Global Positioning System Reference
In-Depth Information
motion of other members of the solar system, was employed for a couple of
centuries after the invention of telescopes to determine longitude, as we
will see. (The phases of the observable moons of Jupiter were used to syn-
chronize astronomical observations taken at di√erent points on the earth.)
The third and fourth methods are entirely modern. Certain atoms vi-
brate at a fixed frequency and so can be harnessed to provide timekeeping
that is independent of the outside world. (Well, not quite independent,
Einstein would remind us.) Pulsars are fast-spinning neutron stars that
emit beams of very-high-energy electromagnetic radiation. They pulse at a
very regular rate, so regular that they rival atomic clocks.
Thus, time and position can be measured very accurately, enabling us
to closely monitor distances, speeds, and rotation rates. Geodesy has ex-
panded its applications as a result of this increased capability. The old aims
still apply: geodesy still deals with the measurement and representation of
planet Earth and its gravitational field, and it still is used for mapping.
However, geodesy is today applied to other fields such as ecology (the
movement of ground due to melting ice caps, mining, or waste disposal);
engineering (e.g., for the planning of dams and the headwaters they cre-
ate); urban management; geophysics; and hydrography. As accurate moni-
toring and measuring of our planet continues and improves, the applica-
tions of geodesy are likely to expand.
