On 2015-03-03 09:23 PM, Steve Allen wrote:
On Wed 2015-03-04T00:04:10 +0000, Tony Finch hath writ:
They have different epochs:
TAI: 1958-01-01 T 00:00:00 Z
PTP: 1970-01-01 T 00:00:00 Z
GPS: 1980-01-06 T 00:00:00 Z
Using ISO 8601 style date and time representation on the TAI timescale
and on the UTC timescale before 1972-01-01T00:00:00Z (UTC) is dangerous
without qualification or explanation.
The "Z" implies its on the UTC timescale. It is controversial when the
term "UTC" came into use. It is controversial if the UTC timescale
existed prior to 1972-01-01T00:00:00Z (UTC), and if it did, exactly what
it is..
"TAI: 1958-01-01 T 00:00:00 Z" - By preceding it with "TAI" we guess
you mean the TAI timescale if we ignore the Z.
1958-01-01T00:00:00 (TAI) is the origin of the TAI timescale, as per
ITU-R Rec 460.
Using pure Gregorian calendar counting method, 1958-01-01T00:00:00 (TAI)
is exactly (1972-1958 = 14 years * 365 = 5110 days + 3 leap year days =
5113 days * 86400 seconds = 441763200 seconds) before
1972-01-01T00:00:00 (TAI).
"PTP: 1970-01-01 T 00:00:00 Z" - The PTP Epoch is defined as
1970-01-01T00:00:00 (TAI) *on the TAI timescale*. Using pure Gregorian
calendar counting method, 1970-01-01T00:00:00 (TAI) is exactly
(1972-1970 = 2 years * 365 = 730 days + 0 leap year days = 730 days *
86400 seconds = 63072000 seconds) before 1972-01-01T00:00:00 (TAI).
"GPS: 1980-01-06 T 00:00:00 Z" - The GPS Epoch is properly and firmly on
the UTC timescale, after 1972-01-01T00:00:00Z (UTC). There's no
controversy there. The GPS Epoch is 1980-01-06T00:00:00Z (UTC) =
1980-01-06T00:00:19 (TAI). From there they count in uninterrupted weeks.
Meantime (always fun to use that expression in a discussion of
timescales :-) ), POSIX "the epoch" is stated as "January 1, 1970
Coordinated Universal Time (UTC)". It is controversial if UTC existed
before 1972. So, to get to that date from 1972-01-01T00:00:00Z (UTC) =
1972-01-01T00:00:10 (TAI) we need to constuct some proleptic timescale
before 1972-01-01T00:00:00Z (UTC). I'm not sure what we'd like to call
this proleptic timescale, lets call it "POSIX" for now. Using Gregorian
calendar counting, 1970-01-01T00:00:00 (POSIX) is (1972-1970 = 2 years *
365 = 730 days + 0 leap year days = 730 days * 86400 seconds = 63072000
seconds) before 1972-01-01T00:00:00Z (UTC).
Similarly for NTP. RFC 5905 states ".. the prime epoch, or base date of
era 0, is 0 h 1 January 1900 UTC". Again, lets call this proleptic
timescale "NTP". So, 1900-01-01T00:00:00 (NTP) is (1972 - 1900 = 72
years * 365 = 26280 days + 17 leap year days = 26297 days * 86400
seconds = 2272060800 seconds) before 1972-01-01T00:00:00Z (UTC).
Our "POSIX" timescale overlaps our "NTP" timescale - they exist on the
same timescale proleptic to 1972-01-01T00:00:00Z (UTC) using the the
Gregorian calendar counting method. I got flamed for calling it
"proleptic UTC". What should it be called? After all its used all the
time, shouldn't we have a name for it?
Getting meaninglessly pedantic, in Survey Review v19 #143 p7 (1967)
A.R. Robins had been talking with Sadler and Smith and with that
information in hand he wrote that atomic time was identical to UT2 at
1958-01-01 T 20:00:00 Z
ITU-R Rec 460 says "TAI .. from the origin 1 January 1958 (adopted by
the CGPM 1971)".
In 'Metrologia - leap second: its history and possible future' -
http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf - we read:
"In conformity with the recommendations of IAU Commissions 4 and 31 in
1967, the CCDS [80] defined the origin so that TAI would be in
approximate agreement with UT2 on 1 January 1958, 0 h UT2. The 14th CGPM
approved the establishment of TAI in 1971."
As I interpret this, while there were previous historic uses of
"1958-01-01" as an epoch for various things, including LORAN and early
development atomic timescales, TAI didn't officially exist until 1971,
and by adopting 1958-01-01T00:00:00 (TAI) as the TAI origin they
acknowledged those precedents and made the definition specific and
official on the TAI timescale. Whatever the values or accuracies may
have been for previous "1958-01-01" epochs, this act established the
modern version accurately tied to the TAI timescale.
Is that how you see it?
This, of course, disagrees with Guinot's memoir, but the various
realizations of UT2 then differed by centiseconds and the different
versions of atomic time were subsequently realigned by milliseconds.
And that date of 1958-01-01 was decided ex post facto at the 1959
August meetings where the US and UK decided to try coordinating their
broadcast time signals using cesium. So there really isn't an epoch
for TAI.
Seems to me there is, as above.
On Tue 2015-03-03T14:31:13 -0800, Hal Murray hath writ:
Since GPS time is a fixed offset from TAI, it's easy to convert.
I believe that BIPM would disagree because of the different kinds of
steering at the nanosecond level. The stance of the BIPM was expressed in
http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf
where TAI "should not be considered as an alternative time reference."
Without the assent of the BIPM it is hard for there to be an agreed
upon name for real-time versions of time scales that are trying to
track the value of TAI (which will not actually be available until the
next issue of Circular T).
Well, really, UTC is a dissemination of TAI encoded with the UTC Leap
Second counting scheme - you need only convert from UTC to TAI, which is
easy if (a big if) you have the Leap Second information and apply it
properly.
-Brooks
--
Steve Allen <s...@ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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