Re: [LEAPSECS] A lurker surfaces

From: M. Warner Losh <imp_at_BSDIMP.COM>
Date: Tue, 02 Jan 2007 22:16:19 -0700 (MST)

In message: <459AC3D8.19739.659AFD99_at_dan.tobias.name>
            "Daniel R. Tobias" <dan_at_TOBIAS.NAME> writes:
: On 2 Jan 2007 at 11:56, Ashley Yakeley wrote:
:
: > GPS is TAI. I'm not proposing abandoning TAI for those applications
: > that need it.
:
: It's a few seconds off from TAI, isn't it? It was synchronized to
: UTC in 1980 (I think), but without subsequent leap seconds, so it's
: now different from both TAI and UTC. They probably should just have
: used TAI if they wanted a time scale without leap seconds, rather
: than ending up creating a different one.

Yes. There's a fixed offset between seconds in TAI and seconds in
GPS. The GPS timescale is tied to seconds in UTC(NRO). TAI is a
paper clock, computed after the fact, so GPS can't ever be TAI time.
However, the differences there are down in the nanosecond or so
range.

There's a big philosophical opposition to using the paper clock that
is TAI in a real-time operational timescale that GPS uses. The
European version of GPS originally specified TAI time, but this was
changed in later revisions to be the same as GPS time. There's an
extreme reluctance in the time community to call something without
leap seconds "TAI" or "TAI + fixed offset". TAI means something very
specific. That's the other problem with just using TAI, btw, but
explaining that point is very hard...

The principle time scientist made me change the description of the
output of measurment time for a product I did. I described it as "TAI
time seconds since 1970." Instead, he descriped it as "Number of PPS
ticks in the UTC time scale since 1972 + 63072000 + 10."
Mathematically, they work out to be the same thing, but he was
extremely resistant to calling it TAI time or using the TAI moniker
for it at all. When I asked him about this, he said that TAI time
isn't realized in real time, but UTC is. UTC is what one can measure
against. Producing a number that corresponded to TAI time was OK, and
likely the least confusing thing to do (we give a second number and
UTC time in 'YYYY-MM-DD HH:MM:SS Z' as well as the channel and the
measurement for that time in out output), but actually calling it TAI
would 'confuse' the really smart time geeks out in the world. I asked
him for a reference where I could read up on this, and he shrugged and
said he just knew it and didn't know of any good write up.

Warner
Received on Tue Jan 02 2007 - 21:19:15 PST