[time-nuts] PTTI 2012, part 2

Tom Van Baak tvb at LeapSecond.com
Mon Dec 3 18:45:12 UTC 2012

Continuing from previous posting...

> Still, there are always a number of talks of more general interest to us time nuts.
> In the next few postings I'll give more details on a couple of topics:

- Neutrino time-of-flight update

Last years' faster-than-light neutrino fiasco is now old news, but I think many of us were curious how other neutrino experiments around the world would react. Not surprisingly, they all got very serious about precision timing! As one presenter mentioned, purchase orders for high-end timing gear were suddenly approved with remarkable ease.

There were a total of 6 talks by people working with neutrino experiments. From what I could tell (having done similar work at home) they totally have their act together now. NIST and USNO were involved with MINOS, the local US effort (neutrinos from FermiLab to the Soudan mine in Minnesota), both in providing gear (high-end dual-frequency or post-processed GPS receivers and local cesium clocks) to absolute calibration using TWSTFT (two way satellite time & frequency transfer).

One talk covered the challenge of measuring distance (presumably there are survey-nuts as well as time-nuts). You think measuring nanoseconds is hard until you hear what it takes to measure distance, including down an opaque slanted mineshaft, and get a final number like 734,286.554 meters!

While PTTI normally concerns itself with national timing laboratories, it was nice to see a whole new community (neutrino physics) get involved in the practical world of nanosecond timing.

- UTCr/Rapid UTC

Some of you know that TAI/UTC is computed from a monthly average of hundreds of atomic clocks around the world. Given the recent improvement in clock performance, intercontinental comparison techniques, and automated communications (internet), BIPM started a pilot program this year to generate a more responsive and fully automated "UTC". One problem with UTC is that physical measurements of, and virtual corrections to, contributing clocks occurs only once a month. This means that your national lab might drift a few ns before being told it is drifting.

Those of you who have built your own GPSDO or explored time constants can relate. So UTCr is an experiment where labs report *daily* and results are computed *weekly* based on a *monthly* moving average, or something like that. I had not heard about this before, but if you google for words like "Rapid UTC" UTCr BIPM you can learn more. The whole subject of time scales is deep and interesting.

- USNO rubidium fountains

While many national labs have developed cesium fountains (for accuracy), USNO has been gradually building rubidium fountain clocks (for stability) and 4 of them are now fully operational. The ADEV of these clocks gets well under 1e-16. The paper will have all the details but the note I made was that with 20 months of data, the stability was near 5e-17 at tau 4 months. That's 100x better than a commercial cesium standard; better than all of USNO's other 70 cesium clocks and 15 H-masers combined. Yes, I've added "rubidium fountain" to my automated eBay searches.

Like always, each of the Rb clocks is a little different. You measure this not by using an external reference (since no better clock is available) but by using the 3-cornered hat technique where you compare clock i against the mean of N clocks.

- WWVB/Xtendwave

This presentation was similar to the one made last year. From the talk, and discussions afterwards, it's clear good progress is being made. As of October 29, 2012 the extended WWVB format is now default, but reception tests are continuing, and the format perhaps slightly tweaked. A prototype receiver was shown in a shoebox. I did not get to see inside.

My guess is that by next year we'll see real hardware and a final spec. I suggested the time-nuts community might be willing to test the new receivers when they are available. Contact me off-list if you want to help. I can't promise anything, but I know that NIST/Xtendwave have embarked on a project that will greatly improve reception quality and totally solve the DST announcement problems. I also believe that a passion-driven, no-cost, geographically-diverse set of time nuts can help make this a well-tuned success.

They also mentioned another cool feature being developed -- a high-rate modulation where, reception permitting, the entire 60 bit message is encoded into a single 1-second frame.

- Loran/UrsaNav

Not sure what to say about this presentation. Apparently this company got the rights to the US Loran transmitters(?) and plans to use them to provide an alternative source of precise time, for example, to the telecom community. CW instead of very low duty cycle Loran pulses would improve S/N and timing accuracy. You can probably read more about it on their web site. Any additional source of precise time is probably a good thing -- when NTP, WWVB, GPS, and your local atomic clock fails. But the presentation raised more questions than answers. I look forward to reading the paper.

I'll finish up in the next posting.


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