Global Positioning System Reference
In-Depth Information
if they are very directional. Any objects that are moving such as road traffic and
objects moving in the wind will also “stir up” the standing waves.
6.1.7
Signatures and Fingerprints
Averaging techniques are the only way to manage the local variability, and a very
effective idea is that of collecting radio signal-level signatures or fingerprints.
Although a route through an environment will have a widely varying level at any
point, on average it will fall within the fade margin. If these margins are collected
along a route, a signature for that route can be found, particularly if the route is
traversed many times. During a particular journey, it is thus possible to compare
the received signal profile with the signature to give an estimate of location.
There are statistical algorithms in the form of filters (i.e., systems that take an
input and produce an output that is a function of the input). The Epilogue includes
details of the Kalman filter and particle filter that are widely used.
Having looked at general principles we will now consider the positioning
potential of the major radio systems available to consumers and businesses.
6.2
Types of Current Radio Systems
6.2.1
System Issues
The section above considered the common basic characteristics of radio relevant
to positioning but now we shall consider the specifics of the more important radio
systems. The sections will be listed in order of range, starting with satellites in
Earth orbit and ending with NFC systems of extreme proximity. There are so
many performance trade-offs that it makes little difference in which order to list
the systems since no one is inherently any “better” that any other. Range of signal
is useful, however, since it brings into focus the very important system issue of
networking. In general, the longer range radio systems are more complex since
they involve multiple base stations, network control systems, multiple frequencies,
and international issues. Indoor low range systems are inherently local and less
expensive (but not necessarily less complex, especially in the future).
The most basic radio system is a simple one-to-one link involving a single
transmitter and single receiver (e.g., a simple telemetry link such as a remote
wireless door bell). The very specific nature of the applications can be useful to
Whereness as it is a means of activity and context confirmation.
Broadcasting, or one-to-many links, is useful and includes systems such as
aircraft navigational beacons that any number of pilots can use simultaneously.
Some interesting new positioning technologies can be piggybacked on broadcast
systems [6] such as local digital TV stations.
