Slashdot Mirror
WWVB Celebrates 50 Years of Broadcasting Time
First time accepted submitter doublebackslash writes "On July 5th, WWVB, NIST's timekeeping radio station transmitting near Fort Collins, will celebrate 50 years of continuous operation. Operating at 60kHz, the signal actually follows the curvature of the Earth via a trick of electromagnetics, allowing nearly the entire globe to receive an accurate time signal, which has in recent years reached an accuracy of 1 part in 70 trillion. Recent upgrades, which came in $15.9 million under budget will allow the station to be better received even in large buildings, giving it an edge on timekeeping that not even GPS can touch, with its need for open skies to receive a signal."
Some 15 years ago, when they were at their original low power, my area was so fringe that my fancy new WWVB wristwatch just wouldn't pick it up.
The protocol is really quite straightforward and well documented at their site. The 60kHz signal sends binary by sending either full power or a bit less (I forget how many dB). I used a computer synced with NTP and a plain old soundcard generating 60kHz from a sound card into an audio amp, and I just did either full on or full off. The output ran into a big coil that I had wound to be roughly resonant around 60kHz.
Much to my amazement, it worked. So I just kept the watch near that coil overnight and it synced perfectly, until WWVB cranked up their power at which point I retired the mess.
You do have a point.
Given that light travels about 0.3 m in one nanosecond, a variation in the signal path-length of about 300 m would induce a smudge on the arrival-times of about a microsecond. Realistic path-length variations could no doubt be larger, and could vary over a time-period of minutes or hours, depending on ionospheric conditions. This would of course be much larger than the inferred time-accuracy of 1.14e-14 s in the single second between each broadcast 'tick'.
However, GPS is subject to the same vagaries of ionospheric conditions, as well as error in signal-interpretation. It has a theoretical accuracy of 14 ns, but more typically it is 100 ns.
No doubt the received accuracy of both WWVB and GPS could be improved by frequently collecting and applying the appropriate ephemera corrections for a given geographical location.
If it weren't for deadlines, nothing would be late.
For some time I plotted the jitter of reception of DCF-77 (a similar transmitter in Germany) and I found there was a clear cycle of increase and
decrease of the jitter of the pulses output by my receiver (measured over one minute) over the day.
At daytime the jitter is around 20us, at nighttime it is more like 200us.
This is most likely explained by path length variations that apparently are depending on propagation.
(although texts about such transmitters often boast that there is no propagation effect like the one seen at shortware at those frequencies)
The claimed accuracy is of course at the source, and maybe when you started receiving WWVB years ago and perform some kind of averaging
over a long interval, you could eventually get an accuracy like that, but there is no way it can be achieved over short intervals, let alone for
individual second pulses.