IEEE Spectrum December, 2017 - 36

For Fridtjof Nansen, 13 April 1895 started well.
Six days earlier, the Norwegian explorer had set a new record for the closest approach to
the North Pole, and now he was moving quickly over unbroken sea ice toward Cape Fligely
and home. But then came a sickening realization: In his eagerness to break camp, he had
forgotten to wind the chronometers. He had lost track of precise time, and thus the ability to track his longitude. * Although Nansen couldn't have lost his position by more than
a few minutes, it forced him to take a circuitously conservative route to avoid being swept
into the North Atlantic. His expedition thus had to endure a hungry winter, camped on an
unknown shore. Not until June the following year did he encounter other explorers and
learn his true position-on Cape Felder, in Franz Josef Land.

Today, anyone with a smartphone can determine their
time and position with ease. Satellites of the Global Positioning System (GPS) broadcast clock signals across the
globe with uncertainties below 100 nanoseconds, or one
ten-millionth of a second. These time signals carry the
information needed for precise navigation: Because radio
waves travel at exactly 0.299,792,458 meters per nano-
second (apart from minuscule variations due to refraction in the atmosphere), comparing signals from different
satellites makes it possible to determine a position within
a few meters. That's why GPS has transformed seismic
monitors, drone delivery, and many other applications.

Reference
medium

Detector

Frequency

Feedback signal

36  | DEC 2017 | NORTH AMERICAN | SPECTRUM.IEEE.ORG

NATIONAL PHYSICAL LABORATORY

Oscillator

Absorption

ATOMIC CLOCKWORK:
An atomic clock begins with
an oscillator [see diagram],
which creates a frequency
close to the energy level of
the atom being used. If the
oscillator deviates from the
reference frequency, the atom's
absorption pattern changes,
the change is detected by a
laser, and the laser's signal is
used as feedback to tune the
oscillator. For the very best
performance, the atoms must
be electromagnetically isolated,
which requires equipment that
can take up entire rooms [see
photo, right].

But GPS can't solve all timing problems. Central to
the system are atomic clocks carried on each satellite.
Although these clocks are extremely stable (and regularly
calibrated by comparing them with ground-based atomic
clocks at national standards laboratories), there are many
ways to go wrong when transferring timing information
to the user-jamming, spoofing, unintentional interference, solar storms, even reflections from buildings. But
what if we could put this precision directly in the hands
of the user by shrinking the atomic clock itself so it could
work inside the GPS receiver? Would we, like Nansen, then
want to carry our very best clocks with us?



http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum December, 2017

IEEE Spectrum December, 2017 - Cover1
IEEE Spectrum December, 2017 - Cover2
IEEE Spectrum December, 2017 - 1
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IEEE Spectrum December, 2017 - Cover3
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