Geoscience Reference
In-Depth Information
in the pursuit of European autonomy (GPS could in principle be turned off at any
time by the US military) the European Union is implementing a European equivalent
under the name of GALILEO.
In practice, the position of a GPS receiver is measured relative to the tracking
stations that determine the orbits of the satellites; the positions of the tracking stations
are measured accurately on the Earth's surface, the Earth's surface is measured
accurately relative to the center of the Earth, the center of the Earth is located among
the satellites, and the satellites and the Earth are located relative to quasars.
It is easily possible to measure the position of a radio receiver to an accuracy of
a few meters with the GPS system and even possible to determine the position of
one GPS receiver relative to another to millimeters if enough care is taken. The GPS
system is used by ships, taxis, delivery drivers, tanks and missiles to navigate to
their destinations. Because of the obvious military interest in GPS, the accuracy
with which GPS satellites broadcast their orbits to ordinary civilian users is delib-
erately worsened by its operators (this is called Selective Availability). In its highest
form of accuracy, though, it is possible with GPS to measure continental drift and
has been used (after considerable scientific effort) to measure how the Indian sub-
continent is pushing into Asia and raising the Himalayan Mountains. In time the
GALILEO system may be sufficiently easy to use, accurate and robust enough to
develop into a means to navigate aircraft through the air from take-off to landing
under automatic control.
If the GPS system measures latitude and longitude, however, what latitude and
longitude does it measure? Its system has a virtual Greenwich Meridian as a zero
point, a result of the exact definitions that it uses for the fundamental parameters of
time, position of the Earth, positions of the fixed quasars, etc.; the GPS system is
called the World Geodetic System, WGS84 and uses as its prime meridian the one
defined by the Bureau International de l'Heure (BIH) at the moment of midnight
on New Year's Day 1984. It also uses various conventions about the shape and rota-
tion speed of the Earth to account for its asphericity and wobbles.
This amazing system enables positions on the Earth's surface to be located very
accurately and under precise assumptions making it possible to detect changes. In
Britain, the latitude and longitude of a given place change relative to WGS84 at the
rate of 25 mm per year. Some parts of the world are moving even faster; Australia,
for example, moves at 1 meter per decade. In saying what the latitude and longitude
of a place are to a high accuracy, it is necessary to say at what date (
epoch
) the
coordinates apply.
Independently of the United States military, scientists have agreed to construct
a system of 500 reference stations around the world whose positions in WGS84 are
accurately measured, including the effects of continental drift. The measurements
are combined by BIH into a positional network called the International Terrestrial
Reference Frame (ITRF). A sub-section of this is maintained in Europe as the
European Terrestrial Reference System (ETRS89). It was identical with WGS84 in
1989, as the name suggests, and has altered since then due to continental drift. It
may be thought symbolic that ETSR89 is drifting eastwards relative to WGS84 or
that Europe is moving apart from the Americas.
