Dave, > > Picking a small nit: WWVB and other Time and Frequency Stations happily > transmit a variety of frequencies for carrier and modulation. That's how > I calibrate my radios.
Sure. The audio tone bursts transmitted by WWV (and WWVH) could be set to anything. However, it would not be so easy to untangle the carrier frequency from the time code on WWVB. The time code generators for WWVB (and for the GPS system, for that matter), assume that the frequency of the carrier and the chipping rate of the time code are locked together in some fixed relationship. This fixed relationship is buried pretty deeply into the design, and I would guess that it would be quite difficult to change the length of the transmitted second without changing the frequency of the carrier and the chip clock. > > Last we talked you said the goal of Ultimate Timekeepers of the World > was to the nanosecond using Two-Way Satellite Transfer. The NIST method > used to wrangle an unruly herd of cesium clocks is described in my book. > The method provides nominal time and frequency offsets between all > clocks in the herd and establish a nominal laboratory timescale. I call > this NTP distributed mode and have threatened to implement it. Absolutely. The frequencies of our internal ensemble of clocks can differ by as much as 1e-11 from the frequency of TAI or UTC. However, if I allowed an offset this large to escape from the clock room I would be sent to bed without dinner for a month. Our serious customers expect (and pay for) fractional frequency stabilities on the order of 1e-14 or 5e-15. These frequencies correspond to time dispersions on the order of nanoseconds per day. > > A review of the IERS data from 1961 to 1972 shows periodic ephemeris > time (ET) offsets declared in both time and frequency. That must have > driven the microsteppers bonkers. You have no idea. As the guy who was out there where the rubber meets the road, it was absolutely crazy. > If I could reinvent the world, I would run the master clocks in barycentric > time and distribute offsets via the web. This is TAI at the mass center > of the solar system where the gravitational potential is zero. Yes, this idea was proposed some time ago, and it is currently being pushed pretty hard by the folks who need to plan for going to Mars. Some version of this idea will probably be adopted if/when space travel becomes more common. However, the idea that the rotating geoid (which is the basis for the current SI defintion) is the center of the universe will not be easy to overcome. > You and I discussed whether NTP should retail TAI (as in POSIX) or UTC > and for a number of reasons decided on UTC. At least for now, I would stick with that decision, since UTC is closer to the definition of legal time and the financial world does not understand TAI and would certainly not want more ambiguity in the time stamp. Judah Levine Time and Frequency Division NIST Boulder _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
