HI Volker Just to be clear: - NTP, GPS and DCF77 all have explicit support for leap seconds so that they are automatically taken care of - in other words equipment that relies on them is using leap seconds automatically, network operators don't need to worry about it, it all works. - Leap seconds are always added at midnight when it takes two seconds to go from 59 to 0 instead of 1. - GPS signals include UTC offset information so the time signal that GPS receivers use (and provide) is UTC by calculation. The only issue with GPS time signals is the deliberate reduction in accuracy of +/- 340 nanoseconds for military purposes. - Global time is synchronised by two-way time and frequency transfer with the BIPM in France. A crude explanation is that the time provided by the various atomic clocks that participate is averaged to make TAI. Hopefully that answers all of your questions. cheers Jay On 23/12/2011, at 2:08 PM, Volker Kuhlmann wrote:
On Fri 23 Dec 2011 11:19:12 NZDT +1300, Jay Daley wrote:
There has been a debate raging for some years on whether or not to change Universal Coordinated Time (UTC) to be the same as International Atomic Time (TAI), which is due for a major vote at the ITU-R in January. Previous votes have seen it defeated but it keeps coming back.
The issue is whether to abolish leap seconds, with the reasoning that they may be more trouble than they're worth. It's an international issue of metrology and I doubt it's decided by ITU-R (standardising wireless communications) and certainly not network operators. What you probably mean is that the ITU is trying to make up their mind which side of the fence they want to put their vote, and is thus asking their members for their opinions. I understand anyone is allowed to put their argument of how they're affected and why things therefore should be one way or the other.
The implication for networks is potentially quite profound. NTP currently supports leap seconds as do GPS satellite time signals and DCF77 radio clocks but if this proposal is agreed that will be turned off.
NTP, GPS and DCF77 are time transfer mechanisms that just transfer the time you feed in. If you were to feed in TAI instead of UTC everyone's lounge clock jumps by a few seconds once (or more likely that will be spread out over 1s every so often). Who'll notice? Did anyone notice the last leap second change? The time transfer mechanisms work just the same.
Practical questions: Do you actually know what you're getting out of your time reference piece? I know DCF77 gives you central European time (i.e. UTC plus time zone offsets). I'm not sure about NTP - at a guess, it gives out whatever the person running the stratum 1 clock puts in. That can be anything. GPS? Hmm. GPS doesn't even run on TAI, for 2 reasons: 1) There is no clock showing TAI, and GPS runs on an American master clock. That's not TAI, but the difference is probably small. Same goes for DCF77, except that the master clock is German and I happen to know it's participating in TAI. 2) GPS uses TAI + some fixed offset of a few seconds (IIRC 12), that being the UTC offset to TAI at the time GPS was rolled out. So GPS showed UTC, which lasted until the next leap second.
When you hook up a GPS receiver to an NTP server, are you sure you know what you're getting? TAI? GPS-time? UTC? Are you sure the receiver applies the current number of leap seconds offset? Does GPS even transmit that information? My computer is NTP synchronised, but I have no way of knowing whether it's TAI, UTC, something in between or something else. If I was doing photography dependent on a certain earth-rotational position I'd use NTP and adjust it so the photo looks good. As long as the time is always the same, it doesn't really matter what it is exactly.
How does this affect networks and network operators? If you're operating a network and time synchronisation is that important for your transmission clock signals aren't you operating on TAI already? Because if not, dealing with all those leap seconds doesn't sound like fun.
Volker
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