[time-nuts] Trimble Thunderbolt no longer determines the correct date
Tom Van Baak
tvb at LeapSecond.com
Fri Aug 11 12:26:26 EDT 2017
> The E911 installation, in the news, is just one of several. Others are hospitals,
> fire stations, etc. using different dispatch systems.
Hey, at least important things like mobile phones, ISP's, Google, Amazon, FedEx and Starbucks aren't affected ;-)
> In a wide-area simulcast-overlap paging system, the transmitters in the same
> coverage area are carefully set to all transmit at exactly the same time.
That's fine. And very clever. But why is this "life safety" system tied to GPS, to a particular vendor, to a particular model of receiver (that clearly states in the documentation that it has a 1024 week / 19.6 year window of valid UTC times)?
> So to me "synchronizing transmitters” means the control system sends the
> traffic out to all the transmitters (over satellite) and tells them all to hold the
> messages in a buffer until “the big hand points straight up” or whatever data
> command the system uses. (excuse the vernacular)
Exactly. In most of the precise timing world the "big hand" is the "top of the second", or the so-called 1 PPS pulse. The idea is that all 1PPS agree with each other, whether from a cesium clock, or WWVB receiver, or NTP, or GPS (or any other GNSS system).
Since the paging system failed it sounds like it was synchronized to some "hand" other than 1PPS. The rare GPS rollover events tend not to disrupt the 1PPS output -- it is still perfectly aligned with UTC -- which is why almost no one else worries about the recent TBolt episode, or any other GPS receiver for that matter.
> The problems being experienced right now appear to be the interface of the ThunderBolt
> with the Zetron Model 620 simulcast controller over TSIP. The Zetron box is also called
> a “wireless data encoder.”
Ah, ok. So do you or anyone have contact within Zetron? The easy fix would be for them to upgrade their firmware and send out a patch. Probably cheaper than supplying new receivers from Trimble. I don't know; for us, a s/w fix is easy compared to a h/w fix or a h/w swap-out. But in the real world, once technicians have driven to a remote installation, maybe there's no real difference between a s/w fix and a h/w swap.
> It is not our goal to blame a particular piece of equipment for this problem.
Right, no need to blame. I think many of us would just want to pinpoint the root cause of the problem, out of engineering curiosity. By root cause I mean actual schematics or lines of source code. It's always been my hope, after every one of these widespread infrastructure events, that the actual source code or design decisions be published eventually so that we can all learn from it.
> The facts are the 1024 roll over happened and just about nobody in the paging
> business knew it was coming.
Ok, now you know about GPS rollovers! Fun, isn't it? Leap seconds are fun too.
When the dust settles, you may want to look into the more general topic of life safety infrastructure vs. free-from-the-sky time & frequency. These days nanosecond precise time is cheaper than water -- but it's also fragile. A lot has been written about this. It's both a wake-up call for naive vendors of products based on GPS alone and also an opportunity for vendors who know how to design and market resilient timing products.
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