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 auto-
matically 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.
VersaSync can be alerted of impending Leap Seconds by any of the following
methods:
GNSS Receiver (if available as an input reference): The GNSS satellite sys-
tem transmits information regarding a Leap Second adjustment at a spe-
cific Time and Date an arbitrary number of months in advance.
Input references other than GNSS: Some of the other available input ref-
erences (e.g., IRIG, ASCII, NTP) can also contain pending Leap Second noti-
fication in their data streams (see chapter below).
Manual user input: VersaSync can be manually configured with the date/-
time of the next pending Leap Second. On this date/time, the System Time
will automatically correct for the Leap Second (unless the System Time’s
timescale is configured as either GPS or TAI).
3.2.3.2 Leap Second Alert Notification
VersaSync will announce a pending Leap Second adjustment by the following
methods:
ASCII Data Formats 2 and 7 (among other formats) from the ASCII Data
option modules contain a Leap Second indicator. During the entire calendar
month preceding a Leap Second adjustment, these Formats indicate that at
the end of the current month a Leap Second Adjustment will be made by
using the character ‘L’ rather than a ‘_ ‘ [space] in the data stream. Note
that this does not indicate the direction of the adjustment as adding or
removing seconds. These formats always assume that the Leap Second will
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CHAPTER 3 • VersaSync User Manual Rev. 12
3.2 System Time
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