Global Positioning System Reference
In-Depth Information
2.4.5
Global Navigational Satellite Systems (GNSS) and GPS
By far the most popular radio positioning system is GPS. It is one of a number of
existing and planned Global Navigational Satellite Systems (GNSS) and is
provided by the U.S. Department of Defense, free for civilian use at reduced
accuracy (about 10m) from that used by U.S. government agencies and their
allies. GPS does suffer from a number of inherent problems, however. At least
four satellites must have a clear line of sight to the receiver, which is not always
possible in dense urban areas and inside buildings; it can be slow to synchronize
so readings only follow power up after several minutes; and there are political
considerations. Increased performance may be achieved by combining the basic
GNSS with additional systems.
2.4.6
Cellular Positioning
There have been many proposals for enhancements to cellular systems to provide
more accurate services than Cell ID. Most are based on the knowledge of signal
timing, which is helped by the fact that digital radios send information in data
packets that are inherently sequenced, highly organized, and time-dependent. It is
unlikely that cellular positioning will ever be as accurate as GPS, but it does have
the advantage of working indoors and in other areas where GPS signals are
difficult to receive.
2.4.7
WiFi Positioning
WiFi positioning is a very useful and increasingly popular method. Most
commercial WiFi positioning systems available today use simple proximity and
are usually combined with communications. An interesting development is the
amateur activity of wardriving, which is mapping WiFi access point location using
GPS and providing the maps freely.
2.4.8
Ultrawideband Positioning
The most exciting new radio technology for positioning uses ultrawideband radio
(UWB), which is classified as a spread spectrum system. It can be thought of as
an indoor secondary radar system. Transponder tags transmit very lowpower
nano-second duration pulses, which are inherently wideband in that they spread
widely across the radio spectrum. Since the duration is so short, the timing
accuracy can be very high leading to positioning accuracies in three dimensions of
typically 10 cm.
