In message: <[EMAIL PROTECTED]> "Daniel R. Tobias" <[EMAIL PROTECTED]> writes: : On 2 Jan 2007 at 11:56, Ashley Yakeley wrote: : : > GPS is TAI. I'm not proposing abandoning TAI for those applications : > that need it. : : It's a few seconds off from TAI, isn't it? It was synchronized to : UTC in 1980 (I think), but without subsequent leap seconds, so it's : now different from both TAI and UTC. They probably should just have : used TAI if they wanted a time scale without leap seconds, rather : than ending up creating a different one.
Yes. There's a fixed offset between seconds in TAI and seconds in GPS. The GPS timescale is tied to seconds in UTC(NRO). TAI is a paper clock, computed after the fact, so GPS can't ever be TAI time. However, the differences there are down in the nanosecond or so range. There's a big philosophical opposition to using the paper clock that is TAI in a real-time operational timescale that GPS uses. The European version of GPS originally specified TAI time, but this was changed in later revisions to be the same as GPS time. There's an extreme reluctance in the time community to call something without leap seconds "TAI" or "TAI + fixed offset". TAI means something very specific. That's the other problem with just using TAI, btw, but explaining that point is very hard... The principle time scientist made me change the description of the output of measurment time for a product I did. I described it as "TAI time seconds since 1970." Instead, he descriped it as "Number of PPS ticks in the UTC time scale since 1972 + 63072000 + 10." Mathematically, they work out to be the same thing, but he was extremely resistant to calling it TAI time or using the TAI moniker for it at all. When I asked him about this, he said that TAI time isn't realized in real time, but UTC is. UTC is what one can measure against. Producing a number that corresponded to TAI time was OK, and likely the least confusing thing to do (we give a second number and UTC time in 'YYYY-MM-DD HH:MM:SS Z' as well as the channel and the measurement for that time in out output), but actually calling it TAI would 'confuse' the really smart time geeks out in the world. I asked him for a reference where I could read up on this, and he shrugged and said he just knew it and didn't know of any good write up. Warner