[LEAPSECS] the big artillery

[Zefram]

Warner Losh wrote:
>Users can only get UTC(foo) or a signal derived from UTC(foo) (e.g.,
>traceable to NIST) and never UTC itself. Of course they can get to a
>putative TAI(foo) trivially (I say putative, because as far as I know, no
>lab generates TAI synchronized signals for reasons you go into). However,
>they cannot get back to TAI(BIPM)
...
>That's the hair I'm trying to split.

I'm not seeing the split.  I think you've just said exactly the same
things about TAI and UTC, and hence agreed with me.  Let's see:

A user can listen to a radio time    :  A user can listen to a radio time
signal that provides PPS markers     :  signal that provides PPS markers
synched to UTC(national_lab)         :  synched to TAI(national_lab)
along with time codes that can       :  along with time codes that can
be decoded to UTC(national_lab).     :  be decoded to TAI(national_lab).
UTC(national_lab) is not exactly     :  TAI(national_lab) is not exactly
the same as UTC(BIPM), but tends     :  the same as TAI(BIPM), but tends
to be within tens of nanoseconds     :  to be within tens of nanoseconds
of it.  UTC(BIPM) (the canonical     :  of it.  TAI(BIPM) (the canonical
UTC) is not precisely available      :  TAI) is not precisely available
in real time: it is determined       :  in real time: it is determined
retrospectively by comparing the     :  retrospectively by comparing the
clocks of the various national       :  clocks of the various national
labs.  UTC(national_lab) timestamps  :  labs.  TAI(national_lab) timestamps
can be corrected to UTC(BIPM)        :  can be corrected to TAI(BIPM)
retrospectively by applying the      :  retrospectively by applying the
offsets given in Circular T.         :  offsets given in Circular T.

Which part of this is wrong, for which time scale?

Of course, I'm referring to the same radio signal in each case, and the
PPS markers are the same.  The decoding process is slightly different,
but with ready availability of the TAI-UTC difference it's not appreciably
more difficult.  The national lab's job of steering clocks and generating
the signal is the same in both views.  The received signal is traceable
to the national lab and ultimately to BIPM in both views.  The only
remaining asymmetry is that the "TAI(national_lab)" notation isn't
officially approved, but that doesn't diminish the concept.

There is no technical difference here, only a political difference.

-zefram