Hello; GPS maintains a set of its own clocks at Falcon AFB and does not really track or steer to TAI - however, they are very close in practice (except that the AF did not know about Leap Seconds when they started out and synced it to UTC in the early 1980's - thus, there is a 19 second offset between the GPS time system and TAI.) Every major time service and most national standards labs maintain a set of clocks of comparable accuracy - US, UK, France, Germany, Russia, Japan, Australia, etc., so there is no shortage of timing info to compare it with. The International GPS Service (IGS - http://igscb.jpl.nasa.gov/ - a collaboration between various geodetic and time service users of GPS - has a rapid service with information including clock offsets with 17 hours latency see http://igscb.jpl.nasa.gov/components/prods_cb.html for data availability. These solutions are NOT based on the official DOD tracking data but instead on the much more accurate carrier phase (and are not affected by either Anti-Spoofing or Selective Availability when these are turned on - see www.timingtechnologies.com/Gpswp1.pdf for a description of these degradations for civilian users). There is no doubt that a major perturbation in the GPS clocks (say, several 100 nanoseconds as is typical with SA) would be detected by the IGS within 24 hours. These was a pilot program set up to use these data for official time transfer - see http://maia.usno.navy.mil/gpst.html for a host of details. I do not know its status since Jim Ray left the USNO. GLONASS maintains another set of clocks and satellites. Of course, once Galileo is launched there will be yet another source of time sync. All of this is important if you need synchronization at 100 nanoseconds or better. LORAN will not give you this by several orders of magnitude - nor will WWVB nor NTP. If you do care about time at this level, get at least a Rubidium clock and sync it to GPS. If you do not, I would not worry about it even at the highest paranoia levels - there are other equally paranoid people who will start screaming well before you notice. On Sunday, June 1, 2003, at 09:57 PM, David G. Andersen wrote:

On Sun, Jun 01, 2003 at 08:13:08AM -0700, Peter Lothberg quacked:

...

...

I don't expect GPS to spin out of control soon..

So GPS tracks TAI and the difference is published (2 months after the fact..)

But it's simple to build a 'jamer' that makes GPS reception not work in a limited area, same for Loran-C used in combination with GPS in many Sonet/SDH S1 devices.

...

but I did wonder how hard it is to find a another reliable clock source of similar quality to GPS to double check GPS.

For NTP purposes, WWVB is actually just fine, as long as you properly configure your distance from the transmitter. The NTP servers list shows several WWVB synchronized clocks. CDMA clocks synch indirectly to GPS, but are typically locally stabalized by a rubidium or ovenized quartz oscillator with decent holdover capabilities for a few days of GPS outages. But they'll suffer the same fate if GPS went just plain wrong.

The NIST timeservers are available over the net, if you can deal with that degree of synch. Lots of them just use ACTS dialup synch to get the offset, and have very good local clocks. ACTS is certainly a good fall-back for GPS, since it uses a wired path instead of a wireless one.

So if you're really paranoid: GPS + WWVB + ACTS + internet to tick/tock or one of the NIST primaries. Ultimately, WWVB, ACTS, and ntp to NIST are all synched from pretty much the same source, but the odds that they'd all go bad are pretty slim. GPS is steered from the USNO time, but the clocks on the satellites are pretty good.

-Dave

-- work: dga@lcs.mit.edu me: dga@pobox.com MIT Laboratory for Computer Science http://www.angio.net/ I do not accept unsolicited commercial email. Do not spam me.

Regards Marshall Eubanks T.M. Eubanks Multicast Technologies, Inc. e-mail : tme@multicasttech.com http://www.multicasttech.com Test your network for multicast : http://www.multicasttech.com/mt/

In a message written on Sun, Jun 01, 2003 at 11:57:21PM -0400, Sean Donelan wrote:

Actually my question wasn't so much about other national standards labs, but that almost every major Internet backbone worldwide seems to trace their time source to GPS. Maybe not that surprising for US/North American providers, but even non-american backbones seem to use GPS.

