Re: [LEAPSECS] What problems do leap seconds *really* create?

From: William Thompson <thompson_at_ORPHEUS.NASCOM.NASA.GOV>
Date: Wed, 29 Jan 2003 15:53:08 -0500

Markus Kuhn wrote:
> John Cowan wrote on 2003-01-29 17:56 UTC:
> > The problem is that they are not announced much in advance, and one needs
> > to keep a list of them back to 1972 which grows quadratically in size.
> Is this a real problem?
> Who really needs to maintain a full list of leap seconds and for what
> application exactly?

        (rest deleted)


Any application which seeks to calculate the difference in time between two
events recorded in UTC time needs to know if there are any leap seconds between
the start and stop time. For example, suppose you were studying solar flares,
and analyzing some data taken in 1998, and you saw a burst of hard X-rays at
23:59:53 UT on Dec 31, followed by a rise in EUV emission at 00:00:10 UT the
next day. You'd think that the delay time between the two would be 17 seconds,
but it's really 18 seconds because of the leap second introduced that day.
That's a vital difference for the scientific analysis of the data.

On another subject, we were asked to introduce ourselves, and explain our
relationship to UTC/TAI issues. My name is William Thompson, and I'm a solar
astronomer working at the NASA Goddard Space Flight Center. I work on various
solar satellite missions, including the Solar and Heliospheric Observatory
(SOHO) and the upcoming Solar Terrestrial Relationships Observatory (STEREO).
My introduction to UTC/TAI issues came about because of the SOHO program. I was
writing the telemetry processing software for one of the instruments, and
realized that the spacecraft operated in TAI time, i.e. the number of seconds
since 1-Jan-1958, while all the science planning and commanding would be done in
UTC. I therefore wrote software to handle the conversions between these two
kinds of time, in Interactive Data Language (IDL) which is a scientific data
analysis package from Reseach Systems Inc., and which is widely used in
astronomy and solar physics. This UTC/TAI conversion software is now used by a
number of science instrumentation teams for spacecraft commanding and telemetry

And yes, part of that software package includes a list of all leapseconds added
since 1 Jan 1972. Currently, my software doesn't handle TAI/UTC conversions
between 1958 and 1972, when UTC seconds had varying lengths.

William Thompson
Received on Wed Jan 29 2003 - 12:53:29 PST

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