Hi Jim,

There have been a number of news articles about leap seconds in recent
years and another batch this week, many with clickbait titles.

Correct, anything synchronizing with UTC already has to know about leap
seconds so that there are no surprises: no accidental extra seconds, no
accidental missing seconds. A leap second is a specific event that
occurs only at the last second of a month. At a minimum you have to know
which month and what sign (positive or negative). Official notices of
leap seconds filter down from BIPM to affected timing systems in months
prior to the event.

The modern version of UTC started in 1972 and from the beginning had the
provision for either positive or negative leap seconds, without favor.
The goal was simply to keep the stable atomic time scale that we all use
roughly in sync with unstable earth rotation time. If a second needs to
be added to UTC it is named 23:59:60 and if a second needs to be deleted
23:59:59 is skipped. On paper it's very simple. Hardware and software to
handle either kind of leap second has been around since the beginning.

It's kind of a historical accident that UTC has had only positive leap
seconds so far. Earth rotation rate is surprisingly erratic [1] and had
UTC been decades or centuries older we would by now have experienced
both kinds of leap seconds frequently. The concern is that many timing
systems today have had relatively little exposure to positive leap
seconds (since they are rare) and even less exposure to negative leap
seconds (none needed yet).

The issue of earth temperature, glacial rebound, ocean level, and mass
distribution is legit, not to mention less well understood effects deep
within the earth. From a time nut perspective, earth is a poor
timekeeper. If you saw its ADEV [2] you'd probably just replace it an
OCXO. ;-)

/tvb

[1] http://leapsecond.com/pages/lod/lod-try-4.png

[2] http://leapsecond.com/museum/earth/

Tom Van Baak via time-nuts writes:

That's because there's a new article out in Nature:

https://www.nature.com/articles/d41586-024-00850-x

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

On 3/28/24 2:03 PM, Tom Van Baak via time-nuts wrote:

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

Hi

There are GPS modules out there that report GPS time rather than UTC. Typically there is a setting buried deep in the docs that controls this. The uBlox folks usually include this on their modules. Furuno does as well.

If you want a packaged device that will do this and chat via ethernet, the Trimble NetRS is not a bad candidate. There are other Trimble devices that also will "do GPS time”.

Bob

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@lists.febo.com> wrote:

--

--Jim Harman

I'd say most receivers do, even my old Motorola. Software wise you can
offset to TAI time using Lady Heather.

On 2024-03-28 12:51, Jim Harman via time-nuts wrote:

On Thu, Mar 28, 2024 at 3:56 PM Jim Harman via time-nuts <
time-nuts@lists.febo.com> wrote:

Indeed, thanks.  That's just what I was looking for.  In fact, I see from
the documentation of one u-blox receiver I checked that they are quite
explicit about just how they handle leap seconds in their UTC reporting
(that's a good sign) as well as offering the option of choosing which time
base of all the different GNSS options they implemented.

Thanks,
Dave

On Thu, 28 Mar 2024 at 19:02, Poul-Henning Kamp via time-nuts <
time-nuts@lists.febo.com> wrote:

Judah Levine is quoted in a WaPo piece.  I have 10 article shares available
for non-subscribers:

Climate change is altering Earth’s rotation enough to mess with our clocks
https://www.washingtonpost.com/science/2024/03/27/leap-second-melting-poles-climate-time/

The nature article (and WaPo) indicate core-mantle coupling would have
necessitated a leap second deletion around 2026 but polar ice melting has
delayed that to 2029.  A lot of systems will have a real-world test for a
novel condition at that point.

Cheers,
Dave Hart

On Thu 2024-03-28T11:03:26-0700 Tom Van Baak via time-nuts hath writ:

Alas, that is the UTC goal stated public presentations.
The actual goal was to have a single international time scale which
would be legal according to the laws of all nations.

Physicists did not want leap seconds.
Radio time signal providers did not want leap seconds.
Astronomers did not want leap seconds.
Astronomers produced a paper pretty much predicting that leap seconds
would become the disaster that we saw in 2012.

All of that was irrelevant because several nations had just made
inconsistent changes to their laws defining their legal time scale.
The only option for a single broadcast time scale which would be legal
to use in all countries was the leap second.

Despite concerns about the effect of leap seconds on navigation it is
not true that the leap second was instituted for navigators.
It would have been easier to produce almanacs for navigation if the
broadcast time signals had gone to purely atomic time without leaps.

Afterwards the principals in the decision process went to the meetings
of international agencies and begged them to approve of the leap
second.  The 1970 IAU meeting erupted into chaos when they learned of
the change, but that was redacted from the proceedings, and they were
coerced to vote for a resolution that approved.  The international
meeting concerned with navigation was forced into a similar approval.

Ever since the IAU approved the existence of Ephemeris Time the
practitioners of astronomical time have tacitly known that UT
(nowadays UT1) is not and never has been the mean solar time of the
Greenwich meridian.  The difference has been increasing.  Since
that inception the pracitioners have taken pains to make sure that
fact is expressed in language so obscure that only a close read of the
mathematical expressions would reveal it and the proceedings
have been edited to omit discussions of it.

If in either the 1950s or the 1970s astronomers had made it clear that
UT1 was not the mean solar time of the Greenwich meridian the likely
outcome would be chaos.  The UK Parliament and other legislative
bodies might have changed their laws to demand that their national
time bureau produce some peculiar version of time.  If different
countries produced different values for the time then each of those
countries would need to compute its own set of almanacs or else tables
of offsets for use with its own time scale.

The most important goal of the folks who are charged by their
governments with producing time scales for navigation and legal
purposes has been to produce values that agree with the values
produced by the pracitioners in other countries.

The meaning of the time scale is less important than the agreement.

UTC with leap seconds is the time scale that was "good enough for
international government work" until it became expedient to rewrite
national laws about time.  That was the goal in 1970.

--
Steve Allen                    sla@ucolick.org              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
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