[LEAPSECS] systems that use GPS time
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.
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