jlevine wrote: > 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. >
We have developers standing by to implement this! :) > 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. How do they get it from your clock room to their target without the error budget overwhelming these dispersions? >> 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. Particularly for us cosmologists who don't believe that there is a center of the universe. From a relativistic point of view it makes no sense. Otherwise we might as well go back to Galileo. Danny _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
