Global Positioning System Reference
In-Depth Information
be used to indicate exact alignment that can be made to within a few centimeters.
Tags used in wet situations, which are encountered in both industrial settings and
also when tags are embedded within living tissue (for example, with animal tags),
require the use of a much lower frequency band. Low-frequency (LF) tags operate
at 150kHz, and significantly above this frequency passage through water
attenuates signals and makes them unusable. The main problem of very low-
frequency tags is the need for an antenna that is wound around a ferrite core. This
is an expensive and bulky component but is necessary to receive enough power at
the very low frequencies used.
The application of RFID is often limited by the physical constraints of the
radio environment. Metal is generally opaque to radio signals and can also
interfere with field patterns. Tag antennae are often bulky and need to be carefully
positioned. Interference, in common with all other radio systems, is another
concern especially if signals are being deliberately intercepted for criminal
purposes or jamming is used to deny service.
Another limitation on tagging technology in general is the issue of personal
privacy. People are increasingly reluctant to trust organizations with personal
information and the automatic nature of tag reading does nothing to help people
have confidence that the information is being used in their best interests. It is
likely that this problem will increase unless the tag industry and corporate
operators of systems inject enough transparency into operations to give the public
confidence. A more general Whereness service, where users can see exactly what
is known about their activities and by whom, might lead to a reduction in their
concerns. An even better situation may result if they are also allowed to edit their
own tag profiles.
6.3
Summary
This chapter was about radio-based positioning and started with a brief
introduction to radio theory to help explain how the main three ways to find
position operate and what radio parameters are important. First, radio can be used
to find distance (lateration) by timing signals. Pulse flight time is the most
common method and it is used in GPS, radar, and many other systems. The other
approach compares the phase of radio frequency carriers. Timing signals between
three links allow position to be found at the intersection of three imaginary
spheres centered on the transmitter. With phase measurements, points of equal
phase are hyperbolae hence the term hyperbolic navigation, which is used to
describe systems such as LORAN.
The second technique (angulation) uses directional antennae to find the angles
between the transmitters and receivers and is widely used in radar and radio
imaging. The third approach, and the one used in many current cellular radios,
WiFi hotspot and ubiquitous computing applications, is using simple proximity.
