Global Positioning System Reference
In-Depth Information
false programming, they will tend to block each other's signals. Only one IR
transmitter can be used unambiguously. Optical fiber communications gets around
this problem by a wavelength division multiplexing (WDM) where each signal is
effectively a different “color” of IR but signals are still received incoherently.
However, by careful pointing and coding it is rarely a problem. True coherent
optical radio is unlikely so IR will remain a cheap, crude, but very useful
positioning tool.
IR transmitters are light-emitting diodes (LEDs) where the two layers of
semiconductors used in the device are chosen to emit IR light when a small
electrical current is passed. They have relatively high bandwidths and can thus be
pulsed on and off very quickly (in the MHz region). IR optics can be used to focus
narrow beams, and large parallel arrays of diodes can generate bright (but
invisible) illumination of scenes. This is a common feature on smaller surveillance
cameras that are sensitive to near IR as well as visible light.
There are three approaches to detecting IR. The first method is using a simple
IR detector diode that is made to pass more current or generate a voltage when
illuminated by an IR beam. The second method is to use a camera sensitive to IR
where a large array of semiconductor detectors, fabricated as an integrated circuit,
has an image focused by a lens. The third approach is passive, using a passive IR
detector (PIR) where far infrared (i.e., heat radiation) is detected by special
material whose surface reacts to changes in intensity. Intruder alarms use this
technology to detect warm bodies and some sophisticated military cameras also
integrate the materials with more conventional silicon chips. In all cases the
presence or absence of IR radiation can be used to sense the presence or absence
of objects, people, and animals in a specific place. Positioning is thus by
proximity with accuracy being determined by the physical geometry of the
system. Overall ranges are low (tens of meters) and are used mostly indoors.
7.2.1
Indoor Positioning
A citywide outdoor ITS experiment that used IR beacons was described in
Sections 2.2.1 and 4.2. The beacons were placed at road junctions and vehicles at
the junctions positioned themselves by proximity to the IR beams using simple IR
detector diodes. By modulating the IR intensity, data was passed from the beacons
to the vehicles indicating which beacon was being used and also included vector
map fragments of routes to other junctions. (Section 8.3 explains the significance
of vector mapping.) In between beacons, a combination of magnetic compass
readings, odometer readings, and map matching was used. It is interesting to note
that the positioning and navigational side of this experiment performed well
without the use of any radio technology. Performance was enhanced by using a
large array of IR LEDs mounted behind a lens, which was part of an additional
“black” lamp mounted on the conventional three lamp traffic signals.
The first large-scale indoor IR positioning experiment was performed by
Want and Hopper [1] with the Active Badge System in 1992 and was aimed,
