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Earth Orientation Parameter (EOP) bulletin handling at the ATNF

This article provides a level of detail appropriate for someone who needs to administer the system and assumes no prior knowledge.

Bulletin source and distribution

The U.S. National Earth Orientation Service (NEOS), a sub-bureau of the International Earth Rotation Service (IERS), produces and disseminates the IERS Bulletin A time service bulletin. NEOS is a joint venture of the National Aeronautics & Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Naval Observatory (USNO), although the bulletins are often loosely said to come from USNO.

IERS Bulletin A contains earth orientation parameters (time, polar motion, and nutation corrections) in the form of 90 day extrapolation tables. These parameters are required by the ephemeris routines in ATELIB which do accurate phase and delay calculations for CAOBS in steering the Australia Telescope Compact Array (ATCA), and also for accurate time keeping at the ATNF observatories.

By prior arrangement NEOS emails the bulletin twice a week (received on Wednesday and Friday, AEST) to the ATNF at 

   eop@atnf.csiro.au
This is defined as an NIS email alias on crux, the NIS master server for ATNF, in /etc/aliases as 
   eop: eop@leon.atnf.csiro.au, mcalabre, mkesteve, observer
In turn, observer is defined as 
   observer: observer@atnarrabri.atnf.csiro.au,
             observer@atparkes.atnf.csiro.au,
             observer@mopra.atnf.csiro.au

Primary bulletin parsing

The copy of IERS Bulletin A forwarded to eop@leon.atnf.csiro.au is received by a special user account called EOP on LEON. LEON is the offline micro-VAX at Narrabri, its disks are visible to the online micro-VAX, NOEL, which controls the array.

Every morning at 0000 (UTC) a perpetual EOP batch job checks its mail and if a bulletin has been received it parses it to extract the prediction tables into a machine-readable form. The result is placed in a directory with logical name AT$EPHEM under the full name name of the bulletin, for example IERSA-06-13.BULL, where the 06 is the volume number, one per year, and 13 is the issue number. A duplicate copy is also written to IERSA.FILE in the same directory and this is the copy used by CAOBS. EOP sends an informative message to MRC/MJK when it has finished processing the bulletin.

The parsed bulletins basically contain two tables, the first contains extrapolated values of MJD, the polar motion in x and y (arcsec), UT1-UTC (sec), and IAT-UTC (the number of leap seconds). The second table contains predictions of the celestial pole offset correction, MJD, dpsi, and deps (mas).

The EOP machinery is located in EOP's home directory. It includes an archive of all IERS email bulletins since late 1989, plus the executables, auxiliary files, and log files. An EOP.README file describes the inner workings of the system in more detail. The bulletin-parsing code resides in a CMS repository in the CODE subdirectory.

The IERSA.FILE format was revised circa Jun/1997 to include the leap second, IAT-UTC, so that a leap second transition would not affect CAOBS while an observation is in progress.

The EOP system has been refined over many years and is now quite robust. However, maintenance is occasionally required in response to changes in the format of IERS Bulletin A which may upset the parser. NEOS usually advises well in advance of forthcoming changes. If a parse error does occur or the EOP batch job goes awry for some other reason, it sends an SOS message to the list defined in EOPMSG.DIS - currently mcalabre, mkesteve, and rwark (others could be added if they wish). The most common SOS arises when NEOS inadvertently sends the same bulletin twice. EOP complains about the second receipt and demands human intervention since NEOS may sometimes send a second bulletin containing corrections meant to override the first. It can also happen, due say to an unfortunate system reboot, that the parser is killed before it has a chance to finish its bookkeeping tasks. This results in an SOS being sent on the next invokation. However, since the prediction table is 90 days long, there is no urgency to the repairs.

Catastrophic errors may occur occasionally without an SOS being sent. For example, in rare circumstances it has happened that the perpetual batch job has been killed before it can resubmit itself and before an SOS can be sent. This can only be detected by the absence of the the regular update messages sent to EOP's maintainers.

The archive of a year's worth of email bulletins and log messages is manually refiled into annual folders at the start of each year (usually by Mark Calabretta).

Secondary bulletin processing

In addition to storing a local copy of the parsed IERS Bulletin A in the VMS domain at Narrabri, we arrange to distribute copies in the unix domain to all the ATNF observatories. After finishing its primary duties, the EOP batch job on LEON mails the parsed bulletin to the following NIS alias list (defined on crux in /etc/aliases): 

   iers_collectors: iers_collector@auriga.atnf.csiro.au,
                    iers_collector@molen.atnf.csiro.au,
                    iers_collector@lynx.atnf.csiro.au,
                    iers_collector@warrum.atnf.csiro.au
The iers_collector NIS alias is defined as a unix pipe 
   iers_collector: "| /nfs/atapplic/EOP/IERS-A/collect"
by means of which sendmail invokes the utility called collect (a Bourne shell script) which receives the mail on stdin, strips the header off and saves the result in a file under the name specified in the subject line of the email. It also recreates a symlink, iersa.file, pointing to this latest bulletin, and invokes a process called send_iersa which sends the new bulletin to instances of the AT distributed clock (ATDC). The ATDC makes use of the EOP data to update DUT1 automatically each day, as well as DUTC as appropriate. send_iersa uses the ATDC clock name stored in /nfs/atapplic/local/clock/atdc.file, a site-specific file.

Several utilities have been provided to allow each site to check the clock and its EOP status. An ATDC display can be obtained via the atclock command on a unix host (+PCS must be enabled in .login.packages to use this).

The collect utility saves its output (stdout and stderr) in a file called collect.log and mails this summary to the NIS mail alias 

   iers_notify: mcalabre,mkesteve
The programs and saved bulletins reside in directory /nfs/atapplic/EOP/IERS-A . Note that /nfs/atapplic/EOP/IERS-A is part of the rdist which runs from Epping to the observatories every evening. That means that the fingerprints left by instances of collect which ran at the observatories are wiped away each evening, replaced by the current state at Epping. The bulletins are owned by daemon as a result of having been created by sendmail.

Summary

Electronic mail is used as the transfer agent for IERS Bulletin A and its by-products, the pathways across the network being as follows: 

   NEOS -> eop@atnf.csiro.au
                |
                |-> (individual recipients)
                +-> LEON::EOP (AT$EPHEM:IERSA.FILE)
                                    |
                                    +-> iers_collectors@atnf.csiro.au
                                                |
                                                |-> Epping (lynx)
                                                |-> Mopra  (warrum)
                                                |-> Narrabri (molen)
                                                |-> Parkes (auriga)

Queries may be addressed to Mark Calabretta (MRC) or Mike Kesteven (MJK) who maintain the system and this page.

Original: Mark Calabretta/Mike Kesteven (22-Jul-1998)
Modified: Mark Calabretta/Mike Kesteven (24-Jul-1998)