On 5/7/2015 11:38 AM, Pete Stephenson wrote: > I recently inquired about taking a tour of the METAS time lab[1] and > they said they'd be willing to show me around. > > Is there anything in particular that fellow time-nuts would be > interested in me asking them about or (if possible) photographing?
Hi all, On Friday I toured the METAS Time & Frequency lab with two others: Patrick, a colleague from work and Atilla, a fellow time-nut. I'd like to report a bit about what we learned today that may be of interest to other time-nuts. UTC(CH) is composed of an ensemble of four commercial HP 5071A cesium clocks and an active hydrogen maser. METAS changed the definition of UTC(CH) several years ago. Details regarding the old and new systems were presented at a conference in 2006. The only archive I can find of the conference paper is, for some reason, available at a US military website[1]. I've mirrored the paper at my site[2]. I'll quickly summarize the two systems below: ===== Old Definition of UTC(CH) ===== UTC(CH) had previously been defined as a computed "paper clock" named UTC(CH.P) which was the weighted average of the four HP 5071As and the maser. This paper clock was steered monthly to track UTC. In the old system, the maser (the reference clock) drives a micro phase stepper and the output of the stepper is steered to the paper clock to form UTC(CH.R), the hardware real-time realization of UTC(CH). UTC(CH.R) is connected to the distribution hardware. Using a paper clock had two main advantages: - It's more stable than any of the individual clocks in the ensemble. - It is tolerant of hardware failure of the non-reference clocks. For example, if the ensemble normally has N clocks and one fails, it continues to work with N-1 clocks. However, there are several disadvantages: UTC(CH.P) was computed for only a single epoch each day so time measurements made at other epochs can only be related to UTC(CH.P) via interpolation. Also, if the reference clock fails the whole system is disrupted. ===== New Definition of UTC(CH) ===== In the new system, UTC(CH) is defined in real-time without interpolation instead of being computed for a single instant each day. UTC(CH) is now defined as a hardware master clock named UTC(CH.RT). To quote the paper, "The UTC(CH.RT) hardware definition of UTC(CH) is chosen from one of two independent master clocks: UTC(CH.A) and UTC(CH.B). Each is the output from a DDS synthesizer, used as a MicroPhase Stepper (MPS). Each is driven by one of the free-running atomic clocks, and steered to track the paper time scale UTC(CH.P). Auto-sense Fault Switches (AFS) are used to choose UTC(CH.RT) between the A/B master clocks. The hardware redundancy between UTC(CH.A) and UTC(CH.B) has two advantages. One is reliability: if one master clock fails, switching to the backup master clock is instantaneous. The second is continuity of service: if some maintenance of one master clock becomes necessary, for example for the purpose of calibration, it is possible to switch to the other master clock at one’s convenience without interruption of service." Typically clock A is the hydrogen maser and clock B is an HP 5071A. In addition to both being reference clocks for UTC(CH.RT), both contribute to the paper time scale. ===== Clock Vault ===== In addition to the maser and four HP 5071A clocks, there are several other clocks that are used for various purposes but which do not contribute to UTC(CH): a passive hydrogen maser, several rack-mountable quartz oscillators, and at least one rubidium oscillator. All the clocks are kept in a single thermally-regulated clock vault in the basement at METAS. While it won't be used to contribute to UTC(CH), a continuous-beam cesium fountain is being constructed in the room next to that containing the UTC(CH) clocks. Details of this clock are available at [3] with a mirror at [4]. Although the Swiss are well-known for their fine watches and other timekeeping devices, METAS is a rather small national time lab (compared to, say, PTB, NIST, or USNO) with comparatively limited resources. Their primary function is to provide a service to customers or users rather than advance the state of the art of timekeeping: for example, they provide NTP service to the public and UTC-traceable calibration service to paying customers. In the past they were the time source for the long-wave time signal radio station HBG, but that service was discontinued in 2011. They are doing some interesting research in regards to time and frequency, but that is not their main focus. ===== Questions & Answers ===== Several fellow time-nuts had sent me questions that they wanted me to ask METAS. Here's the questions and answers, paraphrased from my shorthand notes: 1. Q: How does METAS generate UTC(CH)? A: With four HP5071As and a hydrogen maser that contribute to a "paper clock". See above, [1], or [2]. 