[ntp:questions] Purpose of a leap second file?

David E. Ross nobody at nowhere.invalid
Sat Mar 3 20:35:09 UTC 2012

On 2/15/12 10:22 PM, A C wrote:
> Given the recent thread here about the upcoming leap second, I wanted to 
> know what additional service/value the leap second file provides that 
> isn't handled by ntpd receiving a message to add or remove a second.  I 
> understand the generic ntpd documentation includes a flag to add a leap 
> second so it seems that the leap second file isn't necessary (and that's 
> also borne out in searches) but it must provide something otherwise it 
> wouldn't exist.  The documents mention a "graceful" handling, but what 
> does that mean exactly?

A leap-second file is useful if you are processing archived time-stamped
data and need to convert between TAI and UTC.  When a future leap-second
is announced and added to such a file, it is also useful when projecting
future time-dependent events.

In certain situations, the processing of time-stamped or time-dependent
data requires uniform time intervals.  That is, all minutes must be
exactly 60 seconds in duration; all hours must be exactly 3600 seconds
in duration; all days must be exactly 86,400 seconds in duration; etc.
Such data should use TAI, which exhibits uniform intervals.  To adjust
such data to UTC, a leap-second file is useful.

Why require a UTC adjustment from TAI?  If the data represent a
satellite orbiting the earth and the sub-satellite point on the earth's
surface is needed, the TAI is converted to UTC, which is then converted
through UT2 to UT1 and possibly to UT0 before obtaining the sidereal
time.  Sidereal time then gives the current angle of the earth's
rotation for computing longitude.


David E. Ross

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