EndRun Sonoma D12, User Manual

Page: 156 / 172

S o n o m a U s e r M a n u a l 140
A P P E N D I X F
Background Information
Another way to get the leap second information is to go to the International Earth Rotation Service
(IERS) website. If a leap second is pending it will be posted by the IERS approximately six months
in advance of insertion. This information is available in the latest Bulletin C at the (IERS) website:
http://www.iers.org
Leap seconds are inserted from time-to-time in order to keep UTC, which is derived from atomic
time (TAI), in agreement with the Earth’s rotation rate. Relative to TAI, the Earth’s rotation rate is
slowing down. This means that UTC must be retarded periodically in order to maintain agreement
between UTC and the apparent daylength. If this were not done, eventually UTC would drift out-of-
sync with Earth’s day and many astronomical and navigational problems would ensue.
The International Earth Rotation Service (IERS) is the organization responsible for measuring the
relationship between UTC and the rotation rate of the Earth. When the difference between UTC and
apparent Earth time has exceeded a certain threshold, the IERS coordinates with the Bureau Interna-
tional of the Hour (BIH) to schedule the insertion of a leap second into the UTC time scale.
The IERS publishes Bulletin C about 6 months in advance of each possible leap second insertion
point. Leap seconds may only be inserted at UTC midnight of June 30 or December 31. Bulletin
C confirms either that a leap second will or will not be inserted at the next possible insertion point.
Since the introduction of leap seconds in 1961, they have been added approximately once every 18
months.
The leap seconds which are needed for your Sonoma are actually the difference between GPS and
UTC. The GPS time scale began on January 6, 1980. At that time, the UTC timescale had already
undergone 19 leapsecond insertion events. If you are obtaining your leap second information from
the IERS website, you will need to subtract 19 from the TAI-UTC leap second values published there
to obtain GPS-UTC, the number needed to set the current and future leap seconds for the Sonoma. At
the time of this writing in February 2013, TAI-UTC was 35 seconds and GPS-UTC was 16 seconds.