recent 1pps: 1 Refer to (1)Site 1HZ, (2)GPS, (3)TAC, (4)mix : 3 Select time range: (1)specify dates, (2)last n days : 30 You want to choose Time/Maser, then 1PPS and then TAC (the new GPS reciever). Typically you will want to choose a time range of about 30 days (shorter if the SigmaTau maser has been adjusted in the last 30 days.) logsum will produce a file usually named 1pps.tmp Next you run SigmaTauPlot on the output file from logsum: sigmaTauPlot 1pps.tmp This will produce the same kind of outputs as clockModelPlot. SAVING THE CLOCK MODEL ====================== Once the clock model is determined it is entered into the file /ovlbi/ovlbi/offline/clock/clockModel.dat which has its format specified in /ovlbi/ovlbi/notes/clockModleFile.txt QUALITY OF THE GBES MODEL ========================= If the quality of the GBES clock model is -1 then only the offset of the GBES clock can be used and its value is not very well determined. This mode should be used for the first few days after a clock setting event for the station clock. If the quallity equals zero then the offset and drift are good values for the GBES model but the acceleration of the ST model should be used. Both the drift of the GBES model and ST model are used to check the current clock offset to see if it is still valid. If quality equals 1 then the GBES clock model is completely valid. BEGIN AND END EPOCHS OF THE GBES CLOCK MODEL ============================================ The begin and end epochs of the GBES clock model give the range in time that a particular clock model is valid. The epoch is given in fractions of MJD days. If the end epoch is given as # (i.e. no data) then the model is assumed to be the current valid clock model. If the begin epoch is given as # then the model is assumed to be invalid - usually this will be the case when a better clock model is determined for a given time range which overlaps with a previous clock model.