Overview of Whereness - Ubiquitous Positioning - page 28

2.5 NonRadio and Sensor-Based Positioning
2.5.1 Communications Cable Contact
There are many methods that can be used to find position and location that do not
use radio waves and some could be considered rather obvious. First, the act of
communicating by cable (i.e., using an electrical circuit or an optical fiber) can be
used to show that the users are present at the termination. Telephone areas are a
crude location system. A major problem now exists for emergency call centers
since the new Internet Voice over IP (or VoIP) has no association with a line so
cannot provide a location code. Clearly, it requires a Whereness service provided
by some other means.
2.5.2 Electronic Diaries and Calendars
A second obvious location method is to rely on a diary or calendar entry. If
someone has indicated that he or she plans to be somewhere then there is a chance
that he or she will be there. If a wireless or sensor system can confirm the
presence in some way, then the probability increases greatly.
2.5.3 Infrared
Some of the early experiments in indoor positioning used infrared (IR)
communications. Since all optical systems have the disadvantage of needing a
clear optical path between the transmitter and receiver, these experiments used
electronic badges that had to be worn in a visible fashion. An advantage of these
badges is the extreme cheapness and simplicity of using a IR light-emitting diode
(LED) as the transmitter. Networks of fixed IR receivers can be fixed within
rooms so that when a target enters the room, the code transmitted by the badge can
be detected and a central database updated.
2.5.4 Ultrasonics
More advanced badge systems followed, substituting ultrasonics for the infrared
devices. Although still needing a clear path, the use of sound has an advantage
over any electromagnetic communications. The problem with wireless and optics
is that signal propagation is at the speed of light (300.10 6 ms -1 ), which makes
timing measurements a serious technical challenge involving detecting
nanosecond time periods. The speed of sound is six orders of magnitude slower
and timings are thus well within the capability of the cheapest microprocessors.
With ultrasonic systems centimetric positioning accuracy is both feasible and
inexpensive. Range, however, is limited and to cover a building hundreds or even