Clock Monitoring at Green Bank

Herewith a description of the clock measurement system used at the GBT in Green Bank to record offsets between various clocks.

Related Links

SigmaTau Maser Performance History	Maser Adjustments Log	1pps cable delays	All Timing Logs
Recent Timing plots	Notes on implementing a leap second	Implementing UT1 Corrections	Time Knowledge

How To Adjust the ST Maser Rate

Sources of Time

• GPS receiver: Tom Clark's "Totally Accurate Clock"
  • The TAC detects up to 6 GPS satellites and provides a 1 Hz pulse on the UTC second at the input to its antenna.
• "SiteTime" : the observatory master clock (TRAK Systems model 6464), driven by a 5 MHz reference signal from the Sigma Tau maser.
  • The TRAK provides a 1 Hz pulse which is distributed to many backends (refer to 1pps cable delays ).
  • The TRAK also provides IRIG time signals which are distributed to many devices including the ntp time server on "yed".
• The Sigma Tau Maser provides stable reference frequencies at 5, 10, and 100 MHz, distributed around the site for stabilizing LOs.
  • Log of rate Adjustments, etc.
  • Current maser health monitors.
  • Plots of maser performance.
  • Using the Rubidium Backup standard, Aug 2008.

Logging system

• Site1Hz : the reference 1Hz from the TRAK master clock.
• TAC : the UTC 1Hz from the GPS receiver.
• GBT_VLBA : The 1Hz generated at the VLBA formatter.
• GBTRtn : The 1Hz return from the GBT receiver room (This is twice the one-way delay; typical value 28.9 us).

The various 1Hz inputs can be selected as inputs to a RACAL-DANA 1991 counter. The counter measures the time delay between the arrival of a pulse at its "A" input until a pulse arrives at the "B" input.

Logging System Inputs (as of July 2008)
A	B
Site1Hz	Site1Hz
Site1Hz	TAC
Site1Hz	GBT_VLBA
Site1Hz	GBTRtn

The logging may be viewed in real time with Cleo: cleo sitetime

The logging system records these clock offsets every two minutes and stores them in FITS files in the directory /home/gbtlogs/SiteTime-OnePps-OnePpsDeltas/

Summarizer Programs

• "ppsarchive" runs once a day.
  • Take hourly medians of the clock offsets.
  • Collect all log files for the current month.
  • Write an archive FITS file with hourly medians for all data in the current month up to now.
  • These archive FITS files are written into the directory: /home/ftp/pub/fghigo/OnePpsArchive/
  • file names of the form: yyyy_mm.fits
• "ppssummry" runs immediately following "ppsarchive"
  • From the archive fits file for this month, generate a file with the timing info in ascii tabular form.
  • From the previous file, generate the timing info in the form for pulsar observers: "time_gbt.dat" -- clock offsets are daily medians.
  • copy these ascii files to /home/ftp/pub/fghigo/timer/
  • also copy the pulsar time_gbt.data file to /users/gbpulsar/tempo/clock/