Note: Since the GPS Offset and the TAI Offset have a fixed rela-
tionship, only the GPS Offset can be set. If only the TAI offset is
known, subtract 19 from it, in order to obtain the GPS offset.
Note that the data stream of GPS and several other external references includes inform-
ation about a pending Leap Second, and as such automatically corrects for a Leap Second.
Nevertheless, it is advisable to perform some testing in advance to ensure all system com-
ponents will adjust flawlessly. For more information, see "Leap Seconds" below.
3.2.3 Leap Seconds
3.2.3.1 Reasons for a Leap Second Correction
A Leap Second is an intercalary
1
one-second adjustment that keeps broadcast standards
for time of day close to mean solar time. Leap Seconds are required to synchronize time
standards with civil calendars, thus keeping UTC time in sync with the earth’s rotation.
Leap seconds can be introduced in UTC at the end of the months of December or June.
The INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE
(IERS) publishes a bulletin every six months, either to announce a time step in UTC, or to
confirm that there will be no time step at the next possible date. A Leap Second may be
either added or removed, but in the past, the Leap Seconds have always been added
because the earth’s rotation is slowing down.
Historically, Leap Seconds have been inserted about every 18 months. However, the
Earth's rotation rate is unpredictable in the long term, so it is not possible to predict the
need for them more than six months in advance.
Note: Leap Seconds only apply to the UTC and Local timescales. Leap
Seconds do NOT affect the GPS and TAI timescales. However, a Leap
Second event will change the GPS to UTC, and TAI to UTC time offsets.
When a Leap Second occurs, VersaSync will automatically change these off-
sets by the proper amount, no matter which timescale is currently being
used by the system.
As of 2018 the GPS to UTC Offset is 18 seconds. The last Leap Second occurred on
December 31, 2016.
VersaSync can be alerted of impending Leap Seconds by any of the following methods:
1
Intercalary: (of a day or a month) inserted in the calendar to harmonize it with the solar year, e.g., February 29 in leap
years.
146
CHAPTER 3 • VersaSync User Manual Rev. 7.0
3.2 System Time
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