[time-nuts] Re: question about GPS time

[Jim Harman]

The u-blox receivers have UBX messages that provide the current GPS and UTC
time as well as information about the number of leap seconds that have
occurred and the time, date, and direction of any upcoming leap second.


On Thu, Mar 28, 2024 at 3:01 PM David Bridgham via time-nuts <
time-nuts at lists.febo.com> wrote:

> On 3/28/24 2:03 PM, Tom Van Baak via time-nuts wrote:
>
> > If a second needs to be added to UTC it is named 23:59:60
>
>
> This brings to mind a question I've been meaning to ask one of these
> days.  I guess today is a good day.
>
> I was contemplating building a system that would keep its internal time
> in TAI.  That makes more sense to me than jumping the internal time
> around to keep up with leap seconds; that conversion, if needed, can be
> done in a UI library.
>
> In support of this, I was looking over GPS receiver data-sheets to see
> what I had to work with.  The GPSes I found all liked to report times in
> UTC, rather than TAI or GPS time.  Hmmm.  But one thing I noticed was
> that the seconds field of that UTC time report was defined to be
> [00..59].  Uhhh, so what do they do when a leap second comes along?
> Does the GPS receiver double up with :59 or maybe it rolls over to :00
> and doubles that one up?  Neither of those are very good answers.  Or
> maybe the documentation is just wrong and the receiver actually does the
> right thing and report :60.  I suppose documenting it wrong is better
> than doing it wrong.
>
> So while I'm curious about that, my real question is whether there's a
> way to get GPS time out of a GPS receiver.  Or, lacking that, is there a
> way to reliably get the information out of a GPS receiver as to what
> leap-second offset it's currently using to calculate the UTC that it's
> reporting.  Yeah, I *should* be able to figure that out with my own
> leap-second database but how can I be sure that the GPS is really using
> the same list of leap-seconds that I have?  Better if it just tells me.
> Better still if I could get GPS time without the leap-second offset
> applied.
>
> Thanks,
> Dave
>
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-- 

--Jim Harman