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E-126 TimeVault User’s Manual 6000-100AppE.fm Rev. D
Appendix E: Automated Computer Time Service (ACTS)
the number changes from 50 to the actual number of
days until the time change. It will decrement by “1”
every day, and reach “0” the day the change occurs.
L = a one-digit code that indicates whether a leap second
will be added or subtracted at midnight on the last
day of the current month. If the code is “0”, no leap
second will occur this month. If the code is “1”, a
positive leap second will be added at the end of the
month. This means that the last minute of the month
will contain 61 seconds instead of 60. If the code is
“2”, a second will be deleted on the last day of the
month. Leap seconds occur at a rate of about one
per year. They are used to correct for irregularity in
the earth's rotation.
DUT1 = a correction factor for converting UTC to an older
form of universal time. It is always a number
ranging from -0.8 to +0.8 seconds. This number is
added to UTC to obtain UT1.
msADV = a five-digit code that displays the number of
milliseconds that NIST advances the time code. It is
originally set to 45.0 milliseconds. If you return the
on-time marker (OTM) three consecutive times, it
will change to reflect the actual one way line delay.
UTC(NIST) = a label indicating that you are receiving Coordinated
Universal Time (UTC) from the National Institute of
Standards and Technology (NIST).
OTM = an on-time marker in the form of an asterisk (*).
Once the time code is synchronized within a few
milliseconds of UTC(NIST), the asterisk changes to
a pound sign (#), as described below.
The time values sent by the time code refer to the arrival time of the OTM. In other
words, if the time code says it is 12:45:45, this means it is 12:45:45 when the OTM
arrives. Since the OTM is delayed as it travels from NIST to your computer, ACTS sends
it out 45 milliseconds early. This always removes some of the delay. Better results are
possible if your software returns the OTM to ACTS after it is received. Each time the
OTM is returned, ACTS measures the amount of time it took for the OTM to go from
ACTS to the user and back to ACTS. This quantity (the round-trip path delay) is divided
by 2 to get the one-way path delay. ACTS then advances the OTM by the one-way path
delay and the OTM changes from an asterisk to a pound sign (#). When the # sign
appears, the time code is synchronized within a few milliseconds of UTC(NIST).
For more information about ACTS, visit the following web site from the National Institute
of Standards and Technology: http://www.boulder.nist.gov/timefreq/service/acts.htm