2. Q: How is the Swiss time scale linked to the rest of the world? GPS? Two-way satellite transfer? A: The primary link is a two-way satellite link (TWSTFT). They also have dual-frequency geodetic GPS receivers for all-in-view multi-channel dual-frequency P code time transfer. According to BIPM Circular T, they use a combined smoothing of the TWSTFT and GPS methods. 3. Q: Why was HBG discontinued? A: It was expensive to operate and signal reception in mountainous terrain was often limited. The antennas needed expensive renovation and it was decided that it was easier and cheaper for users to switch to DCF77 rather than repair HBG. 4. Q: What's the explanation for the bug in the transmission of the leap second by HBG in 2005-2006? (See [5] for details.) A: No idea. The scientist giving us the tour worked at METAS in the time and frequency group at the time, but he didn't recall what might have caused the bug. In fact, this was the first he had heard of it. He then expressed dismay at leap seconds in general and wishes TAI was more widely used as a time standard (e.g. computer clocks should use TAI), with the conversion to UTC only occurring for display purposes. ===== Photos ===== Photos are available on Flickr at [6]. Unfortunately, conditions were not ideal for photography and photos of the clock vault were taken from outside the thick, thermally-insulated windows. Nevertheless, if anyone wants the original high-resolution photos, please contact me off-list and I'd be happy to provide them. ===== Interesting Notes ===== 1. The IT department at METAS is concerned about the security of publicly-accessible systems such as NTP servers. As such, none of the master clock systems or the computers that manage them are connected to the internet. Rather than use a typical computer network such as ethernet, the time distribution systems attached to the master clock send IRIG B timecodes over coaxial lines to the public METAS NTP servers[7]. This setup is accurate to within a few microseconds, which is sufficient for NTP, and helps isolate the master clock systems from the internet. 2. The servo loops for the HP 5071As and active hydrogen maser run with time constants of one hour and one day, respectively. (Is this typical?) 3. METAS previously provided a time sync service over analog modems (the actual modems and time code generators can be seen in one of the photos). When the decision was made to discontinue the service, the METAS staff wanted to notify users of the service so they could prepare for the eventual shutdown without any surprises. Unfortunately, Swisscom (the incumbent telephone company) refused to provide METAS information on who was calling the modem lines because their privacy policy (and Swiss law) forbids it. As such, METAS was forced to discontinue the service with no warning. They never received any complaints. 4. Our guide, Dr. Laurent-Guy Bernier, related an amusing story: the Swiss watch industry used to conduct state-of-the-art research on improving the accuracy of wristwatches. Later, they realized that if people wanted accurate watches they were buying cheap quartz ones -- people weren't buying expensive Swiss watches because they used them to tell time, they were buying them as status symbols. Unfortunately, a similar situation came about with METAS: they provide the legal Swiss time scale and offer traceable calibration services to customers, but there's not really a lot of political incentive to spend money on having METAS to offer more services (DCF77 and GPS are run by the Germans and Americans respectively, meet people's needs, and cost the Swiss nothing, so why pay to run HBG?) or do more than a little bit of advanced research. Practical? Perhaps, but I think it's too bad. 5. The exploits of several time-nuts are known to the cloistered researchers at METAS: they find Tom's Project GREAT to be particularly entertaining and interesting. ===== Thanks ===== Dr. Laurent-Guy Bernier at METAS was kind enough to take over an hour out of his busy day to show the three of us around and I would like to thank him for his time and expertise. ===== Links ===== [1] <http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA474255> [2] <https://www.heypete.com/pete/time-and-frequency/archive/ADA474255.pdf> [3] <http://hal-obspm.ccsd.cnrs.fr/hal-00165376/document> [4] <https://www.heypete.com/pete/time-and-frequency/archive/EFTF07_FOCS_2.pdf> [5] <http://phk.freebsd.dk/Leap/20051231_HBG/> [6] <https://flic.kr/s/aHskcPUTko> [7] <http://www.metas.ch/metas/en/home/fabe/zeit-und-frequenz/time-dissemination.html> If anyone has any questions or comments, let me know and I'd be happy to do what I can to answer. Cheers! -Pete _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
