[LEAPSECS] systems that use GPS time

Joe Gwinn joegwinn at comcast.net
Sun Oct 18 16:52:55 EDT 2009

At 6:43 PM -0700 10/11/09, Steve Allen wrote:

>Are there any folks who can comment on the operation of ensembles

>of systems which use GPS time instead of UTC?

I can answer for systems I have worked on.

>What are these systems?

Many large ground-based radars and the like.

>What reasoning went into the choice of GPS time?

It's cheaper and far more reliable to generate a continuous timescale
than to adapt millions of lines of code to handle time jumps.
Trackers in particular don't like time jumps, and negative jumps are
a particular problem.

Given that UTC cannot be used, what is the easiest alternative?
Well, GPS Time is widely disseminated, as many commercially-available
GPS receivers can be configured to provide it.

Naval systems I worked on in the days before GPS generated their own
monotonic local timescales. One common approach was to count
milliseconds since the official birthday of Christ (0h 0m 0s 1
January 0001 AD) in a 48-bit integer. The Gregorian calendar was
used in these calculations, even though it did not exist during
Christ's life. Synchronization to the outside world was by

>How did they reason so as to overcome the implicit mandates

>that POSIX and international standards place on the use of UTC?

>Was the choice made because of an aversion to the handling of

>leap seconds, or were there other concerns?

If there is a requirement (often not the case), it's only a
requirement on messages to and from the system, and not on how time
is handled internally.

In systems where a one-second discontinuity would matter, there is a
distinct aversion to leap seconds, because it's too hard to do enough
tests to ensure that a leap second won't matter.

>Do they use the standard zoneinfo files and just tolerate the fact

>that the broken down time reports are off by a few seconds?

>Or do they install custom zoneinfo files and get reports as true

>UTC and/or local zone time?

None of the above. Time zones are not used at all. They use UTC
directly or they use GPS Time directly.

Joe Gwinn

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