Clock Monitoring at Green Bank
[as of July 2008 -- by F.Ghigo, NRAO]
Herewith a description of the clock measurement system used
at the GBT in Green Bank to record offsets between various clocks.
Related Links
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.
Logging system
The 1Hz pulses
- 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/