> : As I understood it, it was mainly that TAI is a post-factum "postal" > : timescale. > > How is it that UTC can be realized in realtime, but TAI isn't. I > thought the difference between the two was an integral number of > seconds, by definition. Is that understanding flawed? > > Wanrer
Not flawed. But recognize that the words UTC or TAI have more than one meaning. Strictly speaking UTC isn't real-time either. However, individual laboratories produce a real-time realization of UTC, in the form of UTC(k) where k = NIST, USNO, NPL, etc. which is then steered, over time, toward or around the mean. Your own lab or home may have its own realization of UTC, such as Doug's UTC(DWH) or my UTC(TVB). Note there has been strong resistance to the use of the phrase UTC(GPS). True, there exists UTC(USNO) but what a GPS navigation or timing receiver delivers is many layers of uncalibrated phase and frequency offset and uncertainty from UTC(USNO) which it itself just an good estimate of UTC. This is all at the sub-microseconds level. Up at the seconds level the generic terms "UTC time", "TAI time", "GPS time", or, for that matter, "local time", are simply a matter of integer seconds offset; they all nominally follow the same synchronized seconds epoch (1PPS). So when you see "UTC" or "TAI" you have to infer the context. Is the timescale; is it seconds; is it nanoseconds? The danger is mixing the generic use of the acronym (meaning seconds offset) with a more technical use of the word (meaning the realization of the timescale). I can easily set a cesium clock or wristwatch to display in UTC, TAI, GPS, EST, or local time. The ticks occur at the same time. The hands or digits differ. But does this mean my UTC(TVB) clock can produce UTC? or TAI? Yes, it can produce a display that matches TAI to some finite accuracy. No, it does not produce TAI itself. http://www.leapsecond.com/pages/trak6460/ If I had to guess, the resistance to the use of the word TAI by the BIPM or others is that in the past and still today the word TAI represents *first* the notion of a well-organized international ensemble of atomic clocks. Only secondarily does the TAI convey something as mundane as an N second offset on your wristwatch. Mixing the two meanings can lead to trouble. For example, occasionally you hear that Galileo is somehow better than GPS because it uses TAI time instead of GPS time. That's bogus. To the second, any existing WWVB or DCF77 or GPS or Glonass or Galileo receiver can display time as TAI or GPS or UTC, or EST, or sidereal. Clever ones can even handle DST. But at the nanosecond level Galileo can no more deliver TAI than pizza to your receiver. /tvb