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.
How To Adjust the ST Maser Rate
Sources of Time
- NEW as of Feb 2014: CNS GPS receiver (descendant of the TAC)
- the GPS antenna from the roof goes to both the CNS and the TAC. CNS 1 pps output goes to the time logging system.
- The monitor and display computer for the CNS 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)
- 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.
Clock Logging system
The 1Hz pulses
- Site1Hz : the reference 1Hz from the TRAK master clock.
- TAC : the UTC 1Hz from the TAC GPS receiver.
- CNS_GPS : the UTC 1Hz from the CNS GPS receiver.
- GBT_VLBA : The 1Hz generated at the VLBA formatter.
- RA-return: The 1Hz return from the RadioAstron receiver on the 140-foot
- 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 (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) |
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.
Several cron jobs run as fghigo on bratac (as of June 2017)
- 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 runs once a day.
- 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.c" (~fghigo/GBT/fits/)
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.
- flippsrtime.csh creates "time_gbt.dat" and stores it in "/home/pulsar64/tempo/clock", and also in "/home/ftp/pub/fghigo/timer"
- flip3psrtime.csh creates "time3_gbt.dat" and stores it in "/home/pulsar64/tempo/clock", and also in "/home/ftp/pub/fghigo/timer"
- The "flip" is because the sign is flipped from the way we store Site1Hz vs GPS to the way the pulsar people like it.
- "time3_gbt.dat" uses the pulse from the new CNS GPS receiver. This has replaced the TAC receiver.
- some day we will drop time_gbt.dat and store only time3_gbt.dat.
- These scripts use programs "psrtime2.c" and "psrtime3.c"
- "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.
- (set these delays with menu item "data-->set timing parameters")