Each packet receives a destination address so that it
gets to the right place, and a time-stamp so that all of
the packets can be reassembled in the right order to re-create the original message. The more finely a clock can
divide time into billionths and trillionths of seconds,
the more data can be transmitted.
The four rubidium fountain clocks now operating at
the Naval Observatory have increased the Navy’s timing
precision 100-fold, Peil said. But they don’t look a bit like
clocks. They are big silver cylinders and could easily be
mistaken for oversized stainless steel water heaters.
At the heart of each clock is a small vacuum chamber
filled with rubidium atoms. Here, lasers cool the atoms
to “a few millionths of a degree above absolute zero,”
Navy documents explain.
Then a “cloud of atoms” is launched up through the
center of the clock. Near the top of the clock, the atoms
enter a cylindrical chamber called the “drift region.”
There, they begin to spread out and fall like water drops
sprayed into the air by a fountain. The atoms fall through
the drift region for about half a second.
As they travel through this trajectory, the rubidium
atoms pass through two microwave cavities where pulses
of microwave energy cause some of the atoms to oscillate, or jump, between two energy states — a ground
state and an excited state. Lasers detect how many of the
atoms have transferred to the excited state. This is a precise way of measuring the atomic frequency of rubidium
atoms — 6. 8 billion oscillations per second. That frequency serves as a new pendulum for measuring time.
Although the rubidium fountain clocks are considered
the most stable in the world, they alone do not establish
time for the military. That’s done by a “master clock” that
is made up of dozens of very precise clocks, Peil said.
The “total clock ensemble” at the Naval Observatory
includes two dozen hydrogen maser clocks, 70 cesium
atomic clocks and, now, the four rubidium fountains,
Peil said. The different clocks make different contributions to measuring time.
“The masers have very good short-term stability,”
That is, they are very accurate for periods shorter
than about a day. The cesium clocks offer better long-term stability. And the rubidium fountains are more stable still. A single rubidium fountain “is better by a factor
of 10 than all of the cesiums combined,” Peil said.
To establish precise time, Navy timekeepers compile
an average of the outputs from of the masers, the
cesiums and the rubidium fountains.
There’s an alternate master clock, too, a smaller collection of cesium, hydrogen and rubidium clocks that tick
away quietly at Schriever Air Force Base in Colorado.
Precise as it is, the rubidium fountain is not the end of
clock evolution. Next-generation ultra-precise clocks
already are subdividing picoseconds in a handful of laboratories around the world. They’re called “optical clocks.”
Instead of using microwaves to make atoms oscillate,
these clocks use laser light. Microwaves and light are both
part of the electromagnetic spectrum, but visible light has
much shorter wavelengths, thus it can make certain
atoms oscillate thousands of times faster than microwaves
can. And the faster the oscillations, the more accurately
time can be divided into smaller and smaller segments.
Some scientists contend that optical clocks might
lose or gain one second every 3. 7 billion years — just
a little shorter than Earth has existed.
“Optical clocks will be better than fountain clocks,”
Peil said. “But there is a lot of work to be done to get it to
the point where one can be used as an operational device.”
The rubidium fountain clocks are the latest in a 169-
year series of improvements in timekeeping technology
by the Naval Observatory. In 1845, the secretary of the
Navy ordered the Observatory to install a “time ball” on
a pole atop the building housing a 9.6-inch telescope.
The ball was to be dropped down the pole each day precisely at noon so that ships on the Potomac River could
set their chronometers before putting out to sea.
By the end of the Civil War in 1865, a time signal was
being transmitted via telegraph from the Observatory to
the Navy Department downtown. By the early 1870s,
the time signal was being transmitted nationwide via
Western Union telegraph, in part to provide accurate
time for the nation’s expanding railroads.
In 1904, the Navy began transmitting time signals by
radio and, by mid-century, citizens were able to dial an
Observatory number on their rotary phones to learn the
time every 15 seconds. That service continues today, although rotary phones have long since disappeared. ■
WWW.SEAPOWERMAGAZINE.ORG SEAPOWER / MARCH 2014
Personnel with the U.S. Naval Observatory-Detachment
Colorado and 2nd Space Operations Squadron move the rubidium fountain clock into its new home Feb. 4 at Schriever
Air Force Base. The U.S. Naval Observatory monitors the
Global Positioning System constellation and provides time
offsets to the 2nd Space Operations Squadron for its daily
navigation uploads to each individual satellite.