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.
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.
- "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 front panel is set to the UT time manually.
- The Sigma Tau Maser provides stable reference frequencies at 5, 10, and 100 MHz, distributed around the site for stabilizing LOs.
- New "MicroSemi" maser, installed in August 2018, produces its own 1pps.
- MSM Maser adjustments.
- (adjusting rate and acceleration is done with a laptop connected to the maser.)
- The laptop collects monitoring data from the maser.
- Two GPS receivers:
- the GPS antenna on the roof goes to both the GPS receivers.
- The monitor and display computer for the CNS_GPS is windows machine "GBTIME", located behind the main operators console.
- note: the antenna cable delay is set to 57 ns and the internal receiver delay to 37ns (data-->set timing parameters)
- CNS GPS receiver #1 purchased in 2014 (Communication, Navigation, Surveillance)
- CNS GPS receiver #2 purchased in 2018
- These GPS receivers are ntp servers and #2 has an IRIG output.
Questions and maintenance of the CNS GPS receivers:
Richard M. Hambly
CNS Systems, Inc., 363 Hawick Court, Severna Park, MD 21146
410-987-7835 phone, 410-987-7836 Fax, 410-299-2147 cell
firstname.lastname@example.org, www.cnssys.com, www.gpstime.com
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 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) |
| Y0 || GBT_VLBA |
| 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
OnePps FITS files list
FITS file contents
Config file in /home/gbt/etc/config/OnePps.conf
All use the C-program "rfclock_rh7.c" in ~fghigo/GBT/fits)
(rfclock does not use the OnePps.conf)
- IDL script "giplt.pro", in ~fghigo/CLOCKS/dailyplots/
Cron jobs run as fghigo on arcturus
- newtimeplot3.bash (~fghigo/CLOCKS/)
- daily plots
- Runs about every 2 hours.
- The results are put in a web page:
- monmaser (~fghigo/CLOCKS/monmaser.c) runs once a day
- reads the latest maser monitor points from the fits log file (SiteTime-Maser-MaserList) and updates the web page: monmaser.html
- today's maser monitors are appended to a log file : http://www.gb.nrao.edu/~fghigo/timer/monmaser.logs
- maser monitors may be ploted with an idl program: ~fghigo/CLOCKS/plotmaserreg.pro
- monacc1pps_rh7.csh runs once a day. ( in ~fghigo/CLOCKS)
- Read the OnePpsDeltas fits files
- Take hourly medians of the clock offsets.
- Write an archive file with hourly medians for all data in the current month up to now.
- These archive files are written into the directory:
- file names of the form: yyyy_mm.t00new
- Uses the program "rfclock_rh7.c" (~fghigo/GBT/fits/)
- List of monacc1pps_rh7.csh
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.
- ~fghigo/CLOCKS/flippsrtime.csh creates "time_gbt.dat" and stores it in "/home/pulsar64/tempo/clock", and also in "/home/ftp/pub/fghigo/timer"
and also in /home/pulsar_rhel7/src/tempo/clock/
- The "flip" is because the sign is flipped from the way we store Site1Hz vs GPS to the way the pulsar people like it.
- These scripts use program "psrtime2.c" (~fghigo/CLOCKS/src/)
- "psrtime2.c" adds a time offset of -167 ns after 15 Dec 2009 to account for having moved the TAC to a different location and changing the length of cable from the antenna.
- "psrtime3.c" : no time offset is added; the CNS internal timing parameters for cable delay and receiver delay should make the results close to time_gbt.dat.
- CNS : antenna cable delay = 57 ns
- CNS : internal receiver delay = 37 ns.
- (in GBTIME: set these delays with menu item "data-->set timing parameters")
- to match up the CNS delay with the continuation of the previous TAC record,
the "intentional extra early offset" was set to 0.089 on Dec 14, and then to 0.019 on Dec 21, 2017.
- Oct 9: "psrtime2.c" no longer adds the -167 ns offset.