Overview of Whereness - Ubiquitous Positioning - page 26

2.4 Radio Positioning
2.4.1 Communications and Sensing
Radio systems are fundamental to Whereness for two main reasons. First, radio
provides a means to communicate position, and second, it can be used as a sensory
system either with or without the communications functionality. For those
systems that are intended for communications, the sensory aspects may be an
added bonus. WiFi is a good example of a radio standard that is used for
communications but is underused in its potential as a sensory positioning system.
2.4.2 Transparency
An advantage of positioning systems based on radio is that the signals propagate
through many materials and environments with useful ranges that can be colossal.
Most other approaches such as optical and acoustic sensing require an un-
occluded line of sight between transmitter and receiver.
2.4.3 Far-Field and Near-Field Communications
Conventional radio frequency systems use the electromagnetic far field that has
useful propagation ranges constrained only by the power available, the
environment, antennae considerations, and regulations. In contrast, in the last two
decades some very short range systems have been developed using near-field
communication (NFC) for RFID tags. Ranges are typically from a few millimeters
to 2 meters with the near field being used to transfer power as well as information.
2.4.4 Useful Radio Characteristics
Although radio characteristics such as frequency, modulation methods, and
system architectures vary greatly, there are two basic methods to find location.
The simpler involves measuring the strength of the signal, the power of which
falls as the distance between transmitter and receiver increases, and its most basic
form is simply the presence or absence of detectable signals. A more complicated
and (potentially) more accurate approach measures the signal propagation times
and angles.
Additional system elements may also be helpful. For example, in the GSM
cellular radio system, the mobility management subsystem that allocates the radio
resources contains location registers to keep account of the approximate location
(by proximity) of the mobile stations. Known generally as “Cell ID,” this
information can be used for other purposes such as applications to detect fraud or
to offer travel advice based on location.