On any given day, the Navy’s newest and most precise clocks might gain or lose 50 picosec- onds. At that rate, the clocks would creep
one second ahead or lag one second behind every 300
million years, said Steven Peil, a Navy physicist who
helped develop the exotic chronometers.
To most people, that wouldn’t matter. A picosecond
— a trillionth of a second — and even the much larger
nanosecond — a billionth of a second — is too brief to
be noticed. Except at the U.S. Naval Observatory.
There, “we actually care about how many picoseconds it loses per day,” Peil said. That’s because extremely precise timing has become extremely important to the U.S. military, and the Navy is the military’s
official timekeeper. It has been since 1854.
Last summer, RADM Jonathan White, oceanographer
of the Navy, declared an ensemble of four “rubidium foun-
tain clocks” at the Naval Observatory in Washington to be
fully operational. CAPT Tim Gallaudet, the Observatory’s
superintendent, described the rubidium fountains as “the
most precise operational clocks in the world.”
Their picosecond-level precision will help improve
navigation, communication, network operations, data
fusion and other functions that
depend on ultra-precise timing,
Navy officials say.
Perhaps the most immediate need
for the rubidium fountain clocks is
GPS III, the next-generation Global
Positioning System. The first of 32
new GPS satellites is expected to be
launched this year.
For GPS, precise timing is
essential. As the satellites orbit
12,600 miles above Earth, each
transmits data about its current
location and the current time. GPS
receivers on Earth use these signals
to calculate their own locations.
The satellites’ radio signals trav-
el at the speed of light — 186,282 miles per second. A
GPS receiver needs signals from four satellites to accu-
rately calculate its location. Since each satellite is in a
different orbit, each is a different distance from the
receiver. So each satellite’s signal arrives at a slightly
different time. Based on those time differences, the
GPS receiver can calculate its distance from each satel-
lite. And once it knows the distance to at least four
satellites, the GPS receiver can calculate its location.
At 186,282 miles per second, light and radio waves
travel 11. 8 inches per nanosecond. With clocks that are
precise to the nanosecond or better, it will be possible for
GPS receivers to compute their location on Earth to
within about a foot, Peil said. That’s important for navi-
gation and even more so for precision munitions.
Ultra-precise timing also is critical in digital communi-
cation. In digital networks, data sent between phones,
radios and computers is chopped into small packets that
are transmitted individually, making it possible to trans-
mit much more data over a given frequency or channel.
“To fit as many users on a channel as you can, you
divide up the communication into smaller and smaller
units,” Peil said.
Down to the Picosecond
Ultra-precise timing will improve navigation, communication and data fusion
By WILLIAM MATTHEWS, Special Correspondent
The U.S. Military’s Timekeeper
The U.S. Navy — in particular the U.S. Naval Observatory in Washington — has been the military’s official timekeeper since 1854.
■ Rubidium fountain clocks, declared operational last summer,
are the latest in a 169-year series of improvements in timekeeping technology by the Observatory.
■ The most immediate need for the rubidium fountain clocks is
the next-generation GPS (Global Positioning System) III, with the
first of 32 new GPS satellites expected to be launched this year.
■ GPS III will be important for navigation and precision munitions.
Ultra-precise timing also is critical in digital communication.
WWW.SEAPOWERMAGAZINE.ORG 44 SEAPOWER / MARCH 2014