Global Positioning System Reference
In-Depth Information
that are also very cheap and simple. Radio has always been a difficult technology
and the scarce radio spectrum has led to exacting developments. In contrast, many
sensors are single devices connected to the simplest of processors.
The research area of ubiquitous computing was introduced in Section 1.4.6,
and it is important to remember that most networked artifacts of the future (which
some would say would include almost everything) will include three aspects:
computing, wireless communications, and sensing. Recently, a number of new
consumer devices such as the Apple iPhone and the Nintendo Wii game machines
have drawn attention by their ability to sense the position of the user's hand
holding the unit. Robotics is also an advancing area of technology that is
pioneering the use of position sensing (see Section 1.4.13).
The next chapter will discuss the importance of maps but it should be
remembered that until very recently when satellite positioning became available,
cartographers used sensing techniques to map the world. Optical observations of
horizontal and vertical angles with theodolites and distances with tapes, chains, or
distance wheels were taken laboriously in the field.
Sensing movement using mechanical methods and using inertial navigation
do not rely on any external phenomena and are wholly autonomous, which is why
the techniques were developed for military purposes where external signals are a
source of vulnerability. Sophistication can vary enormously from the simple
vehicle odometer or pedestrian pedometer to the optical gyroscopes used in
aerospace and missile systems.
A final class of techniques not involving radio concerns systems associated
with ICT and telephony, where the physical connection to a wired network or its
use is an indication of location. Also, the use of information that is, a priori,
entered in personal calendars and diaries that can then be confirmed, a posteriori,
by network use. The advantage of using basic ICT components for Whereness is
that very few extra investments are needed in infrastructure since most of the
intelligence is based on software.
7.2
Infrared Systems
Infrared (IR) is optical but uses light waves that are longer in wavelength than the
visible spectrum (visible in a rainbow). The longest visible light is at the red end
of the spectrum at around 700nm and the shortest radio wavelength is about 1mm.
Infrared is the portion of the spectrum in between. Most radio signals are created
coherently, which means that the signals at any frequency have continuous phase.
The optical equivalent is lasers but optical receivers are incoherent. They involve
sensing the presence of energy regardless of its format, whereas most radio
receivers involve a coherent demodulation process. This is the reason why optical
systems are cheap and easy but the downside is that spectrally, they are
inefficient. For example, if several IR remote controllers are used at the same time
in a room to control TV and HiFi, although the codes transmitted will prevent
