Re: [LEAPSECS] epoch of TAI, and TAI vis a vis GPS
On Wed 2015-03-04T07:42:46 +, michael.deckers via LEAPSECS hath writ: On 2015-03-04 02:23, Steve Allen wrote on the 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 Well, there is not only personal recollections: RECOMMENDATION S 4 (1970) of the 5th Session of the Consultative Committee for the Definition of the Second: 4. The origin of International Atomic Time is defined in conformance with the recommendations of the International Astronomical Union (13th General Assembly, Prague, 1967) that is, this scale was in approximate agreement with 0 hours UT2 January 1, 1958. A resolution does not change what had been done by the folks running the broadcasts over a decade earlier, nor does it repair the deficiencies in what they had done over that entire interval. A.R. Robins talked with H.M. Smith. Smith had been there making the UK time broadcasts happen. If Smith said that the UK versions of the atomic time scale and the UT2 time scale were aligned at 1958-01-01T20 then that is likely the way the calculations to do that alignment were performed, and likely based on the hour of the day that the ionospheric conditions were most conducive to allowing the best comparison with other transmitters. It is instructive to read BIH Bulletin Horaire for the actual history;, to see the ways that the radio broadcasters attempted, succeeded, and failed at their mandate of maintaining continuously operating transmitters based on continuously operating chronometers; to see the issues with Anna Stoyko's initial, painstaking efforts to reconstruct the history of received radio time signals into the first atomic time scale; to see the number of times that the broadcasters changed their strategies and the number of times that the BIH changed their algorithms and recomputed what past issues should have said the time was. -- Steve Allen s...@ucolick.orgWGS-84 (GPS) UCO/Lick Observatory--ISB Natural Sciences II, Room 165Lat +36.99855 1156 High StreetVoice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] epoch of TAI, and TAI vis a vis GPS
On 2015-03-03 09:23 PM, Steve Allen wrote: On Wed 2015-03-04T00:04:10 +, 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
Re: [LEAPSECS] epoch of TAI, and TAI vis a vis GPS
On 2015-03-04 02:23, Steve Allen wrote on the epoch of TAI: 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 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. Well, there is not only personal recollections: RECOMMENDATION S 4 (1970) of the 5th Session of the Consultative Committee for the Definition of the Second: 4. The origin of International Atomic Time is defined in conformance with the recommendations of the International Astronomical Union (13th General Assembly, Prague, 1967) that is, this scale was in approximate agreement with 0 hours UT2 January 1, 1958. Michael Deckers. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
[LEAPSECS] epoch of TAI, and TAI vis a vis GPS
On Wed 2015-03-04T00:04:10 +, 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 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 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. 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). -- Steve Allen s...@ucolick.orgWGS-84 (GPS) UCO/Lick Observatory--ISB Natural Sciences II, Room 165Lat +36.99855 1156 High StreetVoice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
