Geology Reference
In-Depth Information
ellipsoid must also be specified and that positional shifts of several hundred
metres can be introduced by switching between commonly used versions.
As an example of the importance of this effect, selecting WGS84 instead of
the AGD66 ellipsoid on which the maps of the Bulolo goldfields of Papua
New Guinea are based, would not merely move observers some 200 metres
laterally but put them on the wrong side of the wide Bulolo River!
15.2 SATELLITE NAVIGATION
Satellite navigation is now being used for applications as diverse as personal
navigation devices in smart phones, air and marine traffic control, and guid-
ing ploughs and harvesters. The major users outside the United States are
consequently reluctant to rely entirely on the goodwill of the US military,
and some of the alternatives to GPS, the US system, are expected to become
important during the lifetime of this edition.
15.2.1 Navigation satellites
As the uses of navigation satellites have multiplied, so have the acronyms.
Currently four alternatives to GPS are in various stages of development.
These are Russia's Global Navigation Satellite System (GLONASS), the
European Union's Galileo, China's COMPASS and the Global Indian Nav-
igation System (GINS). Apart from GPS, only GLONASS is currently op-
erational. Galileo and COMPASS should be operational by 2013 and 2015
respectively. The systems are being designed to have some compatibility,
so that it will in theory be possible for signals broadcast from any of the
over 100 satellites to be received by all users, significantly improving both
availability and accuracy.
15.2.2 Locational accuracies
The simplest GPS receivers use only
code-phase
measurements, with dis-
tances from the satellites (
pseudo-ranges
) deduced from the times it takes
unique Pseudo-Random Codes (PRCs) generated by each selected satellite
to reach the measuring point. At least three satellites are required to obtain
a positional (XY) fix, and four if elevation is also required.
Small and reasonably cheap hand-held GPS receivers have been avail-
able since about 1990, but their accuracy was originally no better than a
100 m in position and even less in elevation because of deliberate signal
degradation, known as Selective Availability (SA), implemented by the US
military. On 1 May 2000 the US government turned off SA on all GPS satel-
lites and the positional accuracy on stand-alone receivers improved to a few
tens of metres. The readout precision, for both elevations and co-ordinates,
