Invisible Stars - GPS Declassified: From Smart Bombs to Smartphones - page 78

satellite navigation could become a reality. Just as pendulum clocks were unsuit-
able for use at sea, the first timekeeping approaches that followed pendulums
were not accurate enough to transmit precise time across the vast distance
from a satellite in orbit to a user below. To appreciate how precise the time sig-
nal must be for gps, consider that an error of one nanosecond—a billionth of
a second—results in an error of about one foot. 33 An error of two microseconds—
two millionths of a second—can send a speeding jet of course by a third of a
mile. 34
Pendulum clocks, with the addition of a second pendulum, reached their
performance limit by the early 1920s, keeping accurate time to within ten sec-
onds a year. 35 However, the growing radio industry, which uses frequencies in
the millions of cycles per second, needed more precisely measured seconds
to prevent one station's signal from drifting over another's. 36 The National
Bureau of Standards (nbs; renamed the National Institute of Standards and
Technology, or nist, in 1988) searched for a material that would vibrate—
oscillate—at a constant rate, like electrically driven tuning forks of the era, but
much faster. The answer was to apply an electric current to a thin quartz crys-
tal. Scientists learned to make quartz crystals vibrate at high frequencies that
varied depending on the size and shape of the crystal. Quartz oscillators became
the primary frequency standard at nbs, which set up four 100-kHz quartz oscil-
lators that achieved accuracies of about one second in three years on average. 37
By 1950 quartz accuracy had improved by a factor of one hundred, and crystal
oscillators replaced pendulum clocks at nbs as the primary standard for time
intervals. 38 Quartz oscillators have some drawbacks. No two crystals are exactly
alike or have identical resonant frequencies, and they wear out. Their frequency
changes slowly due to aging and environmental variables such as temperature,
humidity, and pressure. 39
Scientists turned next to the atom. Physicists knew that atoms absorb or
emit energy at specific frequencies, giving each atom a resonant frequency.
Atoms represented an oscillator that would never waver or wear out, and all
atoms of a given element would be identical. News of their potential use in
clocks produced wonderment typical of atomic age discoveries. “'Cosmic Pen-
dulum' for Clock Planned: Radio Frequencies at the Hearts of Atoms Would
Be Used in Most Accurate of Timepieces,” the New York Times announced on
January 21, 1945, a day after Nobel Prize-winning physicist I. I. Rabi proposed
the idea during a talk to the American Physical Society. 40 Seven months later