Geoscience Reference
In-Depth Information
and the southern hemisphere as part of the cycle of summer and winter. As a result,
the stars that are seen above the Prime Meridian at any given moment, according to
a uniformly beating clock, run either late or early.
The direction of the Earth's axis also changes, wobbling rather like the effect of
a sloppy bearing of a rotating machine. Each year the north pole of rotation describes
an anticlockwise circle in the Arctic of radius about 10 meters, and the center of the
circle drifts westward into Canada at the rate of 15 meters per century. Additionally,
continental drift at the rate of millimeters per year moves places relative to the rota-
tion of the Earth. All this means that the system of latitude and longitude, defined by
the axis of the rotation of the Earth and the position of the equator, changes and a
given place on the Earth adjusts its latitude and longitude with time.
These variations in latitude and longitude started to become very clear when it
became possible to compare the positions of places on the Earth and of artificial space
satellites. The potential for using satellites to find position became obvious right at
the dawn of the space age, with the launch of the first space satellite, Sputnik, in 1957.
This satellite carried a beeping radio transmitter, whose sound became famous and
was reproduced around the world as proof of the satellite's existence. The beeping
could be heard as the satellite orbited across the sky, and its frequency was altered
through the Doppler Effect as the satellite approached a radio receiver, passed over-
head and then receded. In order to be able to calculate and predict the position and
the radio frequency characteristics of the Sputnik satellite, scientists had to input the
position of their receiver. It was obvious that you could reverse the problem - if you
know the position of the satellite you can calculate where you are.
NAVIGATIONAL SATELLITES are based on the principles described above.
Their orbits are determined in space relative to the positions of what used to be
called the “fixed stars.” The stars are not really “fixed,” but move relative to each
other as they orbit the Galaxy. Modern astronomers use a better substitute, gener-
ally the same idea but more accurately, by using quasars instead of stars. Quasars
are exploding galaxies, far away across the Universe and they are certainly not fixed
in space but move at high speeds relative to one another. However, any motion they
have is diminished by their enormous distance and they form a “fixed” backdrop
against which to position the satellites. Think of lying in the grass looking up into
the sky. A bee buzzes quickly past your nose and flashes across your field of view,
apparently at high speed. But in the same field of view is an airplane, moving much
faster than a bee, but at such high altitude that it seems to crawl by comparison with
the bee. The airplane is almost fixed. So too are quasars, the positions of which are
measured by radio telescopes of a highly sophisticated nature, while the satellites
are measured by a global network of tracking stations.
The orbits of the satellites in space are measured accurately from Earth and the
orbital information is uploaded into the satellites' computers. It is coded into radio
transmissions from the satellites, so that each one tells you where it is and where it
is going to be. The satellites also broadcast timing signals, and master clocks kept
at the US Naval Observatory control clocks on the satellites which then broadcast
signals in unison. The delays as the timing signals travel from the satellites to a
receiver determine both the time at the receiver and the position of the receiver relative
