Spectracom Corporation
Model 9289
NetClock/NTP Network Time Provider Instruction Manual
4-5
4.3
Leap Second occurrence
4.3.1 Reasons for a Leap Second Correction
A
Leap Second
is an intercalary, one-second adjustment that keeps broadcast standards for
time of day close to mean solar time. Leap seconds are necessary to keep time standards
synchronized with civil calendars, the basis of which is astronomical. They are used to keep the
earth’s rotation in sync with the UTC time.
If it has been determined by the International Earth Rotation and Reference Systems Service
(IERS) that a Leap Second needs to applied, this time correction occurs only at the end of a
UTC month, and has only ever been inserted at the end of June 30 or December 31. 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.
The NetClock can be alerted of impending leap seconds by either of the following methods:
1. GPS Receiver – The GPS satellite system transmits information regarding a Leap
second adjustment at a specific Time and Date an arbitrary number of months in
advance.
2. Modem – (Applicable to only units with Option 03 Modem installed). During a modem
dial-out call, the call service indicates that a Leap second adjustment at the end of this
current calendar month will occur.
4.3.2 Leap Second Alert Notification
The NetClock will announce a pending Leap Second adjustment by the following methods:
1. Data Formats 2 and 7 on the Serial and Remote Ports 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
having a ‘L’ rather than a ‘ ‘ (space) character in the data stream. Note that his does not
indicate the direction of the adjustment as adding or removing seconds. These formats
always assume that the Leap Second will be added, not removed.
2. NTP Packets contain two Leap Indicator Bits. In the 24 hours preceding a Leap Second
Adjustment, the Leap Indicator Bits (2 bits) which normally are 00b for sync are 01b (1)
for Add a Leap Second and 10b (2) for remove a Leap Second. The bit pattern 11b (3)
indicates out of sync and in this condition NTP does NOT indicate Leap seconds. The
Sync state indicates leap seconds by indicating sync can be 00b, 01b, or 02b.
NOTE:
It is the responsibility of the client software utilizing either the Data Formats or NTP
time stamps to correct for a Leap Second occurrence. The NetClock will make the
correction at the right time. However, because computers and other systems may not
utilize the time every second, the Leap second correction may be delayed until the
next scheduled interval, unless the software properly handles the advance notice of a
pending Leap Second and applies the correction at the right time.
Summary of Contents for 9289
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