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 stand-
ards 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 rota-
tion.
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, VelaSync will automatically change these
offsets 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.
VelaSync can be alerted of impending Leap Seconds by any of the following methods:
GNSS Receiver
(if available as an input reference): The GNSS satellite system
transmits information regarding a Leap Second adjustment at a specific Time and
Date an arbitrary number of months in advance.
Input references other than GNSS
: Some of the other available input references
(e.g., IRIG, ASCII, NTP) can also contain pending Leap Second notification in their
data streams (see chapter below).
Manual user input
: VelaSync can be manually configured with the date/time of
the next pending Leap Second. On this date/time, the System Time will
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.
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CHAPTER
3
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VelaSync User's Manual Rev. 3
3.2 System Time
Summary of Contents for VelaSync 1225
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