How An Atomic Clock Works And its Use in the Global Position System (GPS)
I want to show you the most amazing thing.
The world's first commercially available chip scale atomic clock.
Symmetricon's CSAC.
That's right: This tiny device
about the size of a quarter
is an atomic clock.
The most accurate atomic clocks lose about
a second over 138 million years.
The way that atomic clocks work amazes me
let me explain how the very first one worked.
I'll start with Jell-O.
Tap a block of Jell-O and it wiggles back and forth.
Just like the swings of a pendulum in a grandfather clock
the oscillations of this Jell-O keep time.
Now, Jell-O isn't very good for this
but inside an atomic clock there's a chunk of quartz
of a similar shape that it if we tap it
which we do it with a jolt of electricity
it will oscillate some five millions times per second.
It keeps time to about 1 second in 90,000 years
a fraction of the accuracy needed for an atomic clock.
Quartz loses time because it slows down and needs
to be "nudged" to restore its oscillation.
There's where the “atomic” part of an atomic clock comes into play.
We use cesium atoms to control those nudges very accurately.
Every time the quartz's oscillations slow down
just the tiniest bit we give it a tap
an electrical jolt at just the right time
so essentially its oscillations never decay.
Let me show you how we use cesium to do this.
The atoms in pure cesium exist mostly in
two slightly different forms
A low energy form and one with just a bit more energy.
For an atomic clock these two states have two properties
critical to making a clock.
One, they can be separated by a magnet.
And two, the lower energy atoms can be converted
to the higher energy ones if we bombard cesium with the right radiation.
Engineers tie the slowing down of the quartz vibrations
to the precise wavelength of the bombarding radiation
to create a feedback loop.
Let me show you how.
In an oven we heat cesium chloride to create
a gaseous stream of cesium ions.
The stream contains both the low and high energy ions.
We first flow it through a magnet separating the two types,
discarding the high energy ones, allowing the lower energy ions
to pass into a chamber.
Inside the chamber we bombard the ions with
just the right wavelength radiation to make them jump to higher energy.
As these gaseous ions leave the chamber they pass through
another magnet that directs high energy ions toward a detector
this time discarding any lower energy ones.
The detector converts the arriving ions to a current.
The trick here is to tie that current from the detector
to the quartz oscillator.
When the quartz's oscillations decay
that is it slows down a little
then the energy bombarding the cesium ions
in the chambers changes and fewer high energy
ions exit the chamber, so current decreases or stops.
This tells the electronics to "zap" the quartz
oscillator and correct the period of oscillation.
It does this by applying the proper voltage that
via the piezoelectric effect, taps the quartz
and restores its oscillations.
Thus creating a clock that loses less
than a second over many million years.
Our world runs off such accuracy.
For example, the global position system (GPS) requires it.
The global positioning system consists of
24 satellites orbiting the earth.
A GPS receiver uses the position of four
of these satellites to locate itself.
One to correct the time on the receiver
and three to locate its position.
Here's how it works.
A signal is sent to the receiver from the first satellite
that contains that satellites location
and the signal's time of departure.
The receiver then multiplies the signal's travel time
by the speed of light to calculate its distance from the satellite.
With one satellite the receiver knows that
it's located on a sphere around that satellite
with a radius equal to the calculated distance.
So, it does the same calculation with a second satellite.
The intersection of these two spheres narrows
the location to the circumference of a circle.
Then with a third satellite the receiver can reduce
the location to a single point.
Since signals are traveling at the speed of light,
being off by even a millisecond means an error
of about a million feet, or 300 kilometers.
But with atomic clock accuracy
the receiver can locate itself to about 3 feet.
I'm Bill Hammack, the engineerguy.
This video is based on a chapter in the book
Eight Amazing Engineering Stories.
The chapter features more information about this subject.
Learn more about the book at the address below.