Could it be that providers actually have multiple sources, but for some reason GPS is always picked as the primary source for the public facing function? At least a few providers keep their actual sources (the receivers themselves) "hidden", and provide a unix box syncing to all of them as the front end. From my limited knowledge, that front end box will only show the one source it has picked as "best". -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/ Read TMBG List - tmbg-list-request@tmbg.org, www.tmbg.org

You can go out and get yourself a nice Cesium source (like the ones in orbit) that will give you an accurate clock pulse with Stratum 1 quality. Cost: about 45K. Of course, this wont help you with Time Of Day information. It will only keep you completely accurate in case of a loss of outside reference. NTP should properly detect a loss of all outside sync sources. In other words, if all the clocks in the world that you were syncing to all of a sudden said it was tommorrow NTP should properly disregard them as all being out of sync and refuse to sync to them. If you try and start up NTP with your local clock too far out of sync, NTPD will not start up. If they all go away or go out of sync after you have properly disciplined your LO, it should detect the fault and continue to provide accurate time but drift to the degree your local clock isn't of a Stratum 1 quality. The net result is that you will stay accurate as long as your local reference oscillator is of Stratum 1 quality (or better). As an aside, you can check the accuracy of your GPS signal by comparing it to a local Cesium. That's how the government does it. You'll only be able to tell if they agree or not though, not which one is right. -Richard Sean Donelan wrote:

On Sun, 1 Jun 2003, Marshall Eubanks wrote:

...

Every major time service and most national standards labs maintain a set of clocks of comparable accuracy - US, UK, France, Germany, Russia, Japan, Australia, etc., so there is no shortage of timing info to compare it with.

Actually my question wasn't so much about other national standards labs, but that almost every major Internet backbone worldwide seems to trace their time source to GPS. Maybe not that surprising for US/North American providers, but even non-american backbones seem to use GPS.

To be clear, I'm not talking about individuals syncing things to lots of different clocks. Clock.ORG has lots of clock sources around the world. I'm talking about what network providers use.

It was just one of those midnight projects a month or so ago, when I noticed my carefully balanced selection of tickers had slowly over the last few years all changed from other time sources to GPS. Probably not critical, but national standards labs have accidentily flipped the wrong switch in the past and done strange things to their time broadcasts. Yes, lots of people noticed, and it was fixed quickly. NTP has all this great logic for sanity checking time sources, but if they all come from the same origin, what happens?

--On 06/01/2003 11:57:21 PM -0400 Sean Donelan wrote:

On Sun, 1 Jun 2003, Marshall Eubanks wrote:

...

Every major time service and most national standards labs maintain a set of clocks of comparable accuracy - US, UK, France, Germany, Russia, Japan, Australia, etc., so there is no shortage of timing info to compare it with.

Actually my question wasn't so much about other national standards labs, but that almost every major Internet backbone worldwide seems to trace their time source to GPS. Maybe not that surprising for US/North American providers, but even non-american backbones seem to use GPS.

To be clear, I'm not talking about individuals syncing things to lots of different clocks. Clock.ORG has lots of clock sources around the world. I'm talking about what network providers use.

It was just one of those midnight projects a month or so ago, when I noticed my carefully balanced selection of tickers had slowly over the last few years all changed from other time sources to GPS. Probably not critical, but national standards labs have accidentily flipped the wrong switch in the past and done strange things to their time broadcasts. Yes, lots of people noticed, and it was fixed quickly. NTP has all this great logic for sanity checking time sources, but if they all come from the same origin, what happens?

Sean, digging back through some old mail. This is one of those like asking where we would get water after the reservoir dam broke and flooded the town. The planes on GPS instrument approach are going to be much less happy than you are. How about those satellites that use GPS for attitude awareness. Or the rail anticollision systems that are now GPS based. If GPS time gets screwed, internet time is the least of our worries. Some of us remember the false ticker that caused the introduction of the NTP protection code. I don't think that anyone will ever build code like the fuzzball that stopped routing when the time got confused (although I guess I really shouldn't say that.) As for GPS jamming, it's easy to do on a local basis but hard to do on a widespread basis (unless you own the satellites.) For the accuracies that most people care about, dropping one stratum won't be the end of the world. Not too many people care about the microseconds (except us time geeks.) As for why everyone is switching to GPS time: it's cheap, it's much more accurate that anything else and it's available everywhere you can see a reasonable amount of the sky with no service charge. Other than that I can't see why people would do it. jerry