Clock Monitoring at Green Bank

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

Clock Logging system

The 1Hz pulses

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 (revised April 2017)
A B
Site1Hz Site1Hz
Site1Hz TAC
Site1Hz CNS_GPS
Site1Hz GBT_VLBA
Site1Hz RA
Site1Hz GBTRtn
TAC Site1Hz
TAC TAC
TAC CNS_GPS
TAC GBT_VLBA
TAC RA
TAC GBTRtn

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
X1 TAC
X2 not used
X3 CNS_GPS
Y0 GBT_VLBA
Y1 RA (round trip to 140')
Y2 GBTRtn (round trip to GBT)

Site Time

The "SiteTime" computer logs the time offsets, and also logs the maser monitor points, and the round-trip delay 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.

    Summarizer Programs

    Several cron jobs run as fghigo on bratac (as of June 2017)

    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.