[LEAPSECS] the epoch of TAI, with no more doubt

[Michael Deckers]

   On 2019-01-21 00:42, Steve Allen wrote:


> Of course there was a time step.  The BIH had to deal with totally
> hetergeneous data from an ever changing set of contributors.  Almost
> every year for the BIH there was a systematic offset from the times of
> other years.  But until the cesium standard there really is little
> worth in the absolute values; the importance of the numbers in
> Bulletin Horaire liese in seeing and understanding the differences
> between contemporary time services.


    For the internally used and tabulated time scales, yes, there may
    be steps, as convenient. But steps in a time scale used to
    dissmeinate time signals with their own steps and rate offsets
    are highly inconvenient. I was of the incorrect opinion that
    the BIH integrated atomic time scale was aligned with the coordinated
    atomic time scales used by the RGO, NBS, USNO etc since 1960-01-01;
    but it was not, and only joined on 1961-01-01.

    Thanks for the corretion!

> A3 begins 1961-01-01.  It does not exist before then.  Not even when
> Guinot re-interpolated all the atomic time scales in Bulletin
> Horaire ser J no 7 did he extend A3 before then.  He introduced his
> final reconstruction of the old atomic data with
>      It is therefore possible to construct, starting from an arbitrary
>      common origin, scales of Atomic Time ...
> By that 1966 publication Guinot had ceased to mention 1961-01-01, but
> linear interpolation of his new A3 tabulation has the value -1.4123 s
> on 1961-01-01T20, the same as had been used by Anna Stoyko when she
> re-set all of the BIH atomic time scales.


    You are of course right; instead of "A3" I should have said
    "the integrated atomic time scale produced by the BIH for 1957..1960
    and which agrees with UT2 at J1958.0", as described on pages 99..101
    in [https://www.bipm.org/utils/common/pdf/CC/CCTF/CCDS2.pdf].

    From the steps in the WWV time signals as documented in the
    Explanatory Supplement 1992, p 86..87, I compute a decrease
    of 1.465 056 s in the WWV time signals against the underlying
    Cs atomic scale, while this scale ranged over the interval
    from J1958.0 until 1961-01-01, and this applies to all
    continuous time scales with the same rate.

    So, when A3 - UT2 at 1961-01-01 was set to 1.4123 s by the BIH,
    this must amount to a step of about -53 ms at 1961-01-01 in the
    BIH integrated atomic time scales before and after 1961-01-01.
    (And there was no step in UT2 on 1961-01-01.)

    If this step was done to align A3 with the coordinated times
    already in use, I am surprised that such a large deviation
    between integrated atomic clocks could accrue over three
    years -- A and N Stoyko estimated the deviation after 3 years
    to be 10 ms in the paper quoted above.

    Regardless of this difference, there is a thing common to
    all integrated atomic time scales that suggests that they all
    are intended to have J1958.0 as their origin: their difference
    to ET (and later to TT and TDB). In fact, TT - TAI remains very
    close to 32.148 s, which in turn is close to the value ET - UT2
    when UT2 was J1958.0 (but ET - UT2 differs by about 0.5 s for
    the neighboring years). A step of 0.05 s does not change this
    property.

> Guinot also indicates that he retained the jump of 1.6 ms on
> 1962-01-01 in his new tabulation of A3.  These various tabulations
> deserve to be plotted and examined closely for a step, especially
> because 1962-01-01 was also the date of the final change in the
> expression for the seasonal variation of UT2 - UT1.
> https://www.ucolick.org/~sla/leapsecs/seasonal.html
> That change should introduce a step of about 6 ms, and this subject is
> not mentioned in any of the BIH writeups.


    Do you happen to know in which tabulation the jump by 1.6 ms
    occurs? A3 minus which other time scale?

    The 1962 change in the UT2 formula did not apply to prior years;
    a step in UT2 may have influenced the disseminated time signals
    which followed UT2, and the step causes jumps in some differences
    such as A3 - UT2, but it does not not cause a step in UT1 or in
    any (integrated) atomic time scale.

    Michael Deckers.