Global Positioning System Reference
In-Depth Information
on the advance of radio during the 20th century, in a way to which, more recently,
similarly motivated people have pushed forward the Internet, the Web, and even
more recently open mapping (covered in Chapter 8).
For up-to-date background information on digital wireless and mobility, the
BT Technology Journal is an excellent resource that strikes a good balance
between the needs of business people and those more technically inclined. Several
special editions are relevant to wireless and Whereness, as follows. “Localisation
and Personalization” [3] covers location-based services, locating calls for the
emergency services, cellular positioning, and a general overview of positioning
and personalization. “Telecoms Unplugged” [4] covers topics such as wireless
LANs, ultrawideband, 3G and beyond, spectrum, and mobile multimedia. More
recently “Mobility and Convergence” [5] covers some digital wireless
fundamentals, ad hoc wireless, trust, convergence, various Internet topics, and
WiMax.
The words wireless and radio seem interchangeable but this can lead to
confusion. Optical communications are wireless but not usually thought of as a
radio methodology. Radio is usually wireless except that the radio frequency (RF)
electrical signals often pass along wires within equipment. Optical signals (i.e.,
infrared, visible light, and ultraviolet rays), and radio are all forms of
electromagnetic radiation and are physically the same. We describe them as waves
traveling at light speed (300m m/s) in a vacuum or somewhat less through any
other transparent medium. The higher the frequency, the shorter the wavelength,
which is measured in meters for domestic radio, millimeters for microwaves, but
nanometers (billionths of meters) for optics, as shown in Figure 6.1.
Although some areas of physics require that radiation be described as
particles (photons), for the purposes of positioning technology we can avoid that
complication and consider waves alone. Waves normally propagate through a
medium but the concept of the ether was abandoned as unnecessary. Essentially,
radio waves are rapidly changing electrical and magnetic fields that react
magnetically and electrically with materials, in particular with good conductors
(i.e., metals that are usually used to construct antennae). A wave will start to
propagate when a changing electrical or magnetic disturbance is set up in a
transmitting antenna and will continue in a uniform direction in a vacuum without
loss of intensity or power. If the wave encounters any material, it will react by
inducing a similar disturbance to that which caused it. Antennae are made to
resonate at specific wavelengths and to interface efficiently with electronic
equipment so that as much power as possible is directed in the required direction.
Positioning technologies that depend on the directionality of propagation therefore
depend heavily on good antennae.
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