From: William Thompson <>
Date: Thu, 20 Feb 2003 11:22:40 -0500

Tom Van Baak wrote:
> I've been reading a lot of NASA pages on the shuttle
> recently and was reminded once again that NASA
> seems to use "GMT" instead of "UTC" to label their
> timelines. Do any of you know why?

Working in NASA myself, and having read a number of spacecraft interface control
documents, I can state with some certainty that NASA, like many others, tends to
use the terms GMT and UTC interchangeably.

> That brings up another question - has a leap second
> ever occurred during a NASA mission and did it have
> any effect on timing? Or is everything done in Mission
> Elapsed Time (MET) - one more example of a
> leapsecond-less timescale.

Obviously many leap seconds have occured during NASA missions. I've never heard
of a problem caused by leap seconds, but my experience is not very broad. I do
know of at least one instance where an organization within NASA failed to
correctly take leap seconds into account, i.e. still using a value of TAI-UTC=30
when the correct value should have been 31. This caused only a minor
inconvenience, however.

In the few cases where I've been involved, mission planning has always been done
with UTC timestamps, which are then converted into leapsecond-less numbers, such
as TAI or MET, for use by the spacecraft computer. The same conversion is done
in reverse when processing telemetry.

William Thompson
Received on Thu Feb 20 2003 - 08:23:00 PST

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