I can't stand puzzles. A jigsaw puzzle
that others are working on over a Christmas vacation, I might put
in a token piece. Little wooded puzzles (like the one seen here), I refuse to touch. I recently
decided that I do enjoy puzzles: I enjoy designing systems. Systems
(like computer systems, a piece of software, a web application,
etc.) have specific problems to solve, constraints (usually
technological, often psychological, sometimes political as well).
They are some of the more complex puzzles and I quite enjoy
them.
One morning, I found myself wondering how the US Atomic clock
works this morning. More specifically, the synchronization
part.
I have a clock in my home that receives the signal from Denver /
Boulder, CO and keeps its clock correct. I had never really thought
about -how- that works, until this morning. How does it work?
I treated it as a problem that I might be designing a solution
for. I started with some of the constraints and usages of the
"system:"
- The needed receiver electronics must be small
- They must be cheap (cost of $20 / unit)
- It doesn't require any connection (to power, internet, or
other)
- It requires the user to input their timezone (I couldn't
remember if this was the case - did this just "figure out" your
timezone?, I assumed that I had entered it at some point)
I started with my long-held belief that
this was some sort of shortwave / HAM radio message. Shortwave can
travel a long way, as I learned early in my life, but the delay
could be a problem for synchronizing time. I did some mental
calculations based on radio waves traveling at the speed of sound
(which, for some reason, my brain has permanently lodged in my
brain from physics class: 340 m/s) and figured that depending on a
variety of factors, there might 10-12 seconds of delay in a best
case scenario (don't get ahead of me on this one...). I've also
experienced the slowdown of radio waves - talking on a phone via
radio has horribly slow delays (it was the 80s, I was in South
America, technology was different then).
So then, how might it work?
I thought of a few possible solutions:
- You measure decay of the signal to obtain adjustment /
correction.
This would require a signal of known strength (easy), an accurate
measurement (hard - remember the "cheap" requirement), and the
atmospheric interference to be negligible (I don't know, but I
doubt this is true)
- Send multiple messages
If each radio frequency behaves differently (to atmospheric or
other interference that would slow it down), then from the delay on
2 signals coming along different frequencies, you might be able to
accurate obtain decay information and therefore calculate the
distance & correct for it.
This again requires accurate measurement which is likely not
cheap.
I was stumped. But it was a fun exercise.
Google, of course, had the answer (though not in Wikipedia, nor
as easily found as I would have imagined - all about the Atomic
clock, Cesium-133, etc. but little about the signal itself).
Radio waves travel not at the speed of sound, but as all
electromagnetic radiation (microwaves, television waves, waves at
wi-fi frequencies, and light), at the speed of light. It's a bit
higher than the 340 m/s (300 Mm/s). So much for science &
calculations done in the shower. BTW, the signals are broadcast at
60 kHz VLF.
Such a speed means that you simply send a radio signal with the
current time and receive it - no delay and therefore, no
calculation required.
The phone delay I remembered, I'll chalk up to it being old
technology and the delay was likely in the intermediary electronic
processing or the electronic signal rather than the "radio"
portion.
While searching for the answer, I also stumbled across a variety
of interesting items. The most interesting might be the adventures
of a family who, armed with very precise atomic clocks, went up a
mountain to compare the speed of time at a higher elevation
(Einstein's special theory of relativity suggests that they should
- 22 ns over a weekend): http://www.leapsecond.com/great2005/
My point in relating this little story? Well, if I were Aesop, I
might say that
But really, I just enjoy designing systems and I thought you
might too.