Invisible Stars - GPS Declassified: From Smart Bombs to Smartphones - page 79

the world discovered the awesome energy unleashed by splitting atoms when
bombs destroyed Hiroshima and Nagasaki, but atomic clocks involve no fis-
sion or radiation. In 1949 nbs physicists announced they had synchronized a
quartz-crystal oscillator to the natural resonant frequency of ammonia atoms.
They did this by shooting high-frequency microwaves of about 24 billion Hz
(24 gigahertz [GHz]) through a thirty-foot coiled copper tube filled with ammo-
nia gas, causing each ammonia molecule's single nitrogen atom to flip its posi-
tion among three hydrogen atoms, like an inverted pyramid. 41 They could not
see this happen; they measured the effects. At frequencies too high or low, the
microwaves passed through the chamber, hitting a detector that adjusted the
frequency until at 23.8 GHz the gas absorbed the microwaves. This ammonia
device was accurate to within one second in eight months. 42
Atomic clocks (scientists prefer the term atomic frequency standard , or afs)
got smaller and more accurate over time. Cesium, a mercury-like metal that
liquefies just above room temperature, replaced ammonia as the preferred
atomic frequency standard. 43 Instead of shooting microwaves through a cloud
of molecules, second-generation atomic clocks shot a beam of vaporized cesium
atoms through a microwave signal and compared the energy they contained
to another stream that was diverted around the microwaves by magnets. In
1952 a cesium clock achieved an accuracy of one second in three hundred
years—for the first time dividing a second into billionths, creating the nano-
second. 44 Cesium clocks later achieved an accuracy rate of one second in
twenty-ive hundred years, and in 1967 the standard international second was
redefined as the resonant frequency of cesium atoms, replacing the astronom-
ical definition based on a fraction of the solar day. 45 In the 1960s Hewlett-
Packard developed a line of 180-pound cesium clocks portable enough to
transport in planes to synchronize timekeeping facilities around the world. It
was about this time that Roger Easton at the Naval Research Laboratory envi-
sioned placing those clocks in the air continuously aboard satellites.
The gps constellation at this writing uses five cesium and twenty-six rubid-
ium atomic clocks. 46 Rubidium is a silvery-white metal that is easy to vaporize.
Clocks using rubidium in newer gps satellites have demonstrated superior
performance and longer lives than cesium clocks. 47 Although newer clocks are
superior, the military conservatively rates all clocks in the constellation to be
accurate to within one hundred nanoseconds over any three-second interval. 48
Accuracy on the ground depends on a variety of factors, including atmospheric