[LEAPSECS] draft revision of ITU-R TF.460-6

Dennis Ferguson dennis.c.ferguson at gmail.com
Thu Dec 8 23:17:19 EST 2011



On 8 Dec, 2011, at 11:43 , Zefram wrote:

> Steve Allen wrote:

>> http://www.ucolick.org/~sla/leapsecs/draftTF460-7.html

>

> "TAI is not physically realized and consequently is not suitable for

> time dissemination." -- what on Earth is that supposed to mean?


It's a technical argument. I think a physical realization of UTC is UTC(k),
so if you want a real-time estimate of UTC you'll be getting it by tracking
UTC(k), for some value of `k'. Here's how some of those UTC(k)'s did in the
November BIPM Circular T:

1 - Coordinated Universal Time UTC and its local realizations UTC(k). Computed values of [UTC-UTC(k)]
and uncertainties valid for the period of this Circular.
From 2009 January 1, 0h UTC, TAI-UTC = 34 s.

Date 2011 0h UTC SEP 30 OCT 5 OCT 10 OCT 15 OCT 20 OCT 25 OCT 30 Uncertainty/ns Notes
MJD 55834 55839 55844 55849 55854 55859 55864 uA uB u
Laboratory k [UTC-UTC(k)]/ns

CH (Bern) -0.5 -1.2 -2.0 -3.6 -4.5 -6.0 -8.0 0.3 1.7 1.8
IT (Torino) -0.8 -3.1 -3.1 -3.6 -5.0 -5.9 -6.8 0.7 1.9 2.0
JATC (Lintong) 17.4 7.6 -0.2 -0.8 -7.9 -9.5 -14.3 1.4 5.0 5.2
KRIS (Daejeon) 7.2 13.9 10.2 7.2 8.1 6.9 6.1 0.3 5.1 5.1
NICT (Tokyo) 19.0 18.5 18.5 19.2 21.1 22.7 21.8 0.3 4.7 4.7
NIST (Boulder) 10.2 10.8 11.2 9.6 8.9 8.5 7.4 0.3 5.0 5.0
NRC (Ottawa) -19.9 -22.5 -28.0 -33.1 -29.0 -29.0 -28.2 0.3 5.2 5.2
NTSC (Lintong) 1.4 -4.6 -4.8 0.2 4.5 6.6 7.6 1.4 4.9 5.1
ONRJ (Rio de Janeiro) 6.2 0.2 14.2 21.1 10.3 6.9 9.3 3.9 19.6 20.0
PL (Warszawa) 18.5 15.2 10.1 -2.7 -15.6 -19.1 -23.2 1.5 5.1 5.3
PTB (Braunschweig) 4.0 4.6 5.1 3.3 2.8 2.2 1.5 0.2 1.4 1.4
SG (Singapore) 16.6 19.3 24.5 26.2 29.4 27.8 28.7 0.3 5.1 5.1
SU (Moskva) 7.0 9.0 9.6 10.6 9.0 10.4 8.1 1.2 5.2 5.3
TL (Chung-Li) 28.6 29.4 29.6 27.5 28.5 27.9 26.9 0.3 4.9 4.9
USNO (Washington DC) 6.2 6.2 8.5 7.6 6.7 5.8 4.9 0.3 4.1 4.1

So any of those UTC(k)'s would be pretty good. The worst are off by a few 10's of
nanoseconds, the best by 10 ns or so.

Analogously, the physical realization of TAI would be TA(k), so I guess a real-time
estimate of TAI would be some lab's TA(k). Here's how you would have done using the
above labs' TA(k)'s in the same month:

2 - International Atomic Time TAI and Local atomic time scales TA(k). Computed values of [TAI-TA(k)].

Date 2011 0h UTC SEP 30 OCT 5 OCT 10 OCT 15 OCT 20 OCT 25 OCT 30
MJD 55834 55839 55844 55849 55854 55859 55864
Laboratory k [TAI-TA(k)]/ns

CH (Bern) 40261.2 40192.7 40125.0 40056.7 39988.1 39919.7 39851.2
F (Paris) 167705.4 167704.2 167703.1 167697.6 167696.9 167695.8 167693.8
IT (Torino) 106735.2 106835.3 104974.1 103876.8 102400.3 101043.6 99654.0
JATC (Lintong) -50417.8 -50441.2 -50469.5 -50490.8 -50519.3 -50544.8 -50570.2
KRIS (Daejeon) 38299.3 38368.4 38420.0 38473.4 38532.5 38587.7 38643.3
NICT (Tokyo) -126.7 -124.2 -118.4 -114.8 -107.3 -101.9 -96.7
NIST (Boulder) -45367935.4 -45368124.8 -45368314.4 -45368505.8 -45368695.5 -45368884.9 -45369075.0
NRC (Ottawa) 26053.9 26030.2 26003.5 25977.4 25960.6 25939.5 25919.2
NTSC (Lintong) 12928.3 12957.5 12982.3 13014.0 13038.9 13066.6 13094.0
ONRJ (Rio de Janeiro) -9973.7 -10013.8 -10038.7 -10059.4 -10087.4 -10107.9 -10135.7
PL (Warszawa) -9392.6 -9414.5 -9440.9 -9471.9 -9496.6 -9519.0 -9547.6
PTB (Braunschweig) -328739.5 -328727.5 -328714.3 -328698.5 -328685.7 -328681.1 -328673.1
SG (Singapore) 8084.6 8108.3 8134.5 8158.2 8185.4 8207.8 8232.7
SU (Moskva) 27277354.3 27277501.9 27277648.1 27277795.6 27277940.4 27278088.3 27278233.7 (1)
TL (Chung-Li) -473.7 -476.2 -478.7 -480.1 -480.6 -484.9 -485.8
USNO (Washington DC) -35094388.5 -35094680.6 -35094972.9 -35095267.2 -35095560.1 -35095852.7 -35096145.4
- Note on section 2:
(1) SU : Listed values are TAI-TA(SU) - 2.80 seconds.

So instead of 10's of nanoseconds with UTC(k), with TA(k) you get microseconds if
you're lucky and milliseconds if you aren't (and seconds with TA(SU)).

The reason the values of UTC(k) are so close together is that they are sometimes
steered to make that so. UTC(k)'s are the times which are disseminated, so they
try to keep them all as close to each other as possible. The value of an individual
TA(k), on the other hand, is never steered; they don't really give a crap how far off
individual TA(k)'s are from each other since they don't disseminate those values,
so the error accumulated in each TA(k) is the frequency error of that lab's clock
integrated over the entire time that lab has had a clock.

UTC, in the real-time form of UTC(k), is maintained for dissemination. TAI, in the
real-time form of TA(k), is not. If you want realtime TAI you are way better off getting
a UTC(k) and adding 34 seconds.

Dennis Ferguson


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