[LEAPSECS] systems that use GPS time

[Joe Gwinn]

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 
wristwatch.



>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