The International Earth Rotation Service (IERS) periodically introduces an intercalary adjustment to the Coordinated Universal
Time (UTC) time scale to keep it in close agreement with the Universal Time (UT1) time scale. This adjustment— a leap
second—amounts to a one second non-uniformity in the UTC time scale that, in addition to the UT1 time scales, also affects
transformations to other continuous time scales. Since the inception of leap seconds, all adjustments have been at the end of
30 June or 31 December UTC, though a leap second could, by rule, be applied at the end of any month.
AGI products ingest leap second information from the LeapSecond.dat file, an ASCII text file which contains the dates of all
historical and announced leap seconds that would affect the conversion between the UTC time scale and other time scales. The
information in the file is obtained from the U.S. Naval Observatory http://maia.usno.navy.mil/ser7/tai-utc.dat leap second
The format of the LeapSecond.dat file consists of an initial line with an integer value indicating the number of leap second
entries that the file contains, followed by a leap second entry on each subsequent line of the file.
A leap second entry consists of the following space-delimited information:
- The Gregorian calendar 4-digit year, abbreviated month name and day of the date immediately following the leap second which was added.
- The corresponding Julian date.
- The difference between TAI and UTC time scales (the accumulated number of leap seconds) as of the corresponding date.
The modified Julian date of the epoch at which the following rate is established
(Obsolete as of 1 Jan 1972. Since then, the value of this epoch has been defined as 41317.0).
- The rate at which the UTC time scale is slowing with respect to the TAI time scale (Obsolete as of 1 Jan 1972. Since then, the value of this rate has been defined as 0.0).
In addition to the LeapSecond.dat file, AGI products depend upon the proper reflection of the leap second in the Earth
Orientation Parameter data used to fully define the relationship between Earth Centered Inertial (ECI) and Earth Centered
Fixed (ECF) reference frames. The Earth Orientation Parameters, which describe the motion of the spin axis (pole wander) and
the irregular rotation rate of the Earth (UT1 – UTC), are supplied through the file EOP-v1.1.txt for newer versions of the
software and EOP.dat for older versions. At the time of a leap second, the (UT1 – UTC) time history experiences a discontinuity
of 1 second. It is therefore important that the EOP-v1.1.txt, EOP.dat and LeapSecond.dat files be updated together to properly
establish the existence of the new leap second. Proper synchronization is easily achieved through use of the Data Update
Utility provided with STK and ODTK.
AGI software users should also be aware of limitations of particular date formats with respect to representing UTC times across a
leap second. Such limitations, which typically involve expressing the time as the fraction of a day, affect the following date
formats: JDate, JDateOff, ModJDate, YYDDD, YYYYMMDD. UTC dates represented in these formats will appear to jump backward in time
as the leap second is crossed. ITU-R Recommendation 460, which defines UTC, does not provide conventional guidance on representing
UTC time-of-day expressed as a fraction.
and STK Components
users should go here
to learn technical details about how to update for the leap second.