[time-nuts] The TAI zero epoch of 1958

[jimlux]

On 10/24/19 3:32 PM, Steve Allen wrote:
> On Thu 2019-10-24T15:37:38-0400 Scott Stuart hath writ:
>> The context is that someone is going to be communicating timestamps to me
>> in TAI formatted as the number of elapsed seconds since 1958 Jan 1
>> 00:00:00.  To make sure that I understand how this is being calculated, I
>> reviewed the history of the establishment of TAI and UTC in the late 60s
>> and early 70s.  I understand why 1972 Jan 1 00:00:00 UTC == 1972 Jan 1
>> 00:00:10 TAI and why TAI is now 37 seconds ahead of UTC.  I can also see
>> that propagating backwards from the 1961 time conversion formulas to 1958
>> Jan 1 gives TAI - UTC very close to 0 (within about 2 milliseconds).  So,
>> it makes sense to use 1958 Jan 1 00:00:00 UTC == 1958 Jan 1 00:00:00 TAI as
>> the 0 epoch for TAI in seconds.


This stuff is fascinating..


> 
> On 1958-01-01 the USNO time scale systematically differed from the
> international average by 0.035 seconds because USNO had adopted a
> value of its longitude which turned out not to be globally consistent.
> Other sources of time had similarly large offsets plus random errors.
> https://www.ucolick.org/~sla/leapsecs/BHsHn05p142.html
> Try not to demand exactness for time scale before 1972-01-01.
> 
> Nothing that might be called UTC existed before 1959-08 when H.M.
> Smith hosted folks involved with the USNO and UK time broadcasts for
> tea in his living room.  That was when the US and UK informally agreed
> to use cesium chronometers to coordinate their time broadcasts by
> choosing a frequency offset from cesium such that the broadcast
> seconds would attempt to match UT2 seconds.  The formal agreement to
> do that coordination, and the actual coordination, did not occur until
> 1960.  So there was no UTC before 1960, and that name was not used in
> print until 1965.
> 
> In 1960-09 the URSI General Assembly recommended that everyone should
> use this scheme and that BIH should choose the frequency offset.  BIH
> provided a value for frequency offset starting with 1961, and in
> 1961-08 at the IAU General Assembly BIH was officially tasked with
> providing the frequency offsets for radio broadcast time signals.
> 
> TAI did not exist in 1958.  In 1958 several labs were using cesium to
> construct unpublished time scales.  During 1961 BIH took on the task
> of combining all the available atomic time scales.  By 1964 the BIH
> realized that some atomic chronometers were better than others, and
> using the three best they created A3.  The epoch at which A3
> was set 1961-01-01T20:00:00 UT2.
> https://www.ucolick.org/~sla/leapsecs/taiepoch.html
> The calculation of A3 evolved and on 1971-10-04 the 14th CGPM
> authorized the CIPM to define TAI and work with the BIH to realize the
> definition.

So why was 1958-01-01 chosen as an epoch time for TAI (or is it, really, 
maybe TAI defines the time scale, and you use yyyy-mm-dd 
hh:mm:ss.sssss.....   and it's only some other standard that defines 
what "zero" means when you want to represent it as a single "number"

for example CCSDS 301.0-B-4 "Time Codes"  says:
"The CCSDS-Recommended epoch is that of 1958 January 1 (TAI) and the 
recommended time unit is the second, using TAI as reference time scale, 
for use as a level 1 time code. This time code is not UTC-based and 
leap-second corrections do not apply. "

https://public.ccsds.org/Pubs/301x0b4e1.pdf




> 
> For practical purposes the LORAN broadcasts in the 1960s used the time
> scale as the broadcast time signals, so LORAN was using frequency
> offsets to match UT2 with seconds longer than what became the SI
> second.  Therefore by 1972-01-01 the LORAN time scale was 10 seconds
> behind what had then become TAI.
> 
> These gory details are not neatly summarized because of the
> contentious climate during those decades.  
<snip>
It hasn't changed - well maybe for time, but for other standards

  Only by scouring
> contemporary publications for quotes from many folks who were present
> does it begin to become clear how much that attitude prevented clear
> description of the history.
> 

Well, I've always maintained that standards start as someone talking to 
someone else in the hall, or over drinks, tea, and then someone starts 
drawing on a tablecloth, napkin, or board.  And there's a lot of body 
language (if not spoken words) that go into some of the design 
discussions.  This is my contention as to why video conferencing (or 
worse, telecons and ppt) can not replace in person conferences.