Clock Monitoring at Green Bank

[as of July 2008 -- by F.Ghigo, NRAO]
Revised April 2017 (F.Ghigo)
Revised June 2017 (F.Ghigo)
Revised Sept 2018 (F.Ghigo)
Revised June 2020 (F.Ghigo)

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 ST Maser Adjustments Log MSM 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

  • These devices produce pulses at 1 second intervals.
  • The Site time system records differences between the various devices.
  • The timing center is in a rack next to the 2 masers in the equipment room.

    The Counter

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

    The Matrix

    The inputs to the counter are selected from a switching matrix by the sitetime computer.
    The inputs to the matrix are as follows:
    1pps input signal
    X0 Site 1Hz from TRAK master clock
    X1 CNS_GPS1 (1pps output)
    X2 CNS2018 (new CNS_GPS receiver)
    X3 CNS_GPS2 (1/100pps output
    X4 MicroSem (1pps from new Microsemi maser)
    Y1 RA (round trip to 140')
    Y2 GBTRtn (round trip to GBT)

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

  • SiteTime records these clock offsets every minute and stores them in FITS files in the directory /home/gbtlogs/SiteTime-OnePps-OnePpsDeltas/
  • The maser monitor points are stored in: /home/gbtlogs/SiteTime-Maser-MaserList/ -- logged once per hour.
  • The round-trip delays are stored in: /home/gbtlogs/SiteTime-Rtpm140-Rtpm/ -- logged once per 10 seconds.

    The SiteTime computer also collects data from the 3 weather stations and writes Fits log files to /home/gbtlogs/Weather-Weather*

    OnePps FITS files list
    FITS file contents
    Config file in /home/gbt/etc/config/OnePps.conf

    Summarizer Programs

    All use the C-program "rfclock_rh7.c" in ~fghigo/GBT/fits) (rfclock does not use the OnePps.conf)

    Cron jobs run as fghigo on arcturus

    Not cron job: llogsum
    Scripts in /home/astro-util/src/cscrpts/

    Archival File storage


    Pulsar timing files

    Pulsar observers use a "time_gbt.dat" file with the clock offset for each day. These are generated from the "yyyy_mm.t00new" files once per day. Note: Jan 2018: As of Oct 3, 2017, the CNS receiver 1pps is used instead of TAC, because the TAC has gone haywire. The "time_gbt.dat" file is now based on the 1pps from the CNS. There were jumps in the time_gbt.dat due to changing to the new CNS and from adding time offsets in the CNS to try to connect to what the continuation of the old TAC values would be. So as of Jan 2018, the time_gbt.dat file has been modified to remove all the non-real offsets. All time jumps should be real and due to re-synchonizing the 1pps pulse.